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+This eBook, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #64227 (https://www.gutenberg.org/ebooks/64227)
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-The Project Gutenberg eBook of The Evolution Theory, Vol. 1 of 2, by August
-Weismann
-
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org. If you are not located in the United States, you
-will have to check the laws of the country where you are located before
-using this eBook.
-
-Title: The Evolution Theory, Vol. 1 of 2
-
-Author: August Weismann
-
-Translator: J. Arthur Thomson
-            Margaret R. Thomson
-
-Release Date: January 06, 2021 [eBook #64227]
-
-Language: English
-
-Character set encoding: UTF-8
-
-Produced by: Constanze Hofmann, Alan, Marilynda Fraser-Cunliffe and the
-             Online Distributed Proofreading Team at https://www.pgdp.net
-             (This book was produced from images made available by the
-             HathiTrust Digital Library and The Internet Archive.)
-
-*** START OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION THEORY, VOL. 1 OF
-2 ***
-
-
-
-
-                                  THE
-                           EVOLUTION THEORY
-
-                               VOLUME I
-
-
-
-
-                                  THE
-                           EVOLUTION THEORY
-
-                                  BY
-
-                          DR. AUGUST WEISMANN
-    PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU
-
-               TRANSLATED WITH THE AUTHOR'S CO-OPERATION
-
-                                  BY
-
-                           J. ARTHUR THOMSON
-   REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN
-
-                                  AND
-
-                          MARGARET R. THOMSON
-
-                              ILLUSTRATED
-
-                            IN TWO VOLUMES
-
-                                VOL. I
-
-                                LONDON
-                             EDWARD ARNOLD
-                41 & 43 MADDOX STREET, BOND STREET, W.
-
-                                 1904
-
-                         _All rights reserved_
-
-
-
-
-AUTHOR'S PREFACE
-
-
-WHEN a life of pleasant labour is drawing towards a close, the wish
-naturally asserts itself to gather together the main results, and to
-combine them in a well-defined and harmonious picture which may be left
-as a legacy to succeeding generations.
-
-This wish has been my main motive in the publication of these lectures,
-which I delivered in the University of Freiburg in Breisgau. But
-there has been an additional motive in the fact that the theory of
-heredity published by me a decade ago has given rise not only to many
-investigations prompted by it, but also to a whole literature of
-'refutations,' and, what is much better, has brought to light a mass of
-new facts which, at first sight at least, seem to contradict my main
-theory. As I remain as convinced that the essential part of my theory
-is well grounded as I was when I first sketched it, I naturally wish to
-show how the new facts may be brought into harmony with it.
-
-It is by no means only with the theory of heredity by itself that
-I am concerned, for that has served, so to speak, as a means to a
-higher end, as a groundwork on which to base an interpretation of
-the transformations of life through the course of the ages. For the
-phenomena of heredity, like all the functions of individual life, stand
-in the closest association with the whole evolution of life upon our
-earth; indeed, they form its roots, the nutritive basis from which
-all its innumerable branches and twigs are, in the long run, derived.
-Thus the phenomena of the individual life, and especially those of
-reproduction and inheritance, must be considered in connexion with the
-Theory of Descent, that the latter may be illumined by them, and so
-brought nearer our understanding.
-
-I make this attempt to sum up and present as a harmonious whole the
-theories which for forty years I have been gradually building up on
-the basis of the legacy of the great workers of the past, and on the
-results of my own investigations and those of many fellow workers, not
-because I regard the picture as complete or incapable of improvement,
-but because I believe its essential features to be correct, and because
-an eye-trouble which has hindered my work for many years makes it
-uncertain whether I shall have much more time and strength granted
-to me for its further elaboration. We are standing in the midst of
-a flood-tide of investigation, which is ceaselessly heaping up new
-facts bearing upon the problem of evolution. Every theory formulated
-at this time must be prepared shortly to find itself face to face with
-a mass of new facts which may necessitate its more or less complete
-reconstruction. How much or how little of it may remain, in face of
-the facts of the future, it is impossible to predict. But this will
-be so for a long time, and it seems to me we must not on that account
-refrain from following out our convictions to the best of our ability
-and presenting them sharply and definitely, for it is only well-defined
-arguments which can be satisfactorily criticized, and can be improved
-if they are imperfect, or rejected if they are erroneous. In both these
-processes progress lies.
-
-This book consists of 'Lectures' which were given publicly at the
-university here. In my introductory lecture in 1867 I championed the
-Theory of Descent, which was then the subject of lively controversy,
-but it was not till seven years later that I gave, by way of
-experiment, a short summer course with a view to aiding in the
-dissemination of Darwin's views. Then very gradually my own studies and
-researches and those of others led me to add to the Darwinian edifice,
-and to attempt a further elaboration of it, and accordingly these
-'Lectures,' which were delivered almost regularly every year from 1880
-onwards, were gradually modified in accordance with the state of my
-knowledge at the time, so that they have been, I may say, a mirror of
-the course of my own intellectual evolution.
-
-In the last two decades of the nineteenth century much that is
-new has been introduced into biological science; Nägeli's idea of
-'idioplasm'--the substance which determines form; Roux's _Struggle of
-the Parts_, the recognition of a special hereditary substance, 'the
-germ-plasm,' its analysis into chromosomes, and its continuity from
-generation to generation; the potential immortality of unicellular
-organisms and of the germ-cells in contrast to the natural death
-of higher forms and 'bodies'; a deeper interpretation of mitotic
-nuclear division, the discovery of the centrosphere--the marvellous
-dividing apparatus of the cell--which at once allowed us to penetrate
-a whole stratum deeper into the unfathomable mine of microscopic
-vital structure; then the clearing up of our ideas in regard to
-fertilization, and the analysis of this into the two processes combined
-in it, reproduction and the mingling of the germ-plasms (Amphimixis);
-in connexion with this, the phenomena of maturation, first in the
-female and then in the male cell, and their significance as a reduction
-of the hereditary units:--all this and much more we have gained during
-this period. Finally, there is the refutation of the Lamarckian
-principle, and the consequent elaboration of the principle of selection
-by applying it to the hitherto closed region of the ultimate vital
-elements of the germ-plasm.
-
-The actual form of these lectures has developed as they were
-transcribed. But although the form is thus to some extent new, I have
-followed in the main the same train of thought as in the lectures
-of recent years. The lecture-form has been adhered to in the book,
-not merely because of the greater vividness of presentation which it
-implies, but for many other reasons, of which the greater freedom in
-the choice of material and the limiting of quotation to a minimum
-are not the least. That all polemics of a personal kind have thus
-been excluded will not injure the book, but it is by no means lacking
-in discussions of opinion, and will, therefore, I trust, contribute
-something towards the clearing up of disputed points.
-
-I have endeavoured to introduce as much of the researches and writings
-of others as possible without making the book heavy; but my aim has
-been to write a book to be read, not merely one to be referred to.
-
-If it be asked, finally, for whom the book is intended, I can hardly
-answer otherwise than 'For him whom it interests.' The lectures were
-delivered to an audience consisting for the most part of students of
-medicine and natural science, but including some from other faculties,
-and sometimes even some of my colleagues in other departments. In
-writing the book I have presupposed as little special knowledge as
-possible, and I venture to hope that any one who _reads_ the book and
-does not merely skim it, will be able without difficulty to enter into
-the abstruse questions treated of in the later lectures.
-
-It would be a great satisfaction to me if this book were to be
-the means of introducing my theoretical views more freely among
-investigators, and to this end I have elaborated special sections more
-fully than in the lectures. Notwithstanding much controversy, I still
-regard its fundamental features as correct, especially the assumption
-of 'controlling' vital units, the determinants, and their aggregation
-into 'ids'; but the determinant theory also implies germinal selection,
-and without it the whole idea of the guiding of the course of
-transformation of the forms of life, through selection which rejects
-the unfit and favours the more fit, is, to my mind, a mere torso, or a
-tree without roots.
-
-I only know of two prominent workers of our day who have given
-thorough-going adherence to my views: Emery in Bologna and J. Arthur
-Thomson in Aberdeen. But I still hope to be able to convince many
-others when the consistency and the far-reachingness of these ideas
-are better understood. In many details I may have made mistakes which
-the investigations of the future will correct, but as far as the basis
-of my theory is concerned I am confident: _the principle of selection
-does rule over all the categories of vital units_. It does not, indeed,
-create primary variations, but it determines the paths of evolution
-which these are to follow, and thus controls all differentiation, all
-ascent of organization, and ultimately the whole course of organic
-evolution on the earth, for everything about living beings depends upon
-adaptation, though not on adaptation in the sense in which Darwin used
-the word.
-
-The great prominence thus given to the idea of selection has been
-condemned as one-sided and exaggerated, but the physicist is quite as
-open to the same reproach when he thinks of gravity as operative not on
-our earth alone, but as dominating the whole cosmos, whether visible
-to us or not. If there is gravity at all it must prevail everywhere,
-that is, wherever material masses exist; and in the same way the
-co-operation of certain conditions with certain primary vital forces
-must call forth the same process of selection wherever living beings
-exist; thus not only are the vital units which we can perceive, such
-as individuals and cells, subject to selection, but those units the
-existence of which we can only deduce theoretically, because they are
-too minute for our microscopes, are subject to it likewise.
-
-This extension of the principle of selection to all grades of vital
-units is the characteristic feature of my theories; it is to this idea
-that these lectures lead, and it is this--in my own opinion--which
-gives this book its importance. This idea will endure even if
-everything else in the book should prove transient.
-
-Many may wonder, perhaps, why in the earlier lectures much that
-has long been known should be presented afresh, but I regard it as
-indispensable that the student who wishes to make up his own mind in
-regard to the selection-idea should not only be clear as to what it
-means theoretically, but should also form for himself a conception
-of its sphere of influence. Many prejudiced utterances in regard
-to 'Natural Selection' would never have been published if those
-responsible for them had known more of the facts; if they had had
-any idea of the inexhaustible wealth of phenomena which can only be
-interpreted in the light of this principle, in as far, that is, as
-we are able to give explanations of life at all. For this reason I
-have gone into the subject of colour-adaptations, and especially into
-that of mimicry, in great detail; I wished to give the reader a firm
-foundation of fact from which he could select what suited him when
-he wished to test by the light of facts the more difficult problems
-discussed in the book.
-
-In conclusion, I wish to thank all those who have given me assistance
-in one way or other in this work: my former assistant and friend
-Professor V. Häcker in Stuttgart, my pupils and fellow workers Dr.
-Gunther and Dr. Petrunkewitsch, and the publisher, who has met my
-wishes in the most amiable manner.
-
-  FREIBURG-I-BR.,
-  _February 20, 1902_.
-
-
-
-
-PREFATORY NOTE TO ENGLISH EDITION
-
-
-PROFESSOR WEISMANN'S _Evolution Theory_, here translated from the
-second German edition (1904), is a work of compelling interest, the
-fruit of a lifetime of observation and reflection, a veteran's judicial
-summing up of his results, and certainly one of the most important
-contributions to Evolution literature since Darwin's day.
-
-As the author's preface indicates, the salient features of his crowning
-work are (1) the illumination of the Evolution process with a wealth
-of fresh illustrations; (2) the vindication of the 'Germ-plasm'
-concept as a valuable working hypothesis; (3) the final abandonment
-of any assumption of transmissible acquired characters; (4) a further
-analysis of the nature and origin of variations; and (5), above all, an
-extension of the Selection principle of Darwin and Wallace, which finds
-its logical outcome in the suggestive theory of Germinal Selection.
-
-The translation will be welcomed, we believe, not only by biological
-experts who have followed the development of 'Weismannism' during the
-last twenty years, and will here find its full expression for the time
-being, but also by those who, while acquainted with individual essays,
-have not hitherto realized the author's complete system. Apart from
-the theoretical conceptions which unify the book and mark it as an
-original contribution of great value, there is a lucid exposition of
-recent biological advances which will appeal to those who care more
-for facts than theories. To critics of evolutionism, who are still
-happily with us, the book ought to be indispensable; it will afford
-them much material for argumentation, and should save them many tilts
-against windmills. But, above all, the book will be valued by workers
-in many departments of Biology, who are trying to help in the evolution
-of Evolution Theory, for it is characteristic of the author, as the
-history of recent research shows, to be suggestive and stimulating,
-claiming no finality for his conclusions, but urging us to test them in
-a mood of 'thätige Skepsis.'
-
-The translation of this book--the burden of which has been borne
-by my wife--has been a pleasure, but it has also been a serious
-responsibility. We have had fine examples set us by previous
-translators of some of Weismann's works, Meldola, Poulton, Shipley,
-Parker, and others; and if we have fallen short of their achievements,
-it has not been for lack of endeavour to follow the original with
-fidelity, nor for lack of encouragement on the part of the author, who
-revised every page and suggested many emendations.
-
-  J. ARTHUR THOMSON.
-
-  UNIVERSITY OF ABERDEEN,
-  _October, 1904_.
-
-
-
-
-CONTENTS
-
-
-         LECTURE                                                   PAGE
-
-      I. INTRODUCTORY                                                 1
-
-     II. THE DARWINIAN THEORY                                        25
-
-    III. THE DARWINIAN THEORY (_continued_)                          42
-
-     IV. THE COLORATION OF ANIMALS AND ITS RELATION TO THE
-         PROCESSES OF SELECTION                                      57
-
-      V. TRUE MIMICRY                                                91
-
-     VI. PROTECTIVE ADAPTATIONS IN PLANTS                           119
-
-    VII. CARNIVOROUS PLANTS                                         132
-
-   VIII. THE INSTINCTS OF ANIMALS                                   141
-
-     IX. ORGANIC PARTNERSHIPS OR SYMBIOSIS                          161
-
-      X. THE ORIGIN OF FLOWERS                                      179
-
-     XI. SEXUAL SELECTION                                           210
-
-    XII. INTRA-SELECTION OR SELECTION AMONG TISSUES                 240
-
-   XIII. REPRODUCTION IN UNICELLULAR ORGANISMS                      253
-
-    XIV. REPRODUCTION BY GERM-CELLS                                 266
-
-     XV. THE PROCESS OF FERTILIZATION                               286
-
-    XVI. FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS
-         AND ITS IMMEDIATE SIGNIFICANCE                             312
-
-   XVII. THE GERM-PLASM THEORY                                      345
-
-  XVIII. THE GERM-PLASM THEORY (_continued_)                        373
-
-    XIX. THE GERM-PLASM THEORY (_continued_)                        392
-
-
-
-
-LIST OF ILLUSTRATIONS
-
-
-   FIGURE                                                          PAGE
-
-   1. Group of various races of domestic pigeons                     35
-
-   2. Longitudinally striped caterpillar of a Satyrid                67
-
-   3. Full-grown caterpillar of the Eyed Hawk-moth
-      (_Smerinthus ocellatus_)                                       67
-
-   4. Full-grown caterpillar of the Elephant Hawk-moth (_Chærocampa
-      elpenor_)                                                      68
-
-   5. The Eyed Hawk-moth in its 'terrifying attitude'                69
-
-   6. Under surface of the wings of _Caligo_                         70
-
-   7. Caterpillar of a North American _Darapsa_                      71
-
-   8. Caterpillar of the Buckthorn Hawk-moth
-      (_Deilephila hippophaës_)                                      73
-
-   9. _Hebomoja glaucippe_, from India; under surface                76
-
-  10. _Xylina vetusta_, in flight and at rest                        77
-
-  11. _Tropidoderus childreni_, in flying pose                       79
-
-  12. _Notodonta camelina_, in flight and at rest                    80
-
-  13. _Kallima paralecta_, from India, right under side of the
-       butterfly at rest                                        83, 357
-
-  14. _Cœnophlebia archidona_, from Bolivia, in its resting attitude 85
-
-  15. _Cærois chorinæus_, from the lower Amazon, in its resting
-       attitude                                                      86
-
-  16. _Phyllodes ornata_, from Assam                                 87
-
-  17. Caterpillar of _Selenia tetralunaria_, seated on a birch
-      twig                                                      90, 360
-
-  18. Upper surfaces of _Acræa egina_, _Papilio ridleyanus_, and
-      _Pseudacræa boisduvalii_                                      102
-
-  19. Barbed bristles of _Opuntia rafinesquii_                      123
-
-  20. Vertical section through a piece of a leaf of the
-      Stinging-nettle (_Urtica dioica_)                             123
-
-  21. A piece of a twig of Barberry (_Berberis vulgaris_)           124
-
-  22. Tragacanth (_Astragalus tragacantha_)                         125
-
-  23. Bladderwort (_Utricularia grafiana_)                          133
-
-  24. Pitcher of _Nepenthes villosa_                                134
-
-  25. Butterwort (_Pinguicula vulgaris_)                            136
-
-  26. The Sundew (_Drosera rotundifolia_)                           137
-
-  27. A leaf of the Sundew                                          137
-
-  28. Leaf of Venus Fly-trap                                        138
-
-  29. _Aldrovandia vesiculosa_                                      138
-
-  30. _Aldrovandia_, its trap apparatus                             139
-
-  31. Sea-cucumber (_Cucumaria_)                                    148
-
-  32. Metamorphosis of _Sitaris humeralis_, an oil-beetle           150
-
-  33. Cocoon of the Emperor Moth (_Saturnia carpini_)               158
-
-  34. Hermit-crab                                                   163
-
-  35. _Hydra viridis_, the Green Freshwater Polyp                   169
-
-  36. _Amœba viridis_                                               170
-
-  37. Twig of an Imbauba-tree, showing hair cushions                172
-
-  38. A fragment of a Lichen                                        173
-
-  39. A fragment of a Silver Poplar root                            176
-
-  40. _Potentilla verna_                                            181
-
-  41. Flower of Meadow Sage                                         183
-
-  42. Alpine Lousewort (_Pedicularis asplenifolia_)                 184
-
-  43. Flower of Birthwort (_Aristolochia clematitis_)               185
-
-  44. Alpine Butterwort (_Pinguicula alpina_)                       185
-
-  45. _Daphne mezereum_ and _Daphne striata_                        187
-
-  46. Common Orchis (_Orchis mascula_)                              188
-
-  47. Head of a Butterfly                                           190
-
-  48. Mouth-parts of the Cockroach                                  191
-
-  49. Head of the Bee                                               192
-
-  50. Flowers of the Willow                                         194
-
-  51. The Yucca-moth (_Pronuba yuccasella_)                         201
-
-  52. The fertilization of the Yucca                                202
-
-  53. Scent-scales of diurnal Butterflies                           217
-
-  54. A portion of the upper surface of the wing of a male 'blue'
-    (_Lycæna menalcas_)                                             218
-
-  55. _Zeuxidia wallacei_, male                                     218
-
-  56. _Leptodora hyalina_                                           224
-
-  57. _Moina paradoxa_, male                                        225
-
-  58. _Moina paradoxa_, female                                      226
-
-  59. An Amœba: the process of division                             253
-
-  60. _Stentor rœselii_, trumpet-animalcule                         254
-
-  61. _Holophrya multifiliis_                                       256
-
-  62. _Pandorina morum_                                             257
-
-  63. _Volvox aureus_                                               270
-
-  64. _Fucus platycarpus_, brown sea-wrack                          272
-
-  65. Copulation in a Daphnid (Lyncæid)                             276
-
-  66. Spermatozoa of various Daphnids                               277
-
-  67. Spermatozoa of various animals                                278
-
-  68. Diagram of a spermatozoon                                279, 338
-
-  69. Ovum of the Sea-urchin                                   281, 338
-
-  70. _Daphnella_                                                   283
-
-  71. _Bythotrephes longimanus_                                     283
-
-  72. _Sida crystallina_, a Daphnid                                 284
-
-  73. Diagrammatic longitudinal section of a hen's egg before
-      incubation                                                    285
-
-  74. Diagram of nuclear division                                   288
-
-  75. Process of fertilization in _Ascaris megalocephala_           296
-
-  76. Diagram of the maturation divisions of the ovum               299
-
-  77. Diagram of the maturation divisions of the sperm-cell         301
-
-  78. Diagram of the maturation of a parthenogenetic ovum           305
-
-  79. The two maturation divisions of the 'drone eggs'
-      of the Bee                                               307, 337
-
-  80. Fertilization of the ovum of a Gasteropod                     310
-
-  81. Formation of polar bodies in a Lichen                         313
-
-  82. Fertilization in the Lily                                     314
-
-  83. Conjugation of Noctiluca                                      317
-
-  84. Conjugation and polar body formation in the Sun-animalcule    319
-
-  85. Diagram of the conjugation of an Infusorian                   321
-
-  86. Conjugation of an Infusorian                                  323
-
-  87. Diagram to illustrate the operation of amphimixis             348
-
-  88. Sperm-mother-cells (spermatocytes) of the Salamander          350
-
-  89. Anterior region of the larva of a Midge                  364, 393
-
-  90. The Common Shore-Crab, seen from below                        367
-
-  91. Hind leg of a Locustid                                        371
-
-  92. Echinoderm-larvæ                                              387
-
-  93. Development of a limb in the pupa of a Fly                    395
-
-  94. Diagram to illustrate the phylogenetic shifting back of
-      the origins of the germ-cells in medusoids and hydroids       412
-
-  95. Diagram to illustrate the migration of the germ-cells
-      in Hydromedusæ                                                414
-
-
-  COLOURED PLATES
-
-  SOME MIMETIC BUTTERFLIES AND THEIR IMMUNE MODELS
-
-  PLATE I       _to face page_      112
-
-  PLATE II            "     "       114
-
-  PLATE III           "     "       116
-
-
-
-
-LECTURE I
-
-INTRODUCTORY
-
-
-EVERY one knows in a general way what is meant by the doctrine of
-descent--that it is the theory which maintains that the forms of
-life, animals and plants, which we see on our earth to-day, have not
-been the same from all time, but have been developed, by a process
-of transformation, from others of an earlier age, and are in fact
-descended from ancestors specifically different. According to this
-doctrine of descent, the whole diversity of animals and plants owes
-its origin to a transformation process, in the course of which the
-earliest inhabitants of our earth, extremely simple forms of life,
-were in part evolved in the course of time into forms of continually
-increasing complexity of structure and efficiency of function, somewhat
-in the same way as we can see every day, when any higher animal is
-developed from a single cell, the egg-cell, not suddenly or directly,
-but connected with its origin by a long series of ever more complex
-transformation stages, each of which is the preparation for, and leads
-on to the succeeding one. The theory of descent is thus a theory of
-development or evolution. It does not merely, as earlier science did,
-take for granted and describe existing forms of life, but regards them
-as having become what they are through a process of evolution, and it
-seeks to investigate the stages of this process, and to discover the
-impelling forces that lie behind it. Briefly, the theory of descent is
-an attempt at a scientific interpretation of the origin and diversity
-of the animate world.
-
-In these lectures, therefore, we have not merely to show on what
-grounds we make this postulate of an evolution process, and to marshall
-the facts which necessitate it; we must also try to penetrate as far
-as possible towards the causes which bring about such transformations.
-For this reason we are forced to go beyond the limits of the theory of
-descent in the narrow sense, and to deal with the general processes of
-life itself, especially with reproduction and the closely associated
-problem of heredity. The transformation of species can only be
-interpreted in one of two ways; either it depends on a peculiar
-internal force, which is usually only latent in the organism, but from
-time to time becomes active, and then, to a certain extent, moulds it
-into new forms; or it depends on the continually operating forces which
-make up life, and on the way in which these are influenced by changing
-external conditions. Which of these alternatives is correct we can only
-undertake to determine when we know the phenomena of life, and as far
-as possible their causes, so that it is indispensable to make ourselves
-acquainted with these as far as we can.
-
-When we look at one of the lowest forms of life, such as an Amœba or
-a single-celled Alga, and reflect that, according to the theory of
-evolution, the whole realm of creation as we see it now, with Man at
-its head, has evolved from similar or perhaps even smaller and simpler
-organisms, it seems at first sight a monstrous assumption, and one
-which quite contradicts our simplest and most certain observations. For
-what is more certain than that the animals and plants around us remain
-the same, as long as we can observe them, not through the lifetime of
-an individual only, but through centuries, and in the case of many
-species, for several thousand years?
-
-This being so, it is intelligible enough that the doctrine of
-evolution, on its first emergence at the end of the eighteenth century,
-was received with violent opposition, not on the part of the laity
-only, but by the majority of scientific minds, and instead of being
-followed up, was at first opposed, then neglected, and finally totally
-forgotten, to spring up anew in our own day. But even then a host of
-antagonists ranged themselves against the doctrine, and, not content
-with loftily ignoring it, made it the subject of the most violent and
-varied attacks.
-
-This was the state of affairs when, in 1858, Darwin's book on _The
-Origin of Species_ appeared, and hoisted the flag of evolution afresh.
-The struggle that ensued may now be regarded as at an end, at least
-as far as we are concerned--that is, in the domain of science. The
-doctrine of descent has gained the day, and we can confidently say
-that the Evolution theory has become a permanent possession of science
-that can never again be taken away. It forms the foundation of all our
-theories of the organic world, and all further progress must start from
-this basis.
-
-In the course of these lectures, we shall find at every step fresh
-evidence of the truth of this assertion, which may at first seem all
-too bold. It is not by any means to be supposed that the whole question
-in regard to the transformation of organisms and the succession of
-new forms of life has been answered in full, or that we have now been
-fortunate enough to solve the riddle of life itself. No! whether we
-ever reach that goal or not, we are a long way from it as yet, and
-even the much easier problem, how and by what forces the evolution of
-the living world has proceeded from a given beginning, is far from
-being finally settled; antagonistic views are still in conflict, and
-there is no arbitrator whose authoritative word can decide which is
-right. The _How?_ of evolution is still doubtful, but not the _fact_,
-and this is the secure foundation on which we stand to-day: The world
-of life, as we know it, has been evolved, and did not originate all at
-once.
-
-Were I to try to give, in advance, even an approximate idea of the
-confidence with which we can take our stand on this foundation, I
-should be almost embarrassed by the wealth of facts on which I might
-draw. It is hardly possible nowadays to open a book on the minute or
-general structural relations, or on the development of any animal
-whatever, without finding in it evidences in favour of the Evolution
-theory, that is to say, facts which can only be understood on the
-assumption of the evolution of the organic world. This, too, without
-taking into account at all the continually increasing number of facts
-Palæontology is bringing to light, placing before our eyes the forms
-which the Evolution theory postulates as the ancestors of the organic
-world of to-day: birds with teeth in their bills, reptile-like forms
-clothed with feathers, and numerous other long-extinct forms of life,
-which, covered up by the mud of earlier waters, and preserved as
-'fossils' in the later sedimentary rocks, tell us plainly how the
-earlier world of animals and plants was constituted. Later, we shall
-see that the geographical distribution of plant and animal species of
-the present day can only be understood in the light of the Evolution
-theory. But meantime, before we go into details, what may justify my
-assumption is the fact that the Evolution theory enables us to predict.
-
-Let us take only a few examples. The skeleton of the wrist in all
-vertebrate animals above Fishes consists of two rows of small bones, on
-the outer of which are placed the five bones of the palm, corresponding
-to the five fingers. The outer row is curved, and there is thus a space
-between the two rows, which, in Amphibians and Reptiles, is filled by
-a special small bone. This 'os centrale' is absent in many Mammals,
-notably, for instance, in Man, and the space between the two rows is
-filled up by an enlargement of one of the other bones. Now if Mammals
-be descended from the lower vertebrates, as the theory of descent
-assumes, we should expect to find the 'os centrale' even in Man in
-young stages, and, after many unsuccessful attempts, Rosenberg has at
-last been able to demonstrate it at a very early stage of embryonic
-development.
-
-This prediction, with another to be explained later, is based upon the
-experience that the development of an individual animal follows, in a
-general way, the same course as the racial evolution of the species,
-so that structures of the ancestors of a species, even if they are
-not found in the fully developed animal, may occur in one of its
-earlier embryonic stages. Further on, we shall come to know this fact
-more intimately as a 'biogenetic law,' and it alone would be almost
-enough to justify the theory of evolution. Thus, for instance, the
-lowest vertebrates, the Fishes, breathe by means of gills, and these
-breathing organs are supported by four or more gill-arches, between
-which spaces, the gill-slits, remain open for the passage of water.
-Although Reptiles, Birds, and Mammals breathe by lungs, and at no time
-of their life by gills, yet, in their earliest youth, that is, during
-their early development in the egg, they possess these gill-arches and
-gill-slits, which subsequently disappear, or are transformed into other
-structures.
-
-On the strength of this 'biogenetic law' it could also be predicted
-that Man, in whom, as is well known, there are twelve pairs of ribs,
-would, in his earliest youth, possess a thirteenth pair, for the lower
-Mammals have more numerous ribs, and even our nearest relatives, the
-anthropoid Apes, the gorilla and chimpanzee, have a thirteenth rib,
-though a very small one, and the siamang has even a fourteenth. This
-prediction also has been verified by the examination of young human
-embryos, in which a small thirteenth rib is present, though it rapidly
-disappears.
-
-During the seventies I was engaged in investigating the development
-of the curious marking which adorns the long body of many of our
-caterpillars. I studied in particular the caterpillars of our Sphingidæ
-or hawk-moths, and found, by a comparison of the various stages of
-development from the emergence of the caterpillar from the egg on to
-its full growth, that there is a definite succession of different
-kinds of markings following each other, in a whole range of species,
-in a similar manner. From the standpoint of the Evolution theory,
-I concluded that the markings of the youngest caterpillars, simple
-longitudinal stripes, must have been those of the most remote ancestors
-of our present species, while those of the later stages, oblique
-stripes, were those of ancestors of a later date.
-
-If this were the case, then all the species of caterpillar which
-now exhibit oblique stripes in their full-grown stage must have had
-longitudinal stripes in their youngest stages, and because of this
-succession of markings in the individual development, I was able to
-predict that the then unknown young form of the caterpillar of our
-privet hawk-moth (_Sphinx ligustri_) must have a white line along each
-side of the back. Ten years later, the English zoologist, Poulton,
-succeeded in rearing the eggs of _Sphinx ligustri_, and it was then
-demonstrated that the young caterpillar actually possessed the
-postulated white lines.
-
-Such predictions undoubtedly give the hypothesis on which they are
-based, the Evolution theory, a high degree of certainty, and are almost
-comparable to the prediction of the discovery of the planet Neptune
-by Leverrier. As is well known, this, the most distant of all the
-planets, whose period of revolution round the sun is almost 165 of
-our years, would probably never have been recognized as a planet, had
-not Adams, an astronomer at the Greenwich Observatory, and afterwards
-Leverrier, deduced its presence from slight disturbances in the path of
-Jupiter's moons, and indicated the spot where an unknown planet must be
-looked for. Immediately all telescopes were directed towards the spot
-indicated, and Galle, at the Berlin Observatory, found the sought-for
-planet.
-
-We might with justice regard as lacking in discernment those who, in
-the face of such experiences, still doubt that the earth revolves round
-the sun, and we might fairly say the same of any one who, in the face
-of the known facts, would dispute the truth of the Evolution theory. It
-is the only basis on which an understanding of these facts is possible,
-just as the Kant-Laplace theory of the solar system is the only basis
-on which an adequate interpretation of the facts of the heavens can be
-arrived at.
-
-To this comparison of the two theories it has been objected that the
-Evolution theory has far less validity than the other, first, because
-it can never be mathematically demonstrated, and secondly, because
-at the best it can only interpret the transformations of the animate
-world, and not its origin. Both objections are just: the phenomena
-of life are in their nature much too intricate for mathematics to
-deal with, except with extreme diffidence; and the question of the
-origin of life is a problem which will probably have to wait long for
-solution. So, if it gives pleasure to any one to regard the one theory
-as having more validity than the other, no one can object; but there
-is no particular advantage to be gained by doing so. In any case, the
-Evolution theory shares the disadvantage of not being able to explain
-everything in its own province with the Kant-Laplace cosmogony, for
-that, too, must presuppose the first beginning, the rotating nebula.
-
-Although I regard the doctrine of descent as proved, and hold it to be
-one of the greatest acquisitions of human knowledge, I must repeat
-that I do not mean to say that everything is clear in regard to the
-evolution of the living world. On the contrary, I believe that we
-still stand merely on the threshold of investigation, and that our
-insight into the mighty process of evolution, which has brought about
-the endless diversity of life upon our earth, is still very incomplete
-in relation to what may yet be found out, and that, instead of being
-vainglorious, our attitude should be one of modesty. We may well
-rejoice over the great step forward which the dominant recognition of
-the Evolution theory implies, but we must confess that the beginnings
-of life are as little clear to us as those of the solar system. But
-we can do this at least: we can refer the innumerable and wonderful
-inter-relations of the organic cosmos to their causes--common descent
-and adaptation--and we can try to discover the ways and means which
-have co-operated to bring the organic world to the state in which we
-know it.
-
-When I say that the theory of descent is the most progressive step that
-has yet been taken in the development of human knowledge, I am bound
-to give my reasons for this opinion. It is justified, it seems to me,
-even by this fact alone, that the Evolution idea is not merely a new
-light on the special region of biological science, zoology and botany,
-but is of quite general importance. The conception of an evolution of
-the world of life upon the earth reaches far beyond the bounds of any
-single science, and influences our whole realm of thought. It means
-nothing less than the elimination of the miraculous from our knowledge
-of nature, and the placing of the phenomena of life on the same plane
-as the other natural processes, that is, as having been brought about
-by the same forces, and being subject to the same laws. In the domain
-of the inorganic, no one now doubts that out of nothing nothing can
-come: energy and matter are from everlasting to everlasting, they can
-neither be increased or decreased, they can only be transformed--heat
-into mechanical energy, into light, into electricity, and so on. For
-us moderns, the lightning is no longer hurled by the Thunderer Zeus
-on the head of the wicked, but, careless alike of merit or guilt, it
-strikes where the electric tension finds the easiest and shortest line
-of discharge. Thus to our mode of thought it now seems clear that no
-event in the world of the living depends upon caprice, that at no
-time have organisms been called forth out of nothing by the mighty
-word of a Creator, but they have been produced at all times by the
-co-operation of the existing forces of nature, and every species must
-have arisen just where, and when, and in the form in which it actually
-did arise, as the necessary outcome of the existing conditions of
-energy and matter, and of their interactions upon each other. It is
-this correlation of animate nature with natural forces and natural laws
-which gives to the doctrine of evolution its most general importance.
-For it thus supplies the keystone in the arch of our interpretation
-of nature and gives it unity; for the first time it makes it possible
-to form a conception of a world-mechanism, in which each stage is the
-result of the one before it, and the cause of the succeeding one.
-
-How deeply all our earlier opinions are affected by this doctrine will
-become clear if we fix our attention on a single point, the derivation
-of the human understanding from that of animal ancestors. What of the
-reason of Man, of his morals, of his freedom of will? may be asked,
-as it has been, and still is often asked. What has been regarded as
-absolutely distinct from the nature of animals is said to differ from
-their mental activities only in degree, and to have evolved from them.
-The mind of a Kant, of a Laplace, of a Darwin--or to ascend into
-the plane of the highest and finest emotional life, the genius of a
-Raphael or a Mozart--to have any real connexion, however far back,
-with the lowly psychical life of an animal! That is contrary to all
-our traditionary, we might say our inborn, ideas, and it is not to be
-wondered at that the laity, and especially the more cultured among
-them, should have opposed such a doctrine whose dominating power was
-unintelligible to them, because they were ignorant of the facts on
-which it rests. That a man should feel his dignity lowered by the
-idea of descent from animals is almost comical to the naturalist,
-for he knows that every one of us, in his first beginning, occupied
-a much lowlier position than that of our mammalian ancestors--was,
-in fact, as regards visible structure, on a level with the Amœba,
-that microscopically minute unicellular animal, which can hardly be
-said to possess organs, and whose psychical activities are limited to
-recognizing and engulfing its food. Very gradually at first, and step
-by step, there develop from this single cell, the ovum, more and more
-numerous cells; this mass of cells segregates into different groups,
-which differentiate further and further, until at last they form the
-perfect man. This occurs in the development of every human being, and
-we are merely unaccustomed to the thought that it means nothing else
-than an incredibly rapid ascent of the organism from a very low level
-of life to the highest.
-
-Still less is it to be wondered at that the Evolution doctrine met with
-violent opposition on the part of the representatives of religion,
-for it stood in open contradiction to that remarkable and venerable
-cosmogony, the Mosaic story of Creation, which people had been
-accustomed to regard, not as what it is--a conception of nature at an
-early stage of human culture--but as an inalienable part of our own
-religion. But investigation shows us that the doctrine of evolution is
-true, and it is only a weak religion which is incapable of adapting
-itself to the truth, retaining what is essential, and letting go what
-is unessential and subject to change with the development of the
-human mind. Even the heliocentric hypothesis was in its day declared
-false by the Church, and Galilei was forced to retract; but the earth
-continued to revolve round the sun, and nowadays any one who doubted
-it would be considered mentally weak or warped. So in all likelihood
-the time is not far distant when the champions of religion will abandon
-their profitless struggle against the new truth, and will see that the
-recognition of a law-governed evolution of the organic world is no more
-prejudicial to true religion than is the revolution of the earth round
-the sun.
-
-       *       *       *       *       *
-
-Having given this very general orientation of the Evolution problem,
-which is to engage our attention in detail, I shall approach the
-problem itself by the historical method, for I do not wish to bring
-the views of present-day science quite suddenly and directly into
-prominence. I would rather seek first to illustrate how earlier
-generations have tried to solve the question of the origin of the
-living world. We shall see that few attempts at solution were made
-until quite recently, that is, until the end of the eighteenth and the
-beginning of the nineteenth century. Only then there appeared a few
-gifted naturalists with evolutionist ideas, but these ideas did not
-penetrate far; and it was not till after the middle of the nineteenth
-century that they found a new champion, who was to make them common
-property and a permanent possession of science. It was the teaching of
-Charles Darwin that brought about this thorough awakening, and laid
-the foundations of our present interpretations, and his work will
-therefore engross our attention for a number of lectures. Only after
-we have made ourselves acquainted with his teaching shall we try to
-test its foundations, and to see how far this splendid structure stands
-on a secure basis of fact, and how deeply its power of interpretation
-penetrates towards the roots of phenomena. We shall examine the forces
-by which organisms are dominated, and the phenomena produced, and
-thereby test Darwin's principles of interpretation, in part rejecting
-them, in part accepting them, though in a much extended form, and thus
-try to give the whole theoretic structure a more secure foundation.
-I hope to be able to show that we have made some real progress since
-Darwin's day, that deductions have been drawn from his theory which
-even he did not dream of, which have thrown fresh light on a vast range
-of phenomena, and, finally, that through the more extended use of his
-own principles, the Evolution theory has gained a completeness, and an
-intrinsic harmony which it previously lacked.
-
-This at least is my own opinion, but I cannot ignore the fact that it
-is by no means shared by all living naturalists. The obvious gaps and
-insufficiencies of the Darwinian theory have in the last few decennia
-prompted all sorts of attempts at improving it. Some of these were lost
-sight of almost as soon as they were suggested, but others have held
-their own, and can still claim numerous supporters. It would only tend
-to bewilder if I gave an account of those of the former class, but
-those which still hold their own must be noticed in these lectures,
-though it is by no means my intention to expound the confused mass of
-opinions which has gathered round the doctrine of evolution, but rather
-to give a presentation of the theory as it has gradually grown up in my
-own mind in the course of the last four decades. Even this will not be
-the last of which science will take knowledge, but it will, I hope, at
-least be one which can be further built upon.
-
-Let us, then, begin at once with that earliest forerunner of the modern
-theory of descent, the gifted Greek philosopher Empedocles, who,
-equally important as a leader of the state of Agrigentum, and as a
-thinker in purely theoretical regions of thought, advanced very notable
-views regarding the origin of organisms. We must, however, be prepared
-to hear something that is hardly a theory in the modern scientific
-acceptation of that term; and we must not be repelled by the unbridled
-poetical fancy of the speculative philosopher; we have to recognize
-that there is a sound kernel contained in his amusing pictures--a
-thought which we meet with later, in much more concrete form, in the
-Darwinian theory, and which, if I mistake not, we shall keep firm hold
-of in all time to come.
-
-According to Empedocles the world was formed by the four elements of
-the ancients, Earth, Water, Fire, and Air, moved and guided by two
-fundamental forces, Hate and Love, or, as we should now say, Repulsion
-and Attraction. Through the chance play of these two forces with the
-elements, there arose first the plants, then the animals, in such a
-manner that at first only parts and organs of animals were formed:
-single eyes without faces, arms without bodies, and so on. Then, in
-wild play, Nature attempted to put together these separate parts, and
-so created all manner of combinations, for the most part inept monsters
-unfit for life, but in a few cases, where the parts fitted, there
-resulted a creature capable not only of life, but, if the juxtaposition
-was perfect, even of reproduction.
-
-This phantastic picture of creation seems to us mad enough, but there
-slumbers in it, all unsuspected though it may have been by the author,
-the true idea of selection, the idea that much that is unfit certainly
-arises, but that only the fit endures. The mechanical coming-to-be of
-the fit is the sound kernel in this wondersome doctrine.
-
-The natural science of the ancients, in regard to life and its forms,
-reached its climax in Aristotle (died 322 B. C.). A true polyhistorian,
-his writings comprehended all the knowledge of his time, but he also
-added much to it from his own observation. In his writings we find many
-good observations on the structure and habits of a number of organisms,
-and he also had the merit of being the first to attempt a systematic
-grouping of animals. With true insight, he grouped all the vertebrates
-together as Enaimata or animals with blood, and classed all the rest
-together as Anaimata or bloodless animals. That he denied to the latter
-group the possession of blood is not to be wondered at, when we take
-into account the extremely imperfect means of investigation available
-in his time, nor is it surprising that he should have ranked this
-motley company, in antithesis to the blood-possessing animals, as a
-unified and equivalent group. Two thousand years later, Lamarck did
-exactly the same thing, when he divided the animals into backboned and
-backboneless, and we reckon this nowadays as a merit only in so far
-that he was the first, after Aristotle, to re-express the solidarity of
-the classes of animals which we now call vertebrates.
-
-Aristotle was, however, not a systematic zoologist in our sense of the
-term, as indeed was hardly possible, considering the very small number
-of animal forms that were known in his time. In our day we have before
-us descriptions of nearly 300,000 named species wherefrom to construct
-our classification, while Aristotle knew hardly more than 200. Of the
-whole world of microscopic animals he could, of course, have no idea,
-any more than of the remains of prehistoric animals, of which we now
-know about 40,000 named and adequately described species. One would
-have thought that it would have occurred to a quick-witted people like
-the Greeks to pause and ponder when they found mussel-shells and marine
-snail-shells on the hills far above the sea; but they explained these
-by the great flood in the time of Deucalion and Pyrrha, and they did
-not observe that the fossil molluscs were of different species from the
-similar animals living in the sea in their own day.
-
-Thus there was nothing to suggest to Aristotle and others of his time
-the idea that a transformation of species had been going on through the
-ages, and even the centuries after him evoked no such idea, nor did
-there arise new speculations, after the manner of Empedocles, in regard
-to the origin of the organic world. On the whole, the knowledge of the
-living world retrograded rather than advanced until the beginning of
-the Roman Empire. What Aristotle had known was forgotten, and Pliny's
-work on animals is a catalogue embellished with numerous fables,
-arranged according to a purely external principle of division. Pliny
-divided animals into those belonging to earth, water, and air, which is
-not very much more scientific than if he had arranged them according to
-the letters of the alphabet.
-
-During the time of the Roman Empire, as is well known, the knowledge of
-natural history sank lower and lower; there was no more investigation
-of nature, and even the physicians lost all scientific basis, and
-practised only in accordance with their traditional esoteric secrets.
-As the whole culture of the West gradually disappeared, the knowledge
-of nature possessed by earlier centuries was also completely lost,
-and in the first half of the Middle Ages Europeans revealed a depth
-of ignorance of the natural objects lying about them, which it is
-difficult for us now to form any conception of.
-
-Christianity was in part responsible for this, because it regarded
-natural science as a product of heathendom, and therefore felt bound
-to look coldly on it, if not even to oppose it. Later, however, even
-the Christian Church felt itself forced to give the people some mental
-nourishment in the form of natural history, and under its influence,
-perhaps actually composed by teachers of the Church, there appeared a
-little book, the so-called _Physiologus_, which was meant to instruct
-the people in regard to the animal world. This remarkable work, which
-has been preserved, must have had a very wide distribution in the
-earlier Middle Ages, for it was translated into no fewer than twelve
-languages, Greek, Armenian, Syriac, Arabic, Ethiopic, and so on. The
-contents are very remarkable, and come from the most diverse sources,
-that is, from the most different writers of antiquity, from Herodotus,
-from the Bible, and so forth, but never from original observation. The
-compilation does not really give descriptions of animals or of their
-habits, but, of each of the forty-one animals which the _Physiologus_
-recognizes, something remarkable is briefly related in true lapidary
-style, sometimes a mere curiosity without further import, or sometimes
-a symbolical interpretation. Thus the book says of the panther: 'he
-is gaily coloured; after satiating himself he sleeps three days, and
-awakes roaring, giving forth such an agreeable odour that all animals
-come to him.' Of the pelican the well-known legend is related, that
-it tears open its own breast to feed its young with its blood, thus
-standing as a symbol of mother-love. Fabulous creatures, too, appear
-in these pages. Of the Phœnix, that bird whose plumage glitters with
-gold and precious stones, which was known even to Herodotus, and which
-has survived through Eastern fairy-tales on to the time of our own
-romanticists (Tieck), we read: 'it lives a thousand years, because it
-has not eaten of the tree of knowledge'; then 'it sets fire to itself
-and arises anew from its own ashes,' a symbol of nature's infinite
-power of renewing its youth.
-
-But while among the peoples of Europe all the science of the ancients
-was lost, except a few barely recognizable fragments, the old lore was
-preserved, both as regards organic nature and other orders of facts,
-among the Arabs, through whom so many treasures of antiquity have
-eventually been handed down to us, coming in the track of the Arabian
-conquests across North Africa and Spain to the nations of Europe.
-
-It was in this way, too, that the writings of Aristotle again found
-recognition, after having been translated into Latin at Palermo at
-the order of that enthusiast for Science and Art, the Hohenstaufen
-Emperor, Frederick the Second. Our Emperor presented one copy of
-Aristotle's writings to the University of Bologna, and thus the
-wisdom of the ancient Greeks again became the common property of
-European culture. From the thirteenth century to the eighteenth, the
-study of natural science was limited to repeating and extending the
-work of Aristotle. Nothing new, depending upon personal observation,
-was added, and it does not even seem to have occurred to any one to
-subject the statements of the Stagirite to any test, even when they
-concerned the most familiar objects. No one noticed the error which
-ascribed to the fly eight legs instead of six; there was in fact as
-yet no investigation, and all knowledge of natural history was purely
-scholastic, and gave absolute credence to the authority of the ancients.
-
-A revulsion, however, occurred in the century of the Reformation, with
-the breaking down of the blind belief in authority which had till
-then prevailed in all provinces of human knowledge and thought. After
-a long and severe struggle, dry scholasticism was finally overcome,
-and natural science, with the rest, turned from a mere reliance on
-books to original thinking and personal observation. Thenceforward
-interpretations of natural processes were sought for no longer in the
-writings of the ancients, but in Nature herself. Of the magnitude
-of this emancipation, and of the severity of the struggle against
-deep-rooted authority, one could form a faint idea from experience
-even in my own youth. Our young minds were so deeply imbued with the
-involuntary feeling that the ancients were superior to us moderns
-in each and every respect, that not only the hardly re-attainable
-plastic art of the Greeks and the immortal songs of Homer, but all the
-mental products of antiquity seemed to us models which could never be
-equalled; the tragedies of Sophocles were for us the greatest tragedies
-that the world had ever seen, the odes of Horace the most beautiful
-poems of all time!
-
-In the domain of natural science the new era began with the overthrow
-of the Ptolemaic cosmogony, which, for more than a thousand years,
-had served as a basis for astronomy. When the German canon, Nicolas
-Copernicus (born at Thorn, 1473, died 1543), reversed the old
-theory, and showed that the sun did not revolve round the earth, but
-the earth round the sun, the ice was broken and the way paved for
-further progress. Galilei uttered his famous 'e pur si muove,' Kepler
-established his three laws of the movements of the planets, and Newton,
-a century later, interpreted their courses in terms of the law of
-gravitation.
-
-But we have not here to do with a history of physics or astronomy, and
-I only wish to recall these well-known facts, in order that we may
-see how increased knowledge in this domain was always accompanied by
-advances in that of biology.
-
-Here, however, we cannot yet chronicle any such thoroughgoing
-revolution of general conceptions; the basis of detailed empirical
-knowledge was not nearly broad enough for that, and it was in the
-acquiring of such a foundation that the next three centuries, from the
-sixteenth to the end of the eighteenth, were eagerly occupied.
-
-The first step necessary was to collate the items of individual
-knowledge in regard to the various forms of life, and to bring the
-whole in unified form into general notice. This need was met for the
-first time by Conrad Gessner's _Thierbuch_, a handsome folio volume,
-printed at Zurich in 1551, and embellished with numerous woodcuts,
-some of them very good. This was followed, in 1600, by a great work in
-many volumes, written in Latin, by a professor of Bologna, Aldrovandi.
-Not native animals alone but foreign ones also were described in these
-works, for, after the discovery of America and the opening up of
-communication with the East Indies, many new animal and plant forms
-came to the knowledge of European nations by way of the sea. Thus
-Francesco Hernandez (died 1600), physician in ordinary to Philip II,
-described no fewer than forty new Mammals, more than two hundred
-Birds, and many other American animals.
-
-Again, in a quite different way, the naturalist's field of vision
-was widened, namely, by the invention of the simple microscope, with
-which Leeuwenhoek first discovered the new world of Infusorians,
-and Swammerdam made his notable observations on the structure and
-development of the very varied minute animal inhabitants of fresh
-water. In the same century, the seventeenth, anatomists like Tulpius,
-Malpighi, and many others extended the knowledge of the internal
-structure of the higher animals and of Man, and a foundation was
-laid for a deeper insight into the nature of vital functions by the
-discovery of the circulation of the blood in Man and the higher
-animals. In the following century, the eighteenth, this path of active
-research was eagerly followed, and we need only mention such names as
-Réaumur, Rösel von Rosenhof, De Geer, Bonnet, J. Chr. Schäfer, and
-Ledermüller, to be immediately reminded of the wealth of facts about
-the structure, life, and especially the development of our indigenous
-animals, which we owe to the labours of these men.
-
-       *       *       *       *       *
-
-All these advances, great and many-sided as they were, did not at once
-lead to a renewal of the attempt of Empedocles to explain the origin
-of the organic world. This was as yet not even recognized as a problem
-requiring investigation, for men were content to take the world of
-life simply as a fact. The idea of getting beyond the naïve, poetic
-standpoint of the Mosaic story of Creation was as yet remote from the
-minds of naturalists, partly because they were wholly fascinated by
-the observation of masses of details, but chiefly because, first by
-the English physician, John Ray (died 1678), then by the great Swede,
-Carl Linné, the conception of organic 'species' had been formulated and
-sharply defined. It is true enough that before the works of these two
-men 'species' had been spoken of, but without being connected with any
-definite idea; the word was used rather in the same vague sense as the
-word 'genus,' to designate one of the smaller groups of organic forms,
-but without implying any clear idea of its scope or of its limitations.
-Now, however, for the first time, the term 'species' came to be used
-strictly to mean the smallest homogeneous group of individual forms
-of life upon the earth. John Ray held that the surest indication of a
-'species' was that its members had been produced from the same seed;
-that is, 'forms which are of different species maintain this specific
-nature constantly, and one species does not arise from the seed of
-another.' Here we have the germ of the doctrine of the absolute nature
-and the immutability of species which Linné briefly characterized in
-these words: 'Species tot sunt, quot formæ ab initio creatæ sunt,'
-'there are just so many species as there were forms created in the
-beginning.' It is here clearly implied, that species as we know them
-have been as they are from all time, that, therefore, they exist in
-nature as such and unchangeably, and have not been merely read into
-nature by man.
-
-This view, though we cannot now regard it as correct, was undoubtedly
-reasonable, and thoroughly in accordance with the spirit of the time;
-it was congruent with the knowledge, and above all with the scientific
-endeavours of the age. In the eighteenth century there was danger that
-all outlook on nature as a whole would be lost--smothered under the
-enormous mass of isolated facts, and especially under the inundation of
-diverse animal and plant forms which were continually being recognized.
-It must therefore have been regarded as a real deliverance, when Linné
-reduced this chaos of forms to a clearly ordered system, and relegated
-each form to its proper place and value in relation to the whole. How,
-indeed, could the great systematist have performed his task at all, if
-he had not been able to work with definite and sharply circumscribed
-groups of forms, if he had not been able to regard at least the lowest
-elements of his system, the species, as fixed and definite types?
-On the other hand, Linné was much too shrewd an observer not to
-entertain, in the course of his long life, and under the influence of
-the continually accumulating material, doubts as to the correctness
-of his assumption of the fixity and absoluteness of his species. He
-discovered from his own experience, what is fully borne out by ours,
-that it is easy enough to define a species when there are only a few
-specimens of a form to deal with, but that the difficulty increases
-in proportion to the number and to the diversity of habitat of those
-that are to be brought under one category. In the last edition of
-the _Systema Naturæ_ we find very noteworthy passages in which Linné
-wonders whether, after all, a species may not change, and in the course
-of time diverge into varieties, and so forth. Of these doubts no notice
-was taken at the time; the accepted doctrine of the fixity of species
-was held to and even raised to the rank of a scientific dogma. Georges
-Cuvier, the great disciple of the Stuttgart 'Karlschule,' accentuated
-the doctrine still further by his establishment of animal-types, the
-largest groups of forms in the animal kingdom within which a definite
-and fundamentally distinct plan of architecture prevails. His four
-types, Vertebrates, Molluscs, Articulate and Radiate animals, furnished
-a further corroboration of the absolute nature of species, since they
-seemed to show that even the highest and most comprehensive groups are
-sharply defined off from one another.
-
-Let me add that this doctrine of the absolute nature of species was not
-fully elaborated till our own day, when the Swiss (afterwards American)
-naturalist, Louis Agassiz, went so far as to maintain that not only
-the highest and the lowest categories, but all those coming between
-them, were categories established and sharply separated by Nature
-herself. But in spite of much ingenuity and his wide and comprehensive
-outlook he exerted himself in vain to find satisfactory and really
-characteristic definitions of what was to be considered a class, an
-order, a family, or a genus. He did not succeed in finding a rational
-definition of these systematic concepts, and his endeavour may be
-regarded as the last important attempt to prop up an interpretation of
-nature already doomed to fall. But in referring to Louis Agassiz I have
-anticipated the historical course of scientific development, and must
-therefore go back to the last quarter of the eighteenth century.
-
-The first unmistakable pioneer of the theory of descent, which now
-emerged for the first time as a scientific doctrine, was our great
-poet Goethe. He has indeed been often named as the founder of the
-theory, but that seems to me saying too much. It is true, however, that
-the inquiring mind of the poet certainly recognized in the structure
-of 'related' animals the marvellous general resemblances amid all
-the differences in detail, and he probed for the reason of these
-form-relations. Through the science of 'comparative anatomy,' as it
-was taught at the close of the century by Kielmeyer, Cuvier's teacher,
-and later by Cuvier himself, Blumenbach, and others, numerous facts
-had become known, which paved the way for such questions. It had, for
-instance, been recognized that the arm of man, the wing of the bird,
-the paddle of the seal, and even the foreleg of the horse, contain
-essentially the same chain of bones, and Goethe had already expressed
-these relations in his well-known verse,
-
-  'Alle Gestalten sind ähnlich, doch keine gleichet der andern,
-  Und so deutet der Chor auf ein geheimes Gesetz.'
-
-As to what this law was he did not at that time pronounce an opinion,
-though he may even then have thought of the transformation of species.
-At first he contented himself with seeking for an ideal archetype or
-'Urtypus' which was supposed to lie at the foundation of a larger or
-smaller group. He discovered the archetypal plant or 'Urpflanze,'
-when he rightly recognized that the parts of the flower are nothing
-more than modified leaves. He spoke plainly of the 'metamorphosis of
-plants,' meaning by that the transformation of his 'archetype' into
-the endless diversity of actual plant forms. But at first he certainly
-thought of this transformation only in the ideal sense, and not as a
-factual evolutionary process.
-
-The first who definitely maintained the latter view was, remarkably
-enough, the grandfather of the man who, in our own day, made the theory
-of descent finally triumphant, the English physician Erasmus Darwin,
-born 1731. This quiet thinker published, in 1794, a book entitled
-_Zoonomia_, and in it he takes the important step of substituting for
-Goethe's 'secret law' a real relationship of species. He proclaims the
-gradual establishment and ennobling of the animal world, and bases
-his view mainly on the numerous obvious adaptations of the structure
-of an organ to its use. I have not been able to find any passage in
-the book in which he has expressly indicated that, because many of
-the conditions of life could not have existed from the beginning,
-these adaptations are therefore, as such, an argument for the gradual
-transformation of species. But he assumed that such exact adaptations
-to the functions of an organ could only arise through the exercise of
-that function, and in this he saw a proof of transformation. Goethe had
-expressed the same idea when he said, 'Thus the eagle has conformed
-itself through the air to the air, the mole through the earth to the
-earth, and the seal through the water to the water,' and this shows
-that he too at one time thought of an actual transformation. But
-neither he nor Erasmus Darwin were at all clear as to _how_ the use
-of an organ could bring about its variation and transformation. The
-latter only says that, for instance, the snout of the pig has become
-hard through its constant grubbing in the ground; the trunk of the
-elephant has acquired its great mobility through the perpetual use of
-it for all sorts of purposes; the tongue of the herbivore owes its
-hard, grater-like condition to the rubbing to and fro of the hard grass
-in the mouth, and so on. How acute and thoughtful an observer Erasmus
-Darwin was, is shown by the fact that he had correctly appreciated the
-biological significance of many of the colour-adaptations of animals
-to their surroundings, though it was reserved for his grandson to make
-this fully clear at a much later date. Thus he regarded the varied
-colouring of the python, of the leopard, and of the wild cat as the
-best adapted for concealing them from their prey amid the play of light
-and shadow in a leafy thicket. The black spot in front of the eye of
-the swan he considered an arrangement to prevent the bird from being
-dazzled, as would happen if that spot were as snow-white as the rest of
-the plumage.
-
-At the end of the book he sums up his views in the following
-sentences: 'The world has been evolved, not created; it has arisen
-little by little from a small beginning, and has increased through
-the activity of the elemental forces embodied in itself, and so has
-rather grown than suddenly come into being at an almighty word.' 'What
-a sublime idea of the infinite might of the great Architect! the Cause
-of all causes, the Father of all fathers, the Ens entium! For if we
-could compare the Infinite it would surely require a greater Infinite
-to cause the causes of effects than to produce the effects themselves.'
-
-In these words he sets forth his position in regard to religion, and
-does so in precisely the same terms as we may use to-day when we say:
-'All that happens in the world depends on the forces that prevail in
-it, and results according to law; but where these forces and their
-substratum, Matter, come from, we know not, and here we have room for
-faith.'
-
-I have not been able to discover whether the _Zoonomia_, with its
-revolutionary ideas, attracted much attention at the time when it
-appeared, but it would seem not. In any case, it was afterwards so
-absolutely forgotten, that in an otherwise very complete _History of
-Zoology_, published in 1872 by Victor Carus, it was not even mentioned.
-About a year after the appearance of _Zoonomia_, Isidore Geoffrey
-St.-Hilaire in Paris expounded the view that what are called species
-are really only 'degenerations,' deteriorations from one and the same
-type, which shows that he too had begun to have doubts as to the fixity
-of species. Yet it was not till the third decade of the nineteenth
-century that he clearly and definitely took up the position of the
-doctrine of transformation, and to this we shall have to return later
-on.
-
-But as early as the first decade of the century this position was taken
-up by two noteworthy naturalists, a German and a Frenchman, Treviranus
-and Lamarck.
-
-Gottfried Reinhold Treviranus, born at Bremen in 1776, an excellent
-observer and an ingenious investigator, published, in 1802, a book
-entitled _Biologie, oder Philosophie der lebenden Natur_ [_Biology, or
-Philosophy of Animate Nature_], in which he expresses and elaborates
-the idea of the Evolution theory with perfect clearness. We read
-there, for instance: 'In every living being there exists a capacity
-for endless diversity of form; each possesses the power of adapting
-its organization to the variations of the external world, and it is
-this power, called into activity by cosmic changes, which has enabled
-the simple zoophytes of the primitive world to climb to higher and
-higher stages of organization, and has brought endless variety into
-nature.' But where the motive power lies, which brings about these
-transformations from the lowliest to ever higher forms of life, was a
-question which Treviranus apparently did not venture to discuss. To do
-this, and thus to take the first step towards a causal explanation of
-the assumed transformations, was left for his successor.
-
-Jean Baptiste de Lamarck, born in 1744 in a village of Picardy, was
-first a soldier, then a botanist, and finally a zoologist. He won his
-scientific spurs first by his _Flora of France_, and zoology holds him
-in honour as the founder of the category of 'vertebrates.' Not that he
-occupied himself in particular detail with these, but he recognized the
-close alliance of the classes of animals in question--an alliance which
-was subsequently expressed by Cuvier by the systematic term 'type' or
-'embranchement.'
-
-In his _Philosophie zoologique_, published in 1809, Lamarck set
-forth a theory of evolution whose truth he attempted to vindicate by
-showing--as Treviranus had done before him--that the conception of
-species, on the immutability of which the whole hypothesis of creation
-had been based, was an artificial one, read into nature by us; that
-sharply circumscribed groups do not exist in nature at all; and that
-it is often very difficult, and not infrequently quite impossible, to
-define one species precisely from allied forms, because it is connected
-with these on all sides by transition stages. Groups of forms which
-thus melted into one another indicated that the doctrine of the fixity
-of species could not be correct, any more than that of their absolute
-nature. Species, he maintained, are not immutable, and are not so old
-as nature; they are fixed only for a certain time. The shortness of
-our life prevents our directly recognizing this. 'If we lived a much
-shorter time, say about a second, the hour-hand of the clock would
-appear to us to stand still, and even the combined observations of
-thirty generations would afford no decisive evidence as to the hand's
-movement, and yet it had been moving.'
-
-The causes on which, according to Lamarck, the transformation of
-species, their modification into new species, depends, lie in the
-changes in the conditions of life which must have occurred ceaselessly
-from the earliest period of the earth's history till our own day, now
-here, now there, due in part to changes in climate and in food-supply,
-in part to changes in the earth's crust by the rising or sinking of
-land-masses, and so forth. These external changes have sometimes been
-the _direct_ cause of changes in bodily structure, as in the case
-of heat or cold; but they have sometimes and much more effectively
-operated _indirectly_. Thus changed conditions may have prompted
-an animal of a given species to use certain parts of its body in a
-new way, more vigorously, or less actively, or even not at all, and
-the more vigorous use, or, conversely, the disuse, has brought about
-variations in the organ in question.
-
-Thus the whales lost their teeth when they abandoned their fish diet,
-and acquired the habit of feeding on minute and delicate molluscs,
-which they swallowed whole without seizure or mastication. Thus, too,
-the eyes of the mole degenerated through its life in the dark, and a
-still greater degeneration of the eyes has taken place in animals,
-like the proteus-salamander, which always inhabit lightless caves.
-In mussels both head and eyes degenerated because the animals could
-no longer use them after they became enclosed in opaque mantles and
-shells. In the same way snakes lost their legs _pari passu_ with the
-acquisition of the habit of moving along by wriggling their long
-bodies, and of creeping through narrow fissures and holes. On the
-other hand, Lamarck interpreted the evolution of the web-feet of
-swimming birds by supposing that some land-bird or other had formed the
-habit of going into the water to seek for food, and consequently of
-spreading out its toes as widely as possible so as to strike the water
-more vigorously. In this way the fold of skin between the toes was
-stretched, and as the extension of the toes was very frequent and was
-continued through many generations, the web expanded and grew larger,
-and thus formed the web-foot.
-
-In the same way the long legs of the wading birds have been, according
-to Lamarck, gradually evolved by the continual stretching of the limbs
-by wading in deeper and deeper water, and similarly for the long necks
-and bills of the waders, the herons and the storks. Finally we may
-mention the case of the giraffe, whose enormously long neck and tall
-forelegs are interpreted as due to the fact that the animal feeds on
-the foliage of trees, and was always stretching as far as possible, in
-order to reach the higher leaves.
-
-We shall see later in what a different way Charles Darwin explained
-this case of the giraffe. Lamarck's idea is at once clear; it is true
-that exercising an organ strengthens it, that disuse makes it weaker.
-Through much gymnastic exercise the muscles of the arm become thicker
-and more capable, and memory too may be improved, that is to say,
-even a definite part of the brain may be considerably strengthened
-by use. Indeed, we may now go so far as to admit that every organ is
-strengthened by use and weakened by disuse, and so far the foundations
-of Lamarck's interpretations are sound. But he presupposes something
-that cannot be admitted so readily, namely, that such 'functional'
-improvement or diminution in the strength of an organ can be
-transmitted by inheritance to the succeeding generation. We shall have
-to discuss this question in detail at a later stage, and I shall only
-say now that opinions as to whether this is possible or not are very
-much divided. I myself doubt this possibility, and therefore cannot
-admit the validity of the Lamarckian evolutionary principle in so far
-as it implies the directly transforming effect of the functioning of
-an organ. But even if we recognize the Lamarckian factor as a _vera
-causa_, it is easy to show that there are a great many characters
-which it is not in a position to interpret. Many insects which live
-upon green leaves are green, and not a few of them possess exactly the
-shade of green which marks the plant on which they feed; they are thus
-protected in a certain measure from injuries. But how could this green
-colour of the skin have been brought about by the activity of the skin,
-since the colour of the surroundings does not usually stimulate the
-skin to activity at all? Or how should a grasshopper, which is in the
-habit of sitting on dry branches of herbs, have thereby been incited
-to an activity which imparts to it the colour and shape of a dry twig?
-Just as little, or perhaps still less, can the protective green colour
-of a bird's or insect's eggs be explained through the direct influence
-of their usually green surroundings, even if we disregard the fact that
-the eggs are green when they are laid--that is, before the environment
-can have had any influence on them.
-
-The Lamarckian principle of modification through use does not, in
-any case, nearly suffice as an interpretation of the transformations
-of the organic world. It must be allowed that Lamarck's theory of
-transformation was well founded at the time when it was advanced; it
-not only attacked the doctrine of the immutability of species, but
-sought for the first time to indicate the forces and influences which
-must be operative in the transformations of species; it was therefore
-well worth careful testing. Nevertheless it did not divert science from
-its chosen path; very little notice was taken of it, and in the great
-Cuvier's chronicle of scientific publications for 1809, not a syllable
-is devoted to Lamarck's book, so strong was the power of prejudice.
-
-But, although the new doctrine was thus ignored, it did not altogether
-fall to the ground; it glimmered for a while in Germany, where it found
-its champions in the 'Naturphilosophie' of the time, and especially in
-Lorenz Oken, a peasant's son, born at Ortenau, near Offenburg, in 1783.
-
-Oken professed views similar to those of Erasmus Darwin, Treviranus,
-and Lamarck, though they were not clothed in such purely scientific
-garb, being, in fact, bound up with the general philosophical
-speculations which came increasingly into favour at that time, chiefly
-through the writings of Schelling. In the same year, 1809, in which
-Lamarck published his _Philosophie zoologique_, Oken's _Lehrbuch der
-Naturphilosophie_ appeared.
-
-This book is by no means simply a theory of descent; its scope is much
-wider, including the phenomena of the whole cosmos; on the other hand,
-it goes too little into details and is too indefinite to deserve its
-title. Its way of playing with ideas, its conjectures and inferences
-from a fanciful basis, make it difficult for us now to think ourselves
-into its mode of speculation, but I should like to give some indication
-of it, for it was just these speculative encroachments of the
-'categories' of the so-called 'Naturphilosophie' which played a fatal
-part in causing the temporary disappearance of the Evolution-theory
-from science, so that, later on, it had to be established anew.
-
-Oken defines natural science as 'the science of the everlasting
-transmutations of God (the Spirit) in the world': Every thing,
-considered in the light of the genetic process of the whole, includes,
-besides the idea of being, that of not-being, in that it is involved
-in a higher form. 'In these antitheses the category of polarity is
-included. The simpler elementary bodies unite into higher forms,
-which are thus merely repetitions at a potential higher than that of
-their causes. Thus the different genera of bodies form parallel and
-corresponding series, the reasonable arrangement of which results as an
-intrinsic necessity from their genetic connexion. In individuals these
-lowlier series make their appearance again during development. The
-contrasts in the solar system between planets and sun are repeated in
-plants and animals, and, as light is the principle of movement, animals
-have the power of independent movement in advance of the plants which
-belong to the earth.'
-
-Obviously enough, this is no longer the study of nature; it is
-nature-construction from a basis of guesses and analogies rather than
-of knowledge and facts. Light is the principle of motion, and as
-animals move, they correspond to the sun, and plants to the planets!
-Here there is not even a hint of a deepening of knowledge, and all
-these deductions now seem to us quite worthless.
-
-On the other hand, it must be allowed that good ideas are by no means
-absent from this 'philosophy,' nor can we deny to this restlessly
-industrious man a great mind always bent on discovering what was
-general and essential. Much of what we now _know_ he even then guessed
-at and taught, as, for instance, that the basis of all forms of life
-in this infinitely diverse world of organisms was one and the same
-substance--'primitive slime,' 'Urschleim' as he called it, or, as we
-should now say, 'protoplasm.' We can therefore, _mutatis mutandis_,
-agree with Oken when he says,'Everything organic has come from slime,
-and is nothing but diversely organized slime.' Many naturalists of the
-present day would go further, and agree with Oken when he suggests
-that 'this primitive slime has arisen in the sea, in the course of the
-planet's (the earth's) evolution out of inorganic material.'
-
-Thus Oken postulated, as the specific vehicle of life, a primitive
-substance, in essence at least homogeneous. But he went further, and
-maintained that his 'Urschleim' assumed _the form of vesicles_, of
-which the various organisms were composed. 'The organic world has as
-its basis an infinitude of such vesicles.' Who is not at once reminded
-of the now dominant _Cell-theory_? And, in fact, thirty years later,
-when the cell was discovered, Oken did claim priority for himself. In
-so doing, he obviously confused the formulating of a problem with the
-solving of it; he had, quite rightly, divined that organisms must be
-built up of very minute concentrations of the primitive substance, but
-he had never seen a cell, or proved the necessity for its existence, or
-even attempted to prove it. His vesicle-theory was a pure divination, a
-prevision of genius, but one which could not directly deepen knowledge;
-it did not prompt, or even hasten, the discovery of the cell. Here,
-as throughout in his natural philosophy, Oken built, not from beneath
-upwards, by first establishing facts and then drawing conclusions from
-them, but, inversely, he invented ideas and principles, and out of
-them reconstructed the world. In this he differs essentially from his
-predecessors Erasmus Darwin, Treviranus, and Lamarck, who all reasoned
-inductively, that is, from observed data.
-
-Thus the whole evolutionary movement was lost in indefiniteness;
-because men wanted to find a reason for everything, they missed
-even what might then have been explained. Moreover, the theory of
-evolution still lacked a sufficiently broad basis of facts; the
-'Naturphilosophie,' by its want of moderation, robbed it of all
-credit; and it is not to be wondered at that men soon ceased to occupy
-themselves with the problem of the evolution of the living world. A few
-indeed held fast to the doctrine of evolution during the first third
-of the century, but then it disappeared completely from the realm of
-science.
-
-Its last flicker of life was seen in France, in 1830, at the time of
-the July revolution, when the legitimate sovereignty of Charles X was
-overthrown. It is interesting to note the lively interest that Goethe,
-the first forerunner of the theory, and then aged eighty-one, had in
-the intellectual combat that took place in the French Academy between
-Cuvier and Isidore Geoffroy St.-Hilaire. A friend of Goethe's, Soret,
-relates that on August 2, 1830, he went into the poet's room, and was
-greeted with the words: 'Well, what do you think of this great event?
-The volcano is in eruption, and all is in flames. There can no longer
-be discussion with closed doors.' Soret replied: 'It is a terrible
-business! But what else was to be expected with things as they are,
-and with such a ministry, than that it should end in the expulsion
-of the reigning family?' To which Goethe answered: 'We don't seem to
-understand each other, my dear friend. I am not talking of these people
-at all; I am thinking of quite different affairs. I refer to the open
-rupture in the Academy between Cuvier and Geoffroy St.-Hilaire; it is
-of the utmost importance to science.'
-
-In this conflict of opinions, Cuvier opposed Geoffroy's conception of
-the unity of the plan of structure in all animals, confronting him with
-the four Cuvierian types, in each of which the plan of structure was
-altogether different, and strongly insisting on the doctrine of the
-fixity of species, which he maintained to be the necessary postulate of
-a scientific natural history.
-
-The victory fell to Cuvier, and it cannot be denied that there was much
-justification for his opinions at the time, for the knowledge of facts
-at that stage was not nearly comprehensive enough to give security to
-the Evolution theory, and moreover the quiet progress of science might
-have been hindered rather than furthered by premature generalization
-and theorizing. It had now been seen how far the interpretation of
-general biological problems could be carried with the available
-material; the 'Naturphilosophie' had not merely exploited it as far as
-possible, but had burdened it much beyond its carrying power, and the
-world was weary of insecure speculations. The 'Naturphilosophie' was
-for the time quite worked out, and a long period set in, during which
-all energies were devoted to detailed research.
-
-
-
-
-LECTURE II
-
-THE DARWINIAN THEORY
-
- Period of detailed research--Appearance of Darwin's _Origin
- of Species_--Darwin's life--Voyage round the world--His
- teaching--Domesticated animals, dog, horse--Pigeons--Artificial
- selection--Unconscious selection--Correlated variations.
-
-
-THE period of wholly unphilosophical, purely detailed research may be
-reckoned as from about 1830 to 1860, though, of course, many of the
-labours of the earlier part of the century must be counted among the
-investigations which were carried out without any reference to general
-questions, and even after 1860 numerous such works appeared. Nor could
-it be otherwise, for the basis of all science must be found in facts,
-and the thorough working up of the fact-material will always remain
-the first and most indispensable condition of our scientific progress.
-During the period referred to, however, it had become the sole end to
-be striven for; and all energies were concentrated exclusively on the
-accumulation of facts.
-
-The previous century had added much to the knowledge of the inner
-structure of animals, the so-called 'comparative anatomy,' and in
-the nineteenth century this line of investigation was pursued even
-more extensively and energetically, so that the knowledge increased
-enormously. Up till this time it was chiefly the structure of the
-backboned animals and of a few 'backboneless' animals, so called, that
-had been studied, but now all the lower groups of the animal kingdom
-were also investigated, and became known better and in more detail as
-the methods of research improved.
-
-Not content, however, with a knowledge of the adult animal, naturalists
-began to investigate its development. In the year 1814 the first
-great work on development appeared, on the development of the chick,
-by Pander and Von Baer. It was there shown for the first time, how
-the chick begins as a little disk-shaped membrane on the surface of
-the yolk of the egg, at first simply as a pale streak, the 'primitive
-streak,' then as a groove, the 'primitive groove,' by the side of which
-arise two folds, the 'medullary folds,' and further how a system of
-blood-vessels is developed around this primitive rudiment on the upper
-surface of the yolk, how a heart arises before the rest of the body is
-complete, and how the blood begins to circulate; in short, there was
-disclosed all the marvel of development to which we are now so much
-accustomed, that we can hardly understand the sensation it made at that
-time.
-
-Later on, attention was turned to the development of Fishes and
-Amphibians (Agassiz and Vogt, later Remak), then to that of the Worms
-(Bagge), of Insects (Kölliker), and gradually the development of
-all the groups of the animal-kingdom--from Sponges to Man--was so
-thoroughly investigated that it almost seems to-day as if there could
-not be much that is new to discover in this department. This impression
-may indeed be true as far as the less complex processes and the more
-obvious questions are concerned, but it is impossible to predict what
-new problems may confront us, whose solution will depend on a still
-more detailed study of development.
-
-As embryology is a science of the nineteenth century, so also is
-histology, the science of tissues. Its pioneer was Bichat, but its
-real foundations were not laid till Schwann and Schleiden formulated
-the conception of the 'cell,' and proved that all animals and plants
-were composed of cells. What Oken had only guessed at they now proved,
-that there are very minute form-elements of life which build up all the
-parts of animals and plants or produce them by processes of secretion.
-New light was thus shed on embryonic development, and this gradually
-led to the recognition of the fact that the egg, too, is a cell, and
-that development depends on a cell-division process in this egg-cell.
-This led further to the conception of many-celled and single-celled
-organisms, and so on to many items of knowledge to speak of which here
-would carry us too far.
-
-For it is not my intention to attempt a complete review of the
-development of biology in the nineteenth century, or even in the
-period which we have mentioned as devoted to detailed research; it
-is rather my desire to convey a general impression of the enormous
-extent and many-sidedness of the progress that was made in this time.
-Let us therefore briefly recall the entirely new facts which were
-brought to light in this period with regard to the reproduction of
-animals. Asexual reproduction by budding and division was already
-known, but parthenogenesis is a discovery of this period, and so also
-is alternation of generations, so far-reaching in its bearing on
-general problems. It was first observed (1819) by Chamisso in Salpa,
-then by Steenstrup in Medusæ and trematodes, and was later made fully
-clear in its most diverse forms and relations by the researches of
-Leuckart, Vogt, Kölliker, Gegenbaur, Agassiz, and other illustrious
-investigators. Reproduction by heterogony, too, which occurs in
-many crustaceans, and in aphides and certain worms, was recognized
-at that time, and in the sixties Carl Ernst von Baer added to the
-list precocious reproduction, or pædogenesis, which is illustrated in
-certain insects which reproduce in the larval state.
-
-This may suffice to convey some idea of the great mass of new, and in
-some cases startling facts previously unguessed at, which were then
-brought to light in the department of animal biology alone. To this
-must be added the vast increase in the number of known species and
-varieties, their distribution on the earth, and all this, _mutatis
-mutandis_, for plants also. Nor can we omit to mention the rapidly
-growing number of fossil species of animals and plants.
-
-Thus there gradually accumulated a new mass of material; investigation
-became more and more specialized, and the danger became imminent that
-workers in the various departments would be unable to understand
-each other, so completely were they independent of one another in
-their specialist researches. There was lack of any unifying bond, for
-workers had lost sight of the general problem in which all branches
-of the science meet, and through which alone they can be united into
-a general science of biology. The time had come for again combining
-and correlating the details, lest they should grow into an unconnected
-chaos, through which it would be impossible to find one's way, because
-no one could overlook it and grasp it as a whole. In a word, it was
-high time to return to general questions.
-
-       *       *       *       *       *
-
-Though I have called the period from 1830 to 1860 that of purely
-detailed research, I do not mean to ignore the fact that, during that
-time, there were a few feeble attempts to return to the great questions
-which had been raised at the beginning of the century. But the point
-is, that all such attempts remained unnoticed. Thus there appeared, in
-1844, a book entitled _Vestiges of the Natural History of Creation_,
-the anonymous author of which revealed himself much later as Robert
-Chambers, an Edinburgh publisher. In this book the evolution of species
-was ascribed to two powers, a power of transformation and a power of
-adaptation. Two Frenchmen, Naudin and Lecoq, also published a work in
-which the theory of evolution was set forth, and from 1852 to 1854 the
-well-known German anthropologist Schaafhausen was writing on similar
-lines. But all these calls sounded unheard, so deeply were naturalists
-plunged in detailed investigations, and it required a much mightier
-voice to command the ear of the scientific world.
-
-It is impossible to estimate the effect of Darwin's book on _The
-Origin of Species_, published in English in 1858, in German in 1859
-unless we fully realize how completely the biologists of that time
-had turned away from general problems. I can only say that we, who
-were then the younger men, studying in the fifties, had no idea that
-a theory of evolution had ever been put forward, for no one spoke of
-it to us, and it was never mentioned in a lecture. It seemed as if all
-the teachers in our universities had drunk of the waters of Lethe,
-and had utterly forgotten that such a theory had ever been discussed,
-or as if they were ashamed of these philosophical flights on the part
-of natural science, and wished to guard their students from similar
-deviations. The over-speculation of the 'Naturphilosophie' had left in
-their minds a deep antipathy to all far-reaching deductions, and, in
-their legitimate striving after purely inductive investigation, they
-forgot that the mere gathering of facts is not enough, that the drawing
-of conclusions is an essential part of the induction, and that a mass
-of bare facts, however enormous, does not constitute a science.
-
-One of my most stimulating teachers at that time, the gifted anatomist,
-Jacob Henle, had written as a motto under his picture, 'There is a
-virtue of renunciation, not in the province of morality alone, but
-in that of intellect as well,' a sentence which expressly indicated
-the desirability of refraining from all attempts to probe into the
-more general problems of life. Thus the young students of that time
-were nourished only on the results of detailed research, in part
-indeed interesting enough, but in part dry and, because uncorrelated,
-unintelligible in the higher sense, and only here and there awakening a
-deeper interest, when, as in physiology and in embryology, they formed
-a connected system in themselves. Without being fully clear as to what
-was lacking, we certainly missed the deeper correlation of the many
-separate disciplines.
-
-It is therefore not to be wondered that Darwin's book fell like a
-bolt from the blue; it was eagerly devoured, and while it excited in
-the minds of the younger students delight and enthusiasm, it aroused
-among the older naturalists anything from cool aversion to violent
-opposition. The world was as though thunderstruck, as we can readily
-see from the preface with which the excellent zoologist of Heidelberg,
-Bronn, introduced his translation of Darwin's book, where he asks this
-question among others, 'How will it be with you, dear reader, after you
-have read this book?' and so forth.
-
-But before I enter on a detailed examination of the contents of this
-epoch-making book, I should like to say a few words about the man
-himself, who thus revolutionized our thinking.
-
-Charles Darwin was born in 1809, the year of the publication of
-Lamarck's _Philosophie zoologique_, and of Oken's _Lehrbuch der
-Naturphilosophie_. There was thus a whole generation between the first
-emergence of the Evolution theory and its later revival. Darwin's
-father was a physician, and his education was not a regular one. In his
-youth he seems to have devoted much time and enthusiasm to hunting, and
-only very slowly to have taken up regular studies towards a definite
-end. In accordance with his father's wishes, he studied medicine for a
-time, but soon abandoned it to devote himself to botany and zoology.
-Before he had had time to distinguish himself in any special way in
-these subjects, he was offered, in his twenty-first year, the post of
-naturalist on an English war-ship which was to make a voyage round the
-world, and that at a leisurely rate.
-
-This was decisive not only for Darwin's immediate studies, but for
-the work of his life, for, as he tells us himself, it was during this
-voyage on the _Beagle_ that the idea of the Evolution theory first came
-to him. While the vessel made a stay at the Galapagos Islands, west
-of South America, he noticed that quite a number of little land-birds
-occurred there which closely resembled those of the neighbouring
-mainland, but yet were different from them. Almost every little island
-had its own species, and so he concluded that all these might be
-descended from representatives of a few species which had long before
-drifted over from the mainland to these volcanic islands, become
-established there, and in the course of time taken on the character of
-new species. The problem of the transformation of species opened up
-before him, and he made up his mind to follow up the idea after his
-return, in the hope that by a patient collecting of facts, he would by
-and by arrive at some security with regard to this great question.
-
-I need not linger over any detailed account of his travels; one can
-readily understand how a voyage round the world, lasting for five
-years, would offer to the inquiring mind of a Darwin rich opportunities
-for the most varied observations. That he did not fail to make use of
-these is evidenced not only by his book on _The Origin of Species_, but
-by several more special works, published shortly after his return--his
-natural history of those remarkable sessile crustaceans, the barnacles
-or Cirripedia, and his studies on the origin of coral reefs. The
-first-named book still holds its own as a classic monograph on this
-animal group, with its wealth of forms; and the theory of the origin of
-coral reefs which Darwin elaborated has still many adherents, in spite
-of various rival interpretations.
-
-But Darwin would hardly have achieved what he did if he had been
-compelled to secure for himself a professional position in order to
-obtain bread and butter. Such great problems demand not only the whole
-of a man's mental energy, they monopolize his time. Studies of detail
-may well be taken up in leisure hours, but big problems absorb all the
-thoughts and must always be present to the mind, lest the connexion
-between the many individual inquiries, which make up the whole task, be
-lost sight of. Darwin had the good fortune to be a free investigator,
-and to be able to retire, on his return from his travels, to a small
-property at Down in Kent, there to live for his family and his work.
-Here he followed up the idea of evolution which he had already
-formulated, and it has always seemed to me the most remarkable thing
-about him, that he was able to keep in mind and work up the hundreds
-of isolated inquiries that were eventually to be brought together to
-form the main fabric of his theory. When one studies his many later
-writings, one cannot but be surprised afresh by the number of different
-sets of facts he collected at the same time, partly from others,
-partly from personal observation, and continually also from his own
-experiments. He made experiments on plants and on animals, and the
-number of people with whom he carried on a scientific correspondence
-is simply astounding. In this way he brought together, in the course
-of twenty years, an extraordinarily rich material of facts, from the
-fullness of which he was able later to write his book on _The Origin
-of Species_. Never before had a theory of evolution been so thoroughly
-prepared for, and it is undoubtedly to this that it owed a great part
-of its success; not to this alone, however, but still more, if not
-mainly, to the fact that it presented a principle of interpretation
-that had never before been thought of, but whose importance was
-apparent as soon as attention was called to it--the principle of
-selection.
-
-Charles Darwin championed, in the main, the same fundamental ideas as
-had been promulgated by his grandfather, Erasmus Darwin, by Treviranus,
-and by Lamarck: species only seem to us immutable; in reality they can
-vary, and become transformed into other species, and the living world
-of our day has arisen through such transformations, through a sublime
-process of evolution which began with the lowest forms of life, but
-by degrees, in the course of unthinkably long ages, progressed to
-organisms more and more complex in structure, more and more effective
-in function.
-
-It is interesting to note at what point Darwin first put in his lever
-to attempt the solution of the problem of evolution. He started from
-quite a different point from the investigators of the early part of
-the century, for he began with forms of life which had previously
-been markedly neglected by science, the varieties of our domesticated
-animals and cultivated plants.
-
-Previously these had been in a sense mere step-children of biology,
-inconvenient existences which would not fit properly into the system,
-which were therefore as far as possible ignored or dismissed as outside
-the scope of 'the natural,' because it was difficult to know what else
-to do with them. I can quite well remember that, even as a boy, I was
-struck by the fact that one could find nothing in the systematic books
-about the many well-established garden forms of plants, or about our
-domestic animals, which seemed to be regarded as in a sense artificial
-products, and as such not worthy of scientific consideration. But it
-was in these that Darwin particularly interested himself, making them
-virtually the basis of his theory, for he led up from them to the very
-principle of transformation, which was his most important addition to
-the earlier presentations of the Evolution theory.
-
-He started from the existence of varieties which may be observed in
-so many wild species. His line of thought was somewhat as follows: If
-species have really arisen through a gradual process of transformation,
-then varieties must be regarded as possible first steps towards new
-species; if, therefore, we can only succeed in finding out the causes
-which underlie the formation of any varieties whatever, we shall have
-discovered the causes of the transformation of species. Now we find by
-far the greatest number of varieties, and the most marked ones, among
-our domesticated animals and plants, and unless we are to assume that
-each of these is descended from a special wild species, the reason why
-there has been such a wealth of variety-formation among them must lie
-in the conditions which influence the relevant species in the course of
-domestication; and it remains for us to analyse these conditions till
-we come upon the track of the operative factors. With this conviction,
-Darwin devoted himself to the study of domesticated animals and plants.
-
-The first essential was to prove that every variety had not a separate
-wild species as ancestor, but that the whole wealth of our domesticated
-breeds originated, in each case, from one, or at least from a few wild
-species. Of course I cannot here recapitulate the multitudinous facts
-which were marshalled by Darwin, especially in his later works, notably
-his _Animals and Plants under Domestication_, but this is not necessary
-to an understanding of his conclusions, and I shall therefore restrict
-myself to a few examples.
-
-Let us take first the domestic dog, _Canis familiaris_, Linné. We
-have at the present day no fewer than seven main breeds, each of
-which has its sub-breeds, often numerous. Thus there are forty-eight
-sub-breeds which are used as guardians of our houses, 'house-dogs'
-in the restricted sense, thirty sub-breeds of dogs with silk-like
-hair (King Charles dogs, Newfoundland dogs, &c.), twelve of terriers,
-and thirty-five of sporting dogs, among them such different forms as
-the deerhound and the pointer. We have further nineteen sub-breeds
-of bulldogs, thirty-five of greyhounds, and six of naked or hairless
-dogs. Not only the main breeds, but even the sub-breeds often differ
-as markedly from one another as wild species do, and the question must
-first be decided whether each of the very distinct breeds has not a
-special wild species as ancestor.
-
-Obviously, however, this cannot be maintained, for so many species of
-wild dog have never existed on the earth at any time. We know, too,
-that 4,000 or 5,000 years ago a large number of breeds of dogs were
-in existence in India and Egypt. There were Pariah dogs, coursers,
-greyhounds, mastiffs, house-dogs, lapdogs and terriers. It is not
-possible that the products of all lands could, at that time, have been
-gathered into one, and it is inconceivable that so many wild species
-could have existed in the one country of India.
-
-On the other hand, however, it cannot be maintained that all our
-present breeds have descended from _a single_ wild species; it is much
-more probable that several wild species were domesticated in different
-countries.
-
-It has often been supposed that the manifold diversity of our present
-breeds has been brought about by crossing the various tamed species.
-That cannot be the case, however, because crossing gives rise only to
-hybrid mongrel forms, not to distinct breeds with quite new characters.
-It is true that all breeds of dogs can be very readily crossed with
-each other, but the result is not new breeds, but those numberless
-and transient intermediate forms which the dog-breeder despises as
-worthless for his purpose. It must therefore have been through the
-influence of domestication, combined with crossing, that a few wild
-species gave rise to the various breeds of dogs.
-
-The pedigree of the horse is rather more clear than that of the dog.
-Even in this case, indeed, one cannot definitely name the ancestral
-wild form, but it is very probable that it was of a grey-brown colour,
-and similar to the wild horses of our own day. Darwin supposes that
-it must also have had the black stripe on the back which is exhibited
-by the domestic ass, and by several wild species of ass, basing his
-opinion on the fact that the spinal stripe often occurs in foals,
-especially in those of a grey-brown colour.
-
-But though there can be no doubt that this is to be interpreted as a
-reversion to a character of a remote ancestor, it by no means follows
-that the _direct_ ancestral form must have had this stripe. I am
-more inclined to believe that the ancestor which bore this mark was
-considerably more remote, and lived before the differentiation of the
-horse from the ass. Darwin himself noted the remarkable fact that in
-rare cases, especially in foals, not only may the stripe on the back be
-present, but there may be more or less distinct zebra-striping on the
-legs and withers: this, however, must be interpreted as a reversion to
-the character of a very much more remote ancestor, to a common ancestor
-of all our present-day horses and asses, which must have been striped
-over its whole body, like the zebra living in Africa now.
-
-It cannot be proved of any of the wild horses of to-day that they are
-not descended from domesticated ancestors; indeed, we can say with
-certainty that the thousands of wild horses which roam the plains of
-North and South America are descended from domestic horses, for there
-was no horse in America at the time it was discovered by the Europeans.
-In all probability our horse originated in Middle Asia, was there
-first domesticated, and has thence been gradually introduced into
-other countries. In Egypt it appears first on the monuments in the
-seventeenth century B.C., and it seems to have been introduced by the
-conquering Hyksos. On the ancient Assyrian monuments the chase after
-wild horses is depicted, and they were not caught, but killed with
-arrow and lance, like the lion and the gazelle.
-
-But even if two wild species of horse had been tamed in different parts
-of the great continent of Asia, these two domesticated animals would
-have varied much and in the most diverse manner, as we may infer from
-our different breeds of horses at the present day. There are a great
-many of these, and many of them differ very considerably from each
-other. If we think of the lightly built Arab horse, and place beside
-it the little pony, or the enormous Percheron, the powerful cart-horse
-from the old French province of La Perche, which easily draws a load of
-fifty kilograms, we are face to face with differences as great as those
-between natural species. And we may realize how many breeds of horses
-there are now upon the earth if we remember that nearly every oceanic
-island has its special breed of ponies. Not only in the cold Shetland
-Islands, England, Sardinia and Corsica, but in almost every one of the
-larger islands of the extensive Indian Archipelago there is one, and
-Borneo and Sumatra have several.
-
-But the most conclusive proof of descent from a single wild species
-is afforded by the pigeons, and as the production of new breeds among
-them has been, and will continue to be, carried on with particular
-enthusiasm and deliberateness, I propose to deal with them somewhat
-more in detail.
-
-Darwin's work proves beyond a doubt that all our present-day breeds
-of pigeons are descended from a single wild species, the rock-dove,
-_Columba livia_. In appearance, this form, which still lives in a wild
-state, differs little from our half-wild blue-grey field-pigeon. It has
-the same metallic shimmer on the feathers of the neck, the same two
-black cross-bars on the wings as well as the band over the tail, and it
-has also the same slate-blue general colour. Now, all breeds of pigeons
-are without restriction fertile _inter se_, so that any breed can be
-crossed with any other, and it often happens that, in the products
-of such crossing, characters appear which the parents, that is, the
-two or more crossed breeds, did not possess, but which are among the
-characters of the rock-dove. Thus Darwin obtained, by crossing a pure
-white fantail with a black barb, hybrids which were partly blackish
-brown, partly mixed with white, but when he crossed these hybrids with
-others from two breeds which were likewise not blue, and had no bars,
-he obtained a slate-blue rock-pigeon, with bars on the wings and tail.
-We shall inquire later on how far it is correct to regard such cases
-as reversions to remote ancestors, but if we take it for granted in
-the meantime, we have here a proof of the descent of our breeds from a
-single wild species. This is corroborated, too, by everything that we
-know about the distribution of the rock-pigeon and the place and time
-of its domestication. It still lives on the cliff-guarded shores of
-England, Brittany, Portugal, and Spain, and both in India and in Egypt
-there were tame pigeons at a very early period. Pigeons appear on the
-menu of a Pharaoh of the fourth dynasty (3000 B.C.), and of India we
-know at least that in 1600 A.D. there were 20,000 pigeons belonging to
-the court of one of the princes.
-
-The beauty of this bird, and the ease with which it can be tamed,
-obviously called man's attention to it at a very early date, and it has
-been one of man's domestic companions for several thousands of years.
-Now we can distinguish at least twenty main races (Fig. 1), which
-differ from each other as markedly as, if not more markedly than, the
-most nearly allied of the 288 wild species of pigeons which inhabit the
-earth. We have carriers and tumblers, runts and barbs, pouters, turbits
-and Jacobins, trumpeters and laughers, fantails, swallows, Indian
-pigeons, &c.
-
-[Illustration: FIG. 1. Group of various races of domestic pigeons
-(after Prütz). 1. Pouter. 2. Indian barb. 3. Bucharest trumpeter with
-a whorl of feathers (_Nelke_) on its forehead. 4. Nürnberger swallow.
-5. Nürnberger bagadotte. 6. English carrier. 7. Fantail. 8. Eastern
-turbit. 9. Schmalkaldener Jacobin. 10. Chinese owl. 11. German turbit.]
-
-Each of these races falls into sub-races; thus there is a German, an
-English, and a Dutch pouter-pigeon. The books on pigeons mention over
-150 kinds which are quite distinct from one another, and breed true,
-that is, always produce young similar to themselves.
-
-Without entering upon a detailed description of any of these, I should
-like to call attention to the way in which certain characters have
-varied among them. Colour is a subordinate race-character, in so far
-that colour alone does not constitute a race, yet the colouring within
-a particular sub-race is usually very sharply defined, and in every
-breed there are sub-races of different colours. Thus there are white,
-black, and blue fantails, there are white turbits with red-brown wings,
-but also red ones with white heads, and white tumblers with black
-heads, &c. Very unusual colours and colour-markings sometimes occur.
-Thus one sub-race of tumblers exhibits a peculiar clayey-yellow colour
-splashed with black markings, otherwise rare among pigeons, and almost
-suggestive of a prairie-hen; there is also a copper-red spot-pigeon, a
-cherry-red 'Gimpel'-pigeon, lark-coloured pigeons, &c. Then we find all
-possible juxtapositions of colours, limited to quite definite regions
-of the body; thus we have white tumblers with a red head, red tail,
-and red wing-tips, or white tumblers with a black head, red turbits
-with white head, Indian pigeons quite black except for white wing-tips,
-and so on. The distribution of colour is often very complicated, but
-nevertheless, all the individuals of the breed show it in exactly the
-same manner. Thus there are the so-called blondinettes in which almost
-the whole body is copper-red, but the wings white, save that each quill
-bears at the rounded end of its vane a black and red fringe. I should
-never come to an end, if I were to try to give anything like a complete
-idea of the diversity of colouring among the various breeds of pigeons.
-
-Even such an important and, among wild species, unusually constant
-organ as the bill has varied among pigeons to an astonishing degree.
-Carrier-pigeons (Fig. 1, No. 6) have an enormously long and strong
-bill, which is moreover covered with a thick red growth of the cere,
-while in the turbits and owls (Fig. 1, Nos. 8 and 10) the bill is
-shorter than any we find among wild birds. In many breeds even the
-_form_ of the bill deviates far from the normal, as in the bagadottes
-(No. 5) with crooked bill.
-
-Like the bill, the legs vary in regard to their length. The pouters
-(No. 1) stand on their long legs as on stilts, while the legs of the
-'Nürnberger swallow' are strikingly small. Remarkable, too, and very
-different from the wild species, is the thick growth of feathers on the
-feet and toes of the pouters and trumpeters (Fig. 1, No. 1), as well
-as of some other breeds, which suggests the arrangement of feathers on
-a wing.
-
-Furthermore, the number and size of wing and tail-feathers in the
-different breeds often deviate considerably from the normal. The
-fantail (No. 7) in its most perfect form possesses forty tail-feathers,
-instead of the twelve usual in the wild rock-pigeon, and they are
-carried upright like a fan, while the head and neck of the bird are
-bent sharply backwards. In the hen-like pigeons the tail-feathers are
-few and short, so that they show an upright tail like that of a hen. I
-have already referred to the extraordinary carunculated skin-growth on
-the bill of many breeds; such folds also often surround the eye, and,
-as in the Indian barb (No. 3), are developed into well-formed thick
-circular ridges, while in the English carrier (No. 6) they lie about
-the bill as a formless mass of flesh.
-
-Even the skull has undergone many variations, as can be observed
-even in the living bird in many of the breeds with short forehead.
-Differences are to be found, too, in the number and breadth of the
-ribs, the length of the breast-bone, the number and size of the
-tail-vertebræ in different breeds. Of the internal organs, the crop in
-many breeds, but particularly in the pouters (No. 1), has attained an
-enormous size, and with this size is usually associated the habit of
-blowing it out with air, and assuming the characteristically upright
-position.
-
-That variations have taken place, too, in the most delicate structure
-of the brain, is shown by certain new instincts, such as the trumpeting
-of the trumpeters, the cooing of others, and the silence of yet other
-breeds, as well as by the curious habit of the tumblers of ascending
-quickly and vertically to a considerable height, and then turning
-over once, or even several times, in the course of their descent. In
-contrast to this, other breeds like the fantails have altogether given
-up the habit of flying high, and usually remain close to the dove-cot.
-
-Lastly, let me mention that the unusual development of individual
-feathers, or of groups of feathers, has become a race-character, upon
-which depend such remarkable structures as the feather-mantle turned
-over the head in the Jacobins (No. 9), the cap or plume on the head of
-various breeds, the white beard in the bearded tumbler, the collars
-which lie like a shirt-collar on the breast, or run down the sides of
-the neck (Nos. 8 and 10), and the circle of feathers which marks the
-root of the bill in the Bucharest trumpeter (No. 3).
-
-After what has been said, it is hardly necessary to add that the size
-of the whole body differs in different races. But the differences
-are very considerable, for, according to Darwin, one of the largest
-runt-pigeons weighed exactly five times as much as one of the smallest
-tumblers with short forehead, and in the illustration (Fig. 1) the
-pouter looks a giant beside the little barb to its left.
-
-Thus we see that nearly every part of the body of the pigeon has varied
-under domestication in the most diverse ways, and to a high degree;
-and the same is true of several other domesticated animals, poultry,
-horses, sheep, cattle, pigs, and so on, though the matter is not
-altogether so clear in their case, since descent from a single wild
-species cannot be proved, and is in many cases improbable. But in the
-case of pigeons this common descent is certain, and we have now to
-inquire in what manner all these variations from the parent form have
-been brought about.
-
-The answering of this question is rendered easier by the fact that new
-breeds arise even now, and that, to some extent at least, they can be
-caused to arise, consciously and intentionally. In England, as well
-as in Germany and France, there are associations for the breeding of
-birds, and in England especially pigeon and poultry clubs are numerous
-and highly developed. These by no means confine themselves to simply
-preserving the purity of existing breeds, they are continually striving
-to improve them, by increasing and accentuating their characters, or
-even by introducing quite new qualities, and in many cases they succeed
-even in this last. Prizes are offered for particular new variations,
-and thus a spirit of rivalry is fostered among the breeders, and each
-strives to produce the desired character as quickly as possible. Darwin
-says: 'The English judges decided that the comb of the Spanish cock,
-which had previously hung limply down, should stand erect, and in
-five years this end was achieved; they ordained that hens should have
-beards, and six years later fifty-seven of the groups of hens exhibited
-at the Crystal Palace in London were bearded.' The transformation
-does not always come about so quickly, however; thus, for instance,
-it required thirteen years before a certain breed of tumblers was
-furnished with a white head. But the breeders cause every visible part
-of the body to vary as seems good to them, and within the last fifty
-years they have really brought about very considerable changes in many
-breeds. Their method of procedure is carefully to select for breeding
-those birds which already possess a faint beginning of the desired
-character. Domesticated animals have on the whole a higher degree of
-variability than wild species, and the breeder takes advantage of
-this. Suppose it is a question of adding a crown of feathers to a
-smooth-headed breed, a bird is chosen which has the feathers on the
-back of the head a little longer than usual, and mated for breeding.
-Among its descendants there will probably be some which also exhibit
-these slightly prominent feathers, and possibly there may be one or
-other of them which has these feathers considerably lengthened. This
-one is then used for breeding, and by continually proceeding thus,
-and selecting for breeding, from generation to generation, only
-the individuals which approach most nearly to the desired end, the
-wished-for character is at last secured.
-
-Thus it is not by crossing of different breeds, but by a patient
-accumulating of insignificant little variations through many
-generations, that the desired transformations are brought about. That
-is the magic wand by means of which the expert breeder produces his
-different breeds, we might almost say, as the sculptor moulds and
-remoulds his clay model according to his fancy. Quite according to his
-fancy the breeder has brought about all the fantastic forms we are
-familiar with among pigeons, mere variations which are of no use either
-to the bird itself or to man, which simply gratify man's whim without
-in many cases even satisfying his sense of beauty. For many of the
-existing breeds of pigeons, hens, and other domesticated animals, are
-anything but beautiful, the body being often unharmonious in structure
-and sometimes actually monstrous.
-
-Among pigeons, as well as among other domesticated animals, some
-changes have been brought about, which are not only of no use to their
-possessors, but would be actually disadvantageous if they were living
-under natural conditions. Some of the very short-billed breeds of
-pigeons have the bill so short and soft that the young can no longer
-use it to scratch and break the egg-shell, and would perish miserably
-if human aid were not at hand. The Yorkshire pig has become such a
-colossus of fat on weak, short legs, that if it were dependent on
-its own resources, it could not secure its food, much less escape
-from a beast of prey; and among horses the heavy cart-horse and the
-racer are alike unfit to cope with the dangers of a wild life, or the
-vicissitudes of weather.
-
-Breeding has done much to bring about variations useful to man. Thus we
-have breeds of cattle which excel in flesh, or in milk, or as draught
-animals, and sheep which excel in flesh or in wool, and to what a
-height the perfecting of a useful quality can be brought is shown, in
-regard to fineness of wool, by that finest breed of sheep, the merino,
-which instead of the 5,500 hairs borne by the old German sheep on a
-square inch, possesses 48,000.
-
-Not infrequently it is a particular stage of a species that has
-been bred by man, and the other stages have remained more or less
-unaltered. Thus it is with one of the few domesticated insects, the
-silk-moth. Only the cocoon is of use to man, and according to the
-cocoon different breeds are distinguished, differing in fineness,
-colour, &c.; but no breeds can be distinguished in reference to the
-larvæ, or the perfect insects. Among gooseberries there are about a
-hundred varieties distinguished according to the form, colour, size,
-thickness of skin, hairiness, &c., of the fruits, but the little,
-inconspicuous, green blossoms, of which the breeders take no account,
-are alike in them all. In the pansies (_Viola tricolor_), on the other
-hand, it is only by the flowers that the varieties are distinguished,
-while the seeds have remained alike in all.
-
-It may be asked how it could have occurred to any one, when pigeons,
-for instance, first began to be domesticated, to wish to produce
-fantails or pouters, since he could have no mental picture of them
-in advance. Darwin replies to this objection, that it was not always
-conscious and methodical artificial selection, such as is now
-practised, that brought about the origin of breeds, but that they have
-very often resulted, and at first perhaps always, from unconscious
-selection. When savages tamed a dog, they used the 'best' of their dogs
-for breeding, that is, they chose those which had in the highest degree
-the qualities they valued, watchfulness, for instance, or if the dog
-were intended for the chase, keen scent and swiftness. In this way the
-body of the animal would be changed in a definite direction, especially
-if rivalry helped, and if it was the ambition of each to possess a
-dog as good as, or better than those of his tribal companions. That
-perfectly definite changes in bodily form can thus be brought about
-unconsciously is well illustrated by the case of a racehorse. This has
-arisen within the last two hundred years simply because the fleetest
-of the products of crossing between the Arab and the English horse
-were always chosen for breeding. It could not have been predicted that
-horses with thin neck, small head, long rump, and slender legs would
-necessarily be the swiftest runners; but this is the form which has
-resulted from the selection,--a very ugly, but very swift horse. This
-unconscious selection must undoubtedly have played a large part in the
-early stages of the evolution of the breeds of our domestic animals.
-
-But even in the fully conscious and methodical selective breeding of
-particular characters, the breeder rarely alters only the one his
-attention is fixed on; generally quite a number of other characters
-alter apart from his intention as an inevitable accompaniment of the
-desired variation on which attention was riveted. There are breeds
-of rabbits whose ears hang limply down instead of standing erect,
-and in these so-called lop-eared rabbits the ear-muscles are partly
-degenerated, and as a consequence of this lack of muscular strain the
-skull has assumed another form. Thus the variation of one part may
-influence the development of a second and a third organ, and may even
-not stop there, for very often the influence has penetrated much deeper
-and affected quite remote parts of the body.
-
-If any one were to succeed in adding a heavy pair of horns to a breed
-of hornless sheep, there would run parallel with the course of this
-variation, which was directly aimed at, a long series of secondary
-changes which would affect at least the whole of the anterior half of
-the body; the skull would become thicker and stronger to support the
-weight of the heavy horns; the neck-tendon (_ligamentum nuchæ_) would
-have to become thicker to hold up the heavy head, and so also with the
-muscles of the neck; the spinous processes of the cervical and dorsal
-vertebrae would become longer and stronger, and the forelegs, too,
-would need to adapt themselves to the heavier burden. Every organism
-thus resembles, as it were, a mosaic, out of which no individual group
-of pieces can be taken and replaced by another without in some measure
-disturbing the correlation and harmony of the whole: in order to
-restore this, the pieces all round about the changed part must be moved
-or replaced by others.
-
-According to Darwin, it is to this correlation of parts that we must
-refer the variation of other parts besides the one intentionally
-altered in the course of breeding. It must be admitted that the mutual
-dependence of the parts plays a very important rôle in the economy
-and development of the animal body, as we shall see later, and these
-connexions still remain very mysterious to us. Especially is this
-the case with the connexion between the reproductive organs and the
-so-called secondary sexual characters. Removal of the reproductive
-organs or gonads induces, in Man, for instance, if it be effected in
-youth, the persistence of the childish voice and the non-development
-of the beard; in the stag the antlers do not appear, and in the cock
-the comb does not develop perfectly, &c., but we are not yet able to
-understand clearly why this should be so.
-
-
-
-
-LECTURE III
-
-THE DARWINIAN THEORY (_continued_)
-
- Natural selection--Variation--Struggle for existence--Geometric ratio
- of rate of increase--Normal number and ratio of elimination in a
- species--Accidental causes of extinction--Dependence of the strength
- of a species on enemies--Struggle for existence between individuals
- of the same species--Natural selection affects all organs and
- stages--Summary.
-
-
-IN artificial selection, through which, with or without conscious
-intention, our domesticated animals and cultivated plants have arisen,
-there must obviously be three kinds of co-operative factors: first, the
-_variability_ of the species; second, the capacity of the organism for
-_transmitting_ its particular characters to its progeny; and third, the
-_breeder_ who selects particular qualities for breeding. No one of the
-factors can be dispensed with; the breeder could effect nothing, were
-there not presented to him the variations of parts in the particular
-direction in which he wishes them to vary; an indefinite variation,
-that is, a variation not guided by selection, would never lead to the
-formation of new breeds; the species would probably become in time a
-motley mixture of all sorts of variations, but a breed with definite
-characters, transmissible in their purity to its descendants, could
-never be formed. Finally, every process of selective breeding would be
-futile, if the variations which appeared could not be transmitted.
-
-Darwin assumes that processes of transformation quite similar to those
-which take place under the guidance of Man occur also in nature,
-and that it is mainly these which have brought about and guided the
-transformations of species which have taken place in the course of the
-earth's history. This process he calls _natural selection_.
-
-It will readily be admitted that two out of the three factors necessary
-to a process of selective breeding are present also in the natural
-conditions of the life of species. Variability in some degree or
-other is absent from no species of animal or plant, though it may be
-greater in one than in another, and it cannot be doubted that the
-inborn differences which distinguish one individual from another are
-capable of transmission. It is only to untrained observers that all the
-individuals of a species appear alike; for instance, all garden whites,
-or all the individuals of the small tortoiseshell butterfly (_Vanessa
-urticæ_), or all the chaffinches. If the individuals are carefully
-compared it will be recognized that, even in these relatively constant
-species, no individual exactly resembles another; that even among
-butterflies twenty black scales may go to form a particular spot on
-the wings in one individual and thirty or twenty-five in others; that
-the length of the body, the legs, the antennæ, the proboscis exhibit
-minute differences; and it is probable that the same combination of
-quite similar parts never occurs twice. In many animals this cannot, of
-course, be proved, because our power of diagnosis is not fine enough to
-be able to estimate the differences directly, and because a comparison
-of measurements of all the parts in detail is not practicable. So we
-may here confine ourselves to the differences in the human race, which
-we can recognize with ease and certainty. Even as regards the face
-alone, all men differ from one another, and, numerous and complete as
-likenesses may be, it is impossible to find two human beings in which
-even the characters of the face are exactly similar. Even so-called
-'identical twins' can always be distinguished if they are directly
-compared either in person or in a photograph, and if the rest of the
-body be also taken into consideration we find numerous small, sometimes
-even measurable differences.
-
-The same is true of animals, and it is only our lack of practice
-that is at fault if we frequently fail to detect their individual
-differences. The Bohemian shepherds are said to know personally, and be
-able to distinguish from all the rest, every sheep in their herds of
-many thousands. Thus the factors of variability and transmissibility
-must be granted, and it remains only to ask: Who plays the part of
-selecting breeder in wild nature? The answer to this question forms
-the kernel to the whole Darwinian theory, which ascribes this rôle to
-the conditions of life, to definite relations of individuals to the
-external influences which they meet with during the course of their
-lives, and which together make up their 'struggle for existence.'
-
-To make this idea clear I must to some extent diverge.
-
-It is a generally observed fact that, in every species of animals
-or of plants, more germs and more individuals are produced than
-grow to maturity, or become capable of reproduction. Numerous young
-individuals perish at an early stage, often because of unfavourable
-circumstances--cold, drought, damp, or through hunger, or at the hands
-of their enemies. When we ask which of the progeny perish early, and
-which survive to carry on the species, we are at first sight inclined
-to suppose that this is entirely a matter of chance; but this is just
-what Darwin disputed. It is not chance alone, it is, above all, the
-differences between individuals, which enable them to withstand adverse
-circumstances better or worse, and thus decide, according to his view,
-which shall perish and which shall survive. If this be so, then we
-have a veritable process of selection, and one which secures that the
-'best,' that is, the most capable of resistance, survive to breed,
-being thus, so to speak, 'selected.'
-
-It may be asked, however, why so many individuals must perish in youth,
-and whether it could not have been arranged that all, or at least most,
-should survive till they had reproduced. But this is an impossibility,
-unrealizable for this among other reasons, that organisms multiply in
-geometrical progression, and that their progeny would very soon exceed
-the limits of computability. This does not occur, for there is a limit
-set which they can in no case overstep,--which, indeed, as we shall
-see, they never reach--I mean the limits of space and food-supply.
-Every species, by the natural requirements of its life, is restricted
-to a particular habitat, to land or to water, but most are still more
-strictly limited to a definite area of the earth's surface, which alone
-affords the climate suited to them, or where alone the still more
-specialized conditions of their existence can be realized. Thus, for
-instance, the occurrence of a particular species of plant determines
-that of the animal which is dependent on it for its food-supply. If
-they could multiply unchecked, that is, without the loss of many of
-their progeny, every species would fill up its area of occurrence and
-exhaust the whole of its food-supply, and thus bring about its own
-extermination. This seems to be prevented in some way, for as a matter
-of fact it does not happen.
-
-It may, perhaps, be imagined that this might be prevented by a
-regulation of the productivity of the species, and that those which
-have not a large area of distribution, or can only count on a
-relatively limited food-supply, have also a low rate of multiplication,
-but this is not the case; even the lowest rate of multiplication would
-very soon suffice to make any species fill up its whole available space
-and completely exhaust its food-supply. Darwin takes as an example
-the elephant, which only begins to breed at thirty years of age, and
-continues to do so till about ninety, but so slowly that in these
-sixty years only three pairs of young are produced. Nevertheless, in
-500 years an elephant pair would be represented by fifteen millions
-of descendants, if all the young survived till they were capable of
-reproduction. A species of bird with a duration of life of five years,
-during which it breeds four times, producing and rearing four young
-each time, would in the course of fifteen years have 2,000 millions of
-descendants.
-
-Thus, although the fertility of each species is, as a matter of fact,
-precisely regulated, a low rate of multiplication is not in itself
-sufficient to prevent the excessive increase of any species, nor is
-the quantity of the relevant food-supply. Whether this be very large
-or very small, we see that in reality it is never entirely used up,
-that, as a matter of fact, a much greater quantity is always left over
-than has been consumed. If increase depended only on food-supply, there
-would, for instance, be food enough in their tropical home for many
-thousand times more elephants than actually occur; and among ourselves
-the cockchafers might appear in much greater numbers than they do even
-in the worst cockchafer year, for all the leaves of all the trees are
-never eaten up; a great many leaves and a great many trees are left
-untouched even in the years when the voracious insects are the most
-numerous. Nor do the rose-aphides, notwithstanding their enormously
-rapid multiplication, ever destroy all the young shoots of a rose-bush,
-or all the rose-bushes of a garden, or of the whole area in which roses
-grow.
-
-At the same time it must be noted, that the number of individuals in
-a species undoubtedly does bear some relation to the amount of the
-food-supply available; for instance, it is very low among the large
-carnivores, the lion, the eagle, and the like. In our Alps the eagles
-have become rarer with the decrease of game, and where one eagle pair
-make their eyrie they rule alone over a hunting territory of more than
-sixty miles, a preserve on which no others of the same species are
-allowed to intrude. If there were several pairs of eagles in such a
-preserve, they would soon have so decimated the food-supply that they
-would starve. On the other hand, numerous herbivores, e.g. chamois and
-marmots, live within the bounds of the pair of eagles' hunting grounds,
-since the food they require is present in enormously greater quantity.
-
-While it is true that the number of individuals of a given species
-which live in a particular area is not exactly the same year in year
-out, being subject to small, and sometimes, as in the case of the
-aphides and cockchafers, to very great fluctuations, nevertheless we
-may assume that the _average number_ remains the same, that in the
-course of a century, or, let us say, of a thousand years, the number
-of mature individuals inhabiting the particular area remains the
-same. This, of course, only holds true on the supposition that there
-has been no great change in the external conditions of life during
-this period. But before Man began to interfere with nature, these
-external conditions would remain uniform for much longer periods
-than we have assumed. Let us call the average number of individuals
-occurring on such a uniform area, _the normal number_ of the species;
-this number will be determined in the first instance by the number of
-offspring that are annually brought forth, and secondly by the number
-that annually perish before reaching maturity. As the fertility of
-a species is a definite quantity, so also will its elimination be
-definite, or, as we may say, when the normal number under uniform
-conditions of life remains constant, the ratio of elimination will
-also remain constant. Each species is therefore subject to a perfectly
-definite ratio of elimination which remains on the average constant,
-and this is the reason why a species does not multiply beyond its
-normal number notwithstanding the great excess of the food-supply, and
-notwithstanding the fertility which, in all species, is sufficient to
-lead to boundless multiplication.
-
-It is not difficult to calculate the ratio of elimination for a
-particular species, if one knows its rate of multiplication; for if
-the normal number remains constant, it follows that only two of all
-the offspring which a pair brings forth in the course of its life can
-attain to reproductive maturity, and that all the rest must perish.
-
-Suppose, for instance, a pair of storks produced four young ones
-annually for twenty years, of these eighty young ones which are born
-within this period, on an average seventy-eight must perish, and only
-two can become mature animals. If more than two attained maturity
-the total number of storks would increase, and this is against the
-presupposition of constancy in the normal number. It is important, in
-reference to the fact on which we are now focusing our attention, that
-we should consider some other illustrations from the same point of
-view. The female trout yearly produces about 600 eggs; let us assume
-that it remains capable of reproduction for only ten years, then the
-elimination-number of the species will be 6,000 less two, that is,
-5,998, for of the 6,000 eggs only two can become mature animals. But
-in the majority of fishes the ratio of extermination is enormously
-greater than this. Thus a female herring brings forth 40,000 eggs
-annually, the duration of life is estimated at ten years, and this
-means an elimination number of 400,000 less two, that is, 399,998. The
-carp produces 200,000 eggs a year, and the sturgeon two millions, and
-both species live long, and remain capable of reproduction for at least
-fifty years. But of all the 100 million eggs which are produced by the
-sturgeon, only two reach their full development and reproduce; all
-others perish prematurely.
-
-But even with these examples we have not reached the highest
-elimination number, for many of the lower animals--not to speak of
-many plants--produce an even greater number of offspring. Leuwenhoek
-calculated the fertility of a thread-worm at sixty million eggs, and a
-tape-worm produces hardly less than 100 millions.
-
-There exists, therefore, a constant relation between fertility and the
-ratio of elimination; the higher the latter is, the greater must the
-former be, if the species is to survive at all. The example of the
-tape-worm makes this very obvious, for here we can readily understand
-why the fertility must be so enormous, as we are aware of the long
-chain of chances on which the successful development of this animal
-depends. The common tape-worm of Man, _Tænia solium_, does not lay
-its eggs, they remain enclosed within one of the liberated joints or
-'proglottides.' Only if this liberated joint or one of the embryos
-within it happens to be fortuitously eaten by a pig or other mammal can
-there be successful development, and even then under difficulties and
-possible failures, and not right away into adult animals, but first
-into microscopically minute larvæ which may bore their way into the
-walls of the intestine, or, if they are fortunate enough, may get into
-the blood-stream and be carried by it to a remote part of the body.
-There they develop into 'measles,' the so-called bladder-worms, within
-which the head of the tape-worm arises. But in order that this may
-become a complete and reproductive adult worm the pig must die, and the
-next step necessary is that a piece of the flesh of the infected first
-host must happen to be swallowed raw by a man or other mammal! Only
-then does the fortunate bladder-worm--swallowed with the flesh--attain
-the goal of its life, that is, a suitable place to mature in, the
-food-canal of a human being. It is obvious that countless eggs must
-be lost for one that succeeds in getting through the whole course of
-a development depending so greatly on chance. Hence the necessity for
-such enormous productivity of eggs.
-
-In many cases the causes of elimination, which keep a species within
-due bounds, are very difficult to determine. Enemies, that is to say,
-other species which use the species in question as food, play an
-important rôle; often, however, the cause lies in the unfavourableness
-of external conditions, in chance, which is favourable only to one
-of a thousand. The oak would only require to produce one seed in
-the 500 years of its life, if it were certain that that one would
-grow into an oak-tree; but most of the little acorns are eaten up by
-pigs, squirrels, insects, &c., before they have had time to sprout,
-thousands fall on ground already thickly covered with growth where
-they cannot take root, and even if they do succeed in finding an
-unoccupied space in which to germinate, the young plants are still
-surrounded by a thousand dangers--the possibility of being devoured by
-many animals large and small, of being suffocated by the surrounding
-vegetation, and so on. We can thus understand, to some extent, though
-only approximately, why it is that the oak must year by year produce
-thousands of seeds in order that the species may maintain its normal
-number, and not be exterminated; for it is obvious that a constant,
-even though slow diminution of the normal number, a regular deficit, so
-to speak, can end in nothing else than the gradual extinction of the
-species.
-
-But even this prodigality of seeds is not the greatest reach of
-fertility that we meet with in nature; it is, perhaps, amongst the
-simpler flowerless plants that we find the climax. It has been
-calculated that a single frond of the beautiful fern so common in our
-woods, _Aspidium filix mas_, produces about fourteen million spores.
-They serve to distribute the species, and are carried as motes by the
-wind, but comparatively few of the millions ever get the length of
-germinating at all, much less of attaining to full development into
-adult plants. Thus we see that the apparent prodigality of nature is a
-real necessity, an indispensable condition of the maintenance of the
-species; the fertility of each species is related to the actualities of
-elimination to which it is exposed. This is clearly seen when a species
-is placed under new and more favourable conditions of life, in which
-it has an abundant food-supply and few enemies. This was the case, for
-instance, with the horses introduced from Europe into South America,
-where they reverted to a feral state, and are now represented by
-herds of many thousands roaming the great grassy plains. If the small
-singing-birds of a region diminish in number, there is a great increase
-of caterpillars and other injurious insects which form part of their
-food-supply. The colossal destruction which the much-dreaded nun-moth
-from time to time brings about in our woods probably depends in part
-on the diminution of one or another of the many animals inimical to
-insects; but the occurrence of several years of weather-conditions
-favourable to the larvæ must also be taken into account. How
-enormously, indeed almost inconceivably, the number of larvæ may
-increase under favourable conditions is shown by such devastations
-as that in Prussia in 1856, when many square miles of forest were
-absolutely eaten up. The caterpillars were so numerous that even from
-some distance the falling excrement could be heard rustling like rain,
-and ten hundredweights of the eggs were collected, with an average of
-20,000 eggs to the half-ounce!
-
-But it would be a great mistake to conclude, from this enormous and
-sudden increase in the number of individuals, that the normal number
-of individuals is determined by the number of enemies alone. The
-average number of individuals in a species depends on many other
-conditions, especially on the extent of the available area, and on
-the amount of the food-supply in relation to the size of body in the
-species. I cannot dwell on this now, but I wish to point out that,
-for the continuance of a species, it is indifferent whether it is
-'frequent' or 'rare,' if we presuppose that its normal number remains
-on an average constant for centuries, that is, that its fertility
-suffices to make good the continual losses through enemies and other
-causes of elimination. One would be inclined to conclude from such
-cases of sudden and enormous increase in the number of individuals as
-these caterpillar-blights, that enemies and other causes of destruction
-played the major part in the regulation of the normal number of the
-species. But this is only apparently the case. Enemies necessitate
-a certain fertility in the species on which they prey, so that the
-elimination in each generation may be made good; but the number of
-pairs capable of reproduction is not thereby decisively determined.
-We must not forget that the number of enemies is also, on the other
-hand, dependent on the number of victims, and that the normal number of
-enemies must rise and fall with that of the species preyed upon.
-
-For this reason, such an enormous increase as that of the caterpillars
-cannot last long; it carries its corrective in itself. The appearance
-of the caterpillars in such enormous numbers in itself increases the
-host of their enemies; singing-birds, ichneumon-flies, beetle-grubs,
-and predaceous beetles find abundant and available food, and therefore
-reproduce and multiply so rapidly, that, with the help of the
-caterpillar's plant-enemies, especially the insect-destroying fungi,
-they soon reduce the caterpillars to their normal number, or even
-below it. But then the reverse process begins; the enemies of the
-caterpillars diminish because their food has become scarce, and their
-normal number is lowered, while that of the caterpillars gradually
-rises again.
-
-When the number of foxes in a hunting district increases, the number of
-the hares that they prey upon diminishes, and, on the other hand, the
-decimating of the foxes by Man brings about an increase in the number
-of hares in the district. Under natural conditions, that is, without
-the intervention of Man, there would be a constant balancing of the
-numbers of hares and foxes, for every noteworthy increase of the hares
-would be followed by a similar increase of foxes, and this, in its
-turn, would diminish the number of hares, so that they would no longer
-suffice for the support of so many foxes, and these would decrease in
-number again, until the number of hares had again increased because
-of the lessened persecution and elimination. In nature the case is not
-quite so simple, because the fox does not live on hares alone, and the
-hare is not preyed upon only by the fox; but the illustration may serve
-to elucidate the point that a moving equilibrium is maintained between
-the species of a district, between persecutors and persecuted, in such
-a way that the number of individuals in the two species is always
-varying a little up and down, and that each influences the other so
-that a regulative process results. Throughout periods of considerable
-length the average remains the same; that is to say, a _normal number_
-is established. This normal strength of population is the mean above
-and below which the number of individuals is constantly varying. It is,
-of course, seldom that the mutual influences and regulations are so
-simple as in the example given; usually several or even many species
-interact upon each other, and not beasts of prey and their victims
-alone, but the most diverse species of animals and plants, which do
-not stand in any obvious relation to one another at all. Moreover, the
-physical, and especially the climatic conditions, also cause the normal
-number of the species to rise and fall.
-
-The inter-relations between species living together on the same area
-are so intricate that I should like to give two other illustrations.
-Let us first take Darwin's famous instance of the fertility of clover,
-which depends on the number of cats. It is of course only an imaginary
-one, but the facts it is based upon are quite correct. The number of
-cats living in a village to a certain extent determines the number of
-field-mice in the neighbourhood. These again destroy the nests of the
-humble-bees, which live in holes in the ground, and thus the number
-of humble-bees depends on that of the field-mice and cats. But the
-clover must be pollinated by insects if it is to produce fertile seed,
-and only the humble-bee has a proboscis long enough to effect the
-pollination. Therefore the quantity of clover-seed annually produced
-depends on the number of humble-bees, and ultimately upon the number of
-cats. And, as a matter of fact, humble-bees were introduced into New
-Zealand from England, because without them the clover would produce no
-fertile seeds.
-
-On the grassy plains of Paraguay there are no wild cattle and horses,
-because of the presence of a fly which has a predilection for laying
-its eggs in the navel of the newly-born calves and foals, with the
-result that the calves or foals are killed by the emerging maggots. We
-may reasonably assume that the numerical strength of this fly-species
-depends on the distribution of insect-eating birds, whose numbers
-in turn are determined by certain beasts of prey. These again vary
-in number in relation to the extent of the forest-land, and this is
-determined by the number of ruminants which browse on the young growth
-of the woods (Darwin).
-
-That forests can actually be totally destroyed by ruminants is proved
-by the case of the island of St. Helena among others. On its discovery
-the island was covered with thick wood, but in the course of 200 years
-it was transformed into a bare rock by goats and pigs, which devoured
-the young growth so completely that trees which were felled or which
-died were not replaced.
-
-This point is vividly illustrated by Darwin's observation of a
-wide heath on which stood only a few groups of old pine-trees. The
-mere fencing in of a portion of the heath sufficed to call forth a
-thick growth of young seedling pines within the enclosure, and an
-examination of the open part of the heath revealed that the grazing
-cattle had eaten up all the young pine-trees which sprang from seed,
-and that again and again. In one small space thirty-two little trees
-stood concealed in the grass, and several of these showed as many as
-twenty-six yearly rings.
-
-How definitely the number of individuals in different species living on
-the same area mutually limit and thereby regulate each other, Darwin
-sought to illustrate also by the case of the primitive forest, where
-the numerous species of plants occur, not mixed together irregularly,
-but in a definite proportion. We can find examples of the same kind
-wherever the plant-growth of a district has been left to itself. If we
-walk along the banks of our little river, the Dreisam, we see a wild
-confusion of the most diverse trees, shrubs and herbaceous plants. But,
-even though it cannot be demonstrated, we may be certain that these are
-represented in definite numerical proportions, dependent on the natural
-qualities and requirements of each species, on the number of their
-seeds and the facilities for their distribution, on the favourable
-or unfavourable season at which they ripen, and on their varying
-capacity for taking root in the worst ground, and springing quickly
-up, &c. They limit each other mutually, so that the whole flora of the
-river-bank will be made up of one per cent. of this species, one per
-cent. of that, and, it may be, five per cent. of a third, and the same
-combination will repeat itself in the same proportions on the banks of
-other rivers of our country in as far as the external conditions are
-the same. The same must be true of the fauna of such a plant-thicket;
-the animal species also limit one another mutually, and thereby
-regulate the number of individuals, which becomes relatively stable
-over any area on which the conditions remain the same. That is to say,
-a 'normal number' is attained and persists.
-
-Thus we see that the capacity for boundless multiplication inherent in
-every species is limited by the co-existence of other species; there
-is, metaphorically speaking, a continuous struggle going on between
-species, plant and animal alike; each seeks as far as possible to
-multiply, and each is hemmed in by the others and as far as possible
-prevented from doing so. The 'struggle' is by no means only the
-_direct_ limitation of the number of individuals, which consists in
-the use of one species by another as food, as in beasts of prey and
-their victims, or locusts and plants; it is much more the indirect
-limitation--figuratively speaking, the struggle for space, for light,
-for moisture among plants, for food among animals. But all this,
-important as it is, does not yet exhaust the content of that 'struggle
-for existence' to which Darwin and Wallace ascribe the rôle of the
-breeder in the process of natural selection. The struggle, that is,
-the mutual limiting of species, may indeed restrict a species in
-its distribution, and may reduce its normal number possibly to nil.
-In other words, it may bring about extinction, but it cannot make
-a species other than it is. This can only be done by a struggle
-within the limits of the species itself, and this struggle is due
-to the fact that of the numerous offspring, on an average those
-survive--that is, attain to reproduction--which are the most fit,
-whose constitution makes it most possible for them to overcome the
-difficulties and dangers of life, and so to reach maturity. We see,
-in fact, that a large percentage of each generation in all species
-always perishes before attaining maturity. If, then, the decision as to
-which is to perish and which is to reach maturity is _not a matter of
-chance alone_, but is in part due to the constitution of the growing
-individual; if the 'fittest' do _on the average_ survive, and the
-'least fit' are on the average eliminated, we have here a process of
-selection entirely comparable to that of artificial selection, and
-one whose result must be the 'improvement' of the species, whether
-that depends on one set of characters or on another. The victorious
-qualities, which earlier were peculiar to certain individuals, must
-gradually become the common property of the species, if in each
-generation the individuals which attained to reproduction all possessed
-them, and thus could transmit them to their progeny. But those of the
-descendants which did not inherit them would again be at a disadvantage
-in the struggle for existence, or rather for reaching maturity, if in
-each generation a higher percentage of individuals which possess these
-characters reach maturity than of those which do not possess them. This
-percentage must increase in each generation, because, in each, natural
-selection again chooses out the fittest, and it must finally rise to
-100 per cent., that is to say, none but individuals of this fittest
-type will be left surviving.
-
-This does not yet exhaust the process, however, for we can infer
-from the results of artificial breed-forming that the selected
-characters may intensify from generation to generation, and that
-they will continue to do so as long as it gives them any advantage
-in the struggle for existence, for so long will it lead to the more
-frequent survival of its possessors. The increase will only stop when
-it has reached the highest degree of usefulness, and in this way new
-characters may be formed, just as, in artificial selection, the short
-upward-turning feathers of the Jacobin pigeon have been intensified
-into the peruke, a feather canopy covering the head.
-
-A few examples of natural selection will make the process clearer. Our
-hare is well secured from discovery by his fur of mixed brown, yellow,
-white, and black, when he cowers in his form among the dry leaves of
-the underwood. It is easy to pass close to him without seeing him.
-But if the ground and the bushes are covered with snow, he contrasts
-conspicuously with them. Suppose, now, that our climate became colder,
-and that the winter brought lasting snow, the hares which had the
-largest mixture of white in their fur would have an advantage in their
-'struggle for existence' over their darker fellows; they would be less
-easily discovered by their enemies--the fox, the badger, the horned
-owl, and the wild cat. Of the numerous hares which would annually
-become the prey of these enemies, there would be, on an average, more
-dark than light individuals. The percentage of light-coloured hares
-would, therefore, increase from generation to generation, and the
-longer the winter the keener would be the selection between dark and
-light hares, until finally none but light ones would remain. At the
-same time, the colour of the hares would become increasingly light,
-first, because it would happen more and more frequently that two light
-hares would pair, and secondly, because, after a time, the struggle for
-existence would no longer be between light and dark hares, but between
-light hares and still lighter ones. Thus ultimately a race of white
-hares would arise, as has actually happened in the Arctic regions and
-on the Alps.
-
-Or let us think of a herbaceous plant, in appearance something like a
-belladonna, rich in leaves and very juicy, but not poisonous. It would
-doubtless be a favourite food with the animals of the forest, and it
-would not, therefore, attain to more than a sparse occurrence, since
-few of the individuals would be able to form seeds. But now let us
-assume that a stuff of very unpleasant taste develops in the stem and
-leaves of some of the individuals, as may easily happen through very
-slight changes in the chemical metabolism of the plant, what, then,
-could result but that such individuals would be less readily eaten than
-the others? A process of selection must, therefore, ensue, and the
-unpleasant-tasting specimens of the plant would be much more frequently
-spared, and consequently would bear seed much oftener than the
-palatable ones. Thus the number of unpalatable plants would increase
-from year to year. If the stuff in question were not only unpalatable
-but poisonous, or gradually became so, a plant would in time be evolved
-which would be absolutely safe from being devoured by animals, just as
-the deadly nightshade (_Atropa belladonna_) actually is.
-
-Or let us suppose that a stretch of water is inhabited by a species of
-carp, which have hitherto had no large enemy, and so have become lazy
-and slow, and that there migrates from the sea into this stretch of
-water a large species of pike. At first numerous carp will fall victims
-to the pike, and the pike will rapidly increase in number. But if all
-the carp were not equally lazy and dull-witted, if some of them were
-quicker and more intelligent, these would, on an average, become more
-rarely the victims of the pike, and numerous individuals with these
-better qualities would survive in each generation, till ultimately
-there were no others, and the useful characters would gradually become
-intensified, and so a more active and wary race of carp would arise.
-
-Let us suppose, however, that the increased activity and wariness would
-not alone suffice to preserve the colony from extinction; it might
-require also an increased fertility to prevent the normal number from
-being permanently lowered; but even this could eventually be brought
-about by natural selection, if the nature of the species and the
-general conditions of its life permitted. For there are variations of
-fertility in every species, and if the chance of seeing some of its
-eggs become mature animals were greater for the more fertile female
-than for the less fertile, _ceteris paribus_, a process of selection
-must take place, which would result in an increase of fertility as far
-as that was possible.
-
-Obviously, such processes of natural selection can affect all parts
-and characters--size and form of the body, as well as isolated parts,
-the external skin and its colour, every internal organ--and not bodily
-characters alone, but psychical ones as well, such as intelligence and
-instincts. According to this principle, it is only characters which
-are biologically indifferent that cannot be altered through natural
-selection.
-
-Natural selection can also bring about changes at every age, for the
-elimination of individuals begins from the egg, and any kind of egg
-which is in some way better able to escape elimination will transmit
-its useful characters to its descendants, because the resulting young
-animals will thus more frequently reach full development than the
-young from other eggs. In the same way, at every succeeding stage of
-development, every character favourable to the preservation of the
-individual will be maintained and intensified.
-
-We see from all this that natural selection is vastly more powerful
-than artificial selection by Man. In the latter, only one character at
-a time can be caused to change, while natural selection may influence a
-whole group of characters at the same time, as well as all the stages
-of development. Through the weeding out of the individuals which are
-annually exterminated, it is always on an average the 'fittest' which
-survive, that is to say, those which have the greatest number of
-bodily parts and rudiments of parts in the fittest possible condition
-of development at every stage. The longer this process of selection
-continues, the smaller will be the deviations of the individual from
-this standard, and the more minute will be the differences of fitness
-determining which is to be eliminated and which is to survive to
-reproduce its characteristics. In the immeasurable periods of time
-which are at the disposal of natural selection, and in the inestimable
-numbers of individuals on which it may operate, lie the essential
-causes of superiority of natural selection over the artificial
-selection of Man.
-
-To sum up briefly: Natural selection depends essentially on the
-cumulative augmentation of the most minute useful variations in the
-direction of their utility; only the useful is developed and increased,
-and great effects are brought about slowly through the summing up of
-many very minute steps. Natural selection is a self-regulation of the
-species which secures its preservation; its result is the ceaseless
-adaptation of the species to its life-conditions. As soon as these
-vary, natural selection changes its mode of action, for what was
-previously the best is now no longer so; parts that before had to be
-large must now perhaps be small, or vice versa; muscle-groups which
-were weak must now become strong, and so on. The conditions of life
-are, so to speak, the mould into which natural selection is continually
-pouring the species anew.
-
-But the philosophical significance of natural selection lies in
-the fact, that it shows us how to explain the origin of useful,
-well-adapted structures purely by mechanical forces and without
-having to fall back on a _directive_ force. We are thus for the first
-time in a position to understand, in some degree, the marvellous
-adaptation of the organism to an end, without having to call to our
-aid any supernaturally intrusive force on the part of the Creator. We
-understand now how, in a purely mechanical way, through the forces
-always at work in nature, all forms of life must conform to, and adapt
-themselves precisely to the conditions of their life, since only the
-best possible is preserved, and everything less good is continually
-being rejected.
-
-Before I go on to expound in detail the phenomena which we refer to
-natural selection, I must briefly state that Darwin did not ascribe to
-natural selection by any means all the changes which have taken place
-in organisms in the course of time. On the one hand, he ascribed a not
-inconsiderable importance to the correlated variations we have already
-mentioned; still more, however, he relied on the direct influence of
-altered conditions of life, whether these consist in climatic and other
-changes in the environment, or in the assumption of new habits, and
-the increased or diminished use of individual parts and organs thereby
-induced. He recognized the principle so strongly emphasized by Lamarck,
-of use and disuse as a cause of heritable increase or decrease of the
-exercised or neglected part, though he did so with a certain reserve.
-I shall return later to these factors of modification, and shall then
-attempt to show that these too are to be referred to processes of
-selection, which are, however, of a different order from the phenomena
-which the Darwin-Wallace principle of natural selection serves to
-interpret. But, in the first instance, it appears to me to be necessary
-to show how far the Darwin-Wallace interpretation will suffice, and
-in the next lectures we shall occupy ourselves with this question
-exclusively.
-
-
-
-
-LECTURE IV
-
-THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION
-
- Biological significance of colours--Protective colours of
- eggs--Animals of the snow-region--Animals of the desert--Transparent
- animals--Green animals--Nocturnal animals--Double
- colour-adaptation--Protective marking of caterpillars--Warning
- markings--Dimorphism of colouring in caterpillars--Shunting back
- of colouring in ontogeny--'Sympathetic' colouring in diurnal
- Lepidoptera--In nocturnal Lepidoptera--Theoretical considerations--The
- influence of illumination in the production of protective colouring,
- _Tropidoderus_--Harmony of protective colouring in minute
- details--_Notodonta_--Objections--Imitation of Strange objects,
- _Xylina_--Leaf-butterflies, _Kallima_--_Hebomoja_--Nocturnal
- Lepidoptera with leaf-markings--Orthoptera resembling
- leaves--Caterpillars of the Geometridæ.
-
-
-WE have seen what Darwin meant by natural selection, and we understand
-that this process really implies a transformation of organisms by
-slow degrees, in the direction of adaptive fitness--a transformation
-which must ensue as necessarily as when a human selector, prompted
-by conscious intention, tries to improve an animal in a particular
-direction, by always selecting the 'fittest' animals for breeding.
-In nature, too, there is selection, because in every generation
-the majority succumb in the struggle for life, while on an average
-those which survive, attain to reproductive maturity, and transmit
-their characters to their descendants, are those which are best
-adapted to the conditions of their life--that is, which possess those
-variations of most advantage in overcoming the dangers of life.
-Since individuals are always variable in some degree, since their
-variations can be inherited by their progeny, and since the continually
-repeated elimination of the majority of those descendants is a fact,
-the inference from these premisses must be correct; there must be a
-'natural selection' in the direction of a gradually increasing fitness
-and effectiveness of the forms of life.
-
-We cannot, however, directly observe this process of natural selection;
-it goes on too slowly, and our powers of observation are neither
-comprehensive nor fine enough. How could we set about investigating
-the millions of individuals which constitute the numerical strength
-of a species on a given area, to find out whether they possess
-some variable character in a definite percentage, and whether this
-percentage increases in the course of decades or centuries? And
-there is, furthermore, the difficulty of estimating the biological
-importance of any variation that may occur. Even in cases where we know
-its significance quite well in a general way, we cannot estimate its
-relative value in reference to the variation of some other character,
-though that other may also be quite intelligible. Later on, we shall
-speak of protective colouring, and in so doing we shall discuss the
-caterpillars of one of the Sphingidæ, which occur in two protective
-colours, some being brown, others green. From the greater frequency of
-the brown form we may conclude that brown is here a better adaptation
-than green, but how could we infer this from the character itself,
-or from our merely approximate knowledge of the mode of life of the
-species, its habits, and the dangers which threaten it? A direct
-estimation of the relative protective value of the two colours is
-altogether out of the question. The survival of the fittest cannot be
-proved in nature, simply because we are not in a position to decide,
-_a priori_, what the fittest is. For this reason I was forced to try
-to make the process of natural selection clear by means of imagined
-examples, rather than observed ones.
-
-But though we cannot directly follow the uninterrupted process of
-natural selection which is going on under natural conditions, there is
-another kind of proof for this hypothesis, besides that which consists
-in logically deducing a process from correct premisses; I should like
-to call this the practical proof. If a hypothesis can be made to
-explain a great number of otherwise unintelligible facts, it thereby
-gains a high degree of probability, and this is increased when there
-are no facts to be found which are in contradiction to it.
-
-Both of these criteria are fulfilled by the selection-hypothesis, and
-indeed the phenomena which may be explained by it, and are intelligible
-in no other way, present themselves to us in such enormous numbers,
-that there can be no doubt whatever as to the correctness of the
-principle; all that can be still disputed is, how far it reaches.
-
-Let us now turn our attention to this practical way of proving the
-theory by the facts which it serves to interpret, beginning with a
-consideration of the external appearance of organisms, their colour and
-form.
-
-
-_The Colour and Form of Organisms._
-
-Erasmus Darwin had in many cases already rightly recognized the
-biological significance of the colouring of an animal species, and
-we may be sure that many of the numerous good observers of earlier
-times had similar ideas. I can even state definitely that Rösel von
-Rosenhof, the famous miniature-painter and naturalist of Nürnberg in
-the middle of the eighteenth century, recognized clearly, and gave
-beautiful descriptions of what we now call colour-adaptation. It is
-true that he gave them only as isolated instances, and was far from
-recognizing the phenomenon of colour-adaptation in general, or even
-from inquiring into its causes. From the time of Linné, the endeavour
-to establish new species overshadowed all the finer observation of
-life-habits and inter-relations, and, later on, after Blumenbach,
-Kielmeyer, Cuvier, and others, the eager investigation of the internal
-structure of animals also tended to divert attention from these
-œcological relations. In systematic zoology, colour ranked only as a
-diagnostic character of subordinate value, because it is often not very
-stable, and indeed is sometimes very variable; it was therefore found
-preferable to keep to such relatively stable differences as are to be
-found in the form, size, and number of parts.
-
-Charles Darwin was the first to redirect attention to the fact that the
-colouring of animals is anything but an unimportant matter; that, on
-the contrary, in many cases it is of use to the animal, e.g. in making
-it inconspicuous; a green insect is not readily seen on green leaves,
-nor a grey-brown one on the bark of a tree.
-
-It is plain that the origin of such a so-called 'sympathetic'
-coloration, harmonizing with the usual environment of the animal, can
-be easily interpreted in terms of the principle of selection; and it is
-equally evident that it cannot be explained by the Lamarckian principle
-of transformation. Through the accumulation of slight useful variations
-in colour, it is quite possible for a green or a brown insect to arise
-from a previous colour, but a grey or a brown insect could not possibly
-have become a green one simply by getting into the habit of sitting on
-a green leaf; and still less can the will of the animal or any kind
-of activity have brought the change about. Even if the animal had any
-idea that it would be very useful to it to be coloured green, now that
-it had got into the habit of sitting on a leaf, it could not have done
-anything towards attaining the desirable green colour. Quite recently
-the possibility of a kind of colour-photography on the skin of the
-animal has been suggested, but there are many species whose colouring
-is in contrast to their environment, so that the skin in these cases
-does not act as a photographic plate, and it would, therefore, have
-to be explained how it comes to pass that it functions as such in
-the sympathetically coloured animals. I do not ask for proof of the
-chemical composition of the stuff which is supposed to be sensitive to
-light. Whether this be iodide of silver or something quite different,
-the question remains the same: how comes it that it has only appeared
-in animals to which a sympathetic colouring is advantageous in the
-struggle for life? And the answer, from our point of view, must read:
-it has arisen through natural selection in those species to which a
-sympathetic colouring is useful. Thus even if the supposition that
-sympathetic colouring is due to automatic photography on the part of
-the skin were correct, we should still have to regard it as an outcome
-of natural selection; but it is not correct--at least in general--as
-the above objection shows, and as will be further apparent from many of
-the phenomena of colour-adaptation which I shall now adduce.
-
-To explain sympathetic coloration, then, we must assume, with Darwin
-and Wallace, a process of selection due to the fact that, as changes
-took place in the course of time in the colouring of the surroundings,
-those individuals on an average most easily escaped the persecution of
-their enemies which diverged least in colour from their surroundings,
-and so, in the course of generations, an ever greater harmony with this
-colouring was established. Variations in colouring crop up everywhere,
-and as soon as these reached such a degree as to afford their
-possessors a more effective protection than the colouring of their
-fellows, then natural selection of necessity stepped in, and would only
-cease to act when the harmony with the environment had become complete,
-or, at least, so nearly so that any increase of it could not heighten
-the deception.
-
-Of course, it is presupposed in the working out this selective process
-that the species has enemies which see. This is the case, however, with
-most animals living on the earth or in the water, unless they are of
-microscopic minuteness. Many animals, too, are subject to persecution
-not only in their adult state, but at almost every period of their
-life, and so, in general, we should expect that many of them would have
-attained at each stage that coloration of body that would render them
-least liable to discovery by their enemies.
-
-And this is in reality the case: numerous animals are protected in some
-measure by so-called sympathetic colouring, from the egg to the adult
-state.
-
-Let us begin with the egg, and of course there is no need to speak of
-any eggs except those which are laid. Of these many are simply white
-in colour, e.g. the eggs of many birds, snakes, and lizards, and this
-seems to contradict our prediction; but these eggs are either hidden in
-earth, compost, or sand, as in the case of the reptiles, or they are
-laid in dome-shaped nests, or concealed in holes in trees, as in many
-birds; thus they require no protective colouring.
-
-In other cases, however, numerous eggs, especially of insects and
-birds, possess a colouring which makes it very difficult to distinguish
-them from their usual surroundings. Our large green grasshopper
-(_Locusta viridissima_) lays its eggs in the earth, and they are
-brown, exactly like the earth which surrounds them. They are enough
-in themselves to refute the hypothesis that sympathetic colouring has
-arisen through self-photography, for these eggs lie in total darkness
-in the ground. Insect-eggs which are laid on the bark of trees are
-often grey-brown or whitish like it, and the eggs of the humming-bird
-hawk-moth (_Macroglossa stellatarum_), which are attached singly to the
-leaves of the bedstraw, have the same beautiful light-green colour as
-these leaves, and, in point of fact, green is a predominant colour of
-the eggs in a very large number of insects.
-
-But the eggs of many birds, too, exhibit 'sympathetic' colouring; thus
-the curlew (_Numenius arquata_) has green eggs, which are laid in the
-grass; but the red grouse (_Lagopus scoticus_) lays blackish-brown
-eggs, exactly of the colour of the surrounding moor-soil; and it has
-been observed that they remain uncovered for twelve days, for the
-hen lays only one egg daily, and does not begin to brood until the
-whole number of twelve is complete. Herein lies the reason of the
-colour-adaptation, which the eggs would not have required, if they had
-always been covered by the brooding bird.
-
-The eggs of birds are frequently not of one colour only; those of the
-Alpine ptarmigan (_Lagopus alpinus_), for instance, are ochre-yellow
-with brown and red-brown dots, resembling the nest, which is carelessly
-constructed of dry parts of plants. Sometimes this mingling of colours
-reaches an astonishing degree of resemblance to surroundings, as in the
-golden plover (_Charadrius pluvialis_), whose eggs, like those of the
-peewit (_Vanellus cristatus_), are laid among stones and grasses, not
-in a true nest, but in a flat depression in the sand, and, protected
-by a motley speckling with streaking of white, yellow, grey and brown,
-are excellently concealed. Perhaps the eggs of the sandpipers and gulls
-are even better protected, for their colouring is a mingling of yellow,
-brown, and grey, which imitates the sand in which they are laid so
-perfectly, that one may easily tread on them before becoming aware of
-them.
-
-But let us now turn from eggs to adult animals. Darwin first pointed
-out that the fauna of great regions may exhibit one and the same
-ground-colouring, as is the case in the Arctic zone and in the deserts.
-The most diverse inhabitants of these regions show quite similar
-coloration, namely, that which harmonizes with the dominant colour of
-the region itself. It is not only the persecuted animals, which need
-protection, that are sympathetically coloured in these cases, the
-persecutors themselves are likewise adapted, and this need not surprise
-us, when we remember that the very existence of a beast of prey
-depends on its being able to gain possession of its victims, and that
-therefore it must be of the greatest use to it to contrast as little as
-possible with its surroundings, and thus be able to steal on its quarry
-unperceived. Those that are best adapted in colour will secure the most
-abundant food, and will reproduce most prolifically; and they will thus
-have a better prospect of transmitting their usual colouring to their
-offspring. The Polar bear would starve if he were brown or grey, like
-his relatives; among the ice and snow of the Polar regions his victims,
-the seals, would see him coming from afar.
-
-In the Arctic zone the adaptation of the colouring of the animals to
-the white of the surroundings is particularly striking. Most of the
-mammals there are pure white, or approximately white, at least during
-the long winter; and it is easily understood that they must be so if
-they are to survive in the midst of the snow and ice,--both beasts of
-prey and their victims. For the latter the sympathetic colouring is of
-'protective' value; for the former, of 'aggressive' value (Poulton).
-Thus we find not only the Polar hare and the snow-bunting white, but
-also the Arctic fox, the Polar bear, and the great snowy owl; and
-though the brown sable is an exception, that is intelligible enough,
-for he lives on trees, and is best concealed when he cowers close to
-the dark trunk and branches. For him there would be no advantage in
-being white, and therefore he has not become so.
-
-Desert animals are also almost all sympathetically coloured, that
-is, they are of a peculiarly sandy yellow, or yellowish-brown, or
-clayey-yellow, or a mixture of all these colours; and here again the
-beasts of prey and their victims are similarly coloured. The lion must
-be almost invisible from a short distance, when he steals along towards
-his prey, crouching close to the ground; but the camel too, the various
-species of antelope, the giraffe, all the smaller mammals, and also the
-horned viper (_Vipera cerastes_), the Egyptian spectacled snake (_Naja
-haje_), many lizards, geckos, and the great Varanus, numerous birds,
-not a few insects, especially locusts, show the colours of the desert.
-It is true that the birds often have very conspicuous colours, such as
-white on breast and under parts, but the upper surface is coloured like
-the desert, and conceals them from pursuers whenever they cower close
-to the ground. It has even been observed that a locust of the genus
-_Tryxalis_ is of a light sand-colour in the sandy part of the Libyan
-desert, but dark brown in its rocky parts, thus illustrating a double
-adaptation in the same species.
-
-Another group, which agrees in colour with the general surroundings,
-is that of the 'glass-animals,' as they have been called, though
-perhaps 'crystal animals' is a better term. A great number of simple
-free-swimming marine forms, and a few fresh-water ones, are quite
-colourless, and perfectly transparent, or have at most a bluish or
-greenish tinge, and on this account they are quite invisible as long as
-they remain in the water. In our lakes there lives a little crustacean
-about a centimetre in length, of the order of water-fleas (_Leptodora
-hyalina_), a mighty hunter among the smallest animals, which swims
-forward jerkily with its long swimming-appendages, and widely spreads
-its six pairs of claws, armed with thorny bristles, like a weir basket,
-to seize its prey. We may have dozens of these in a glass of water
-without being able to see a single one, even when we hold the glass
-against the light, for the creatures are crystal-clear and transparent,
-and have exactly the same refractive power as the water. It requires a
-very sharp scrutiny and a knowledge of the animals to be able to detect
-in the water little yellowish stripes, which are the stomachs of the
-animals filled with food in process of digestion, for which, as we can
-readily understand, invisibility cannot very well be arranged. If the
-water be then strained through a fine cloth, a little gelatine-like
-mass of the bodies of the _Leptodora_ will remain on the sieve.
-
-A great many of the lower marine animals are equally transparent,
-and as clear as water; most of the lower Medusæ, the ctenophores,
-various molluscs, the barrel-shaped Salpæ, worms, many crustaceans of
-quite different orders, and above all an enormous number of larvæ of
-the most diverse animal groups. I can remember seeing the sea at the
-shore at Mentone so full of Salpæ, that in every glass of sea-water
-drawn at random there were many of them, and sometimes a glass held a
-positive animal soup. But one did not see them in the glass of water,
-and only those who knew what to look for recognized them by the bluish
-intestinal sac that lies posteriorly in the invisible body. But when
-the water was poured off through a fine net, there remained on the
-filter a large mass of a crystalline gelatinous substance.
-
-It is obvious that this must serve as a protective arrangement, for the
-animals are not seen by their pursuers; but it is not an _absolute_
-protection, for they have many pursuers who do not wait till they see
-their prey, but are almost constantly snapping the mouth open and
-shut, leaving it to chance to bring them their prey. _No protective
-arrangement, however, affords absolute security_; it protects against
-some enemies, perhaps against many, but never against all.
-
-But now let us turn to a group of a different colouring, the green
-animals. We are familiar with our big grass-green grasshopper, and
-we know how easily it is overlooked when it sits quietly on a high
-grass-stem, surrounded by grasses and herbage; the light grass-green of
-its whole body protects it most effectively from discovery: for myself,
-at least, I must confess that in a flowery meadow I have stood right
-in front of one, and have looked close to it for a long time without
-detecting it. In the same way countless insects of the most diverse
-groups--bugs, dipterous flies, sawflies, butterflies--and especially
-the larvæ (caterpillars) of the last, are of the same green as the
-plants on which they live, and this again applies to the predaceous
-species, as well as the species preyed upon. Thus the rapacious
-praying-mantis (_Mantis religiosa_) is as green as the grass in which
-it lurks motionless for its victim--a dragonfly, a fly, or a butterfly.
-
-There are also green spiders, green amphibians like the edible frog,
-and especially the tree-frog, green reptiles like lizards and the
-tree-snakes of tropical forests. It is always animals which live among
-green that are green in colour.
-
-We may wonder, for a moment, why there are so few green birds, since
-they spend so much of their time among the green leaves. But this
-paucity of green birds is only true of temperate climates. In Germany
-we have only the green woodpecker, the siskin, and a few other little
-birds, and even these are not of a bright green, but are rather
-greyish-green. The explanation lies in the long winter, when the
-trees are leafless. In the evergreen forests of the tropics there are
-numerous green birds belonging to very diverse families.
-
-Yet another group with a common colour-adaptation deserves mention--the
-beasts of the night. They are all more or less grey, brown, yellowish,
-or a mixture of these colours, and it is obvious that, in the
-duskiness of night, they must blend better with their environment on
-this account. White mice and white rats cannot exist under natural
-conditions, since they are conspicuous in the night, and the same would
-be true of white bats, nightjars, and owls; but all of these have a
-coloration suited to nocturnal habits.
-
-A very remarkable fact is that in many animals the colour-adaptation
-is a double one. Thus the Arctic fox is white only in winter, while in
-summer he is greyish-brown; the ermine changes in the same way, and the
-great white snowy owl of the Arctic regions has in summer a grey-brown
-variegated plumage. Many animals which are subject to persecution
-also change colour with the seasons, like the mountain hare (_Lepus
-variabilis_), which is brown in summer and pure white in winter, the
-Lapland lemming, and the ptarmigan (_Lagopus alpinus_), which do the
-same. It has been doubted whether natural selection can explain this
-double coloration, but I do not know where the difficulty lies, and
-there is certainly no other principle whose aid we can evoke. The
-mountain hare must have had some sort of colour before it attained to
-seasonal dimorphism. Let us assume that it was brown, that the climate
-became colder and the winter longer, then those hares would have most
-chance of surviving which became lighter in winter, and so a white race
-was formed. Poulton has shown that the whiteness is due to the fact
-that the dark hairs of the summer coat grow white as they lengthen at
-the beginning of winter, and the abundance of new hairs which complete
-the winter coat are from the first white throughout. If the white hairs
-were to persist throughout the summer it would be very disadvantageous
-to their wearer; so a double selection must take place, in summer the
-individuals which remain white, in winter those which remain brown,
-being most frequently eliminated, so that only those would be left
-which were brown in summer and white in winter. This double selection
-would be favoured by the fact that there would be, in any case, a
-change of fur at the beginning of summer; the winter hairs fall out
-and the fur becomes thinner. The process does not differ essentially
-from that which takes place in any species when two or more parts or
-characters, which are not directly connected, have to be changed, such
-as, for instance, colour and fertility. The struggle for existence will
-in this case be favourable, on the one hand, to the advantageously
-coloured, and on the other to the most fertile, and though the two
-characters may at first only occur separately, they will soon be united
-by free crossing, until ultimately only those individuals will occur
-which are at once the most favourably coloured and the most fertile.
-So in this case there remain only those which are brown in summer and
-white in winter.
-
-We must ascribe to the influence of the processes of selection the
-exact regulation of the duration of the winter and summer dress, which
-has been carefully studied in the case of the variable hare. In the
-high Alps it remains white for six or seven months, in the south of
-Norway for eight months, in Northern Norway for nine months, and in
-Northern Greenland it never loses its white coat at all, as there
-the snow, even in summer, melts only in some places and for a short
-time. But apart from concealment there is certainly another adaptation
-involved here--namely, the growth of the hair as a protection against
-the cold. From an old experiment made in 1835 by Captain J. Ross, and
-recently brought to light again by Poulton, we learn that a captive
-lemming kept in a room in winter did not change colour until it was
-exposed to the cold. The constitution of animals which become white in
-winter is thus so organized that the setting in of cold weather acts
-as a stimulus which incites the skin to the production of white hairs.
-This predisposition also we must refer to the influence of natural
-selection, since it must have been very useful to the species that the
-winter coat should grow just when it was necessary as a protection
-against cold. This explains at the same time why the predisposition
-to respond to the stimulus of cold by a growth of winter fur finds
-expression earlier in those colonies of Arctic animals, such as the
-hare, which live in Lapland, than in those which live in the south of
-Norway.
-
-But that it is not the _direct_ influence of cold which colours the
-hair of a furred animal white we can see from our common hare (_Lepus
-timidus_), which, in spite of the winter's cold, does not become white,
-but retains its brown coat, and not less so from the mountain hare
-(_Lepus variabilis_), which in the south of Sweden also remains brown,
-although the winter there may be exceedingly cold. But as the covering
-of the ground with snow is not so uninterrupted there as in the higher
-North, a white coat would be not a better protection than a brown
-one, but a worse. The white colouring of Arctic animals is therefore
-not directly due to the influence of the climate, as has often been
-maintained, but is due to it indirectly, that is, through the operation
-of natural selection. I have tried to make this clear by means of this
-example, so that we may not have to repeat it in considering those
-which are to follow.
-
-       *       *       *       *       *
-
-But all attempts at any other explanation are even more decidedly
-excluded when we turn our attention to more complicated cases of
-colour-adaptation, which are not confined to the simple, general
-coloration, but are helped by markings and colour-patterns, that is, by
-schemes of colour.
-
-Thus numerous caterpillars exhibit definite lines and spots on their
-ground-colouring, which, in one way or another, aid in protecting them
-from their enemies.
-
-The green grass-eating caterpillar of many of our _Satyridæ_ has two or
-more darker or lighter lines running down the sides of its body, which
-make it much less conspicuous among the grasses on which it feeds than
-if it were a uniform green mass (Fig. 2). Not infrequently the colour
-and form present a remarkably close resemblance to the inflorescences
-or fruit-ears of the grasses. Caterpillars marked thus are never found
-on the leaves of trees, where they would immediately catch the eye. It
-is true that longitudinal striping often occurs on caterpillars which
-live on other plants besides grass, but as these other plants grow
-among the grasses the protective efficacy is just the same. This is
-the case with the Pieridæ (Garden Whites).
-
-All the caterpillars of our Sphingidæ, on the other hand, which live
-on bushes and trees, have on the sides of the segments light oblique
-stripes, seven in number, which are disposed to the longitudinal axis
-of the body at the same angle as the lateral veins of a leaf of their
-food-plant have to the mid-rib. It cannot of course be said that the
-caterpillar thereby gains the appearance of a leaf, indeed, if one sees
-it apart from its food-plant it does not look in the least like a leaf,
-but among the leaves of a bush or tree this marking secures it in a
-high degree from discovery. Thus the caterpillar of the eyed hawk-moth
-(_Smerinthus ocellatus_), when it is sitting among the crowded foliage
-of a willow, is often very difficult to find, because its large green
-body does not appear as a single green spot, but is divided by the
-oblique lateral stripes into sections like the half of a willow leaf,
-so that even a searching glance is led astray, there being nothing to
-focus attention on the animal as distinguished from its surroundings
-(Fig. 3). As a boy I often had the interesting experience of
-overlooking a caterpillar which was sitting just before me, until after
-a time I chanced to hit upon the exact spot in the field of vision.
-
-[Illustration: FIG. 2. Longitudinally striped caterpillar of a Satyrid.
-After Rösel.]
-
-[Illustration: FIG. 3. Full-grown caterpillar of the Eyed Hawk-moth,
-_Smerinthus ocellatus_. _sb_, the subdorsal stripe.]
-
-In the majority of these caterpillars with oblique stripes, the
-likeness to the half of a leaf is heightened by the fact that the light
-oblique row is accompanied by a broader coloured band, suggesting the
-shade of the leaf's mid-rib. The caterpillar of _Sphinx ligustri_
-has a lilac band, and that of _Sphinx atropos_ a blue one. In both
-cases it is difficult to believe that such striking colours can
-secure the animals from discovery, yet among the blending shadows
-of the leaf-complex of their food-plant they greatly increase their
-resemblance to a leaf-surface. Of the death's-head caterpillar
-(_Sphinx atropos_) this sounds almost incredible, for this form is
-chiefly a bright golden yellow, and the narrow white oblique stripes
-have sky-blue borders becoming darker towards the under side; but it
-must not be forgotten that the potato is not the true food-plant of
-the species, for it lives, in its true home in Africa, and also in the
-south of Spain, on wild solanaceous plants, which, we are informed
-by Noll, have precisely these colours--golden-yellow and blue in the
-blossom, the fruit, and in part also in the leaves and stem. There the
-caterpillars sit the whole day long on the plants, while with us they
-have formed the habit of feeding only in the twilight and at night, and
-concealing themselves in the earth by day, a habit that is found in
-other caterpillars also, and which we must again ascribe to a process
-of natural selection.
-
-[Illustration: FIG. 4. Full-grown caterpillar of the Elephant Hawk-moth
-(_Chærocampa elpenor_) in its "terrifying attitude."]
-
-Some caterpillars exhibit other, more complex markings, which do not
-protect them by rendering them difficult to detect, but by terrifying
-the enemy who has discovered them, and warning him away. Such
-terrifying or aggressive colours are to be found, for instance, in the
-caterpillars of the Sphingid genus _Chærocampa_ in the form of large
-eye-like spots, which occur in pairs close together on the fourth
-and fifth segments of the animal. Children and those unfamiliar with
-animals take these for true eyes; and as the caterpillar, when it is
-threatened by an enemy, draws in the head and anterior segments, so
-that the fourth one is greatly distended, the eye-spots seem to stand
-on a thick head (Fig. 4), and it cannot be wondered at that the smaller
-birds, lizards, and other enemies are so terrified that they refrain
-from attacking. Even hens hesitate to seize such a caterpillar in its
-defiant attitude, and I once looked on for a long time in a hen-coop
-while one hen after another rushed to pick up a caterpillar I had
-placed there, but, when close to it, hastily drew back the head already
-prepared to strike. Even a gallant cock was a long time in making
-up his mind to attack the terrible beast, and drew back repeatedly
-before he at length ventured to strike a deadly blow with his bill.
-After the first stroke the caterpillar, of course, was lost. Thus even
-this disguise is only a _relative_ protection, effective only against
-smaller enemies. But that these are really frightened away, I had once
-an opportunity of observing, when I put a caterpillar of the common
-elephant hawk-moth (_Chærocampa elpenor_) in the feeding-trough of a
-hencoop, and a sparrow flew down to feed from the trough. It descended
-at first with its back to the caterpillar and fed cheerily. But when by
-chance it turned round, and spied the caterpillar, it scurried hastily
-away.
-
-Among Lepidoptera, too, eye-spots often occur on the wings, and to
-some extent, at least, they have in this case also the significance of
-warning marks. Take, for instance, the large blue and black eye-spots
-on the posterior wings of the eyed hawk-moth (_Smerinthus ocellatus_).
-When the insect is sitting quietly the two spots are not visible, as
-they are covered by the anterior wings, but as soon as the creature is
-alarmed it spreads all four wings, and now both eyes stand boldly out
-on the red posterior wings and alarm the assailant, as they give the
-impression of the head of a much larger animal (see Fig. 5). There are
-also eye-like spots which have not this significance and effect, as,
-for instance, the 'eye-spots' on the train-feathers of the peacock and
-the Argus pheasant, or the little eye-like spots on the under surface
-of many diurnal butterflies. In the first case, it is a matter of
-decoration; in the second, perhaps of the mimicry of dewdrops, which
-increases still further the resemblance to a withered leaf; but there
-are undoubtedly many cases in which the eye-spots serve as means of
-frightening off enemies, and these cases are especially common among
-butterflies.
-
-[Illustration: FIG. 5. The Eyed Hawk-moth in its 'terrifying attitude.']
-
-Such warning marks are in no way contradictory to the sympathetic
-colouring of the rest of the body, and indeed we usually find them
-in combination with it. In some cases the eye-spot, though very
-conspicuous, is covered, as in the eyed hawk-moth, when at rest, by
-the sympathetically coloured parts--in this instance the anterior
-wings. In other cases eye-spots of considerable size lie clearly
-exposed, but exhibit the same sympathetic colours as the whole of the
-rest of the wing-surface. In this case they do not interfere with
-the protective influence of general colouring, because they are only
-visible from a very short distance. This is the case in the large
-_Caligo_ species of South America, which only fly for a short time in
-the early morning and in the evening, remaining concealed throughout
-the day in dark shadowy places, where the mingled colouring of brown,
-grey, yellow, and black on the under surfaces of the wings prevents
-their being recognized from a distance as butterflies at all. But even
-the best sympathetic colouring is not an absolute protection, and when
-the insect is discovered by an enemy near at hand, the terrifying mark,
-a large deep-black spot on the posterior wing, comes into effect, and
-scares the assailant away.
-
-[Illustration: FIG. 6. Under surface of the wings of _Caligo_.]
-
-In such cases the sympathetic colouring was probably the first to
-arise, and the eye-spot was developed later by a new process of
-selection, brought about by the necessity of protecting the species
-more effectively than by mere inconspicuousness alone. In many cases
-it can be proved that the power of scaring off an enemy did not begin
-with the formation of the eye-spot, but with the development of a new
-instinct. When the caterpillar of _Chærocampa elpenor_ is attacked
-it immediately assumes the defiant attitude described above, but the
-same striking attitude is assumed by the caterpillars of the allied
-American genus _Darapsa_, as I learn from an old illustration by Abbot
-and Smith, although this form possesses no eye-spots (Fig. 7). Thus,
-then, metaphorically speaking, the caterpillar at first attempted
-to scare off its enemy by a terrifying attitude alone, and it was
-only subsequently, in the course of the phyletic evolution, that the
-eye-spots were added, in the elephant hawk-moths and other species,
-to heighten the terrifying effect. But that the eye-spot did not make
-its appearance suddenly is proved by several American species of
-_Smerinthus_, in which they are much less perfectly developed than in
-the European species. In these Sphingidæ, too, the defiant attitude
-was evolved earlier than the eye-spots, as we may see from our poplar
-hawk-moth (_Smerinthus populi_), which, when alarmed, spreads out all
-four wings in the same peculiar manner which in the eyed hawk-moth
-(_Smerinthus ocellatus_) displays the eye-spots; it strikes about with
-its wings as if to scare off the enemy, an effect which will certainly
-be more surely achieved if, at the same time, a pair of eyes becomes
-suddenly visible.
-
-Sympathetically coloured caterpillars are, however, by no means the
-only ones; there are some with such striking, glaring colours that,
-far from rendering their possessors inconspicuous, they make them
-visible from a long way off; but this apparent contradiction of the
-theory of the colour-adaptation of animals that require protection
-has been explained by the acuteness of Alfred Russel Wallace. We know
-that among insects, and also among caterpillars, there are many which
-have a repulsive taste. In any case, certain caterpillars are rejected
-by many birds and lizards. Such species are, therefore, relatively
-safe from being devoured. If they were protectively coloured, or if,
-moreover, they resembled caterpillars with an agreeable taste, they
-would gain little advantage from their unpalatability; for the birds
-would at first take them for eatable, and would only discover their
-repulsiveness on attempting to eat them. But a caterpillar which has
-received a single stroke from a bird's bill is doomed to death. It must
-therefore be of the greatest advantage for unpalatable caterpillars,
-and unpalatable animals generally, to be in their colouring as
-conspicuously distinguishable as possible from the edible species.
-Hence, then, the glaring colours, which we can now refer without any
-further difficulty to the process of natural selection, for every
-individual of an ill-tasting species that is more conspicuously
-coloured than its fellows must have an advantage over them, and must
-have a better chance of surviving, because it will be less easily
-mistaken for a member of an edible species.
-
-[Illustration: FIG. 7. Caterpillar of a North American _Darapsa_ in its
-"terrifying attitude" (after Abbot and Smith).]
-
-I should like to discuss one other phenomenon, which is well calculated
-to give us a deeper insight into the transformation processes of
-organisms--I refer to the remarkable dimorphism of colour which occurs
-in many of the species of caterpillar just described.
-
-The caterpillar of the convolvulus hawk-moth (_Sphinx convolvuli_) is
-in its full-grown stage green, like the wild convolvulus on which it
-lives, or brown like the ground on which its food-plant grows. It thus
-shows a double adaptation, each of which is capable of protecting it
-to a certain extent, and we might think to the _same_ extent. But that
-is not so, the brown colouring is a more effective protection than the
-green, as we may learn from two facts. In the first place, the four
-young stages of the caterpillar are green, and it only becomes brown in
-the last stage, though sometimes even then it remains green. This shows
-that the brown is a relatively modern adaptation, and it could not have
-arisen had it not been better than the original green. In the second
-place, the green-coloured caterpillars of the convolvulus hawk-moth
-are nowadays much less numerous than the brown ones, and this implies
-that the latter survive oftener in the struggle for existence. We have
-here an interesting case of an easily recognizable process of selection
-still going on between the old green and the newer brown variety.
-
-It is hardly necessary to ask why the brown colour should in this case
-be a better protection than the green, for it is obvious that such a
-large green body as that of the full-grown convolvulus-caterpillar
-would be but badly concealed among the little leaves of the convolvulus
-plant in spite of its green colour; while the brown caterpillar, on
-the brown soil, with its pebbles, hollows, and irregular shadows, is
-excellently protected, especially if it passes the day concealed in the
-ground, as is actually the case.
-
-Our view is materially strengthened by the fact that the same
-phenomenon of double colouring occurs in several allied species of
-Sphingidæ, but in a manner which shows us that we have to do with a
-similar process of transformation, only at a more advanced stage. The
-caterpillar of _Chærocampa elpenor_ (Fig. 4) shows the same state of
-things as that of the convolvulus hawk-moth; it is brown or green,
-and the green form is the less common. But in the two other European
-species of _Chærocampa_ the full-grown caterpillar is always brown,
-and indeed it becomes brown in the fourth stage, instead of, like
-_Chærocampa elpenor_, only in the fifth and last. Another indigenous
-sphingid species, _Deilephila vespertilio_, only remains green during
-the first two stages, and assumes in the third stage the grey-brown
-colour which it afterwards retains. The dark colour has obviously
-prevailed among the full-grown caterpillars for a considerable
-length of time, for it is in this, the largest and most conspicuous
-stage, that the change of colour must have been most necessary, and
-consequently the process of selection must have begun in it, and only
-after the more protective brown became general would it have extended
-to the next stage below, if it were of use there too, and, later on,
-to still earlier stages in the life-history.
-
-One might be inclined to ascribe this shunting back of a new character
-from the later to the earlier stages of development to purely internal
-forces, which brought it about of necessity, and quite independently
-of whether the extension of the character was useful or injurious.
-We shall come back to this later, and try to find out how far this
-is the case, but in the meantime we may regard at least so much as
-established, that this shunting back does not take place everywhere and
-without limits, but that natural selection calls a halt as soon as its
-effect would be injurious.
-
-[Illustration: FIG. 8. Caterpillar of the Buckthorn Hawk-moth,
-_Deilephila hippophaës_. _A_, Stage III. _B_, Stage V. _r_, ring-spots.]
-
-There could be no continuance of insect-metamorphosis if every
-character of the final stage had to be shunted back to the one next
-below, for then, for instance, the characters of the butterfly must,
-in the course of the phyletic evolution, be carried back to the pupa
-and larva. But even in the larval stage alone it can be seen that
-this carrying back is kept within well-defined limits. Thus, for
-instance, in the dimorphic caterpillars of the Sphingidæ the brown of
-the full-grown stage never comes so far down as the earliest stages,
-for the little caterpillars are all green, like the leaves and stems
-on which they sit. On the other hand, there are species in which the
-green persists, as apparently the most advantageous colour. Thus in
-the buckthorn hawk-moth (_Deilephila hippophaës_) (Fig. 8), which
-lives in the warm valleys of the Alps, and especially in Valais, the
-caterpillars are grey-green in all stages, and are exactly of the
-shade of the lower surface of the buckthorn leaves; they possess no
-oblique lines, for these would not make them more like the leaves, as
-the full-grown caterpillars are much bigger than an individual leaf
-of buckthorn, on which, moreover, the lateral veins are not very
-conspicuous. Nevertheless the caterpillar enjoys very fair security, as
-it does not feed through the day, but only in twilight and at night;
-it passes the daytime concealed in the dry leaves and earth about the
-base of the bush. Its resemblance to the leaves is very great, and is
-increased by the fact that it bears on the last segment a comparatively
-large orange-coloured spot (_r_), exactly the colour of the buckthorn
-berry, which ripens just at the time that the caterpillar attains its
-full growth.
-
-But butterflies are as much persecuted, and have as much need of
-protection, as caterpillars, and among them, too, we find many
-instances of protective colouring, which are the more interesting in
-that they occur, as a rule, only on such parts of the body as remain
-visible when the insect is at rest, which is exactly what we should
-expect if the coloration has been wrought out in the course of natural
-selection. But it is well known that the resting position of diurnal
-Lepidoptera is quite different from that of the nocturnal forms, and is
-not even the same among all families, and in accordance with this we
-find the sympathetic colouring occurs on quite different areas in the
-different families.
-
-The reason why the butterflies only require to be protected by their
-colour in the sleeping or resting position is that no colour whatever
-could make a flying butterfly invisible to its enemies, because the
-background against which its body shows is continually changing during
-its flight, and, moreover, the movement alone is enough to betray it,
-even if it is of a dull colour.
-
-Thus, in general, only those parts of a butterfly's wing that are
-invisible at rest could safely bear bright or conspicuous colour, while
-the visible portions had to acquire sympathetic coloration through
-natural selection.
-
-As the diurnal butterflies, when at rest, turn their wings upward and
-bring them together, it is only the under side which is sympathetically
-coloured, and that only as far as it is visible, that is, the whole of
-the posterior wing, and as much of the anterior one as is not covered
-by it. Many diurnal butterflies, when at rest, fold the anterior wing
-so far back that only its tip remains visible, and in such cases only
-this tip is protectively coloured, while in other forms, which have not
-this habit, almost the whole surface of the wing is sympathetically
-coloured.
-
-A very simple protective colouring is exhibited by our 'lemon
-butterfly' (_Rhodocera rhamni_), in which the under surface is a
-whitish yellow, which protects the insect well when it settles on
-the dry leaves on the ground in the light woods which it is fond of
-frequenting.
-
-Our gayest diurnal butterflies, the species of _Vanessa_, all have the
-under surface of a dusky colour, sometimes passing into a blackish
-brown, as in the peacock-butterfly, _Vanessa_ (_v. io_), sometimes
-more into greyish brown, or brown-yellow, or reddish brown. They are
-never simple colours, but always consist of mixtures of different
-colour-tones--indeed, there is often a complex mingling of many
-colours, as grey, brown, black, white, green, blue, yellow, and red,
-made up of dots, strokes, spots, and rings, into a wonderful and
-very constant pattern, which, taken as a whole, has the effect of
-being uniform, and harmonizes with the soil, or with the highway, on
-which the species loves to settle, with much greater accuracy than a
-monochrome grey or brown would do. When the 'painted lady' (_Vanessa
-cardui_) settles on the ground it is hardly distinguishable from
-it, and this species in particular has a preference for settling
-on the ground. Other species of _Vanessa_, such as the peacock and
-the Camberwell beauty (_Vanessa antiopa_), are underneath of a dark
-blackish grey, or even black; when resting they press themselves into
-the darkest corners and crevices, and are thus most effectively secured
-from discovery.
-
-Many diurnal Lepidoptera, on the other hand, especially the
-wood-butterflies of the family Satyridæ, have the habit of resting
-on the trunks of trees, as _Satyrus proserpina_ does on the great
-beech-trunks of the forest clearings. These large butterflies, coloured
-conspicuously on the upper surface in deep velvety black and white, are
-marked on the under surface exactly to match the whitish bark of the
-great beech, covered over with white, grey, blackish-brown, and yellow
-spots, and the butterfly whose flight one has just been carefully
-following disappears as it suddenly alights on such a tree-trunk. As I
-have already stated, the protective colour only extends over as much
-of the insect as is seen when it is at rest. As the anterior wings are
-folded far back between the posterior ones, the protective colouring is
-limited to the whole surface of the posterior wing, and the tip of the
-anterior one, as far as that is visible in the resting attitude; the
-protectively coloured area is somewhat sharply bounded, and it is often
-of very different extent in quite nearly allied species, according
-to whether the species folds the anterior wing far back or not. Thus
-in our common small tortoiseshell-butterfly (_Vanessa urticæ_) the
-protective area is considerably wider than in the large tortoiseshell
-(_Vanessa polychloros_), much as the two resemble each other in other
-details.
-
-This harmony between the wing tips and the posterior wings is nowhere
-wanting, where the under side is protectively coloured at all, but in
-many cases the protective colouring spreads over almost the whole of
-the anterior wings, and these are then not folded far back when at
-rest, as will be seen later in the so-called leaf-butterflies.
-
-There is one genus of diurnal butterflies which seems to contradict
-the law that all the surface that is visible in the resting position
-exhibits the protective coloration--the South American wood-butterflies
-of the genus _Ageronia_. They have on the upper surface a very
-complicated bark-like pattern of mingled grey on grey, and this
-confirms the usual rule, for we know that these butterflies--a striking
-exception among all the other diurnal forms--settle with outspread
-wings on the trunk of a tree in exactly the same attitude as many of
-the nocturnal Lepidoptera of the family of the Loopers or Geometridæ,
-in which the upper surface is also deceptively like the bark of the
-tree on which they rest.
-
-[Illustration: FIG. 9. _Hebomoja glaucippe_, from India; under surface.
-_A_, in flight. _B_, in resting attitude.]
-
-In all the nocturnal Lepidoptera it is the _upper_ side of the wing
-which is sympathetically coloured, if protective coloration has been
-developed at all. In all the Sphingidæ, many 'Owls' and Bombycidæ, the
-anterior wings are grey banded with darker zigzag lines, and mottled
-with many shades of black, grey, yellow, red, and even violet. As the
-anterior wings cover the body and the posterior wings like a roof,
-they make the resting insect very inconspicuous when it has settled
-on wooden fences, trunks of trees, or even old timber. When bright
-colours--red, yellow, or blue--occur in these moths it is always on
-the posterior wings, which are covered when at rest. This can best be
-observed in the species of the genus _Catocala_.
-
-Let us now, however, interrupt our survey of the facts for a moment,
-and let us inquire whether all the cases of protective colouring in
-Lepidoptera we have considered can be referred to natural selection, or
-whether it is not conceivable that other causes may have evoked them.
-
-[Illustration: FIG. 10. _Xylina vetusta_, after Rösel. _A_, in flight.
-_B_, at rest.]
-
-The first thing to be said is that the Lamarckian principle of the
-inherited effects of use and disuse cannot here be taken into account,
-as the colours of the surface of the body do not exercise any active
-function at all; their effect is due simply to their presence, and
-it is for them quite indifferent whether and how often they have
-opportunity to protect their bearers from enemies, or whether no
-enemies ever chance to appear. It has frequently been suggested, too,
-that these colorations are associated with the differences in the
-strength of the illumination to which the different parts and surfaces
-are exposed. But this again is untenable, as is proved even by the
-dimorphism frequently occurring in caterpillars, for the green and
-the brown individuals are exposed to precisely the same light; and
-still more clearly by the sympathetic colouring, which is so exactly
-defined and yet so different on the under surface of the diurnal
-butterflies. Yet there are isolated cases in which it seems as if
-the direct influence of the light had brought about certain striking
-differences in the colouring of the parts of an insect, and I shall
-describe perhaps the prettiest of these cases, to which Brunner von
-Wattenwyl directed attention. It concerns one of the Orthoptera of
-Australia, a Phasmid, _Tropidoderus childreni_, Grey, which has a
-general colouring of leaf-green, but with singular deviations from it
-on certain areas of the body. In this insect the anterior wings which
-form the wing covers or elytra (Fig. 11, _V_) are so short that they
-scarcely cover the half of the long abdomen. Their place is taken by
-the anterior margin of the posterior wing (_H. horn_), which is hard
-and horny like the elytra, and in the resting position protects the
-whole abdomen. All these covering parts are grass-green, except at the
-places where they overlap; on these areas they have a faded look, and
-are yellowish instead of green. Brunner says of this: 'The phenomenon
-gives the impression that the more brilliant colour is a character due
-to daylight. If several sheets of white paper of unequal dimensions be
-placed one above the other, ... and exposed to the sun, after a short
-time silhouettes of the smaller sheets will appear on the larger ones,
-either in a lighter or in a darker colour. Probably this "fading"
-of the covered parts in the Phasmid also belongs to this "category
-of photographs."' This seems convincing, but analogous phenomena in
-other insects prevent our regarding the pretty comparison with the
-photograph as a sufficient explanation. If it were a question of a
-diurnal butterfly, such an assumption would have to be rejected on this
-ground alone, that the wing colouring is developed in the pupa, and
-appears perfect and unalterable as soon as the perfect insect emerges.
-But in the pupa the position of the wings is exactly the reverse
-of that seen in the resting attitude of a butterfly, that is, the
-protectively coloured under side of the wing is not turned towards the
-light but away from it. Moreover, in the pupa the anterior wings cover
-the posterior ones completely, no matter what the wing position may be
-later in the perfect insect. Furthermore, the thick and often darkly
-coloured sheath of the pupa prevents the light having any effect, and
-not a few species pass their pupal stage in such dark places--for
-instance, under stones, as in the case of many 'Blues'--that the light
-can hardly reach them. And if the light did exercise an influence, how
-could it produce such diverse coloration as the protective colours of
-diurnal butterflies, on the one side dark, even to blackness, on the
-other side, yellow, reddish, and even white and pure green; and how
-should the same rays of light call forth complicated colour patterns
-on one and the same surface, for instance, the white, sprinkled with
-green, of the Aurora butterfly (_Anthocharis cardaminis_)? Finally, we
-have only to remember that numerous nocturnal Lepidoptera pass through
-their pupa stage underground, although they exhibit brilliant as well
-as protective colours in the most appropriate distribution, to reject
-once for all the hypothesis that the influence of light plays any
-decisive rôle in determining the distribution of the colours on the
-wings of Lepidoptera.
-
-But it is otherwise with _Tropidoderus_. In this case the wings grow
-gradually during the slow growth of the animal, which takes place in
-full light, and the wings of the young insect probably lie one above
-the other, in exactly the same position, and cover the same places as
-in the full-grown form; we might, therefore, from the facts of the
-case, admit the possibility that the yellow of the covered portions is
-due to the exclusion of light.
-
-[Illustration: FIG. 11. _Tropidoderus childreni_, after Brunner von
-Wattenwyl, in flying pose. _V_ anterior wing. _H. häut_, membranous
-part of posterior wing. _H. horn_, horny portion.]
-
-But as soon as the conditions that obtain among Lepidoptera are
-also taken into consideration we recognize the insufficiency of the
-interpretation suggested, for among butterflies we have precisely the
-same phenomenon--sharp limitation of the protective colouring to the
-parts visible in the resting position, a fact which, in the case of
-the said butterflies, admits of no other interpretation than that of
-natural selection. Let us therefore see if we cannot, in the case of
-_Tropidoderus_, arrive at some better understanding of the phenomenon
-than that implied in the theory of direct light-influence. Obviously,
-the yellow parts of the animal do not require to be green, since they
-are not visible in the sitting position, and the locust in flight could
-not by any device be made invisible. It therefore only remains to be
-explained why the yellow parts are not colourless, and why they are
-not also green. We cannot at present answer with any confidence; it is
-possible that the colouring matter which causes the green only becomes
-green under the influence of direct sunlight, and otherwise remains
-yellow; it is possible, too, that, as in Lepidoptera (see Fig. 9), the
-full protective colour is only developed by natural selection in the
-places which are visible in the sitting position, and that the covered
-parts take on any indifferent colour, which might be readily afforded
-by the metabolism of the insect. But this much is certain, that the
-covered parts would be green, if that were advantageous to the survival
-of the species, just as the under surface of the wings of some diurnal
-butterflies is green. Had it been required, the green colour would have
-resulted in the course of natural selection, just as it has resulted
-in the most different parts of the most diverse insects, even in those
-whose development takes place entirely removed from the influence of
-light. Therein lies the difference between our interpretation and that
-of Brunner von Wattenwyl: without natural selection no explanation of
-this case is possible.
-
-[Illustration: FIG. 12. _Notodonta camelina_, after Rösel. _A_, in
-flight. _B_, at rest.]
-
-Hitherto I have spoken only of the diurnal butterflies in which the
-anterior wings show an extension of the protective colouring which
-marks the whole surface of the posterior wings, and it was always the
-tips of the anterior wings that were thus coloured. But among the
-nocturnal Lepidoptera there are corresponding cases, in which a little
-tip of the posterior wing forms the continuation of the protective
-surface of the anterior wing. Some species of _Notodonta_ and allied
-genera show in the posterior corner of the otherwise whitish posterior
-wings a little grey spot, and a hair tuft which in colour, and--when it
-is big enough--in marking, exactly resembles the protectively coloured
-anterior wings (Fig. 12). The 'why' is at once clear, when one looks at
-the insect in the resting position, for only this little corner of the
-wing projects beyond the covering anterior wing. This has been regarded
-as telling against natural selection, for such a little spot could not
-possibly, by its colour, turn the scale as to the life or death of the
-individual, and so could not be selected. But one might say the same of
-the tip of the anterior wing in the diurnal forms, although there the
-protective surface is larger, often much larger. But who is to decide
-how large an exposed, unprotected spot must be in order to attract
-the attention of an enemy on the look out for food? Or who can prove
-that the best and most familiar protective colouring really protects
-its possessors? What if, after all, it is all a game, a joke, which
-the Creator is playing with us poor mortals? Did not a trustworthy
-observer recently watch carefully, and see how a pair of sparrows daily
-cleared a wooden fence on which moths of the genus _Catocala_ and other
-species of nocturnal Lepidoptera, excellently furnished with protective
-colours, were wont to settle by day? They did their work thoroughly,
-and hardly overlooked a single individual. But who has a right to see
-anything more in this than--what surely goes without saying--that the
-best protective colouring is not an absolute protection, and never
-preserves all from destruction, but always only some, and it may be
-very few.
-
-How else could there be such a high ratio of elimination, and such a
-constancy in the number of individuals of a species on any unchanging
-area? These sparrows had simply made full use of an experience,
-probably acquired by chance to begin with, and their vision had become
-sharpened for this particular species on the almost similarly coloured
-wooden fence, just as that of the expert butterfly collector does. It
-certainly does not follow from this that the protective colouring was
-useless, nor can we regard the harmony between the protruding tip of
-the anterior or posterior wing and the large protectively coloured
-surface of the covering wing as of no importance. On the contrary, if
-the tips were white or conspicuously coloured like the rest of the
-posterior wing, they would assuredly attract the sharp eye of hungry
-enemies to the spot, and so betray the victim. Instead of this, the
-spot in question is not only dark, but, in the case of _Notodonta_, is
-furnished with a tuft of hairs, which, in the insect's resting position
-(Fig. 12, _B_), lies on the back, and looks like a dark, somewhat
-curved projecting tooth, in front of which there stands another, quite
-similar, which arises from the anterior wing, and behind there are
-other seven, rather smaller, dark teeth of the same kind, springing
-from the outer edge of the anterior wing. Taken altogether, they mimic
-the dentated edge of a withered leaf, and thus, in spite of their
-diverse origins, form a unified picture, and one with a considerable
-protective value. How is it possible to doubt that each of these
-hair-tufts has arisen under the influence of natural selection, and
-that its absence or imperfect development might result in the discovery
-and elimination of the insect concerned?
-
-These cases seem to me particularly beautiful proofs of the productive
-efficiency of selection. The wing is protected just as far as it
-protrudes from beneath the other--not a millimetre further! How should
-it be otherwise, when the colouring of the parts just beside these is
-indifferent for the species, so that any variations in these parts in
-the direction of protective colouring never survive to be transmitted
-and accumulated?
-
-It is precisely this restriction to what is absolutely necessary that
-is the surest sign, here and elsewhere, that the character in question
-has been brought about by natural selection. And if this is the only
-possible, and at the same time quite sufficient explanation of the
-remarkably well-defined colour deliminations in all Lepidoptera,
-there can be no reason why we should try to drag in any other factor
-to explain the case of _Tropidoderus_, the less so as here again
-selection alone can account for the green of the exposed surfaces;
-and furthermore, the modification, common in other Phasmidæ, of the
-most anterior green stripe of the posterior wing into a firm cover
-protecting the soft abdomen, also points to natural selection; the
-cover-wings proper have here become too short, and so the edge of
-the posterior wing has been modified into a hard rib, which protects
-the soft body of the insect (Fig. 11, _H. horn_). No differences in
-illumination, and no _direct_ effect of any external influence whatever
-could have brought that about.
-
-How much more I might adduce in this connexion! The manifold diversity
-of colour and form adaptation is so great among insects, to which
-protection from their enemies is so necessary, and especially among
-butterflies, that I should never come to an end if I were to try to
-give even an approximate idea of it. Let us, therefore, turn now from
-such cases to a higher--the highest--grade of adaptation, that in which
-there is not only a mimicry of special and complex coloration, but in
-which the whole animal has become like some external object, and is
-thereby secured from discovery.
-
-We must first consider the case of our lappet moth (_Gastropacha
-quercifolia_), which in its copper-red colour and in the remarkable
-shape and dentated edges of its wings, and finally in the quite
-extraordinary clucking-hen-like attitude of the wings when at rest,
-greatly resembles some dry oak-leaves lying one above the other.
-
-Not unlike this is a 'shark' moth found in this country, _Xylina
-obsoleta_, which, as the name indicates, looks when at rest like a
-broken bit of half-rotten wood (Fig. 10, p. 77). It 'feigns death,' as
-we commonly say, that is, it draws the legs and antennæ close to the
-body, and does not move; indeed, one may lift it up and throw it on the
-ground without its betraying by a single twitch that it lives. Only
-after it has been left undisturbed for some time does it show signs of
-life again, and makes off hastily, to find a better hiding-place. The
-colouring of this moth is so curiously mingled--brown, whitish, black,
-and yellow--and traced with acute-angled lines and curves, that one
-cannot distinguish it at sight from a bit of rotten wood. I experienced
-that myself once when, passing a hedge, I thought I saw a _Xylina_
-sitting on the ground, and picked it up to examine it. I thought it was
-a bit of wood, and, disappointed, I threw it down again on the grass,
-but then I felt uncertain, and picked it up once more--to find that it
-was a moth after all[1]!
-
-[1] Rösel says in this connexion: 'The marvellous form of this Papilio
-preserves it from injuries, for, when he hangs freely on a trunk of a
-tree, he would be taken ten times sooner for a piece of bark than for
-a living creature. By day, too, he is so little sensitive, that if he
-be thrown down from his resting-place he falls to the ground as if
-lifeless, and remains lying motionless. One may also throw him into
-the air, or turn him about, and he will rarely give a sign of life.
-I have impaled many of them on needles, without seeing any sign of
-sensitiveness on their part. This is the more remarkable that these
-birds (sic), after they have submitted to all the torment and misery
-one can inflict on them, without showing any sign of feeling, will,
-whenever they are left in peace and have no further disturbances
-to fear, quickly creep off to a dark corner and attempt to conceal
-themselves from future attacks.'--_Insektenbelustigungen_, Nürnberg,
-1746, vol. i. p. 52.
-
-This case of _Xylina_ is hardly less remarkable, and its likeness to
-the mimicked object is scarcely less wonderful than that of the often
-discussed mimicry of a leaf, with stalk, midrib, and lateral veins, by
-many of the forest butterflies of South America and India.
-
-[Illustration: FIG. 13. _Kallima paralecta_, from India, right under
-side of the butterfly at rest. _K_, head. _Lt_, maxillary palps. _B_,
-limbs. _V_, anterior wing. _H_, posterior wing. _St_, 'tail' of the
-latter, corresponding to the stalk of the leaf. _gl_^1 and _gl_^2,
-transparent spots. _Aufl_, eye-spots. _Sch_, mould-spots.]
-
-The best known of these is the Indian _Kallima paralecta_, which, when
-it settles, is deceptively like a dead leaf, or rather like a dry or
-a half-withered one, on which brown alternates with red, and on which
-there are one or two translucent spots, without scales, presumably
-representing dewdrops. The upper surface of this butterfly is simply
-marked, but gorgeously coloured--blue-black with a reddish yellow,
-or bluish white band--and quite constant. The under surface, on the
-other hand, although it always resembles a dead leaf, shows very
-varied ground colours, being sometimes greyish, sometimes yellowish,
-or reddish yellow, or even greenish. Often it shows the lateral
-veining of the leaf quite as distinctly as in Fig. 13, but often quite
-indistinctly, and the black, mouldy spots (_Sch_) of our figure may
-be more strongly marked, or they may be absent. It would seem as if
-the mimicry of different kinds of leaves was here aimed at--so to
-speak--just as in the case of the varied and numerous species of the
-South American genus _Anæa_, which usually live in the woods, and are
-all more or less leaf-like, but each species is like a different leaf,
-or like a leaf in a different condition, dry, moist, or decomposing.
-It is simply astounding to see this diversity of leaf mimicry, and
-the extraordinary faithfulness with which the impression of the leaf
-is reproduced. But it is by no means always the venation which causes
-the resemblance, for this is often inconspicuous; the high degree
-of deceptiveness is due to the silvery-clear yellow, dark yellow,
-red-brown to dark black-brown ground-colouring, which is never quite
-uniform, and over which there usually spreads a whitish ripple,
-combined with the remarkable imitation of the sheen of many leaves. The
-upper side of this butterfly is almost always conspicuously decorated
-with violet, dark blue or red, but always without any relation to the
-under surface. Not in all, but in many of the species of this genus,
-we find the round, translucent mirrors on the wing already mentioned
-in the case of _Kallima_, and in some species quite remarkable means
-are made use of to make the resemblance to a leaf thoroughly deceptive.
-Thus _Anæa polyxo_, when sitting, looks like a leaf out of the edge
-of which a caterpillar has eaten a little piece; in reality there is
-nothing missing from the wing, but on the front margin of the anterior
-wing a semicircular spot of a bright, soft, yellow colour stands out so
-sharply from the rest of the chestnut-brown wing surface, that it has
-the effect of a hole in the leaf.
-
-[Illustration: FIG. 14. _Cœnophlebia archidona_, from Bolivia, in its
-resting attitude. _mr_, midrib of the apparent leaf. _st_, the apparent
-stalk.]
-
-A modern opponent of the selection theory (Eimer) has suggested
-that the marking of the lateral veins, and other resemblances to a
-leaf in _Kallima_, represent nothing more than the pattern which
-was present in any case, inherited from ancestors, and which in the
-course of time arranged itself in a particular manner according to
-internal developmental laws. Not selection--that is, adaptation to
-surroundings--but the internal developmental impulse has brought about
-the resemblance to the leaf. It is astonishing how a preconceived
-idea can blind a man and weaken his judgment! It goes without saying
-that the adaptations do not start from a _tabula rasa_, but from
-what is already present; of course, natural selection makes use of
-the markings inherited from ancestors; it takes what already exists,
-and alters or extends it as suits best. Thus it is easy to prove
-that the clear mirrors (Fig. 13, _gl_^1 and _gl_^2) on the wings of
-_Kallima_ have arisen from a modification of the nuclei of eye-spots,
-just as the dark mould-spots which often occur, frequently develop
-in association with the inherited eye-spots; not always however, for
-many such accumulations of black scales occur in spots on which there
-has never been an eye-spot. Thus, too, the 'midribs' of the butterfly
-have in part arisen from a gradual displacing, extending, and altering
-of the direction of inherited stripes as, for instance, is clearly
-recognizable in the posterior wing of Fig. 13, but sometimes they are
-new formations. But the veining of a leaf is never found on the wing
-of any butterfly of a species which has not the habit of resting among
-leaves, or which has not had it at one time, and it never corresponds
-to the natural marking of any genus which does not live in forests.
-This impression of leaf-venation has obviously arisen from quite
-different patterns of markings, and it has been reached now by one
-way, now by another. We can see this from the fact that, in different
-butterflies, it lies in quite different positions on the wing. In
-the _Kallima_ species the stalk of the leaf lies in the tail of the
-posterior wing, the tip of the midrib lies near the tip of the wing; in
-_Cœnophlebia archidona_ it is exactly reversed, the tip of the anterior
-wing (Fig. 14) is prolonged, and forms the stalk, while a broad,
-dark, stripe, the midrib (_mr_), runs from there across the middle of
-both wings, and seems to give off two or three lateral ribs running
-outwards. If it be asked whether this butterfly always sits down so
-artistically that the 'upward turning leaf-stalk is in juxtaposition
-to a twig,' we may answer that a bird flying fast is not likely to
-look to see whether every leaf in the profusion of foliage in the
-primitive forests is properly fastened to its stalk or not, any more
-than we should do in the case of a painted bush, on which many a leaf
-has the appearance of floating in the air, just as in nature, or in its
-faithful copy, the photograph.
-
-[Illustration: FIG. 15. _Cærois chorinæus_, from the lower Amazon, in
-its resting attitude. _V_, anterior wing. _H_, posterior wing. _mr_,
-midrib of the apparent leaf. _sr_, lateral veins. _st_, hint of a
-leaf-stalk.]
-
-Quite different from the leaf-marking either of _Cœnophlebia_ or
-_Kallima_ is that of one of the Satyrides of the lower Amazon valley,
-_Cærois chorinæus_ (Fig. 15). If one spreads this butterfly out in the
-usual way it does not look in the least like a leaf, and one only sees
-a number of curiously placed disconnected stripes on the under surface
-of the wing. But if the wings be folded together to correspond with
-the sitting position of the butterfly, there appears the figure of a
-leaf, of which, however, only half is present, and whose midrib (_mr_)
-runs obliquely forward from the inner angle of the posterior wing.
-Here, again, it is not difficult to guess that this straight stripe has
-arisen, by displacement and straightening, from a curved line inherited
-from some remote ancestor, and it is these precise changes which are
-the work of the adaptive processes of natural selection. The same
-applies to the lateral ribs (_sr_), which are here four in number.
-
-But even the division of the wing surface by a single dark line, such
-as that which crosses the middle of the posterior wing of _Hebomoja_
-(Fig. 9), an Indian butterfly, heightens not inconsiderably the
-resemblance of the resting butterfly to a leaf, a resemblance which
-has already been shown in the form and colour. Indeed, even the sharp
-division of the wing surface into a darker inner and a lighter outer
-portion, which occurs in many species of _Anæa_, gives a very vivid
-impression of a leaf crossed by a midrib.
-
-It is not without a purpose that I have lingered so long over the
-leaf-butterflies. I wished to make it clear that we have by no means
-to do with a few exceptional cases, but with a great number, in all
-of which resemblance to a leaf has been aimed at, although it has
-been attained in varying degrees, and by very diverse ways. Whoever
-surveys this wealth of fact must certainly receive the impression,
-that, wherever it was advantageous to the existence of the species, the
-evolution of such a deceptive resemblance has also been possible. In
-any case one cannot but be convinced that it is not a case of chance
-resemblance, as some naturalists have recently tried to maintain.
-
-But I have not yet quite finished my outline-survey of the facts, for I
-must not omit to mention that, in the evergreen tropical forests, there
-are also large nocturnal Lepidoptera, which mimic leaves, sometimes
-green ones, sometimes brown, dead ones.
-
-[Illustration: FIG. 16. _Phyllodes ornata_, from Assam. Upper surface
-with leaf-like marking only on the anterior wing, which is the only
-part visible when at rest; ⅔ nat. size.]
-
-Fig. 16 gives a good picture, reduced to two-thirds, of such a species,
-_Phyllodes ornata_, from Assam. The posterior wings are conspicuously
-coloured in deep black and yellow; in the resting position they are
-covered by the anterior wings, and these are red-brown with black
-markings which precisely and clearly mimic the ribs of a leaf. The
-midrib begins near the tip of the anterior wing, but breaks off
-half-way across the wing at two silvery white spots, similar to those
-in many of the diurnal forms, which also mimic decaying leaves. Three
-pairs of side veins go off backwards and forwards with remarkable
-regularity from the midrib, almost at the same angle, and parallel
-to one another, and three more are indicated by vague shading. Then
-the midrib begins again in the internal half of the wing, though only
-represented by a broad shading. The whole suggests two torn, rotten
-leaves, one partly covering the other; and the deception will certainly
-be perfect when the moth rests on the ground or among decaying leaves.
-
-That all these extremely favourable protective colorations find their
-explanation in the slow and gradually cumulative effects of natural
-selection cannot be disputed; it is beyond doubt that they cannot be
-explained, so far as we know, in any other way.
-
-If, however, it were possible for a species of butterfly living in
-the forest and among leaves to become, through natural selection,
-in any degree, and in a continually increasing degree, like a leaf,
-surely many insects living in the woods, and especially in the
-tropical woods, would also have followed such an advantageous path of
-variation--at least, so we should be inclined to think. And this is
-indeed the case; numerous insects, of different orders, if they are
-as large as a leaf, have taken on the colour, form, and usually also
-the markings, of a leaf. Thus green and also decaying and dead leaves
-are most realistically imitated by many tropical Locustidæ. Besides
-_Tropidoderus_, figured on p. 79, a _Pterochroa_ of South Brazil
-affords a particularly fine illustration of this, for not only does
-the ground-colour, brown or green, harmonize with that of a dead or
-fresh leaf, but, at the same time, all sorts of details are marked on
-the insect, which help to heighten the deceptive impression. Even the
-outline of the wings is leaf-like, and leaf-veins are marked on the
-wing-covers with the most beautiful distinctness, and finally there
-is, especially in the light-green individuals, a spot at the wing tip
-which, by means of a mixture of brown, yellow, reddish, and violet
-colour-tones, mimics a decaying spot with astonishing fidelity. Here,
-again, the origin of this special adaptation can be clearly recognized,
-for the vaguely concentric arrangement of the colours indicates
-that, in the ancestors of the species, an eye-spot had occurred on
-this area, of the same kind as we still see on the posterior wing,
-which is covered in the resting position. Thus we can again look
-back on the history of the species and conclude that the dissolution
-and degeneration of the eye-spot began at the time when the leaf
-resemblance was evolved, and this was probably caused by some change of
-habitat, which we can now no longer guess at.
-
-Many species of leaf-like Orthoptera, both in the Old and New World,
-have tough, green, parchment-like wing-covers which bear a remarkable
-resemblance to the thick Magnolia-like leaves of tropical plants. Along
-with these we must also mention the 'walking leaf,' which has been well
-known for centuries. In its case, not the wing-covers alone, but the
-head and thorax, and even the legs, are of the colour and shape of a
-leaf.
-
-The stick-insects, too, must not remain unnoticed; those quaint
-inhabitants of warm countries, whose elongated brown body looks like a
-knotted twig, and whose long legs, likewise stick-like, are stretched
-out irregularly at different angles to the body, and usually remain
-motionless when the insect is resting. These creatures are vegetarian,
-and generally keep so still, that even the naturalist who is on the
-look-out for them may easily overlook them. Even such an experienced
-student of insects as Alfred Russel Wallace was deceived, for a native
-of the Phillipines once brought him a specimen as a 'walking-stick'
-insect, which he rejected, saying that this time it was no animal
-but really a twig, until the native showed him that it was an insect
-whose likeness to a twig was increased by the fact that it bore on
-its back a ragged green growth, which looked exactly like a liverwort
-(_Jungermannia_), which occurs on the twigs of the trees in that region.
-
-We must also notice here the thorn-bugs, which are numerous on the
-prickly shrubs of tropical deserts and plateaux, especially in Mexico.
-These bear on the relatively very small body two or three large spines,
-which make them look like a part of the thorny bush on which they sit.
-But this masking by mimicry of thorns is not confined to insects, it is
-seen in lizards as well, notably in _Moloch horridus_, a lizard that
-lives in the Australian bush, and is covered all over with thorn-like
-scales.
-
-These examples should be enough to show that mimicry of the usual
-surroundings on the part of animals which are in need of protection,
-or are wont to lurk on the watch for their prey, are not isolated
-exceptions, chance resemblances, or, as they used to be called, 'freaks
-of nature,' but that, on the contrary, they are the rule, depending on
-natural causes, and always occurring when these causes are operative.
-That such protective resemblances seem to be much more frequent in
-warmer climates than with us is probably a fallacy due to the fact that
-the number of species (especially of insects) is very much greater
-there, and that many insect types have their representatives of
-considerable size of body, which not only makes them more conspicuous
-_to us_, but makes some protective device in relation to their enemies
-or victims much more necessary.
-
-But we must here take account of one more example which occurs in our
-fauna in many modifications: the caterpillars of Geometridæ. Many
-of these soft and easily injured caterpillars resemble closely, in
-colour and shade, the bark of the tree or shrub on which they live
-(Fig. 17). At the same time they have the habit, when at rest, of
-stretching themselves out straight and stiff, so that they stand out
-free, at an acute angle from the branch, thus seeming like one of its
-lateral twigs. In many species the resemblance is heightened by the
-extraordinary pose of the head (_K_) and of the claw-like feet (_F_),
-which, partly pressed close to the head, partly standing out from it,
-give the anterior end of the caterpillar the appearance of two terminal
-buds, while various little pointed, knotlike warts, scattered over the
-body, represent the sleeping buds of the little twig. Who has not at
-one time or other taken such a caterpillar for a little branch, and not
-inexpert observers only, but even trained naturalists? Many a time I
-have not been able to make quite sure of what I had before me until I
-touched it!
-
-[Illustration: FIG. 17. Caterpillar of _Selenia tetralunaria_, seated
-on a birch twig. _K_, head. _F_, feet. _m_, tubercle, resembling a
-'sleeping bud'; nat. size.]
-
-
-
-
-LECTURE V
-
-TRUE MIMICRY
-
-Mimicry: its discovery by Bates--Heliconiidæ and
-Pieridæ--Danaides--_Papilio merope_ and its five females--The females
-lead the way--Species with mimicry in both sexes--Objections--Enemies
-of butterflies--The immunity of the models--Poisonousness of the
-food-plants of immune species--Several mimics of the same immune
-species--Persecuted species of the same genus resemble quite different
-models--_Elymnias_--Degree of resemblance--Differences between the
-caterpillars of the model and the copy--The same resemblance arrived
-at by different ways--Transparent-winged butterflies--The gradually
-increasing resemblance points to causes operating mechanically--Rarity
-of the mimetic species--Danger to the existence of the species not a
-necessary condition of mimetic transformation--_Papilio meriones_ and
-_Papilio merope_--Comparison with the dimorphic caterpillars--_Papilio
-turnus_--'Mimicry rings' of immune species--_Danais erippus_ and
-_Limenitis archippus_--Marked divergence of mimetic species from their
-nearest relatives--Mimicry in other insects--Imitators of ants and bees.
-
-
-LET us now turn to the most remarkable of all protective form- and
-colour-adaptations, the so-called Mimicry, including all cases of the
-imitation of one animal by another, which we came to know first through
-Bates, and to a fuller understanding of which A. R. Wallace and Fritz
-Müller have especially contributed.
-
-While the English naturalist, Bates[2], was collecting and observing
-on the banks of the Amazons--as he did for twelve years--it sometimes
-occurred that, among a swarm of those gaily coloured, quaintly shaped
-butterflies, the Heliconiidæ (Pl. II, Fig. 13), he caught one which,
-on closer examination, proved to be essentially different from its
-numerous companions. It was certainly like them both in colour and
-form, but it belonged to quite a different family of butterflies,
-that of the Pieridæ or Whites (Pl. II, Fig. 19). These whites with
-the colours of the Heliconiidæ always occurred singly in swarms of
-the latter form, and Bates found that, in the different districts of
-the Amazon, they always resembled in a striking manner the species of
-Heliconiidæ there prevalent. Many of them had been previously known
-to entomologists, and because they diverged so far from the usual
-type of the Pieridæ, especially in the form of the wing, the name
-Dysmorphia, the 'mis-shapen,' had been given to them, although the
-meaning of this 'mis-shapenness' long remained a mystery. The French
-Lepidopterist, Boisduval, went a step further when he pointed out as
-something remarkable that nature sometimes makes several species of
-quite different families exactly alike, and called attention to three
-African butterflies, of which we shall have to speak later in detail.
-But even he was too much fettered by the old views of the immutability
-of species to arrive at a correct interpretation. Thus it was reserved
-for Bates to take the decisive step. Observing that the Heliconiidæ
-occurred frequently, and usually in large swarms, he concluded
-that they must have few enemies, and as he never saw the numerous
-insectivorous birds and insects hunting them, he further concluded
-that they must have something disagreeable which secured them from the
-attacks of these predaceous forms. On the other hand, he found that the
-heliconid-like Whites were always rare, and he took this as a sign that
-they were much persecuted, and that they must, therefore, be palatable
-tit-bits for the insectivores. If it were possible, then, that a
-species of Whites with the usual white colour of the family should give
-rise to variations, which would make them in any degree resemble the
-Heliconiidæ, which are secure from persecution, and if, in addition,
-those that exhibited the profitable variation attached themselves to
-swarms of the mimicked form, then these variants would be to a certain
-extent secured from attack, and more and more so in proportion as the
-resemblance to the protected model increased. The great likeness of
-these Whites to the Heliconiidæ, Bates further argued, would depend on
-a process of selection, based on the fact that, in each generation,
-those individuals would on the average survive for reproduction
-which were a little more like the model than the rest, and thus the
-resemblance, doubtless slight to begin with, would gradually reach its
-present degree of perfection.
-
-[2] _Contributions to an Insect Fauna of the Amazon Valley_, Trans.
-Linn. Soc., Vol. XXIII, 1862.
-
-Bates's hypotheses have been subsequently confirmed in the most
-striking way. The Heliconiidæ do possess a disagreeable taste and
-odour, and are utterly rejected by birds, lizards, and other animals.
-It has been directly observed that puff-birds, species of _Trogon_,
-and other insectivorous birds, looking down from the tops of trees in
-search of food, allowed to pass unheeded the swarms of gaily coloured
-Heliconiidæ which were fluttering among the leaves, and experiments
-with various insectivorous animals yielded the same result: _the
-Heliconiidæ are immune_. We can, therefore, not only understand that
-it must be advantageous to resemble them, we can also appreciate many
-of their peculiar characters, such as their gay coloration, which must
-serve as a sign of their disagreeable taste, and their slow, fluttering
-flight, as well as their habit of flocking together, which must make
-it easier for the birds to recognize them as uneatable. Everything
-which marks out these unpalatable morsels, and makes them more readily
-recognizable, must be to their advantage, and therefore must have been
-favoured by natural selection (Pl. II, Fig. 13).
-
-In the same way, every increase of resemblance on the part of the
-mimics would increase their chances of escaping notice, and any one
-who is accustomed to observe butterflies in nature can well understand
-that even very slight resemblances may have formed the beginning of
-the selection process; perhaps even a mere variation in the manner
-of flight, combined with the habit of associating with the swarms of
-Heliconiidæ. I myself have many times been momentarily deceived in our
-own woods by a White of unusually majestic flight, so that I took it
-for an _Apatura_ or a _Limenitis_. If, therefore, individual Whites
-occurred here and there in the Amazon valley, which flew somewhat after
-the manner of the Heliconiidæ, and associated with them, they might
-possibly have attained a certain degree of security through that alone,
-and it would be greatly increased if at the same time they varied
-somewhat in colour in the direction of their companions.
-
-In any case there can be no doubt whatever that in these cases a real
-transformation of the species in colour and marking, and perhaps often,
-too, in form of wing, has taken place, and that within comparatively
-modern times--let us say during the distribution of a species which
-required protection over a large continent, or since the last breaking
-up of an immune species into local species. Various facts prove this;
-above all, the circumstance that it is often only the females which
-exhibit this protective mimicry; and that one and the same species may
-mimic a different immune species in different areas, but always the one
-occurring abundantly in that area, and so on.
-
-Definite examples will make this clearer, and I will only say in
-advance that, since the discovery of Bates, numerous cases of mimicry
-in butterflies have been found, not only in South America, but in all
-tropical countries which have a rich Lepidopteran fauna. And it is not
-only between the Heliconiidæ and the Pieridæ that such relations have
-been evolved; many much-persecuted, unprotected species of different
-families everywhere mimic species which are rejected on account of
-their nauseous taste, and these, too, belonging to different families.
-The Heliconiidæ are a purely American group, but in the Old World
-and in Australia their place is taken by the three great families of
-Danaides, Euplœides, and Acræides, since, as it seems, they all taste
-unpleasantly, and are rejected by all, or at least by most, of the
-insectivorous birds. Numerous species of the genus _Danais_ (Pl. I,
-Fig. 8), _Amauris_ (Pl. I, Fig. 5), _Euplœa_ (Pl. III, Fig. 25, 27),
-and _Acræa_ (Pl. II, Fig. 2), and also many species of _Papilio_ and
-other genera, enjoy the advantage of unpleasant taste, if not even of
-poisonousness; they are, therefore, secure from pursuit, and are, in
-consequence, much mimicked by palatable butterflies.
-
-As a further example, I now select a diurnal butterfly from Africa,
-_Papilio merope_ Cramer[3], which was shown by Trimen in 1868 to be
-mimetic. The species has a wide distribution, for, if we except slight
-local differences in the marking of the male, its range extends over
-the greater part of Africa, from Abyssinia to the Cape, and from East
-Africa to the Gold Coast.
-
-[3] The West African form of _Papilio merope_ has been quite recently
-distinguished from the southern form and regarded as a distinct
-species, the latter being now called _Papilio cenea_. The differences
-in the males are very slight--somewhat shorter wings, shorter
-wing-tail, and so on--differences which seem relatively unimportant in
-comparison with the differences between the males and the females.
-
-The male is a beautiful large butterfly, yellowish white, with a touch
-of black, and with little tails to the posterior wings (Pl. I, Fig.
-1), like our own swallowtail. A very nearly related species occurs
-in Madagascar, and there the female is similarly coloured, though it
-may be distinguished by having a little more black on the wing. On
-the mainland of Africa, however, the females of _Papilio merope_ are
-so different in colour and form of wing that it would be difficult to
-believe them of the same species as the male had not both sexes more
-than once been reared from the eggs of one mother. The females (Pl. I,
-Fig. 6) in South Africa imitate a species of _Amauris_, _A. echeria_
-(Pl. I, Fig. 7), of a dark ground-colour with white, or brownish-white,
-mirrors and spots, and they resemble it most deceptively. But what
-makes the case more interesting in its theoretical aspect is that
-_Danais echeria_ of Cape Colony is markedly different from _Danais
-echeria_ of Natal, and the female of _Papilio merope_ has followed
-those two local varieties, and has likewise a Cape and a Natal local
-form. Even this is not all, for in Cape Colony there are two other
-females of _Papilio merope_. One of them has a yellow ground-colour,
-and resembles _Danais chrysippus_, which is extremely abundant there
-(Pl. I, Fig. 3); the other is entirely different (Pl. I, Fig. 4), for
-it closely mimics another Danaid occurring in the same districts of
-Africa, and also immune, _Amauris niavius_ (Pl. I, Fig. 5), not only in
-the beautiful pure white and deep black of the wing surface, but also
-in the distribution of these colours to form a pattern.
-
-We have thus in Africa four different females of _Papilio merope_,
-each of which mimics a protected species of Danaid. They are not
-always locally separate, so that each is exclusively restricted to a
-particular region, for their areas of distribution often overlap, and,
-at the Cape for instance, one male form and three different forms of
-female have been reared from one set of eggs. In addition, we have the
-fact that between the two local forms of _Danais echeria_ transition
-forms occur, and that the mimetic females of _Papilio merope_ show
-the same transition forms locally, and we must admit that all these
-facts harmonize most beautifully with the selection interpretation,
-but defy any other. And that the last doubt may be dispelled, nature
-has preserved _the primitive female form_ on the continent of
-Africa--namely, in Abyssinia, where, along with the mimetic females,
-there are others which are tailed like the males (Pl. I, Fig. 1), and
-are like them in form and colour, a few minor differences excepted.
-
-Thus we have in _Papilio merope_ a species which, in the course of
-its distribution through Africa, has scarcely varied at all in the
-male sex, but in the female has almost everywhere lost the outward
-appearance of a _Papilio_, and has assumed that of a Danaid, which is
-protected by being unpalatable, and not even everywhere the appearance
-of the same species, but in each place that of the prevailing one, and
-sometimes of several in one region. These females thus show at the
-present day a polymorphism which consists of four chief mimetic forms,
-to which has to be added the primitive form--that resembling the male.
-This has survived in Abyssinia alone, and even there it is not the only
-one, but occurs along with some of the mimetic forms.
-
-To the question why only the females are mimetic in this and other
-cases, Darwin and Wallace have answered that the females are more in
-need of protection. In the first place, the males among butterflies
-are considerably in the majority, and, secondly, the females must
-live longer in order to be able to lay their eggs. Moreover, the
-females, which are loaded with numerous eggs, are heavier in flight,
-and during the whole period of egg-laying--that is, for a considerable
-time--they are exposed to the attacks of numerous enemies. Whether
-one of the abundant males is devoured sooner or later is immaterial
-to the persistence of the species, since one male is sufficient to
-fertilize several females. The death of a single female, on the other
-hand, implies a loss of several hundred descendants to the species.
-It is, therefore, intelligible that, in species already somewhat
-rare, the female must first of all be protected; that is to say, that
-all variations tending in the direction of her protection would give
-rise to a process of selection resulting in an augmentation of the
-protective characters.
-
-But there are also butterflies in which both sexes mimic a protected
-model. Thus many imitators of the unpalatable Acræides (Pl. II, Fig.
-21) resemble the model in both sexes, and of the South American Whites
-which mimic the Heliconiidæ there are some which have the appearance
-of the Heliconiidæ even in the male sex (Pl. II, Fig. 18, 19), while
-others look like ordinary Whites (for instance, _Archonias potamea_).
-But in many of these species, which are mimetic in the female sex, we
-find also in the male some indications of the mimetic colouring, but
-in the first instance only on the under surface. Thus the females of
-_Perhybris pyrrha_ (Pl. II, Fig. 17) resemble in their black, yellow,
-and orange-red colour-pattern the immune American Danaid, _Lycorea
-halia_ (Pl. II, Fig. 12), but their mates are, on the upper surface,
-like our common Whites, though they already show on the under surface
-the orange-red transverse stripes of the _Lycorea_ (Pl. II, Fig. 16).
-In other mimetic species of Whites a similar beginning is even more
-faintly hinted at, and in others, again, the upper surface of the male
-is also provided with protective colours, and only a single white spot
-on the posterior, or sometimes even on the anterior wing as well, shows
-the original white of the Pieridæ (Fig. 18).
-
-I do not know how any one can put any other construction on these
-facts than that the females first assumed the protective colouring,
-and that the males followed later, and more slowly. Whether this is
-due to inheritance on the female side, and thus ensues as a mechanical
-necessity, in virtue of laws of inheritance still unknown to us, or
-whether it arose because there was a certain advantage in protection
-to the males--though not such a marked one--and that these, therefore,
-followed independently along the same path of evolution as the females,
-has yet to be investigated. Personally, I incline to the latter view,
-because there are protected mimetic species, in which the female
-mimics one immune model, and the male another, quite different from
-the female's. A case in point is that of an Indian butterfly, _Euripus
-haliterses_, and also _Hypolimnas scopas_, in the latter of which
-the male resembles the male of _Euplœa pyrgion_, and the female is
-like the somewhat different female of the same protected species. The
-Indian _Papilio paradoxus_, too, seems to show the independence of the
-processes of mimetic adaptation, for the male is like the blue male
-of the immune _Euplœa binotata_ (Pl. III, Fig. 25), while the female
-resembles the radially-striped female of _Euplœa midamus_ (Pl. III,
-Fig. 27), and this double adaptation is repeated in another of the
-persecuted butterflies, _Elymnias leucocyma_ (Pl. III, Fig. 26, 28).
-
-Many objections have been made to the interpretation of mimicry by
-selection. It has been asserted that butterflies are exposed to injury
-from birds only to an inconsiderable extent, not sufficient to account
-for such an intense and persistent process of selection, because they
-are not very welcome morsels, on account of the large and uneatable
-wings and the relatively small body. Doubt has also been raised as to
-the immunity of the models, which has not been proved in many of the
-species in regard to which it is assumed. Finally, it is maintained
-that the advantage which resemblance to an immune model brings is
-not proved, but is purely hypothetical; and that it is probable that
-the birds do not distinguish the colours and markings of the flying
-butterflies at all, but are at the most only deceived by resemblances
-in their manner of flight.
-
-The last objection contains a certain amount of truth, inasmuch as
-the manner of flight always plays a part in the mimicry of a strange
-species. We shall see later how much the instincts of a species
-contribute to the deception in all cases of protective colouring.
-It is, therefore, not improbable that, in many cases, the imitation
-of the flight of an immune species, and a gradually increasing
-familiarity with the habitats of the same immune species, preceded the
-modification of the colour. Indeed, the slow flight of immune species
-(Heliconiidæ) has been unanimously emphasized by observers, as a factor
-in facilitating the recognition of the butterflies by the sharp-sighted
-birds.
-
-That it was not only in earlier ages of the world's history that
-butterflies were much persecuted, as some have supposed, but that they
-are so still, seems to me indisputable in view of the observations
-of the last quarter of a century. Even in this country, where both
-butterflies and insect-eating birds are being more and more crowded out
-through cultivation, a considerable number of butterflies in flight
-fall victims to the birds. Kennel gives observations on this point in
-regard to the white-throat; Caspari for the swallows. The latter let
-about a hundred little tortoiseshell butterflies (_Vanessa antiopa_)
-fly from his window, 'but not ten of them reached the neighbouring
-wood,' all the rest being eaten by swallows, 'which congregated in
-numbers in front of his window.' Kathariner observed, in the highlands
-of Asia Minor, a flock of bee-eaters (_Merops_) which caught in flight
-and swallowed a great many individuals of a very beautiful diurnal
-butterfly (_Thais cerisyi_).
-
-Finally, Pastor Slevogt has collected much evidence to show that
-our indigenous butterflies have a great deal to suffer in the way
-of persecution from birds. And in regard to tropical countries, the
-chase of butterflies by insectivorous birds has long been known. Thus
-Pöppig says that in the primitive forests one can easily recognize
-the place which has been selected by one of the Jacamars (Galbulidæ)
-as its favourite resting-place, for the wings of the largest and most
-beautiful butterflies, whose bodies alone are eaten, lie on the ground
-in a circle for a distance of several paces. We owe direct observations
-on the hunting of insects by birds of the primitive forest especially
-to Dr. Hahnel, who found many opportunities for observation in the
-course of his enthusiastic collecting journeys in Central and South
-America. He writes: 'No other family of butterflies suffered so much
-from birds as the Pieridæ (Whites), and these freebooters often snapped
-away the prettiest and freshest specimens from quite close to me. Every
-time I was amazed anew at the unfailing security of their flight, and I
-gladly paid for the spectacle by the loss of a few specimens.' Of the
-pursuit of one of the large _Caligo_ species, whose leaf-like under
-surface, marked with eye-spots, I have already described, (Fig. 6, p.
-70), he says: 'With incredible skill this fairly large insect avoided
-every blow of the bill of the bird which followed it in close chase,
-and saved itself by flying from one shrub to another, till at last it
-was lost to sight in the thickest tangle of branches, and the exhausted
-bird gave up further attempts at pursuit.'
-
-But, in addition to the birds, the butterflies of the primitive forest
-have to dread the persecution of other insects, especially of the large
-predaceous dragon-flies, which throw themselves upon them in the midst
-of their flight. Hahnel often saw a specimen of the large, beautiful,
-blue _Morpho cisseis_, which was fluttering peacefully about the crown
-of a tree, suddenly shoot head downwards, 'like an ox with horns
-lowered, and then reascended apparently with difficulty, after it had
-torn itself free from its sudden assailant, whose jaws left distinct
-short scars.'
-
-In addition to birds and predatory insects the butterflies are
-persecuted by the whole army of lizards. In order to entice the
-butterflies, Hahnel laid bait in the wood, 'sugar-cane, little sweet
-bananas, and such like.' Various kinds of butterfly settled on it,
-'Satyrides, Ageroniæ, _Adelpha_ and other Nymphalidæ.' He saw that they
-'were persistently stalked and attacked by greedy lizards, which, in
-spite of their plump figure and uncouth gait, showed themselves able
-to spring suddenly out and snatch their prey with great adroitness. It
-is, however, very wonderful to see the agility such a persecuted insect
-displays in evading the repeated attacks of these marauders.' Thus on
-one occasion an _Adelpha_ was driven off a dozen times from the exposed
-bait by a lizard, which pounced upon it, but it always settled down
-for a short time on a leaf, and soon returned to its repast, whereupon
-the enemy 'instantaneously rushed upon it in a fury, until at last he
-was obliged to give in,' abandoning the attempt to catch a creature so
-adept in retreat.
-
-Many butterflies assemble at midday on sandbanks in the middle
-of the river, in order to drink, and there, too, the lizards are
-always lurking about. Hahnel gives a pretty and undoubtedly accurate
-description of the protective value of the long tail borne by many
-of the sail-like Papilios at the end of the posterior wing; they
-'quite obviously' afford protection against the lizards, 'which, after
-snapping, often find themselves obliged to be content with the tail
-alone, while the rest of the animal flies away practically uninjured.'
-
-Not only is the great persecution of the butterflies a fact, the
-immunity of the known species, which are models for mimicry, is
-also certain. For numerous species, at any rate, this has now been
-established. First of all--as has already been said--this is true of
-the Heliconiidæ, in regard to which Wallace long ago showed that, if
-the thorax be pressed, they exude a yellowish juice of unpleasant
-smell. This is probably the blood of the insect, but that does not
-hinder the repulsive odour of the living butterfly being perceptible at
-a distance of 'several paces,' as Seitz observed in _Heliconius besei_.
-
-Repeated experiments have been made, which have shown that such
-butterflies are rejected not only by the insectivorous birds of the
-primitive forest, but also by tame turkeys, pheasants and partridges,
-usually so greedy. Hahnel has recently repeated these experiments in
-Brazil with hens, and he obtained the same result. The hens, 'which
-otherwise devoured all butterflies eagerly,' rejected all Ithomidæ,
-Heliconiidæ, the white Papilios, as also some of the gaily coloured
-Heliconiid-like moths which fly by day, such as _Esthema bicolor_ and
-_Pericopis lycorea_. Obviously, the gay or conspicuous colour of these
-Lepidoptera acts as a warning signal of their unpalatability, and
-protects them from attempts on the part of the birds to investigate
-their flavour. Hence we find that the under surface of these insects
-is coloured like the upper. Even the numbers of these species which
-fly about indicates that they must be little decimated, and, in point
-of fact, we never find the wings of Heliconiidæ lying on the ground in
-the forests of South America, while those of the Nymphalidæ and other
-butterflies are by no means uncommonly seen as the remains of birds'
-meals.
-
-There is just as little room for doubt, as in the case of the
-Heliconiidæ and their allies, that the Danaidæ, Acræidæ, and the
-Euplœidæ in the tropical regions of the Old World enjoy a certain
-immunity on account of their repulsive odour and taste. Here, too,
-observation and experiment have shown that birds, lizards, and
-predaceous insects leave the butterflies of these families unmolested.
-I need only mention the observation of Trimen that, under an acacia
-much visited by butterflies, on which Mantides--the so-called
-praying-insects--caught and devoured large numbers, the wings of an
-_Acræa_ or a _Danais_ were never found. These unpalatable butterflies
-also possess a motley or at least striking dress, recognizable from
-afar, and alike on both surfaces; and they also have a slow flight,
-by which they are readily recognized. They, too, usually assemble
-in large swarms, and both sexes are alike, or resemble each other
-closely in colouring, or at least they are both equally conspicuous.
-But even these cases do not complete the list of butterflies which are
-protected by their unpalatability; among the otherwise much-persecuted
-and therefore palatable Pieridæ (Whites) there is an Asiatic genus,
-_Delias_, which in all probability belongs to the immune butterflies,
-as their gaily coloured under surface indicates, and among the
-nocturnal Lepidoptera of different countries and families there are
-isolated generations which are very gaily and conspicuously coloured,
-and which are rejected by birds, their unpleasant odour being
-perceptible at a distance of several feet (Chalcosiidæ and Eusemiidæ).
-The latter no longer fly under cover of night, like their relatives,
-but have assumed diurnal habits.
-
-It is to be supposed that the repulsiveness of such 'unpalatable'
-butterflies is associated with the food-plant on which the caterpillar
-lives. Acrid, nauseous, astringent, and actually poisonous substances
-are produced in many plants, and we shall see later that this is to
-their own advantage; these substances pass into the insect, and they
-do so probably in part unaltered, in part certainly altered, but still
-they are protective, perhaps even in an increased degree. This is
-borne out by the fact that many caterpillars of immune butterflies
-live on more or less poisonous plants: the Acræidæ and Heliconiidæ on
-Passiflores, which contain nauseous substances; the Danaidæ on the
-poisonous Asclepiadæ, which are rich in milky juice or latex; the
-Euplœæ on the poisonous species of _Ficus_, the Neotropinæ on the
-Solanaceæ, and so on. But there are many genera, rich in species,
-and distributed over the whole earth, the caterpillars of which live
-on plants of very various families and characters, and of these the
-majority of species are palatable, though a few are repulsive in
-taste and odour, and therefore immune. This is the case in the genus
-_Papilio_. As far back as the sixties Wallace discovered that there
-were immune species of _Papilio_, and that these were mimicked
-by other species. Later it was shown that these immune species
-live chiefly on poisonous plants (in the wide sense), on various
-Aristolochiæ; and Haase has recently grouped these together as
-poison-eaters (Aristolochia-butterflies or Pharmacophagæ). They are
-distinguished by a conspicuous red on the body. In some of them, as in
-_Papilio philoxenus_, a repulsive odour as of decomposing urine has
-been detected in the living animal.
-
-We see, then, that the much-persecuted and easily injured butterflies
-make use of a poisonous substance (in the widest sense), prepared in
-the plant for its own protection, and, wherever their own metabolism
-makes it possible, they use it to protect themselves. We need not
-wonder, therefore, that so many butterflies are immune, nor that among
-the numerous palatable species a small proportion have endeavoured to
-become like the protected species, as far as natural selection was able
-to bring such a resemblance about.
-
-There is hardly any adaptation phenomenon so widely distributed and
-diverse in its manifestations, which has been at the same time so much
-observed and followed out into all its details, as Mimicry; and it must
-surely be regarded as a justification of the validity of interpreting
-it in terms of Natural Selection that all the observed phenomena tally
-so beautifully with the deductions from the theory. I at least know of
-no facts which contradict the theory, but of many which might have been
-predicted from it.
-
-For instance, it might have been predicted from the theory alone that
-an immune species would often have several mimics, as, in point of
-fact, is frequently the case, and it would be easy to give numerous
-examples of this. Thus the two Danaids of South and Central Africa,
-_Amauris echeria_ and _Amauris niavius_, are mimicked, not only
-by the two female forms of _Papilio merope_, as we have already
-described in detail, but the latter is also mimicked by Nymphalid,
-which requires protection, _Diadema anthedon_, and the former by two
-diurnal butterflies of different families, _Diadema nuina_ and _Papilio
-echerioides_.
-
-Similarly, the black-and-red coloured _Heliconius melpomene_ in Brazil
-is mimicked both by the female of a White (_Archonias teuthamis_),
-and by a _Papilio_, which has received the name of _P. euterpinus_ on
-account of this resemblance. Thus, too, the immune _Methona psidii_,
-Cr. of Brazil, with its half-transparent wings marked with black bands,
-has five mimics, belonging to five different genera, and one of these
-is not a true diurnal butterfly at all, but one of the day-flying
-species of the genus _Castnia_, whose systematic position is doubtful.
-
-[Illustration: FIG. 18. Upper surfaces of _A_, _Acræa egina_, from the
-Gold Coast, immune. _B_, _Papilio ridleyanus_, from Gaboon, not immune.
-_C_, _Pseudacræa boisduvalii_, from the Gold Coast, not immune.]
-
-The West African immune Acræid, _Acræa gea_ (Pl. II, Fig. 21), is
-deceptively mimicked, both as to the narrow, long shape of the wing
-and its blackish-brown and white mottled markings, by a Nymphalid,
-_Pseudacræa hirce_, by the female of a Papilio (_P. cynorta_) whose
-mate is quite different, and by the female of a Satyrid (_Elymnias
-phegea_) (Pl. II, Fig. 20). In the _Papilio_ the resemblance extends to
-the peculiar pitch-black shining spot on the under side of the base of
-the posterior wing, and all three are like the model on both surfaces,
-and therefore in flight as well as in the resting attitude.
-
-On the same West African coast occurs the strange greyish-black _Acræa
-egina_, with brick-red spots and bands, and coal-black dots (Fig. 18,
-_A_). This immune species is deceptively mimicked in its native country
-by two other butterflies--a Nymphalid, _Pseudacræa boisduvalii_ (Fig.
-18, _C_), and by a female _Papilio_ (_P. ridleyanus_) (Fig. 18, _B_),
-by the latter not so exactly as by the former, but quite sufficiently
-to be confused with its model in flight.
-
-It would have been less easy to predict with certainty from the theory
-that, conversely, the different species of a genus which stood in need
-of protection would be able to mimic quite different immune models, for
-who would have ventured to prophesy how far the capacity of a species
-for variation might go, and how many different kinds of coloration it
-was able to assume? But the facts teach us that there is a wide range
-of possibility in this respect.
-
-Most interesting in this respect is, perhaps, the Asiatic-African
-genus _Elymnias_, a Satyrid whose numerous (over thirty) species
-all seem to be in need of protection, for many of them mimic immune
-butterflies, while the rest are inconspicuous and are provided with
-protective colouring on the under surface. On Plates II and III some
-of the former are depicted beside their models. The single African
-species (_Elymnias phegea_) (Pl. II, Fig. 20) mimics, as has been
-already mentioned, the prevalent _Acræa gea_ (Pl. II, Fig. 21). Many
-of the Asiatic Elymniidæ are mimics of the immune Euplœæ, especially
-the dark-brown species with steel-blue shimmer, such as _E. patna_ in
-India, _E. beza_ in Borneo, and _E. penanga_ in Borneo. In Amboina
-there flies an _E. vitellia_, the female of which mimics accurately the
-plain, light-brown, inconspicuous _Euplœa climena_ which occurs there.
-The male of _Elymnias leucocyma_ (Pl. III, Fig. 26) resembles the brown
-and blue shimmering _Euplœa binotata_ (Pl. III, Fig. 25), while the
-female mimics the dusky, radially-striped female of _Euplœa midamus_
-(Pl. III, Figs. 27 and 28): the male of _Elymnias cassiphone_ resembles
-the blackish-brown and deep-blue iridescent _Euplœa claudia_, while the
-female is like the female of _Euplœa midamus_. A number of species of
-_Elymnias_ copy Danaids: thus both sexes of _E. lais_ are like _Danais
-vulgaris_ (Pl. III, Figs. 29 and 30), and _E. ceryx_ and _E. timandra_
-are like another similar Danaid, _D. tytia_. The female only of _E.
-undularis_ of Ceylon mimics the brown-yellow _D. genutia_ (Pl. II, Fig.
-22) in general appearance, though not minutely, while the male (Pl.
-II, Fig. 24) seems to attempt an imitation of the blue EuplϾ. A rare
-form, not often represented in collections, _Elymnias künstleri_, bears
-a striking resemblance to the Danaid, _Ideopsis daos_ Boisd., with its
-white wings spotted with black, while three species mimic the probably
-immune Pierid genus _Delias_, especially on the under surface, which
-is decorated with yellow and red. Perhaps the one which has diverged
-farthest from the original type is _Elymnias agondas_ Boisd. (Pl. II,
-Fig. 32) of the Papua region and the island of Waigeu, for it bears
-two large blue eye-spots on the posterior wings, and thus, especially
-in the case of the almost white female, closely resembles _Tenaris
-bioculatus_ (Pl. III, Fig. 31). There are thus seven or eight types
-of marking and colouring differing from one another, and belonging to
-six different genera and a much greater number of species, which are
-mimicked by this one genus _Elymnias_.
-
-It is most interesting to note how these mimetic species give up, more
-or less, the original sympathetic colouring of the under surface,
-and use in establishing their mimicry the marking elements which
-were originally directed towards concealment. According to the
-beautiful observations of Erich Haase on this genus _Elymnias_, the
-ground-colouring on the under surface must have been 'a grey, darkly
-mottled protective one,' as still occurs, for instance, in several
-mimetic species, such as _Elymnias lais_ (Pl. III, Fig. 30). This
-leaf-colouring disappears more and more the more perfect the mimicry of
-the model becomes, until, finally, the model is repeated on the under
-surface also. Compare, for instance, Figs. 30 and 32. From this we
-may conclude that a dress which makes Lepidoptera appear unpalatable
-morsels is a more effective protection than resemblance to a leaf. That
-might indeed be deduced even from the theory, for resemblance to a leaf
-never protects _absolutely_, and does so, in any case, only during
-rest, while apparent unpalatability repels assailants at all times.
-
-Those unversed in butterfly lore usually ask, when these mimetic
-relations are expounded to them, how we know that copies which are
-so like their models really belong to a different genus, or even
-family. There are certainly cases in which model and copy resemble
-each other so closely that even a zoologist cannot tell one from the
-other without close examination, as, for instance, in the case of
-certain transparent-winged Heliconiidæ of Brazil (Ithomiides) and their
-mimics belonging to the family of Whites. But even in such cases the
-likeness only extends as far as is theoretically requisite, that is,
-only to those characters that make the butterfly appear to the eye of
-its pursuer like another species, known to it to be unpalatable. The
-likeness does not extend to details, which can only be seen with a
-magnifying-glass or a microscope, and above all, it does not extend
-to the caterpillar, pupa, or egg. Thus, in the case cited, we may be
-certain that the caterpillar of _Ithomia_ is quite different from
-that of the mimicking White, since the former will be, in structure,
-of the type of _Ithomia_ caterpillar, and the other of the usual type
-of Whites. As yet, indeed, these two species are not known in their
-caterpillar stages, but other cases are known. A species belonging to
-the same genus as our indigenous 'kingfishers' (_Limenitis populi_), a
-diurnal butterfly of North America, _Limenitis archippus_ (Pl. I, Fig.
-9), strongly resembles the brown-yellow, immune _Danais erippus_ (Pl.
-I, Fig. 8), while the caterpillars of both species are quite different,
-that of _Danais erippus_ possessing the remarkable, soft and flexible
-horn-like processes of the Danaid caterpillars (Pl. I, Fig. 10_a_),
-while the caterpillar of _Limenitis archippus_ (Pl. I, Fig. 11_a_) is
-at once recognizable by its blunt, club-shaped and spinose papillæ
-as a _Limenitis_ caterpillar. The adaptation of the butterfly to its
-protected model has thus exercised no influence upon the caterpillar.
-Nor has it affected the pupa, which in both cases exhibits the very
-different and quite characteristic form of the _Danais_ pupa and the
-_Limenitis_ pupa respectively (Pl. I, Fig. 10_b_, and 11_b_).
-
-But even in the butterfly itself nothing is altered, except what
-increases the resemblance to the model. All else has remained
-unchanged, above all, the venation of the wings. Since the painstaking
-and valuable work of Herrich-Schäfer the venation has been made the
-basis of the whole systematic arrangement of butterflies, and it
-enables us, in point of fact, to distinguish with precision, not the
-families alone, but often even the genera, for the course of the
-veins in the different species of a single genus is the same, and
-that is true for the mimetic species as well as for others. Thus the
-Danaid-like _Limenitis_ has the usual _Limenitis_ venation, of the kind
-seen in our own indigenous species of _Limenitis_, and the already
-described _Elymnias_ species of the African and Indian forests and
-grassy plains have always the venation characteristic of this genus,
-whether they be protected only by sympathetic colouring or imitate
-an immune _Euplœa_, a _Danais_, an _Acræa_, or a _Tenaris_. However
-much the contour of the wing may vary, the venation is unaffected,
-and we can distinguish model from copy by this means alone, so that,
-even when there is the closest resemblance, no doubt is possible.
-In its theoretical aspect this constancy of venation is obviously
-important, for as nothing about the organism is incapable of variation,
-the veining of the wings might have varied, as indeed it has varied
-from genus to genus in the course of the phylogenetic history; but as
-changes in venation could not be detected by the butterflies' enemies,
-however sharp-sighted, there has been no reason in these cases for
-variation in this respect.
-
-In this connexion Poulton has brought forward interesting facts showing
-that the mimics of one model, belonging to different genera, often
-secure the same effect in quite different ways. Thus the glass-like
-transparency of the wings in the Heliconiidæ of the genus _Methona_
-depends on a considerable reduction of the size of the scales, which
-ordinarily cover both sides of the wing as thickly as the tiles on a
-roof, and produce the colour. In another quite similar species, also
-transparent-winged, the Danaid _Ituna ilione_, the transparency is due
-to the absence of most of the scales, and in a third mimic, _Castnia
-linus_, var. _heliconoides_, the scales are not altered either in size
-or number, but have become absolutely unpigmented and transparent.
-In a fourth mimic, a Pierid, _Dismorphia crise_, the scales have not
-decreased in number, but have become quite minute, while in a fifth
-case, the nocturnal _Hyelosia heliconoides_ Swains., the same thing
-has happened as in _Castnia_, but the scales are also fewer in number.
-Thus in each of the mimics the changes which have taken place in the
-scales are quite different, but they bring about the same effect, the
-glass-like transparency of the wings, on which the resemblance to the
-model depends: what we have before us is, therefore, not a similarity
-of variation, but only an appearance of similarity in external features.
-
-In the face of such facts there can be no further question of the often
-repeated objection, that the resemblance of model and copy depend on
-the similarity of external influences upon species living in the same
-latitude, even if that were not already sufficiently refuted by the
-frequent restriction of the mimicry to the female. And that mimicry
-should be a mere matter of chance is negatived even by the single
-fact that model and copy always live in the same area, and that the
-local varieties of the model are faithfully followed by the mimic.
-An interesting example of this is furnished by _Elymnias undularis_,
-already mentioned, for in this case the female (Pl. II, Fig. 23)
-mimics the brown-yellow _Danais plexippus_ (Pl. II, Fig. 22), not
-wherever _E. undularis_ occurs, but only in Ceylon and British India.
-In Burmah, where another Danaid, _D. hegesippus_, is common, it mimics
-that; and in Malacca it does not copy a Danaid at all, but resembles
-the male of its own species, which in India is very different from
-it, since there the female mimics one of the blue iridescent EuplϾ
-(Pl. III, Fig. 24). It cannot therefore be a matter of 'chance,' and
-we should have to give up all attempt at a scientific interpretation
-if we were not prepared to accept that of natural selection. Even the
-interference of a purposeful Power can hardly be seriously considered
-in this case, even by those who are inclined to such a view, for the
-_gradual_ approximation to the model, which is a matter of course in a
-process of evolution, could only appear, if referred to the benevolent
-intelligence of a Creator, as an unworthy trick, designed to lead
-humanity astray in its strivings after knowledge. On the other hand,
-this gradual increase of resemblance, which becomes apparent when we
-compare several mimetic species--this carrying over, step by step,
-from the female to the male--and many other facts point to the working
-of natural forces according to law, and, if there is to be found
-anywhere in living nature a complicated process of self-regulation,
-it certainly lies before us here, clearer and less open to objections
-than almost anywhere else. I do not mean to say, however, that we
-can verify it statistically in detail, as has been demanded by the
-fanatical opponents of natural selection. A direct testing of natural
-selection is, as has been already shown, nowhere possible: we can never
-exactly estimate how great the advantage is which a species requiring
-protection derives from a slight increase in the resemblance to an
-immune model; and I for one do not know how we could even definitely
-prove that a certain species needed a greater degree of protection than
-it had previously enjoyed in order to ensure its persistence in the
-struggle. It would be necessary to know the total number of individuals
-living on a certain area for many generations. If it appeared that
-there was a progressive diminution in the number of individuals, we
-should be justified in concluding that the species had not an adequate
-power of persistence, and that it therefore required a more effective
-protection. But it is impossible for us to collect such exact data
-for any species living under natural conditions, although we can
-often say approximately that a species is progressively decreasing in
-numbers. Even this, however, we can usually do only in cases which
-are influenced directly or indirectly by the interference of Man in
-nature, and in which the falling off in the species occurs so rapidly
-that there is no time for the slow counteractive influence of natural
-selection. We shall see later that in this way many species have been
-eliminated even within historic times.
-
-I have just spoken of the 'need of protection,' and I have a few
-remarks to add on that subject. It is a mistake to believe that every
-'rare' species, that is, one represented by few individuals, is
-already in process of disappearing. It is not the absolute number of
-individuals that determines the survival of a species, but the fact
-of the number remaining the same. It is equally mistaken to suppose
-that an amelioration of the conditions of existence for any species
-by natural selection is possible only when its persistence is already
-threatened; that is, when the number of individuals (the 'normal
-number') is steadily decreasing. On the contrary, it is of the essence
-of natural selection that every favourable variation which crops up is,
-_ceteris paribus_, preserved, and becomes the common possession of the
-species, quite independently of whether this improvement is absolutely
-necessary to its preservation or not. In the latter case it will simply
-become a commoner species instead of a rare one; and every species
-is, so to speak, striving to become common and widely distributed,
-since every advantageous variation that can possibly be produced is
-accumulated and made the common property of the species. But this has
-its limits, not only in the constitution and the structure of each
-species, but also in the external conditions of its life. If a species
-of butterfly be restricted, in the caterpillar stage, to a single, rare
-species of plant, its normal number will be, and must remain, a small
-one. But if there arise within it a variation in the food-instinct
-whereby a second and it may be a commoner plant becomes available, then
-the normal number of the species will rise, and perhaps the original
-number of individuals may be more than doubled. It is, however, by no
-means necessary to assume that the species was previously in process of
-decadence; on the contrary its normal number may have remained quite
-constant.
-
-So, in the case of the mimetic butterflies, we do not need to assume
-that they all previously required protection in the sense that they
-would have become extinct had they not assumed a likeness to an
-immune species. We may indeed conclude, on other grounds, that it
-was the rarer species which increased their number of individuals
-by the mimetic protection, and in doing so they certainly enhanced
-at the same time their chance of survival as a species. In the more
-abundant species mimetic resemblance to species whose unpalatability
-rendered them immune could not have been evolved, as it would have been
-disadvantageous, not only for the model, but for the mimicking species
-itself, while in species less rich in individuals, such resemblance
-would necessarily have a protective value, no matter whether the
-species was in danger of extinction or not. The process of selection
-must have started simply because the mimetic individuals survived more
-frequently than the others, and the mimetic resemblance must have
-gone on increasing as long as the increase brought with it a more
-effective protection. It is, therefore, a fallacious objection to say
-that a species, whose existence was threatened, would, considering the
-slowness of the process of selection, have died out altogether before
-it could have acquired effective protection by mimicking an immune
-species. The assumption is false--the widespread, hazy idea that the
-process of natural selection can only begin when the existence of the
-species is threatened. On the contrary, every species utilizes every
-possibility of improvement; and every improvement for which variation
-supplies the necessary material is possible. The augmentation of the
-profitable variations follows as a necessity from the more frequent
-survival of the best-adapted individuals, and this 'more frequent
-survival' will be not only a relative one, due to the fact that the
-better adapted individuals will be less decimated, it will also be
-absolute, because more individuals of the species will survive than
-before. Of this _Papilio merope_ may serve as an example; in Madagascar
-it now flies about only slightly varied from the original form, var.
-_meriones_. Here, therefore, the species is maintained, without the aid
-of mimetic protection. We do not know if the reason for this lies in
-the absence of an immune model, or in the non-appearance of suitable
-mimetic variants, or in other conditions; but we know that without
-mimicry the species holds its own against its enemies. But if, in
-Abyssinia, a female of this butterfly exhibited variations which would
-make her resemble, in any degree, the unpalatable _Danias chrysippus_,
-these mimetic variants would be less decimated than the original form
-of female, and would, therefore, gain stability, and gradually increase
-both in mimetic resemblance and in the number of individuals. But is
-this any reason why the original form of the female should diminish
-in numbers? In itself, certainly not; the red mimetic females could
-increase in number without causing any decrease of the yellow ones,
-for the red are in no way in conflict with the yellow, and we must not
-think of the number of individuals as so fixed for each species that it
-cannot increase. On the contrary, it _must_ increase, as soon as the
-conditions of existence are permanently improved, and this happens, in
-this case, through the mimetic protection of the red female. We can
-thus easily understand how mimetic and non-mimetic females can live
-side by side in Abyssinia.
-
-In all the rest of Africa, however, there are only mimetic females
-of _Papilio merope_, and none of the colour of the male; these last,
-therefore, have been crowded out by the mimetic form, not actively, but
-through the more frequent survival of the mimetic form, so that those
-like the male became gradually rarer, and finally died out--that is,
-ceased to occur. The matter is not so simple as it seems, and we shall
-best understand it by thinking of the dimorphism of the caterpillars
-of our hawk-moths, which we discussed before, in which the green form
-in the full-grown caterpillar is less well protected than the brown.
-In many species the brown form has crowded out the green, in others
-brown and green occur side by side, but the green is less abundant, and
-in some species very rare. This must be regarded as the simple result
-of the circumstance that a higher percentage of the green than of the
-brown caterpillars fall victims to enemies, and thus, in the course of
-generations, the green form becomes slowly but steadily rarer. This
-will be the case even if the newer and better adaptation raises the
-number of individuals (the 'normal number') in the species, for this
-increase must always be a limited one, even if it be very great, which
-is hardly likely in this case. For the normal number is not determined
-by the mortality at one stage, but by that at all the stages of life
-taken together. Thus a normal number always persists, notwithstanding
-the improved conditions for the species, and, on this assumption, the
-form under less favourable conditions cannot permanently hold its own
-with that under better conditions, but must gradually disappear. We
-can understand, then, that the primitive form of the _Papilio merope_
-female may persist even for a long time side by side with the mimetic
-form in certain habitats. It is, probably, not a mere chance, that
-this should have happened just in Abyssinia, for, in that region, the
-mimetic female is still tailed--that is, she has not yet reached the
-highest degree of resemblance to her immune model. In the whole of the
-rest of Africa the process of the transformation of the female has
-already reached its highest point, and on the east and west coasts,
-as well as in South Africa, the primitive form of the species is now
-represented only by the male.
-
-The gradual dying out of the less favourably conditioned forms of a
-species is a law which follows as a logical necessity from the essence
-of the process of selection, but its reality may be inferred from the
-phenomena themselves. On it depends, as far at least as adaptations are
-concerned, the transformation of species.
-
-A beautiful example of the crowding out of a less favoured form of a
-species by a more favoured one is afforded by a butterfly of North
-America, of which the two female forms have long been known, although
-the reason for their dimorphism was not understood. A yellow butterfly,
-_Papilio turnus_, not unlike our swallow-tail, has yellow females in
-the north and east of the United States, but black ones in the south
-and west. There was much guessing as to what the cause of this striking
-phenomenon might be, and it was for a time thought that this difference
-was directly due to the influence of climate, and, later, the black
-form of female was regarded as protectively coloured, because of the
-supposed greater persecution by birds in the south, since the female
-would be less easily recognized if of a dark colour, and would thus be
-better protected. This last explanation could hardly be looked upon as
-satisfactory, for a black butterfly in flight would be very easily seen
-by sharp-sighted birds; indeed, against a light background, it would be
-even more readily seen than a light one.
-
-Since we have acquired a more exact knowledge of the immune species
-of _Papilio_ this case has become clear to us. For on those stretches
-of country on which the black female of _Papilio turnus_ lives there
-occurs another _Papilio_ which is black in both sexes, _Papilio
-philenor_, and this is one of those species which are protected by
-their unpleasant taste and odour. Here, therefore, we have a case of
-mimicry, the female of _Papilio turnus_ imitates the immune _Papilio
-philenor_, and thereby secures protection for itself; but as the immune
-model only occurs in the southern half of the distribution of _Papilio
-turnus_ a somewhat sharp separation of the two forms of female
-has been evolved; the black, mimetic form, being the most fit, has
-completely crowded the primitive yellow form out of the area inhabited
-by _Papilio philenor_, while beyond this area, to the north and west,
-the yellow form alone prevails. The extensive and careful studies of
-Edwards have shown that the two forms occur together only in a very
-narrow transition region.
-
-We thus see that the facts, wherever we scrutinize them carefully,
-harmonize with the theory. Of course we can only penetrate to a
-certain depth with the theory of selection, and we are still far from
-having reached the fundamental causes of the phenomena. Indeed, our
-understanding must in the meantime stop short before the causes of
-variations and their accumulation, but up to that point the theory
-gives us clearness, and discloses the causal connexion of phenomena
-in the most beautiful way. Although we do not yet understand how the
-southern female _Papilio turnus_ was able to produce the advantageous
-black, we do see why a black variation, when it did occur, should
-increase and be strengthened, until it crowded out the yellow form from
-the area of the immune model, and we are able in a general way to refer
-the whole complicated phenomena of mimicry to their proximate causes.
-
-This is true also of other phenomena which have had no part in
-establishing the theory, since attention was only directed to them
-later, and it is true even of some which, at first sight, seem to
-contradict the theory altogether. To this class belongs, for instance,
-the phenomenon that immune species not unfrequently mimic each other,
-as was first observed among the Heliconiid-like butterflies of South
-America. In four different families, the Danaidæ, the Neotropidæ, the
-Heliconiidæ, and the Acræidæ, there are species, distributed over the
-same area, which resemble each other in their conspicuous colouring
-and marking, and also in the peculiar shape of the wings. After what
-has been said one might be inclined to regard one of these species as
-the unpalatable model and the others as the palatable mimics, but they
-are all unpalatable, and are not eaten by birds. The puzzle of this
-apparent contradiction was solved by Fritz Müller[4], who pointed out
-that the aversion to non-edible butterflies is not innate in birds, but
-must be acquired. Each young bird has to learn from experience which
-victim is good to eat, and which bad. If every inedible species had
-its particular and distinctive colour-dress a considerable number of
-individuals of each species would fall victims to the experiments of
-young birds in each generation, for a butterfly which has once been
-pecked at, or squeezed by the bill of a bird, is doomed to die. But if
-two inedible species which resemble each other inhabit the same area
-they will be regarded by the birds as one and the same, and if five or
-more inedible species resemble each other all five will present the
-same appearance to the bird, and it will not require to repeat on the
-other four the experience of unpalatability it has gained from one.
-Thus the total of five species will be no more severely decimated by
-the young birds than each of them would have been if it had occurred
-alone; the same number of victims of experiment, which are necessary
-every year in the education of the young birds, will, when all five
-species look alike, be divided among the whole 'mimicry ring,' as we
-may say. The advantage of the resemblance is thus obvious, and we
-can understand why a process of selection should develop among such
-inedible species which should result in their being readily mistaken
-for one another; we can understand why, in the neighbourhood of Fritz
-Müller's home, Blumenau, in the province of Santa Catarina in South
-Brazil, the Danaidæ, species of _Lycorea_; the Heliconiidæ, _Heliconius
-eucrate_ and _Eueides isabella_; and the Neotropinæ, _Mechanitis
-lysimnia_ and species of _Melinæa_, should all exhibit the same
-colours, brown, black and yellow, in a similar pattern, on similarly
-shaped wings. The agreement is by no means perfect in detail, but it
-can be noticed in all parts of South America inhabited by species of
-these genera, and the same differences which distinguish, for instance,
-the two species of _Heliconius_ flying in two different regions,
-also distinguish the two species of _Eueides_ and the two species
-of _Mechanitis_. In Honduras we find the same mutually protective
-company of inedible genera as in Santa Catarina, but represented by
-other species, which all differ from the species in Santa Catarina
-in the same characters, as, for instance, that they have two instead
-of one pale yellow cross-stripe on the anterior wings. The species
-are: _Lycorea atergatis_, _Heliconius telchinia_, _Eueides dynastes_,
-_Mechanitis doryssus_, and _Melinæa imitata_[5]. In the environs of
-Bahia this mimicry ring consists of the following species: _Heliconius
-eucrate_, _Lycorea halia_, _Mechanitis lysimnia_, and _Melinæa ethra_,
-as figured on Pl. II, Fig. 12, iv, and such a mutual assurance society
-has always one or other edible species as mimic. The larger the mimetic
-assurance company is, the less harm can mimics do to it. In the case
-figured it is two Pieridæ already known to us that have fairly well
-assumed the Heliconiid guise, namely, _Dismorphia astynome_ (Pl.
-II, Figs. 18 and 19) and _Perhybris pyrrha_ (Pl. II, Figs. 16 and 17).
-In the latter of these the male still has, on the upper surface, just
-the appearance of one of our common Garden-whites, while the female is
-coloured quite like the Heliconiidæ, but without having lost the form
-of wing of the Whites. The larger the mimetic company is the greater
-will be the protection afforded to its palatable mimics, since they
-will be the more rarely seized by way of experiment. It is, of course,
-obvious that in this kind of mimicry--that is, in the imitation of an
-unpalatable and rejected species for protection--it is presupposed as
-a general postulate that the edible mimics are considerably in the
-minority, as Darwin showed; for if it were otherwise their enemies
-would soon discover that among the apparently unpalatable species there
-were some which were pleasant to taste. Here, too, the facts bear out
-the theory, although exceptions can easily be imagined, and do seem to
-occur.
-
-[4] _Kosmos_, vol. v, 1881, p. 260 onwards.
-
-[5] According to Poulton's report in _Nature_, July 6, 1889, of 'Sykes,
-Natural Selection in the Lepidoptera,' _Trans. Manchester Microscop.
-Soc._ 1897, p. 54.
-
-PLATE I
-
- FIG
-
- 1. PAPILIO MEROPE, MALE, AFRICA.
-
- 2. THE SAME SPECIES, ONE FORM OF MIMETIC FEMALE.
-
- 3. DANAIS CHRYSIPPUS, AFRICA, IMMUNE MODEL OF FIG. 2.
-
- 4. PAPILIO MEROPE, SECOND FORM OF MIMETIC FEMALE, S. AFRICA.
-
- 5. AMAURIS NIAVIUS, S. AFRICA, IMMUNE MODEL OF FIG 4.
-
- 6. PAPILIO MEROPE, THIRD FORM OF MIMETIC FEMALE, S. AFRICA.
-
- 7. AMAURIS ECHERIA, S. AFRICA, IMMUNE MODEL OF FIG. 6.
-
- 8. DANAIS ERIPPUS, IMMUNE MODEL OF FIG. 9, CENTRAL N. AMERICA.
-
- 9. LIMENITIS ARCHIPPUS, CENTRAL N. AMERICA, MIMICS THE FOREGOING
- SPECIES.
-
- 10. DANAIS ERIPPUS, (_a_) CATERPILLAR, (_b_) PUPA.
-
- 11. LIMENITIS ARCHIPPUS, (_a_) CATERPILLAR, (_b_) PUPA.
-
-_To face Plate I_
-
-[Illustration: PLATE I. LONDON: EDWARD ARNOLD.]
-
-This comparative rarity is true of the imitators of the Heliconiidæ
-and their great mimicry ring of unpalatable species, and is very
-general. Thus, for instance, there is a series of palatable mimics of
-the beautiful blue _EuplϾ_ of the Indo-Malayan region (Pl. III, Figs.
-25 and 27), but each of these mimics is rare compared with the hosts
-of the blue unpalatable company, for these immune butterflies also
-occur in many species, all similar to _Euplœa midamus_ or _binotata_
-(Pl. II, Figs. 1 and 3); and the same applies to the mimics of the
-Indo-Malayan Danaidæ. There are a great many _Danais_ species, all of
-them resembling _Danais vulgaris_ (Pl. III, Fig. 20), which, when they
-occur together, form an inedible ring, and this ring is imitated by a
-whole series of edible species, each of which is comparatively rare.
-And there are no fewer than six species of _Papilio_ which resemble
-these Danaids to the point of being easily mistaken for them, while
-another rare _Papilio_ effectively copies the iridescence of the blue
-_EuplϾ_--a coloration so unusual in the genus that the species has
-received the name of _Papilio paradoxus_.
-
-But even in single species of butterflies immune through unpalatability
-there is usually a great abundance of individuals. Thus _Danais
-chrysippus_, which is distributed over the whole of Africa, is a very
-common butterfly wherever it can live at all; and in North America,
-in which country there are only two widely distributed species of
-_Danais_, these often occur in enormous numbers. The beautiful large
-_Danais erippus_ Cramer (Pl. I, Fig. 8), is distributed over almost all
-America, and in many places is not only frequent, but occurs in great
-swarms. Usually it peoples the broad, open stretches of the western
-prairies of the United States, but when violent winds blow, as they
-do there in September especially, the insects are driven together
-into the small wooded spots of the prairie, and then they cover the
-trees in incredibly large crowds, often so thickly that the leaves are
-entirely hidden, and the trees look brown instead of green. Millions of
-butterflies go to make up such swarms, which have been observed in many
-parts of the United States, even quite in the East, in New Jersey, and
-elsewhere.
-
-Considering this extraordinary abundance of the immune species, it
-is not surprising that its palatable copy, _Limenitis archippus_
-(Pl. I, Fig. 9), should also be widely distributed in North America,
-and in many places it is not rare, but even abundant. The enormous
-majority of _Danais erippus_ will protect the species which resembles
-it so closely, even though it is not rare. Any doubt as to this being
-a case of mimicry disappears in face of the fact that, in Florida,
-there flies a second very similar but much darker brown North American
-_Danais_, and that it is accompanied there by an equally dark variety
-of _Limenitis archippus_ (_L. eros_).
-
-To prove the correctness of the hypothesis of an actual process of
-selection--which we assume in our interpretation of mimicry--I mean
-the assumption that the disguise of the species seeking protection
-really deceives the enemy, and thus actually affords protection, I
-need only cite the evidence of an acute and experienced entomologist
-who was himself deceived by it. Seitz[6], to whom we owe many valuable
-biological observations on butterflies, relates that, while he was
-collecting in the neighbourhood of the town of Bahia, he was surrounded
-by swarms of _Catopsiliæ_, similar to our lemon butterfly, especially
-the common _Catopsilia argante_, but he took no notice of these, as he
-'had already collected as many of them as he wanted.' It was only when
-he saw a pair _in copula_ that he caught them in his net. But to his
-extreme surprise he found that he had not caught a _Catopsilia_, but a
-butterfly of the family Nymphalidæ, one of those _Anææ_ whose numerous
-species are distributed over South America. These _Anææ_ are dark, or
-beautifully bright on the upper surface, but on the under side are
-leaf-coloured, and one of them bears the name _Anæa opalina_, because
-it is quite clear and pale, and of opal-like brilliance. The captive
-was nearly related to this species. Seitz was so much surprised
-by the discovery that the male, which had quickly detached itself from
-the female, escaped him, and he could only make out that, 'as it flew
-away, it unfolded dark wings, which certainly bore little resemblance
-to those of the lemon butterfly.' In the hope of securing more of
-this rare booty he then hunted only for _Catopsilia argante_, without
-however securing another coveted specimen--he caught no more _AnϾ_,
-which shows that in this case, too, the mimetic species was much rarer.
-
-[6] In citing this observation of Seitz, I do not mean to assert that
-there is true mimicry between _Anæa opalina_, or its allied species
-in Bahia, and the _Catopsilia_, though I regard this as extremely
-probable, because of the marked dimorphism between the male and the
-female, in conjunction with the very striking resemblance of the female
-to the _Catopsilia_. The example was given only to show how very
-deceptive such resemblances may be. To assert with confidence that it
-is a case of mimicry we should require to know that _Catopsilia_ is
-immune, and on that point we have as yet no information.
-
-PLATE II
-
- FIG.
-
- 12-15 REPRESENT A 'MIMICRY-RING' COMPOSED OF FOUR IMMUNE SPECIES
- BELONGING TO THREE DIFFERENT FAMILIES AND FOUR DIFFERENT GENERA.
-
- 12. HELICONIUS EUCRATE, BAHIA.
-
- 13. LYCOREA HALIA, BAHIA.
-
- 14. MECHANITIS LYSIMNIA, BAHIA.
-
- 15. MELINÆA ETHRA, BAHIA.
-
- 16, 17. PERHYBRIS PYRRHA, MALE AND FEMALE, S. AMERICAN 'WHITES'
- (PIERIDÆ). THE FEMALE MIMICS AN IMMUNE HELICONIID, WHILE THE MALE
- SHOWS ONLY AN INDICATION OF THE MIMETIC COLOURING ON THE UNDER SURFACE.
-
- 18, 19. DISMORPHIA ASTYNOME, MALE AND FEMALE, ALSO BELONGING TO THE
- FAMILY OF 'WHITES,' AND MIMICKING IMMUNE HELICONIIDS; A WHITE SPOT ON
- THE POSTERIOR WING OF THE MALE IS ALL THAT REMAINS OF THE ORIGINAL
- 'WHITE' COLORATION.
-
- 20. ELYMNIAS PHEGEA, W. AFRICA, OF THE FAMILY SATYRIDES, MIMICS THE
- FOREGOING SPECIES.
-
- 21. ACRÆA GEA, AN IMMUNE W. AFRICAN SPECIES.
-
- 22. DANAIS GENUTIA, AN IMMUNE DANAID FROM CEYLON.
-
- 23. PLYMNIAS UNDULARIS, FEMALE, ONE OF THE MIMICS OF FIG. 22. THE
- MALE, WHICH IS QUITE DIFFERENT, IS FIGURED ON PLATE III (FIG. 24).
-
-_To face Plate II_
-
-[Illustration: PLATE II. LONDON: EDWARD ARNOLD.]
-
-We see, then, that the need for protection in butterflies has a great
-influence on their external appearance, especially as regards their
-colour and marking. First, because the resting insect frequently has
-the visible surfaces sympathetically coloured, and also, because there
-are numerous species, indeed whole families, which contain nauseous,
-perhaps even actually poisonous, juices, and these have been subject
-to a double process of selection, directed towards the increase of the
-nauseousness, and at the same time towards acquiring as conspicuous
-a dress as possible. Thus the whole surface of these butterflies
-became gaily coloured, and often--as in many of the tropical nocturnal
-Lepidoptera which fly by day, the Agaristidæ, Euschemidæ, and
-Glaucopidæ--quite glaringly bright. We thus understand the striking
-or at least readily recognizable colours of the Heliconiidæ, the
-Euplœæ, the Danaidæ, and the Acræidæ. Finally, the unpalatable species
-influence many others which are edible, since the latter strive to
-resemble an immune species; and how considerable the variations and
-colour transformations thus induced can be is shown by the Whites of
-the genus _Perhybris_ (Pl. II, Figs. 16 and 17) and _Archonias_, in
-which the male has wholly or partially retained the primitive dress of
-the Whites, and in which, side by side with wholly mimetic species,
-other species occur in which both sexes exhibit the garb of the Whites
-unaltered. Such cases tell decidedly against the often expressed view
-that mimetic species must have had from the outset a great resemblance
-to the model; they show rather that very great deviations in form,
-but more especially in colour, have been brought about solely by the
-necessity for mimetic adaptation, and that they have come about only
-slowly and step by step, as the different grades of resemblance to the
-model in different species of the same genus clearly show.
-
-Lepidoptera are by no means the only insects which exhibit the
-phenomenon of mimicry, nor are insects the only animals in which it
-occurs; and unpleasant taste and odour are not the only protective
-characters; there are many others, as, for instance, among insects, the
-hardness of the chitinous cuticle.
-
-One of the most beautiful examples of mimicry was discovered by
-Gerstäcker, not in free nature, but in the entomological collection at
-Berlin. There he found beside a green, metallic weevil-beetle, one of
-the Pachyrhynchidæ from the Philippines, two other insects with the
-same metallic sheen and very similar form of body. They had been put in
-beside the weevil as duplicates, but more careful observation showed
-that they were delicate Gryllidæ, which mimicked the hard beetles
-so deceptively that even the practised eye of the entomologist was
-misled by them. Later on it was shown that these Gryllids live in the
-Philippines beside the weevils, and even on the same leaves with them,
-and that the beetles are protected from the attacks of birds and other
-enemies by the extraordinary hardness of their cuticle. The case is
-especially remarkable because in general the Gryllidæ have no metallic
-shimmer, and the form of body must have been considerably altered to
-make them resemble the beetle. The usually broad head of the Gryllids
-is in this case narrower, the usually flat wing-covers are arched and
-pear-shaped, and the legs have become quite beetle-like. The security
-enjoyed by the weevil must be very perfect, for it is mimicked by three
-other species of beetle in the Philippines.
-
-Animals can also be protected from attack by the possession of
-dangerous weapons. To this class belong insects with poisonous
-stings, like the bees, wasps, and ants, and in some degree also the
-ichneumon-flies. We cannot wonder, therefore, that these dreaded
-species find imitators. In this case it is not of so much importance
-that the copy should be rarer than the model, for anything that looks
-like a dangerous insect will be avoided, since close investigation is
-in this case attended with danger. So we find that hornets, wasps,
-and bees are frequently imitated by other insects, by beetles, flies,
-and butterflies; and these must derive a certain advantage, even
-when the resemblance is only a general one. Many Longicorns, which
-visit flowers, are striped black and yellow, like a wasp, and so are
-many flies, like the species of _Syrphus_, and so on. The Longicorn
-_Necydalis major_ bears a strong resemblance to a large ichneumon-fly;
-it has the same long-drawn-out body, the same swellings on the femur
-and tibia, the curved antennæ, the glossy brown colour, and its
-wing-covers are quite short, leaving the wings free, so that the
-deception is very complete.
-
-Bees, too, are sometimes so well imitated that they are hardly to be
-distinguished from their mimics, not in flight only, but also when
-visiting flowers. The best and commonest mimic of our honey-bee is
-a perfectly harmless fly of the same size and colour, the drone-fly
-(_Eristalis tenax_). The two are often to be seen together on the
-same flowering shrub, as, for instance, in autumn, on the Japanese
-buckwheat of our gardens (_Polygonum sieboldii_), both busily seeking
-for honey. I once noticed a boy catching the flies with a net in order
-to imprison them, but a bee stung him severely in the finger. He
-immediately abandoned the chase, and gave up the flies, perceiving the
-dangers of confusion. So the animal enemies of _Eristalis_ will often
-prefer to leave it in peace rather than run the risk of being stung.
-
-PLATE III
-
- FIG.
-
- 24. ELYMNIAS UNDULARIS, MALE OF THE SPECIES OF WHICH THE MIMETIC
- FEMALE IS DEPICTED IN FIG. 23.
-
- 25. EUPLŒA BINOTATA, IMMUNE INDIAN SPECIES, MIMICKED BY
-
- 26. ELYMNIAS LEUCOCYMA, MALE, OF WHICH
-
- 27. EUPLŒA MIDAMUS.
-
- 28. THE FEMALE MIMICS FAIRLY CLOSELY
-
- 29. DANAIS VULGARIS, IMMUNE INDIAN DANAID.
-
- 30. ELYMNIAS LAIS, MIMETIC OF THE FOREGOING SPECIES, BUT ONLY ON THE
- UPPER SURFACE. THE LOWER SURFACE RETAINS THE ORIGINAL PROTECTIVE
- COLOURING REPRESENTING A DECAYING LEAF.
-
- 31. TENARIS BIOCULATUS, FROM THE PAPUA REGION.
-
- 32. ELYMNIAS AGONDAS, MIMICS THE FOREGOING SPECIES FROM THE SAME
- LOCALITY.
-
-_To face Plate III_
-
-[Illustration: PLATE III. LONDON: EDWARD ARNOLD.]
-
-There is still another relation between two species which can be
-induced by mimicry--namely, parasitism, when, for instance, the
-so-called cuckoo-bees and parasitic humble-bees deceptively resemble
-in colour, arrangement of hair, and form of body, the species into
-whose nests they smuggle their eggs, to have them brought up at the
-expense of the bee or humble-bee in question. In the same way, among
-the numerous parasites of ant nests, there are some which copy the ants
-themselves, and so secure themselves from molestation, although they
-devour the ants' eggs and pupæ. Thus, among the hosts of South American
-driver-ants (_Eciton prædator_) there lives a predaceous beetle of the
-family Staphylinæ, which has received the name _Mimeciton_ because it
-resembles the ant in form and in the nature of the external surface,
-though not in colour, which is to be explained by the fact that this
-ant has no compound eyes, and is therefore almost blind, or at any rate
-cannot see colours.
-
-I should never come to an end were I to attempt to exhibit the great
-wealth of observations now available in regard to mimicry. But this
-at least may be added, that isolated cases of mimicry have been found
-even among Vertebrates. Thus, according to Wallace, the red-and-black
-striped poisonous coral snake of South America (_Elaps_) is most
-realistically imitated by a non-poisonous snake (_Erythrolampus_) of
-the same region. Among birds, Wallace cites a few cases which may be
-regarded as mimicry, but none are known among mammals, which is not
-to be wondered at when we consider how very much less numerous in
-individuals the species are which live together on one area, and how
-much less likely it is that two species should be, to begin with, so
-near each other in size, habit, and form that the process of natural
-selection could bring about a deceptive degree of resemblance.
-Without doubt it is among insects that the conditions for mimicry are
-especially favourable, partly because of the enormous number of species
-which live together and have interrelations on the same area, even in
-our latitudes and much more so in the tropics, and also because of
-their usually great fecundity, and their rapid multiplication, both of
-which are factors favourable to starting and continuing the processes
-of natural selection. Furthermore, we have to take into account the
-hosts of enemies which depend wholly or in great part on insects for
-food, and destroy them in enormous numbers, eliminating them in inverse
-proportion to the perfection of their adaptation. Finally, there is the
-extreme susceptibility of many insects to injury. This makes it very
-desirable that they should have some disguise sufficient to protect
-them from even the first attempt at an attack, since that would in many
-cases prove fatal.
-
-
-
-
-LECTURE VI
-
-PROTECTIVE ADAPTATIONS IN PLANTS
-
- Protection against large animals--Poisons--Ethereal oils--Spines and
- thorns--Sharp and stinging-hairs--Felt-hairs--Position of the thorns:
- buckthorn--Tragacanth shrub--Prigana scrub--Alpine shrubs--Protection
- against small enemies--Chemical substances--Mechanical protective
- arrangements--Raphides--Conclusion.
-
-
-WE have seen in how many different ways animals are able to adapt
-themselves to the conditions of life, both protectively and
-aggressively; how they approximate in their colour to that of their
-surroundings so that they harmonize with it; how they copy lifeless
-objects, or parts of plants, leaves, or twigs, or even mimic, in form
-and colour, other animals which are in some way protected. When we
-consider that by far the greater number of species find protection in
-some degree through their colouring, and often through their form, and
-when, at the same time, we remember how different this colouring is in
-nearly related species, and even within the same species (dimorphism),
-we can scarcely avoid the impression that the forms of life are made of
-a plastic material, which, like the sculptor's clay, can be kneaded at
-will into almost any desired form.
-
-This impression is corroborated when we turn our attention to plants,
-and consider the different ways in which they are able to protect
-themselves from the attacks of animals.
-
-That plants stand in need of some protection is obvious enough, since
-their leaves and other green parts contain much nourishment, and an
-endless army of animals, large and small, depends upon these alone for
-sustenance. Indeed, the existence of animals depends altogether on the
-occurrence of plants, for carnivorous and saprophytic animals could
-only arise after vegetarian forms had been already in existence. But if
-the green parts of the plants were left defenceless at the mercy of the
-multitude of herbivorous animals, it would not be long before they were
-exterminated from the face of the earth, for the animals would devour
-unsparingly whatever was within their reach, and, as their increase
-does not depend on their ratio of elimination alone, but also on their
-fertility, and on their rapidity of multiplication, they would go on
-increasing in numbers at the expense of the superabundant nourishment
-until the plants on which they depended were themselves consumed.
-
-When we inquire into the means whereby plants evade such a fate we are
-astonished at the endless diversity of the devices employed.
-
-Let us consider first of all the menace to plants from the larger
-herbivores, from elephants and cattle down to the hare and the
-roe-deer; we find that many plants are protected by poisons, which
-develop in the sap of their stems, leaves, roots, and fruits. The
-juicy and beautifully leaved Belladonna (_Atropa belladonna_) is never
-touched by roe-deer, stags, or other herbivores, and the same is true
-of the thorn-apple (_Datura stramonium_), the henbane (_Hyoscyamus
-niger_), the spotted hemlock (_Conium maculatum_), the danewort of
-our woods (_Sambucus ebulus_), and many others; they all contain a
-poison. Like the unpalatable butterflies, these unpalatable plants are
-also furnished with a warning sign of their undesirability, namely, a
-disagreeable odour, perceptible even by man, which scares off animals
-from touching them. The development of this through natural selection
-presents no very serious difficulty.
-
-But, strangely enough, there are not a few poisonous plants in which
-we, at least, are unable to detect any such warning sign. Among these
-are the blue aconite (_Aconitum_), the black hellebore (_Helleborus
-niger_), the meadow-saffron (_Colchicum autumnale_), species of
-Gentian, of spurge (_Euphorbia_), and others. Yet these are avoided by
-deer, roe-deer, chamois, hares, and marmots, and our cattle, horses,
-and sheep also usually leave them untouched. A case has, however, been
-reported from the valley of the Aur, on the lower Rhine, which seems to
-contradict this. On the rocky grass-slopes of the valley the poisonous
-hellebore (_Helleborus viridis_) grows in great abundance, and the
-sheep of that region, which were wont to graze on the slopes, avoided
-these plants. But some sheep from another part were imported into the
-valley, and these ate the hellebore, with the result that many died.
-If these poisonous plants, then, were furnished with a warning sign
-such as a disagreeable odour, not perceptible to us, we should have to
-assume that the imported sheep had a less acute sense of smell than
-the others, which is not impossible in domesticated animals. If there
-were no such warning sign, then it must have been not an instinct but a
-continuous _tradition_ which prevented the native sheep from touching
-the inedible plants.
-
-A more naïve interpretation of nature than that of our day would have
-regarded the fragrant ethereal oils developed in the seeds of many
-plants, as in those of fennel, cummin, and other Umbelliferous plants,
-as a peculiarity designed for the use and profit of man. But these
-ethereal substances are obviously a means of protection against the
-depredations of seed-eating birds, for a sparrow which was allowed to
-eat three or four seeds of cummin died very soon afterwards.
-
-Many plants produce bitter substances in their green parts, and so
-secure at least some measure of protection, as is the case with the
-majority of mosses, the ferns, and species of _Plantago_ and _Linaria_.
-Others, again, deposit silicic acid in their cell-walls, or develop in
-addition a very thick epidermis, so that they afford at the best an
-unpleasant food, e.g. many grasses, the horse-tails, the rhododendron,
-and the bilberry. Others, again (_Alchemilla vulgaris_), have
-cup-shaped leaves, which retain rain and dew for a long time, and this
-protects them from grazing animals, which are unwilling to touch wet
-grass and plants.
-
-Especially widely distributed and diverse is the protection of plants
-by sharp thorns and spines. It is extremely interesting to note in how
-many different and advantageous ways this armature is disposed.
-
-Obvious at once is the fact that thorns and spines only occur on
-those parts which are naturally exposed to attack. Thus we find them
-particularly strong in young plants, and on the lower parts of older
-ones. The holly, for instance, has crenate, spinose leaves only to the
-height to which grazing animals can reach; beyond that the leaves are
-smooth-edged and spineless, like those of the camelia. It is almost the
-same with some wild pear-trees, which are quite covered with thorns as
-long as they are low, but afterwards grow a thornless crown.
-
-Similarly, low bushes, when they are armed with thorns or the like at
-all, are covered with them all over, like the rose-bush.
-
-When the leaves of a plant are spinose the spines are disposed on
-the parts usually attacked; and thus we understand why the enormous
-floating leaves of _Victoria regia_ should have on their under surface
-long, pointed spines which, especially at the upturned margin, attain a
-length of several inches; it is from water animals--water snails--that
-danger threatens them.
-
-Thorns are developed in the most diverse ways. In many of the bushes
-on the coast of the Mediterranean true leaves are wanting altogether,
-the green branches and twigs being themselves the assimilating parts,
-and these are so stiff and rigid, so like some kind of thorn, that they
-suffice to scare off any greedy herbivore. Among our own bushes the
-Broom (_Spartium scoparium_) may be taken as an example of this class.
-
-In other cases the spines are found on the leaves themselves, but
-there is great diversity in their mode of arrangement. In many
-tropical plants, such as the Yucca and the Aloe, the point of the
-long, reed-shaped leaf is transformed into a spine, and this is the
-case in many of our native grasses. Kerner von Marilaun notes that, in
-the Southern Alps, two such grasses, _Festuca alpestris_ and _Nardus
-stricta_, occur frequently in certain localities, and they prick the
-muzzles of the cattle so badly that they return bleeding from the
-pasture. This prevents these Alpine runs from being made full use of,
-so the grasses are as far as possible extirpated by man, and, curiously
-enough, also by the cattle themselves, for they seize the grass at the
-base of the tuft with their teeth, pull it out, and let it fall, so
-that it withers. Kerner saw thousands of such pieces of turf which had
-been pulled up by the cattle lying dried and bleached by the sun on
-some of the Alpine grazing grounds in the Tyrolese Stubaithal.
-
-Again, in many plants the whole leaf-edge is transformed into a spiny
-wall, which may be enlarged by indentations and lobate projections,
-as in the holly, and, in a much higher degree, in the thistles
-(_Carduus_), in _Eryngium_, in _Acanthus_, and in many Solanaceæ.
-Often, too, there are barbed hooks on the leaf-edge, which work like
-a saw; or the leaf-edge, though without spines, may be made sharp by
-deposits of silicic acid, as in the sedges, whose sharp edges are
-moved to and fro in the mouths of ruminants, and thus injure the
-mucous membrane. The hook-bristles of the fig-cactus (_Opuntia_),
-which, though small, are abundantly provided with barbs, must also be
-mentioned; for they are to be found in great numbers surrounding the
-buds of these plants, and most effectively protect them from being
-eaten away by animals (Fig. 19).
-
-To this category, too, belong the short, prickly bristles of the
-rough-leaved plants, which cover the whole plant as with an overcoat
-of sharp needles; of these we may mention the adder's tongue (_Echium
-vulgare_), the comfrey (_Symphytum officinale_), and the borage
-(_Borago officinalis_).
-
-Very well known are the stinging-hairs of the Urticaceæ, long hairs
-(Fig. 20) with an elastic base, but with glass-like, brittle, rounded
-heads, which break off at the lightest touch, whereupon the sharp
-point of the broken hair penetrates the skin of the creature which has
-touched it, and the poisonous contents of the hair are poured into
-the wound. Even our large stinging-nettle (_Urtica dioica_) can cause
-intense irritation, and evoke the 'nettle-rash,' named after it, on
-the human skin; but there are many tropical species of nettle, e.g.
-_Urtica stimulata_ in Java, and others, which have an effect similar
-to that of snake-poison and produce tetanoid spasms, and so on. In
-addition to formic acid these hairs contain an undefined ferment, a
-so-called Enzyme. It need scarcely be said that these stinging-hairs
-must have much more severe effects on the mucous membrane of the mouth
-of grazing animals than on the human skin, and that they are therefore
-an excellent protection for the plants. As a matter of fact we never
-find our nettle patches eaten away, and even the donkey, which eats
-thistles freely, turns away from the stinging-nettle. But even these
-stinging-hairs, like all other protective devices, do not afford an
-_absolute_ protection. The caterpillars of several of our diurnal
-butterflies feed exclusively on the stinging-nettle, and they eat up
-the leaves, stinging-hairs and all. This is the case with five species
-of the genus _Vanessa_, namely: _Vanessa io_, the 'peacock,' _Vanessa
-urticæ_, the small tortoiseshell, _Vanessa prorsa_, _Vanessa C. album_,
-the C. butterfly, and _Vanessa atalanta_, the admiral.
-
-[Illustration: FIG. 19. Barbed bristles of _Opuntia rafinesquii_;
-enlarged.]
-
-[Illustration: FIG. 20. Vertical section through a piece of a leaf of
-the Stinging-nettle _(Urtica dioica_), bearing two stinging-hairs;
-magnified 85 times; adapted from Kerner and Haberlandt.]
-
-We are all familiar with our mulleins (_Verbascum_), those beautiful
-flower-spikes with the thick, soft felt leaves, which grow on stony or
-sandy soil. Harmless as they look, they are much disliked by animals
-as food, for the thick hairy felt which covers them breaks up in the
-mouth, and sticks in the folds of the mucous membrane, causing burning
-sensations and other discomforts. They, too, are therefore spared by
-grazing animals, but they have smaller enemies, like the caterpillars
-of the genus _Cucullia_, which, however, never completely destroy them,
-but only eat large holes in their leaves.
-
-Let us now consider in somewhat greater detail the true thorns, the
-most conspicuous protection of many plants. It is very remarkable that
-these are always so placed, and so regulated as to their length and
-character, as to afford protection to the most important and the most
-exposed parts of the plant. Thus many bushes, which would otherwise be
-in danger of being completely devoured by cattle, are stiff with thorns
-which are nothing else than pointed, hard twigs without, or with very
-little foliage. Among these are the sloes, the buckthorn (_Rhamnus_),
-the sea-buckthorn (_Hippophäe_), and the barberry (_Berberis_). In the
-last-named three thorns arise in a group, and protect the young bud
-from danger in three directions (Fig. 21).
-
-[Illustration: FIG. 21. A piece of a twig of Barberry (_Berberis
-vulgaris_) in spring; after Kerner.]
-
-The fine-leaved mimosas of the tropics have similar but very long and
-sharp thorns, and their leaves are movable and sensitive, so that, when
-they are touched, they shut up and draw back behind the rampart of
-stiff thorns, which are just of the right length to protect them.
-
-In many thorny bushes only the young shoots of each spring remain
-green through the summer, and in autumn they become transformed into
-thorns, under whose protection the shoots of the following spring will
-develop. Sometimes, too, the leaf-stalks are modified in the course of
-the summer into thorns, as in Tragacanth (_Astragalus tragacantha_).
-In this case the young leaves are protected by a circle of thorns,
-consisting of the leaf-stalks of the preceding year which have not
-fallen off (Fig. 22, _A_, _B_, _C_).
-
-I should have to go on for a long time with my exposition, even if I
-were to confine attention to the essential facts; we shall, therefore,
-only recall the well-known phenomenon of the Cactuses, in which the
-leaves are entirely transformed into spines, which may attain a length
-of eight centimetres, while the fleshy stem alone represents the
-green--that is, the assimilating parts of the plant. The species of
-Cactus are almost the only plants which grow on the stony, hard, and
-hot plateaux of Mexico, and they are protected from desiccation by the
-thickness of their epidermis. But, enticing as is the food promised
-by the juicy stem, animals rarely venture to approach them, and it is
-only when tortured by thirst that horses and asses occasionally knock
-off the spines with their hoofs, and so reach the soft tissues rich in
-water. For this attempt, however, as Alexander von Humboldt pointed
-out, they often suffer, as the sharp spines are apt to pierce the hoof.
-In any case, the cactuses are effectively protected from the danger of
-extermination by grazing animals.
-
-[Illustration: FIG. 22. Tragacanth (_Astragalus tragacantha_). _A_,
-two spring shoots. _B_, a single leaf, from which the three uppermost
-leaflets have fallen off. _C_, leaf midrib, from which all the leaflets
-have fallen off. After Kerner.]
-
-It must certainly strike every one that many districts, especially
-those which are dry, hot, and stony, are conspicuously rich in thorny
-plants, and it has often been supposed that the production of thorns
-must be a direct result of these peculiar conditions of life; indeed,
-the hard, thorny habit of many of these plants has even been regarded
-as a protection against desiccation. This, however, is contradicted
-by all those thorny plants which, like the cactuses, possess tissues
-extremely rich in sap, and in which desiccation is prevented, not
-by the thorns, but by the thick epidermis. The only satisfactory
-explanation is that afforded in terms of natural selection. In such
-hot, and at the same time dry regions, the plant-growth is often very
-scanty, and the food available for the grazing animals is, at least at
-times, very scarce; on this account, if the plants are to survive there
-at all, they must be armed with the most perfect means of protection
-possible against the attacks of hungry and thirsty animals. The
-struggle for existence in relation to such enemies is much more severe
-than in more luxuriant regions, and the protection by thorns has been
-developed to the highest possible pitch of perfection; species which
-were unable to develop this protection died out altogether. Hence the
-cactuses of Mexico, and the many thorny bushes and shrubs of the hot,
-and, in the summer, dried-up stony coast-lands of the Mediterranean in
-Spain, Corsica, Africa, and other countries. This so-called 'Prigana
-scrub' embraces a number of species, whose nearest relatives in our
-climate are not provided with spines, as, for instance, _Genista
-hispanica_, _Onobrychis cornuta_, _Sonchus cervicornus_, _Euphorbia
-spinosa_, _Stachys spinosa_, and others.
-
-Why do so few thorny plants grow on the rich and well-watered Alpine
-pastures? Probably because there is to be found there a rich and
-luxuriant plant-growth which can never be wholly exterminated by
-the grazing of animals, so that an individual species would not, by
-developing thorns, have gained any advantage in the way of increased
-capacity for existence.
-
-But these Alpine grazing grounds serve well to illustrate how great
-may be the advantage which protective devices give to a species.
-Much to the annoyance of the herdsmen, who endeavour to extirpate
-them as far as possible, enormous masses of rhododendrons often
-cover whole stretches, because their hard silicious leaves cannot be
-eaten, and many other plants despised of cattle flourish and increase
-on the grazing runs, like the repulsively bitter, large _Gentiana
-asclepiadea_, the malodorous _Aposeris fœtida_, and various ferns of
-disagreeable taste.
-
-The advantage derived by plants from the possession of any kind of
-protective device against grazing animals is perhaps best of all seen
-in the 'shrubbery,' which on every Alp is to be found in the immediate
-neighbourhood of the herdsman's hut. There, where the cattle daily
-assemble, and where the soil is continually being richly manured by
-them, we always find a large, luxuriantly growing company of the
-poisonous aconite, the bitter goosefoot (_Chenopodium bonus henricus_),
-the stinging-nettle, the thistle (_Cirsium spinosissimum_), the
-ill-smelling _Atriplex_, and some other inedible species, while the
-palatable herbs are gradually exterminated by the cattle which daily
-gather round the hut (Kerner).
-
-To sum up. We have seen that there is among plants an extraordinary
-diversity of protective adaptations, which secures them from
-extermination by the larger herbivores.
-
-Since all useful contrivances, or, as we say, all adaptations, are
-capable of interpretation in terms of the process of selection, we
-must refer this great array of the most diverse protective devices
-to natural selection; and again, as among animals, we receive the
-impression that the organism is, to a certain extent, really capable
-of producing every variation necessary to its maintenance. Literally
-speaking, this would not be correct, but at any rate the number of
-adaptations possible to each form of life must be an enormous one, so
-great, indeed, that ultimately every species does secure protection
-for itself in some manner and in some degree, whether it be by the
-production of a poison or a nauseous substance within itself, or by
-surrounding itself with thorns or spines. And if it be, in a certain
-sense, a matter of 'chance' whether a plant has taken to one method of
-defence or to another, according as its innate constitution favoured
-the production of one rather than of any other, yet it would not
-be easy to prove, even in the case of the purely chemical means of
-protection, that these would have occurred in the same distribution
-and concentration as a necessary result of the metabolism of the
-plant, even if they had not been useful and consequently augmented
-by selection. But in the case of the mechanical means of protection
-this mode of explanation fails as utterly as that of the direct
-effect of the conditions of life. Why the holly should have spinose
-leaves beneath and smooth ones above can never be deduced from the
-constitution of the species.
-
-While the protective adaptations of plants against the larger
-herbivores always point to natural selection, our appreciation of the
-adaptability of plants, and at the same time of the potency of natural
-selection, will be strengthened still more if we turn our attention for
-a little to the arrangements which prevent the extermination of plants
-by the lower and small animals.
-
-It might indeed be supposed that extermination by these could hardly
-be an imminent danger, but if we think of the cockchafer blight, or of
-the destruction of whole woods by the caterpillar of the 'white nun,'
-or even of the destruction of several successive plantings of young
-salad plants which the snails often cause in our gardens, it cannot be
-doubted that all plants would be exterminated by insects and snails
-alone unless they were protected against them in some degree.
-
-We owe our detailed knowledge of the means by which plants protect
-themselves against the menace of the greedy and prolific snails to
-the beautiful investigations of Stahl, Professor of Botany in the
-University of Jena.
-
-In this case, too, both chemical and mechanical means are made use of.
-The minute quantity of tannic acid which is contained in the leaves of
-the clover prevents many snails from eating them, as, for instance,
-the garden snail (_Helix hortensis_). If the leaves be soaked so as
-to wash out the tannin the snail readily accepts them as food. It is
-true that the small, whitish field-slug (_Limax agrestis_) does not
-object to the presence of the tannin, and eats the fresh leaves of
-the clover; indeed, there is no such thing as absolute protection.
-In discussing the herbivorous mammals I have already mentioned that
-many trees and shrubs, mosses and ferns are effectively protected by
-the large amount of tannin they contain; this protection is effective
-also against snails, for all these plants are fairly free from their
-attacks; and the same is true of many other tannin-containing plants,
-species of saxifrage and sedum, the strawberry, many water-plants, like
-the pond-weeds (_Potamogeton_), the horn-nut (_Trapa_), the mare's tail
-(_Hippuris_). All these plants are only eaten by snails in case of
-necessity, or in the washed-out state.
-
-In other plants protection is gained by means of some acid, especially
-oxalic acid, like the wood-sorrel (_Oxalis acetosella_), the sorrel
-(_Rumex_), and the species of Begonia. When Stahl smeared slices of
-carrot, which is a favourite food of snails, with a weak (one per
-cent.) solution of oxalate of potassium, they were refused by the
-snails, and this is not surprising when we remember that even the
-external skin of the snail is very sensitive, and the mucous membrane
-of the mouth is not likely to be less so.
-
-Similarly, many plants develop ethereal oils in the hairs which cover
-them, as in the herb-Robert (_Geranium robertianum_). Even the almost
-omnivorous field-slug (_Limax agrestis_) does not touch this plant, and
-if it be placed upon it, escapes with all dispatch from the ethereal
-oil, which burns its naked skin, by covering itself with mucus and
-letting itself down to the ground by a thread. The mints (_Mentha_) and
-the dittany (_Dictamnus albus_) also produce such oils.
-
-Among chemical means of protection must be named the pure bitter
-stuffs, such as are found in the species of gentian, the milkwort
-(_Polygala amara_), and in many other plants, and also the curious
-'oil-bodies' of the liverworts.
-
-But some plants also defend themselves against the attacks of snails by
-mechanical means.
-
-First there are the various kinds of bristle arrangements, which
-prevent the snails from creeping up the stalks. We never find the
-comfrey (_Symphytum officinale_) of our meadows eaten by snails,
-for it is thickly covered over with stiff bristles, which are most
-disagreeable to the snail, and the stinging-nettle (_Urtica dioica_) is
-similarly protected by bristle hairs, while, as we have already seen,
-its stinging-hairs secure immunity from the attacks of larger animals.
-
-And although it is true that the majority of plants do not prevent the
-snails from creeping up their stalks, yet they do not serve them in any
-great degree as food, since the green parts often offer resistance to
-mastication and digestion. Thus the lime encrustations which cover the
-stoneworts (_Chara_) prevent snails from eating them. If the lime be
-dissolved by means of acids, and the plants then offered to the snails,
-they will eat them greedily. The same is true of the silicifying of
-the cell-walls, so widely distributed among mosses and grasses, and
-when this occurs in a high degree it forms an effective protection
-even against the large herbivores. Our slightly siliceous grasses are
-secure from snails, and that it is really the presence of the silicic
-acid which deters them from an otherwise welcome kind of food is proved
-by Stahl's experiment of growing maize in pure water, and so obtaining
-plants poor in silica. These were devoured without ceremony by the
-snails.
-
-Of the many other protective peculiarities which make it difficult for
-snails to eat plants I shall only recall the so-called 'Raphides,'
-those microscopic crystal-like needles of oxalate of lime, pointed
-at both ends, which lie close together in the tissues of many
-plants. Cuckoo pint (_Arum maculatum_), the narcissi, the snowdrops
-(_Leucojum_), the squill (_Scilla_), and the asparagus contain them,
-and all these plants are spared by snails obviously because during
-mastication they are unpleasantly affected by the raphides. Even the
-voracious field-slug rejects these.
-
-Of course it cannot be said that these raphides protect against all
-other enemies. They are effective against rodents and ruminants,
-and also against locusts, but a number of caterpillars seek out by
-preference just those plants which contain raphides. Thus certain
-caterpillars of the Sphingidæ feed on species of _Galium_ and
-_Epilobium_, the leaves of the vine, and the wild balsam (_Impatiens_).
-The caterpillar of _Chærocampa elpenor_, which especially prefers
-_Vitis_ and _Epilobium_, has transferred its affections to the fuchsias
-in our gardens, which came from South America; the butterfly not
-infrequently lays its eggs on these plants, and the caterpillars devour
-them readily; but the fuchsias may also contain raphides.
-
-We may say, indeed, that almost all wild Phanerogams are protected in
-some degree against snails, and this almost suggests the question: What
-then is left for the snails to feed on if everything is thus armed
-against them? But, in the first place, there remain our cultivated
-plants, which, like the garden lettuce (_Lactuca_), are quite without
-defence; and secondly, the snails often eat the plants only after
-they have been rooted up and lie rotting on the ground, that is,
-when the protective ingredient has been dissolved out by the rain;
-finally, no means of protection, as I have often said already, is
-absolute or effective against all snails. Many of these are, as Stahl
-calls them, 'specialists.' Thus, the large slug of our woods eats the
-poisonous fungi which are rejected by other snails, and in the same
-way there are many other specialists which, however, are not likely to
-eliminate unaided the plants to which they have adapted themselves.
-There are certainly also omnivorous forms, like the field-slug
-(_Limax agrestis_), to which we have referred so often, and _Arion
-empiricorum_, the red slug, but just because these eat so many kinds of
-plant they are less dangerous to any one species.
-
-These manifold devices for protecting plants against the depredations
-of snails afford another proof that innumerable details in the
-organization of plants, as of animals, must be referred to natural
-selection, since they are capable of interpretation in no other way.
-If these protective devices were to be found only in isolated plants,
-we might perhaps talk of 'chance'; we might refer them to the inborn
-constitution of the plant, which made the production of bristles, or
-bitter stuffs, or the deposition of silicic acid a necessity, and which
-'happened' to make the plants distasteful to certain snails. But as
-it appears that all plants are protected against snails, one in this
-way, another in that, this objection cannot be sustained. Furthermore,
-some of the beautiful experiments made by Stahl to prove the protective
-effect of these devices showed, at the same time, that they were not
-in themselves indispensable to the existence of the plant; maize, for
-instance, develops a plant perfectly capable of life, even though
-silicic acid be withheld, and the acid is, therefore, not an element
-essential to its constitution, but a means of protection against
-voracious animals. The clearest proof of this is afforded by plants
-like the lettuce (_Lactuca_), which formed protective stuffs in the
-wild state, but have lost them altogether under cultivation, through
-disuse, as we shall see more precisely later on. As the eyes of animals
-which live in darkness have degenerated, so the plants which have been
-taken under the protection of man have lost their natural means of
-defence, because these were no longer necessary to the maintenance of
-the species. Even the protective bitter substances (tannin-compounds)
-are not essential to the constitution of the genus _Lactuca_; their
-formation may be discontinued without the plant being otherwise
-affected. And in this case it is not a question of the withdrawal
-of something which has to be taken in from outside, it is the
-non-development of what is purely a product of the internal metabolism.
-
-The adaptations of plants against snails are instructive in another
-way, namely, in their extraordinary diversity. Here again we see how
-great is the plasticity of organic forms, and how precisely, though
-in many very different ways, they adapt themselves to the conditions
-of their life, in this case the weaknesses of their greedy enemies,
-and all to attain the same end, the security of their existence as a
-species. We see at the same time that innumerable minute details in the
-structure and character of a species, which may appear unimportant, may
-yet have their definite uses--hairs, bristles, and raphides, as well as
-bitter substances, ethereal oils, acids, and tannin-compounds. But we
-must, of course, have minute and exhaustive investigations, like those
-of Stahl, in regard to the biological relations of these peculiarities
-before their utility can become clear to us.
-
-
-
-
-LECTURE VII
-
-CARNIVOROUS PLANTS
-
- Introduction--The Bladderworts or Utriculariæ--Pitcher-plants,
- Nepenthes--The Toothwort, Lathræa--The Butterwort, Pinguicula--The
- Sundew, Drosera--The Flytrap--Aldrovandia--Conclusions.
-
-
-THAT the principle of selection dominates, to a large extent at least,
-all the structural characters of plants, and moulds these in direct
-relation to the prospects of greater success which may be offered in
-the vicissitudes of the life-conditions of a single species or group
-of species, is nowhere more apparent than in the case of the so-called
-'insectivorous' or 'carnivorous' plants. Here again it was Charles
-Darwin who led the way, for while many plants had long been known on
-the sticky leaves of which insects were often caught and killed, it
-had occurred to no one to regard this as of any special use for the
-plant, much less to look on the peculiar dispositions of such leaves
-as especially determined for this purpose. Darwin was the first to
-show that there is no small number of plants--we now know about
-500--which secure only a portion of their nutritive material by the
-usual method of assimilation, and gain another and smaller portion by
-dissolving and utilizing animal protoplasm, especially nitrogenous
-muscle substance. The correctness of this interpretation was at first
-disputed, but Darwin showed that pieces of muscle, or any nitrogenous
-organic substance, were really dissolved by the relevant parts of the
-plant, and were afterwards absorbed. It can therefore no longer be
-doubted that the remarkable contrivances by which animals are laid
-hold of by plants--are in a certain sense caught and killed--have
-arisen with reference to this particular end; or, to speak less
-metaphorically, that existing structural and functional peculiarities
-in a plant which caused animals to be held fast were of advantage to
-the nutrition of the plant, and were therefore augmented and perfected
-by natural selection. That this was possible is obvious from the number
-of insectivorous plants which now live upon the earth, and that these
-processes of selection ran their courses quite independently of one
-another, and even that they started from different parts of the plant,
-is shown by the diversity of the contrivances which occur in plants of
-several different families. A few of these I wish to discuss in some
-detail.
-
-[Illustration: FIG. 23. _Utricularia grafiana_, after Kerner. _A_, a
-plant in its natural position, floating in the water. _FA_, traps. _B_,
-a trap enlarged four times. _sz_, suctorial cells. _kl_, valve, which
-closes the entrance to the trap. _C_, suctorial cells on the internal
-wall of the trap, enlarged 250 times.]
-
-The marshes of European countries, and also those of warmer lands,
-often contain bladderworts, or Utriculariæ (Fig. 23)--floating
-water-plants, without roots, and with horizontally spread,
-long-drawn-out, tendril-like shoots, in part thickly covered with
-whorls of delicate, needle-shaped leaves, in part bearing sparse
-leaves of quite peculiar structure. These are stalked, hollow
-bladders (Fig. 23 _A_, _FA_), with quite a narrow entrance at the
-apex, which is closed, as far as larger animals are concerned, by
-projecting bristle-like hairs (_B_). Small animals, such as water-fleas
-(_Daphnia_), species of _Cyclops_, and Ostracods, can swim in between
-the bristles, and they then come in contact with a valve which opens
-easily inwards (_B_, _kl_) and allows them to penetrate into the
-interior of the trap. Once inside they are captives, for the valve
-does not open outwards; therefore they soon die and decompose, and are
-then taken up by special absorptive cells (_B_, _C_, _sz_) and utilized
-as nourishment for the plants. In this way the Utriculariæ catch
-numerous little crustaceans and insect larvæ, which slip into their
-traps, presumably for concealment.
-
-[Illustration: FIG. 24. Pitcher of _Nepenthes villosa_, after Kerner.
-_St_, stalk of the leaf. _Spr_, its apex. _Fk_, the pitcher. _R_, the
-margin beset with incurved spines.]
-
-Another example is found in the marsh plants of the genus _Nepenthes_,
-some species of which live as climbers on the outskirts of tropical
-forests, climbing up the trees and letting their long, thin tendrils
-hang downwards, often over ponds and stagnant pools, where swarms of
-small flying insects abound. These plants have developed exceedingly
-remarkable contrivances for catching insects and using them as food
-(Fig. 24). The long stalks (_St_) of their leaves (_Spr_) are first
-bent downwards, then they suddenly turn sharply upwards, and the
-upturned portion is modified into a pitcher-like structure, in the
-bottom of which a fluid gathers, acid in taste, containing pepsin,
-and therefore a digestive fluid. Nitrogenous substances, such as
-flesh, dissolve in this fluid, and insects which fall into the pitcher
-from the rim are killed and dissolved. There are many species of
-_Nepenthes_, but not all of them possess the trap-structure in equal
-perfection, so that we are able, to some extent, to follow the course
-of its evolution, from a broad leaf-stalk, somewhat bent over at the
-edges, to the marvellous closed pitcher shown by _Nepenthes villosa_
-(Fig. 24) of Borneo. In this species the pitchers attain a length of
-fifty centimetres, and are beautifully coloured, resembling in that
-respect, as well as in their form, the tobacco-pipe-like flowers of
-the tropical Aristolochiæ. When we come to discuss the origin of
-flowers, we shall see that the bright, conspicuous colour possesses
-a very considerable value in attracting insects; and in the case of
-the pitcher-plant, too, the gorgeous colour probably allures insects
-to settle on the rim of the pitcher, and they are tempted to dally
-the longer since it secretes honey. But the thick, swollen rim of the
-pitcher is as smooth as if it were made of polished wax, and resembles
-the petals of those magnificent large orchids, the Stanhopeæ; the
-inner surface of the pitcher below the margin is also smooth, so that
-insects which creep about seeking honey are apt to slip and fall to the
-bottom. Even if many of them are not at once killed by the digestive
-fluid, but are able to climb up the smooth wall again, they cannot
-escape, for beneath the swollen rim, which projects inwards, there is a
-circle of strong bristles or teeth, with the points directed downwards,
-which, like thorns, prevent the captive's escape. Thus the pitchers
-of _Nepenthes_ secure and digest a large number of insects, and we
-can easily understand that the plant acquires a considerable amount
-of valuable nourishment in this way, for ready-made protoplasm is a
-convenient food to which the plant has to do but little in order to
-convert it into its own particular kind of living matter.
-
-The toothwort (_Lathræa squamaria_) must also be briefly noticed
-here, because it does not catch insects through the medium either of
-air or of water, but through the earth. As is well known, this plant
-is parasitic on the roots of various foliage-trees. It is of a pale
-yellowish colour, and has no green assimilating parts. For such a plant
-it must be of particular value to be able to catch animals and to
-use them as food. To this end the short, pale leaves, which surround
-the creeping, underground stem in the form of closely appressed
-scales, have been modified into snares for minute animals. The leaves
-have their upper parts recurved downwards, and the edges have grown
-together, so that only a small opening is left at the base, and this
-leads into a system of tunnels. Aphides, rotifers, bear-animalcules,
-but especially springtails (Podurids), creep into these hollow leaves,
-are held fast by a sticky secretion, and are dissolved and absorbed.
-
-Another example, also indigenous, is that graceful marsh plant, the
-butterwort (_Pinguicula vulgaris_), whose broad, tongue-shaped leaves,
-arranged in the form of a rosette, have been modified into an insect
-trap by the turning up of their edges, while the middle is deepened
-into a longitudinal groove (Fig. 25). The whole upper surface of the
-leaf is covered with an enormous number of little mushroom-shaped
-glands (_B_, _C_, _Dr_), which secrete a viscid slime. Insects which
-settle on the leaf stick fast, and as the glands continue to pour out
-more and more slime, while at the same time the edges of the leaf,
-stimulated by the struggling of the insect, curl over still farther,
-the victims are drowned in the slime, and ultimately absorbed; for
-this secretion is so powerful that even fragments of cartilage are
-dissolved by it in forty-eight hours. Midges and mayflies in particular
-fall victims to this plant, which is common in marshy places both in
-mountain and plain.
-
-[Illustration: FIG. 25. Butterwort (_Pinguicula vulgaris_). _A_, the
-entire plant, showing the incurved margins of the leaves and some
-insects caught by the secretion. _B_, cross-section through a leaf,
-enlarged 50 times. _r_, the margin. _Dr_, _Dr_^l, two kinds of glands.
-_C_, a portion of the leaf-surface, magnified 180 times.]
-
-We must also mention the sundew (_Drosera rotundifolia_), which takes
-its name from the seeming dewdrops that sparkle in the sun on the
-leaves, or rather on the rounded extremities of long and rather thick
-cilia-like hairs which cover the whole upper surface of the leaf. In
-reality the apparent dewdrops consist of a sticky, clear, viscid slime,
-which is secreted by the glandular ends of the pin-shaped hairs or
-'tentacles.' Insects which settle on the leaf are caught by the slime,
-and in this case also an acid, pepsin-containing fluid is secreted,
-which gradually effects the digestion of the soluble parts of the
-insect. It is especially noteworthy that it is not only those tentacles
-which are in contact with the insect that take part in its digestion
-and absorption, for all the others gradually alter their position from
-the moment when any nitrogenous body, be it a fragment of flesh or an
-insect, touches any of them. All begin to curve slowly towards the
-stimulating object (Fig. 27), so that, after one to three hours, all
-the tentacles have their heads towards it, and collectively pour out
-their digestive juice upon it.
-
-[Illustration: FIG. 26. The Sundew (_Drosera rotundifolia_), after
-Kerner.]
-
-[Illustration: FIG. 27. A leaf of the Sundew, with half of the
-tentacles curved in upon a captured insect; enlarged 4 times.]
-
-The sundew grows in marshes, as, for instance, those of the Black
-Forest, and also on the moss-covered ridges there, and it is easy to
-observe that a leaf often shows not merely a single gnat, midge, or
-little dragon-fly, but several, sometimes as many as a dozen. In this
-case, again, the value of the arrangement from the point of view of
-nourishment can be no inconsiderable one.
-
-In the case of the sundew we are obviously face to face with an
-exceedingly complex adaptation, for not only is there a secretion of
-the peculiar digestive juices, which occur only in carnivorous plants,
-but the secreting tentacles are actively motile. That the tentacles
-more remote from the captive may be excited to curve towards it, it is
-necessary that the stimulus exerted by it on the heads of the tentacles
-connected with it be conveyed to the base, and thence to the tips of
-the other tentacles, for they curve throughout their whole length.
-The utility of the contrivance is obvious, but that an arrangement so
-divergent from the ordinary dispositions of plants could be brought
-about points to the length of time that the processes of natural
-selection must have gone on, preserving every new little variation, and
-adding it to the rest.
-
-[Illustration: FIG. 28. Leaf of Venus Fly-trap (_Dionæa muscipula_),
-after Kerner. _A_, leaf-blade (_Spr_) open. _St_, leaf-stalk. _Stch_,
-sensitive hairs. _B_, vertical section through the closed leaf-blade.]
-
-[Illustration: FIG. 29. _Aldrovandia vesiculosa_, a branch with the
-traps _FA_.]
-
-Two plants remain to be noticed in conclusion, both possessing movable,
-closing traps for catching animals. The so-called Venus fly-trap
-(_Dionæa muscipula_) is a marsh plant of North America, the leaves of
-which, like those of _Pinguicula_ and _Drosera_, are arranged in a
-rosette on the ground. The individual leaf has a spatula-like stalk
-and a blade in two halves (Fig. 28, _A_), each edged with long and
-strong spinous processes, directed obliquely inwards. The halves of
-the blade, when the necessary stimulus is applied to the surface, can
-close together in a very short time, from 10 to 30 seconds. The two
-rows of marginal spines then cross, as the interlocking fingers of
-the hands do, and thus form a cage out of which the imprisoned insect
-cannot escape. The appropriate stimulus to set the mechanism in motion
-is a light touch, while a more violent shock, or strong pressure,
-or a current of air, does not cause the trap to close. But if a fly
-comes to creep about on the leaf, and in doing so touches one of six
-short jointed hairs rising erect from a minute cushion of cells, then
-the leaf closes, quickly indeed, but at the same time so gently and
-imperceptibly that the fly is unaware of danger and does not try to
-escape. Then numerous purple mucous glands begin to surround the victim
-with pepsin-containing, acid, digestive juice which gradually dissolves
-it.
-
-One of the water-plants of Southern Europe, _Aldrovandia vesiculosa_,
-which is also to be found in swamps on the northern ridge of the
-Alps, possesses, in addition to the capturing and digesting apparatus
-proper, an active motile apparatus, which is set in motion through
-sensitive hairs. When I found the plant for the first time in a swamp
-at Lindau, on the Lake of Constance, I took it at first sight for an
-_Utricularia_, for the two plants resemble each other in external
-appearance (cf. Figs. 22 and 29), but the modification of the leaves
-into traps is quite different. On both halves of the leaf-blade there
-are numerous bristles (Fig. 30, _A_), and the lightest touch on these
-by a little water animal acts as a releasing stimulus to the motile
-elements of the leaf (_Stch_). As in the Venus fly-trap, the two halves
-of the leaf close together somewhat quickly, but quite quietly, and the
-animal is caught. Fig. 30 shows a section of one of these traps in its
-closed state. The captive animals cannot escape, because the margins of
-the leaf shut quite tightly on one another, and are beset with little
-teeth. Numerous little glands (_Dr_) secrete a digestive juice, and
-after some days, or even weeks, the insoluble remains of the minute
-animals may be found inside the trap.
-
-[Illustration: FIG. 30. _Aldrovandia_: its trap apparatus. _A_, open.
-_St_, stalk of the leaf. _Spr_, blade of the leaf. _Stch_, sensitive
-bristles. _Dr_, glands. _B_, closed, a cross-section.]
-
-Many more cases of animal-catching plants might be adduced, but
-it is far from my intention to try to describe all the existing
-contrivances; those already mentioned may suffice to give an idea of
-the diversity and of the detailed effectiveness of these adaptations.
-They amplify--so it seems to me--our conception of the scope of natural
-selection, by showing us that adaptations may arise which are quite
-foreign to the original mode of life of the organism in question,
-and stand, indeed, in apparent contradiction to its fundamental
-physiological processes. It is hardly necessary to enter into a
-special argument to show that they can only have been brought about
-in the course of natural selection, since every other interpretation
-of their occurrence fails. Neither climatic nor any other external
-direct influence could have effected these modifications of the parts
-of plants, which are all so different, yet all so well suited to
-their purpose; they are different even in plants growing quite close
-together, like the sundew and the butterwort. The Lamarckian principle
-of use and disuse hardly enters into the question at all, since plants
-do not possess a will, and we can hardly speak of 'chance' where we
-have to do with such complex and diversely combined transformations.
-A process of selection actually operative in each of these cases can
-easily be thought out, and I shall leave it to my readers themselves
-to do this, and shall only indicate that we have to do with increasing
-elaboration in two different directions: first, improvements in the
-ability to utilize animal substances which happened to stick to the
-leaves, and second, an increase in the probability of animals sticking
-to the leaves, and so becoming available. Thus there arose, on the one
-hand, dissolving and digestive juices, and arrangements for absorption;
-and, on the other hand, viscid slime, and traps of various kinds to
-secure the animals, as well as honey and bright colours to attract them.
-
-But it is not merely transformations in the form of the stems and
-leaves which have come about; there are also important physiological
-changes. The sensitiveness to stimulus of various parts of the leaf is
-greatly increased, to a certain extent in the butterwort, the edges of
-whose leaves turn inwards in response to stimulus, still more in the
-sundew, in which the stimulus is conveyed from the tentacles touched to
-all the others, but most wonderfully of all in the Venus fly-trap and
-_Aldrovandia_, whose sensitive hairs so transmit the stimulus that the
-whole leaf is affected by it, and is set in motion, in a manner quite
-comparable to the effects of a nerve-stimulus in animals.
-
-Thus the case of carnivorous or insectivorous plants shows us that,
-in the course of natural selection, quite new organs can be produced
-in a plant by a thoroughgoing transformation of old ones, as, for
-instance, the pitchers of _Nepenthes_, and that, furthermore, even
-the physiological capacities of the plant may be changed in the most
-far-reaching manner, increasing and varying until they come to resemble
-the functions of the animal body.
-
-
-
-
-LECTURE VIII
-
-THE INSTINCTS OF ANIMALS
-
- The robber-wasp--Statement of the problem--Material basis of
- instincts--Instincts are not 'inherited habits'--Instinct of
- self-preservation--Fugitive instinct: death-feigning--Masking
- of crabs--Nutritive instinct--Monophagous caterpillars--Diverse
- modes of acquiring food: May-flies, sea-cucumbers, fishes that
- snare--'Aberration' of instinct--Change of instinct during
- metamorphosis: Eristalis, Sitaris--Imperfection of adaptation points
- to origin through natural selection--Instinct and will--Instincts and
- protective coloration--Leisurely flight of Heliconiidæ--Rapid flight
- of Papilionidæ--Instincts which act only once in a lifetime--Pupation
- of butterflies--Pupation of the Longicorns--Pupation of the
- silk-moth--The emperor moth--The cocoons of Atlas--Oviposition of
- butterflies.
-
-
-WE have hitherto considered animals with especial regard to the
-variation and re-adaptation of morphological characters, e.g.
-modifications of form and colour; and we have now to ask whether their
-behaviour also is to be referred as to its origin, in whole or in
-part, to the principle of selection. All around us we can see that
-animals know how to use their parts or organs in a purposeful manner:
-the duckling swims at once upon the water; the chicken which has just
-been hatched from the egg pecks at the seeds lying on the ground; the
-butterfly but newly emerged from the pupa, as soon as its wings have
-dried and hardened, knows how to use them in flight; and the predatory
-wasp requires no instruction to recognize her victim, a particular
-caterpillar, a grasshopper, or some other definite insect; she knows
-how to attack it, to paralyse it by stings, and then hesitates not
-a moment as to what she has to do next; she drags it to her nest,
-deposits it in one of the cells already prepared for her future brood,
-lays a single egg upon it, and roofs the cell carefully over. It is
-only because all these complex acts are so precisely performed, as
-precisely as if the wasp knew why she performed them, that the species
-is able to maintain its existence, for only thus can the rearing of the
-next generation be secured. Out of the egg there slips a little larva,
-which at once makes for the paralysed victim, feeds upon it, and grows
-thereby, then, within the shelter of the closed cell, passes through
-the pupa stage and is transformed into a perfect wasp. Many species
-of these predatory wasps do not lay the egg directly beside or upon
-their prey, but lest its movements should endanger their offspring,
-they hang the egg above it by a silken thread. It is thus in security,
-and the young larva, too, when it appears, can withdraw to its safely
-swinging resting-place as soon as danger threatens from the convulsive
-struggles of the unfortunate victim at whose body it is gnawing.
-
-Every animal has a great many such 'instincts,' which lead it, indeed
-force it, to act appropriately towards an end, without having any
-consciousness of that end. For how should the butterfly know what
-flying is, or that it possessed the power of flight at all, or who
-could have shown the predatory wasp, when she wakened from the pupa
-sleep to quite a new kind of life, all that she had to do in order to
-procure food for herself and to secure shelter and nourishment for
-the brood which was still enclosed within her ovary? Since species
-have developed from other species, these regulators of the body, the
-instincts, cannot have been the same in earlier times; they must have
-evolved out of the instincts of ancestors, and the questions we have to
-ask are: By what factors? In what way? Has the principle of selection
-been operative here too, or can we refer instincts to the inherited
-effects of use and disuse?
-
-Before I enter upon this question it is necessary to consider for a
-little the physiological basis of instinct. We can distinguish three
-kinds of actions: purely reflex, purely instinctive, and purely
-conscious actions. In the case of the first, we see most clearly that
-they depend on an existing mechanism, for they follow of necessity on
-a particular stimulus, and cannot always be suppressed. Bright light
-striking our eye makes the pupil narrower by a contraction of the iris,
-and in the same way our eyelids close if a finger be thrust suddenly
-towards them. We know, too, the principle of these reflex mechanisms;
-they depend on nerve connexions. Sensory nerves are so connected in the
-nerve-centres with motor nerves, that a stimulus affecting the former
-at the periphery of the body, as at the eye, is carried to certain
-nerve-cells of the brain, and from these it excites to activity certain
-motor centres, so that definite movements are set up. It is rarely
-only one muscle that is thus excited to activity, there are usually
-several, and here we have the transition to instinctive action, which
-consists in a longer or shorter series of actions, that is, of motor
-combinations. These, too, are originally, at least, set a-going by a
-sense impression, an external stimulus which affects a sensory nerve
-exactly in the same way as in the reflex mechanism, and this stimulus
-is carried to a particular group of sensory nerve-cells in the central
-nervous organ, and from these transmitted by very fine inter-connexions
-to motor centres. There are extraordinarily complex instinctive
-actions, and in these the completion of one action is obviously the
-stimulus to the second, the completion of the second to the third, and
-so on, until the entire chain of inter-dependent movements which make
-up the whole performance has been completed.
-
-Instincts have thus a material basis in the cells and fibres of
-the nervous system, and through variations in the connexions and
-irritability of these nervous parts they too can be modified, like any
-of the other characters of the body, such as form and colour.
-
-Conscious actions depend directly on the will, and they have a close
-connexion with instinctive actions in as far as these also can be
-controlled by the will, that is, can be set a-going or inhibited, and
-also, on the other hand, in as far as purely voluntary actions may
-become instinctive through frequent repetition. The first case is
-illustrated, for instance, when the suckling of a child at the mother's
-breast is continued into the second year of life, as not infrequently
-happens in the southern countries of Europe. Such a child knows exactly
-why it wants the breast, and its action is a conscious one, while the
-newborn child seeks about with the mouth instinctively, and when it has
-found what it sought performs the somewhat complex sucking movements
-automatically. The second case is illustrated, when, for instance, we
-have made a habit of winding up a watch on going to bed, and do it when
-we happen to change our clothes through the day, although it is then
-purposeless and would have been omitted if the action had required
-a conscious effort of will. One can often observe on oneself in how
-short a time a conscious action may become instinctive. I once sent my
-keyless watch to a watchmaker for repairs, and received from him for
-the time an ordinary watch, which had to be wound with a key, which
-key I kept for safety in my purse. At the end of eight days I got
-back my own watch, and on undressing the first night I found myself
-'instinctively' taking my purse from my pocket in order to get the key,
-which, as I very well knew, I no longer needed. And that a long series
-of complex movements, originally performed only consciously, may be
-gone through instinctively, is shown by the fact that pieces of music
-which have been learnt by heart can often be played without mistake
-from beginning to end while the player is thinking of quite other
-things. The complex instinctive actions of animals are performed in
-quite a similar manner.
-
-There is thus no sharp boundary line between reflex and instinctive
-actions, nor between instinctive and conscious actions, but one passes
-over into the other, and the thought suggests itself, that in the
-phyletic development also transitions from one kind of action to the
-other must have taken place. As long as one believes the Lamarckian
-principle to be really operative one can suppose that actions, which
-were originally dependent on the will, when they were often repeated,
-became instinctive, or, in other words, that instincts, many of them at
-least, are inherited habits.
-
-I shall endeavour to show later on that this assumption, plausible as
-it seems at first sight, cannot be correct; but in the meantime I must
-confine myself to saying that there are a great number of instincts
-which must be referred to the process of selection, and that the rest
-can be similarly interpreted in their essentials at least.
-
-The instinct of self-preservation is universally distributed, and it
-is exhibited in many animals by flight from their enemies. The hare
-flees from the fox and from men; the bird flies away at the approach of
-the cat; the butterfly flies from even the shadow of the net spread to
-catch it. These might be regarded as purely conscious actions, and in
-the case of the hare and the bird experience and will have undoubtedly
-some part in them, but even in these the basis of the action is an
-organic impulse; this, and not reflection, causes the animal to flee
-at sight of an enemy. In the butterfly, indeed, this must be purely
-instinctive, since it is done with the same precision immediately on
-leaving the pupa state, before the animal has had any experience. But
-even in the case of the hare and the bird, taking to flight would in
-most cases come too late if reflection were necessary first; if it is
-to be effective it must take place as instantaneously as the shutting
-of the lids when danger threatens the eye.
-
-The hermit-crab (Fig. 34, p. 163), which conceals its soft abdomen
-in an empty mollusc shell, and drags that about with it on the floor
-of the sea, withdraws with lightning-like rapidity into its house
-as soon as any suspicious movement catches its eye, and it is very
-difficult to grip one of its legs with the forceps in time to draw it
-out of its shell. The same is the case with the so-called Serpulids,
-worms of the genus _Serpula_, and its allies; it is not easy to seize
-them, because, however quick one is with the forceps, their instinct
-of fugitive self-preservation acts more quickly still, and they shoot
-back into their protecting tubes before one has had time to grasp them.
-But this impulse to flee from enemies, though it seems almost a matter
-of course, is by no means common to all animals, for in quite a large
-number the instinct of self-preservation finds expression in an exactly
-contrary manner, namely, in the so-called 'death-feigning,' that is,
-remaining absolutely motionless in a definite position precisely
-prescribed to the animal by its instinct. In speaking of protective
-colouring, I drew attention to the 'wood-moth' (_Xylina_), which
-resembles a broken fragment of half-decayed wood so deceptively, and
-I pointed out that the colour-resemblance to wood would be in itself
-of but little use to the insect if it were not combined with the
-instinct to remain motionless in danger, to 'feign death.' The antennæ
-and legs are drawn close to the body, so that they rather heighten
-the disguise, and, instead of running away, the insect does not move
-a muscle until the danger is past. This instinct must have evolved
-hand in hand with the resemblance to a piece of wood, and, just as
-we sought to interpret the latter from the fact that the moths which
-most resembled the wood had always the best chance of surviving, so we
-maintain that those moths would profit most by their resemblance which
-drew in their legs and antennæ closely and lay most perfectly still.
-Thus the brain-mechanism, which effected the keeping still whenever the
-senses announced danger, would be more and more firmly established and
-perfected in the course of selection.
-
-Even nearly related animals may have quite different instincts
-which secure them against danger. Thus in the group of pocket crabs
-(Notopoda) there are some species which run away when danger threatens,
-but others which anticipate the risk of discovery by masking themselves
-to a certain extent. With the last pair of legs they hold over
-themselves a large piece of sponge, which then grows till it often
-leaves only the limbs and frontal region uncovered. Of course there
-can be no question of consciousness in what the crab does, as is
-proved by the fact that these crabs will, in case of necessity, take a
-transparent piece of glass instead of the sponge; but the impulse to
-cover themselves with something is strong in them, and finds expression
-not only when they see a really protective substance, but even when
-they see one which is transparent and therefore wholly useless for the
-purpose. Crabs from which the sponge has been taken away wander about
-until they find another; the impulse is thus set up not only by the
-sight of the sponge or of a stone, but also by the feeling that their
-back is uncovered. The large spider-crab of the Mediterranean (_Maja
-squinado_) effects its disguise in a somewhat different manner. It has
-peculiar hooked bristles on the back, and on these it hooks little
-bunches of seaweed, often many of them, so that it is entirely covered
-and looks like a bunch of wrack rather than like an animal. Here again
-a bodily variation has gone hand in hand with the development of the
-instinct to cover itself: the bristles of the back have become hooked.
-Many instincts are accompanied by structural modifications, and in the
-crabs which cover themselves with sponge or stone this is the case, for
-the last pair of thoracic legs is turned towards the back, instead
-of being set at the side of the body, as is usual among crabs. They
-are thus enabled to hold the sponge much better and more permanently,
-and as this is advantageous we may well ascribe the change to natural
-selection.
-
-Let us now turn our attention to another category of instincts, the
-most common and most indispensable of all, those which lead to the
-seeking and devouring of food.
-
-The chicken just emerged from the egg picks up the seeds thrown to it
-with no experience of what eating is, or what can be made to serve it
-as food; its instinct for food expresses itself in picking up, and it
-is awakened or stimulated to action by sight of the seeds. As Lloyd
-Morgan in his excellent book on _Habit and Instinct_ well says, 'It
-does not pick at the seeds because instinct says to it that this is
-something to be picked up and tested, but because it cannot do anything
-else.'
-
-In the same way the instinct to seek for food wakes in the kitten at
-the sight of a mouse. I once set before a kitten which had never seen
-a mouse a living one in a trap. The kitten became greatly excited, and
-when I opened the trap and let the mouse run away she overtook and
-caught it in a few bounds. The instinct in this case does not express
-itself as in the chicken by the rapid lowering of the head and seizing
-the food, but in quite a different combination of movements, in running
-after and grasping the fleeing victim. But that is not all that is
-included in the instinctive action in the case of the cat, for there
-is also the whole wild and gruesome play, the familiar letting go and
-catching again, the passionate growling of satisfaction which, in its
-wildness, reminds us much more of a blood-thirsty tiger than of a tame
-domestic animal.
-
-As the egg-laying instinct of the female butterfly is excited only by
-the sight and odour of a particular plant, so also is the food instinct
-of the caterpillar. If we put a silkworm caterpillar (_Bombyx mori_)
-just out of the egg upon a mulberry leaf it will soon begin to gnaw at
-it; but put it on a beech leaf or on that of any other indigenous tree,
-shrub, or herb and it will not touch it, but simply die of hunger.
-And yet it could quite well eat many of these leaves, and thrive on
-them too, but the smell and perhaps also the sight of them is not the
-appropriate stimulus to liberate the instinct of eating. There are
-many species of caterpillar which are 'monophagous,' that is to say,
-restricted to a single species of plant in a country. One may ask how
-such a restriction of the liberating stimulus to a single species
-could have been brought about by natural selection, since it could not
-possibly be advantageous to be so much restricted in food. The answer
-to this will be found in the following facts. On the Belladonna plant
-there lives a little beetle whose feeding instinct is aroused by this
-plant alone. But as _Atropa belladonna_ is avoided entirely by other
-animals on account of its poisonousness, this beetle is, so to speak,
-sole proprietor of the Belladonna; no other species disputes its food,
-and in this there must assuredly be a great advantage, as soon as the
-other instincts, above all that of egg-laying, are so regulated as to
-secure that the larva shall have access to its food-plant; and this
-is the case. The monophagy of many caterpillars is to be understood
-in the same way; it is an adaptation to a plant otherwise little
-sought after, and it is combined with a more or less complete loss
-of sensitiveness to the stimulus of other plants. The establishment
-of such a specialized food-instinct depends on its utility, and has
-resulted from the preference given, through natural selection, to those
-individuals in which the food-instinct responded to the stimulus of
-the smallest possible number of plants, and at the same time to those
-which showed themselves best adapted to a plant especially favourable
-to their kind, whose food-instinct was not only most strongly excited
-by this one plant, but also whose stomach and general metabolism made
-the best use of it. So we understand why so many caterpillars live
-on poisonous plants, not only some of our indigenous Sphingidæ, like
-_Deilephila euphorbiæ_, but whole groups of tropical Papilionidæ,
-Danaides, Acræides, and Heliconiidæ. With this again is connected the
-poisonousness or nauseousness of these butterflies.
-
-How diversely the instinct to procure food may be developed in one and
-the same group of animals is shown by the fact that not infrequently
-plant-eating, saphrophytic, and flesh-eating animals occur in a single
-group of organisms, as, for instance, in the order of water-fleas or
-Daphnidae, or in the class of Infusorians. Many species find their
-food by making an eddy in the water, which brings a stream towards the
-mouth, and with it all sorts of vegetable or dead particles; others
-live by preying upon other animals like themselves.
-
-But even when the food-instinct in all the species of a group is
-directed towards living prey, the procuring of it may be achieved by
-means of quite different instincts. Such finer graduations of the
-food-instinct are found not infrequently within quite small groups of
-animals, as in the Ephemeridæ or Day-flies. All their larvæ live by
-preying on other animals, but those of one family, represented by the
-genus _Chloëon_, seek to secure their victims by agility and speed,
-while the larvæ of the second family, with the typical genus _Baëtis_,
-have the instinct to press their smooth broad bodies, with large-eyed
-head, close to the brook pebbles on which they sit. They are exactly
-like these in colour, and thus they lurk almost invisible, until a
-victim comes within their reach, when they throw themselves upon it
-with a bound. The third group, with the typical genus _Ephemera_,
-follows its instinct to dig deep tubes in the mud at the bottom of
-the water, and to lurk in these for their prey. We have thus within
-this small group of Day-flies three distinct modifications of the
-food-instinct, which differ essentially from one another, are made
-up of quite different combinations of actions, and, consequently,
-must have their foundation in essentially different directive
-brain-mechanisms. All these cases have only one feature in common; the
-animals all throw themselves upon their prey as soon as they are near
-enough.
-
-[Illustration: FIG. 31. Sea-cucumber (_Cucumaria_), with expanded
-tentacles (_a_), and protruded tube-feet (_b_); after Ludwig.]
-
-But even this common feature is not everywhere part of the
-food-instinct. The sea-cucumber (_Cucumaria_) (Fig. 31), according to
-the observations made on it by Eisig in the Aquarium of the Zoological
-Station at Naples, gets its food in the following manner. The animal
-sits half or entirely erect on a projecting piece of rock and unfolds
-its ten tree-like tentacles which surround the mouth. These are
-branched, and have quite the effect of little tufts of seaweed. They
-are probably taken for such by many minute animals; for larvæ of all
-kinds, Infusorians, Rotifers and worms settle down on them. But the
-sea-cucumber bends inwards first one tentacle and then another, so
-slowly as barely to be noticeable, brings the point to its mouth, lets
-it glide gradually deeper into the gullet, until the whole tentacle is
-within, and after a time draws it out again equally slowly and unfolds
-it anew. Obviously it wipes the tentacle inside the gullet, and retains
-everything living that was upon it. This performance is repeated
-continually, day and night, and it is usually the only externally
-visible sign of life which the animal displays.
-
-This remarkable instinct is associated with a structural peculiarity,
-for without the arborescent tentacles the fishing would not be nearly
-so successful. Other sea-cucumbers or Holothurians have different
-tentacles, and use them in quite a different manner, filling the mouth
-with mud by means of them.
-
-Very frequently, indeed, there are visible structural changes
-associated with the modified food-instinct. Most predatory fishes chase
-their prey, like the pike, the perch, and the shark, but there are also
-lurkers, and these show in addition to the lurking instinct certain
-definite bodily adaptations, without which this instinct could not have
-such full play.
-
-Thus in a marine fish known as the 'star-gazer' (_Uranoscopus_) the
-eyes are situated not on the sides but on the top of the head, and the
-mouth is also directed upwards. Its instinct leads it to bury itself in
-the sand so that only the eyes are uncovered. It lies in wait in this
-way until a suitable victim comes within reach, and then snaps at it
-with a sudden movement. But it also possesses a decoying organ, a soft
-worm-shaped flap, which it protrudes from the mouth as soon as little
-fishes draw near. They make for this bait, and are thus caught.
-
-Such ingenious fishing, which is quite suggestive of the human method
-of catching trout with artificial bait, occurs in many predatory
-fishes; but in every case the fish acts instinctively, without
-reflection, on becoming aware of approaching prey. The suitability of
-the action to the end does not depend upon consciousness of the end,
-or upon reflection, but is a purely mechanical action, performed in
-response to the stimulus of a sense-impression.
-
-This is best shown by the fact that the instincts may lead their
-possessors astray, which always happens when an animal is transferred
-to an unnatural situation, to which its instincts are not adapted, so
-to speak. The mole-cricket, which is in the habit of escaping pursuit
-by burrowing in the earth, makes violent motions with the forelegs,
-even if it be placed upon a plate of glass into which it could not
-possibly burrow; an ant-lion (_Myrmeleo_), whose instinct impels
-it to bore into loose sand by pushing backwards with the abdomen,
-goes backwards on a plate of glass as soon as danger threatens, and
-endeavours, with the utmost exertions, to bore into it. It knows no
-other mode of flight, and its intellect is much too weak to suggest any
-novel mode. Even the mode of escape most universal among animals, that
-of simply running away, does not occur to it; it acts as it must in
-accordance with its inborn instinct; it cannot do otherwise.
-
-The change of instincts in the different stages of development of
-one and the same animal have always seemed to me very remarkable; for
-instance, the change of the food-instinct in the caterpillar and the
-butterfly, where the food-instinct is liberated in the caterpillar
-by the leaf of a particular plant, but in the butterfly by the sight
-and fragrance of a flower, the nectar of which it sucks. In this case
-everything is different in the two stages of development, the whole
-apparatus for seeking and taking food, as well as the nerve-mechanism
-which determines these modes of action. And how far apart often are the
-stimuli which liberate the instinct! The larva of the flower-visiting,
-honey-sucking _Eristalis tenax_ is the ugly, white, so-called
-rat-tailed larva, well described by Réaumur, which lives swimming
-in liquid manure, and feeds on that! What complete and far-reaching
-changes, not only in the visible structure, but also in the finer
-nervous mechanisms, which we cannot yet verify, must have taken place
-in the vicissitudes of time and circumstance during the life-history of
-this insect!
-
-[Illustration: FIG. 32. Metamorphosis of _Sitaris humeralis_, an
-oil-beetle, after Fabre. _a_, first larval form, much enlarged. _b_,
-second larval form. _c_, resting stage of this larva (so-called
-'pseudo-pupa'). _d_, third larval form. _e_, pupa.]
-
-Not the food-instinct alone, but the instinct of self-preservation,
-of mode of motion, in short, every kind of instinct, may vary in the
-course of an individual life. Let us follow the somewhat complex
-life-history of a beetle of the family of the Blister-beetles or
-Cantharides, as we learnt it first from Fabre. The female of the
-red-shouldered bee-beetle (_Sitaris humeralis_) lays its eggs
-on the ground in the neighbourhood of the underground nest of a
-honey-gathering burrowing-bee (_Anthophora_). The larvæ, when they
-emerge, are agile, six-legged, and furnished with a horny head and
-biting mouth-parts, as well as with a tail-fork for springing (Fig. 32,
-_a_). The little animals have at first no food-instinct, or at least
-none manifests itself, but they run about, and as soon as they see a
-bee of the genus _Anthophora_ they spring upon it and hide themselves
-in its thick, hairy coat. If they have been fortunate the bee is a
-female, who founds a new colony and builds cells, in each of which she
-deposits some honey and lays an egg upon it. As soon as this has been
-done the _Sitaris_ larva leaves its hiding-place, bites the egg of the
-bee open, and gradually eats up the contents. Then it moults, and takes
-the form of a grub with minute feet and imperfect masticating organs;
-the tail-fork, too, is lost, for all these parts are now useless, since
-it can obtain liquid nourishment without further change of place, from
-the honey in the cell, in exactly the quantity necessary to its growth.
-Then it spends the winter in a hardened, pupa-like skin, and it is
-not till the next year (the third), after another short larval stage
-(_d_) and subsequent true pupahood (_e_), that the fully-formed beetle
-emerges. This again possesses biting mouth-parts, and eats leaves, and
-has legs to run with and wings to fly with.
-
-In this beetle, then, the food-instinct changes three times in the
-course of its life; first the egg of the bee is the liberating
-stimulus, then the honey, and finally leaves. The instinct of moving
-about varies likewise, expressing itself first in running and jumping
-and in catching on, then in lying still within the cell, and, lastly,
-in flying and running about on bushes and trees.
-
-We can well understand that, in the course of innumerable generations
-and species of insects, the various stages of development would,
-by means of selection, become more and more different from each
-other, both structurally and in their instincts, as they adapted
-themselves better to different conditions of life; and thus ultimately
-instincts frequently and markedly divergent have been developed in
-the successive stages of life. No other interpretation is possible;
-through natural selection alone can we understand even the principle
-of such adaptations. An animal can thus very well be compared to a
-machine which is so arranged that it works correctly under all ordinary
-circumstances, that is to say, it performs all the actions necessary to
-the preservation of the individual and of its kind. The parts of the
-machine are fitted together in the best possible way, and work on each
-other so ingeniously that, under normal circumstances, a result suited
-to the end is achieved. We have seen how precisely the liberating
-stimulus for an action may be defined, and this secures a far-reaching
-specialization of instincts. But as every machine can work only with
-the material for which it was constructed, so the instinct can only
-call forth an action effectively adjusted to its end when the animal
-is under natural conditions. Its specialization has its limits, and
-in this lies the reason of its limited purposiveness. For instance,
-if the larva of _Sitaris_ were not impelled by the sight of every
-bee to spring on it and cling to it, but only by the _females_, then
-many of them would be saved from the fate that awaits them if they
-attach themselves to male bees, which make no nest, or even to other
-flying insects, in which case also there is no possibility of further
-development. But both these things happen, although the latter has
-not yet, to my knowledge, been recorded of _Sitaris_, but only of its
-relative, the larva of _Melöe_.
-
-'Instinct goes astray,' it is often said; but in truth it does not
-go astray, but is only not so highly specialized in relation to the
-liberating stimulus of the action as seems to us necessary for perfect
-purposiveness. But in this very imperfection there lies, as it seems to
-me, another proof that we have to do with the results of a process of
-selection, for it is of the very nature of these never to be perfect,
-but only relatively perfect, that is to say, just as perfect as is
-necessary to the maintenance of the species. At the moment at which
-this grade of perfection is reached every possibility of a further
-increase in the effectiveness of adjustment to the end ceases, because
-it would then no longer directly further the end. Why, for instance,
-should the liberating stimulus in this case be more highly specialized,
-since enough of the _Sitaris_ larvæ already succeed in attaching
-themselves to female bees? It is not for nothing that the beetles of
-this family are so prolific; what is lacking in the perfection of the
-instinct is made up for by the multitude of young larvæ. A single
-female of the oil-beetle (_Melöe_) lays several hundred eggs.
-
-In speaking of the animal as a machine, it must be added that it
-is a machine which can be altered in varying degrees, which can be
-regulated to work at high or low pressure, slowly or quickly, finely or
-roughly. This regulating is the work of the intelligence, the limited
-'thinking-power,' which must be ascribed to the higher animals in a
-very considerable degree, but which in the lower animals becomes less
-and less apparent, until finally it is unrecognizable. That instinctive
-actions can be modified or inhibited by intelligence and will is proved
-by any trained beast of prey which masters its hunger and the impulse
-to snap at the piece of flesh held before it, because it knows that
-if it does not control itself painful blows will be the consequence.
-In a later lecture I shall return to the connexion between will and
-instinct; all that concerns us here is to regard instincts as the
-outcome of the processes of selection, and as an indirect proof of the
-reality of these.
-
-From what I have already said at least so much must be clear, that
-nothing, in principle, stands in the way of referring instincts to
-selection, since their very essence is their adaptation to an end, and
-such purposive changes are precisely those that are preserved in the
-struggle for existence. It might, however, be supposed that in all this
-the principle of use and disuse also had a share, and that without it
-no changes in instincts could have come about.
-
-There are, however, numerous instincts in considering which this can be
-entirely excluded.
-
-At an earlier stage we discussed in detail the protective colourings
-which secure insects, and especially butterflies, from extermination
-by their numerous enemies, and it was mentioned that this was always
-accompanied by corresponding instincts, without which the protective
-colouring and the deceptive form would have profited nothing, or at any
-rate not nearly so much. If the caterpillar of the _Catocala sponsa_,
-which resembles the bark of an oak so deceptively, did not possess at
-the same time the instinct to creep away from the leaves and hide in
-the clefts of the bark on the trunk of the oak-tree, its disguise would
-be of very little use to it; and if the predatory and grass-coloured
-praying mantis was not impelled by instinct to lie in wait among the
-grass for its prey, instead of pursuing it, it would rarely succeed in
-seizing any of its victims, because of its somewhat leisurely mode of
-movement. This adaptation of the instincts to the protective colouring
-is carried into the most minute and apparently trifling details. Thus
-different observers have established the fact that the nauseous,
-sometimes even poisonous, butterflies, which are distinguished by their
-glaring or sharply contrasted colour-pattern, are all slow fliers.
-This is the case with the Danaides and Euplœides of the Old World and
-the Heliconiides of the New; many of their mimetic imitators also fly
-slowly.
-
-If we inquire how this instinct of fluttering, careless flight has
-come to be, we may leave habit as _primum movens_ out of the question
-altogether, for there are no external conditions which could have
-induced the butterfly to take to slower flight than its ancestors
-exhibited. That it is now advantageous for it--since it acts as a
-signal of its nauseousness--to be as clearly seen and recognized as
-possible can exercise no direct influence on its manner of flight,
-since it knows nothing about it. Even if we assume that individual
-variations cropped up which had an instinct for slower flight, there
-would still, without selection, be no reason why this variation in
-particular should multiply, still less why the originally slight
-slowing of the flight should become more marked in the course of
-generations. On the contrary, the butterflies fly a great deal, just
-as all other diurnal butterflies do; they exert their power of flight
-as long as the sun shines, and if the exercise of one generation
-influences the next, they ought to become gradually more capable of
-rapid flight. In this case exactly the opposite takes place to what is
-ascribed to the Lamarckian principle; more constant use must here have
-brought about a diminution of the activity of the relevant parts. It is
-quite otherwise when we look at it from the point of view of selection.
-The variants which cropped up by chance with slower flight survived
-because they were most easily recognized and avoided; they are the most
-frequent survivors; they leave descendants which inherit the slower
-flight-instinct, and this goes on increasing in them as long as the
-increase carries any advantage with it. As soon as this ceases to be
-the case the variation comes to a standstill, for it is adapted to the
-average of the conditions at a given time.
-
-We may picture to ourselves the thousand kinds of regulations of animal
-movements through instinct as having come about in a similar way; in
-the majority of cases we _must_ picture it thus. For it is only in the
-case of those with high intelligence that we can ask whether the animal
-did not by deliberation help in establishing the purposive variation
-in its movements. Among insects in any case this could only be taken
-into account to a very limited extent, although I do not dispute that
-the more intelligent among them may learn, and may make experiments,
-and can modify their actions accordingly. But in fleeing from an enemy
-experience has nothing to do with it, for the first time it is caught
-it pays the penalty with its life. Without care, and with no idea of
-the dangers surrounding them on all sides, the butterflies float about,
-guided only by their instinct, which, however, is so exactly adapted
-to the conditions of their life that a sufficient number of them to
-preserve the species always happily escapes all the many dangers. I
-may remind you of Hahnel's case of the butterfly, already mentioned,
-which escaped the agile lizard by flying rapidly up from the sweet
-bait, but settled again upon it without fear immediately afterwards, to
-fly from the lizard as before, and did so several times in succession.
-We usually judge such actions far too much from the human standpoint;
-the butterfly does not wish to escape the death which threatens it; it
-knows nothing about death; it is not with it as it was with Dr. Hahnel
-himself, who when he was once in danger from a jaguar in a thicket
-was so affected by the thought of the death he had happily escaped
-that he never cared to pass the place again, but made a long circuit
-to his home. The butterfly does not act according to reflection and
-imagination; it flies up with lightning-like rapidity when the lizard
-rushes at it, because this rapid movement, which it _sees_, acts as
-the stimulus which liberates the flight-instinct, and this works so
-promptly that in most cases the insect is rescued from danger. Its
-disposition, however, is not otherwise affected by its narrow escape,
-and it obeys anew the food-instinct which impels it to settle again
-on the bait, until the flight-instinct is again set a-going by the
-visual impression of the re-advance of the lizard. It is the plaything
-of its instincts, a machine which works exactly as it must. That it
-is only sense-impressions and not conceptions which here liberate the
-actions can be well seen in the case of shy species of butterfly like
-our purple emperor (_Apatura iris_), which flies up like lightning from
-the moist wood-paths on which it loves to settle as soon as any rapidly
-moving visual image, even if it be only a shadow, strikes its eyes. For
-this reason the collector tries to approach it so as not to throw his
-shadow before him, for then the insect lets the advancing enemy get
-quite close, and only flies up when the net is quickly thrust towards
-it. In all probability the eye of this insect is particularly well
-adapted for perceiving movements, and certainly the flight-instinct
-reacts very promptly to such visual impressions, and we can understand
-that it must have been so regulated if, as we assume, the regulation
-came about through processes of selection, for the enemies of the
-butterflies, such as birds, dragon-flies, and lizards shoot quickly
-out on their prey, and therefore those butterflies must always have
-survived whose instinct impelled them to take to flight most quickly.
-
-In this, then, as in a thousand other cases, the instinct of flight,
-or indeed any other mode of movement, cannot be interpreted as an
-'inherited habit,' because there is no evidence of the possession of
-that degree of intelligence which could have induced the variation in
-the previous habit, that is, in manner of movement. The same is true
-of animals of low intelligence in regard to all the other instincts,
-which otherwise might seem to be explicable in terms of the Lamarckian
-principle.
-
-In addition, there is a whole large group of instincts in regard to
-which the idea of the Lamarckian principle cannot be entertained, as
-I showed years ago, and it consists of all those instincts which are
-only exercised once in the course of a lifetime. These cannot possibly
-depend on practice in an individual lifetime, and transmission of
-the results of this exercise to the following generation; they can
-therefore only be interpreted in terms of selection, unless we are to
-give up all attempts at a scientific interpretation, and simply accept
-them as 'marvels.'
-
-To this class belong all the diverse instincts by which insects
-protect themselves against attack during the pupa stage. Even the way
-in which the caterpillars of many diurnal butterflies hang themselves
-up in pupation is not by any means a very simple instinctive action.
-The caterpillar first spins, in a suitable place, a small round disk
-of silk threads, to which it then attaches the posterior end of its
-body, so securely that it cannot be easily torn away. More complicated
-still is the securing of the pupa when it does not hang freely, but
-is to remain pressed against a wall or a tree, as is the case in the
-Papilionidæ and the Pieridæ. In this case the caterpillar must, in
-addition to the usual cradle, spin a thread of silk, in an ingenious
-way, diagonally across the thorax, so that it may cross about the
-middle of the wing rudiments, and not be too loose, lest the pupa fall
-out, yet not too tight, lest the thread cut too deeply into the wing
-rudiments and hinder their development. When one remembers that it is
-the caterpillar that does all this, before it has taken the form of the
-pupa, and that it must all be adapted to the pupa's form, we are amazed
-at the extraordinary exactness with which instinct prescribes all the
-individual movements which make the whole of the complex performance
-effective. And yet, as each caterpillar only accomplishes this
-performance once in its life, it could at no time in the development
-of the species have become a habit in the case of any individual
-caterpillar, and it cannot therefore be an 'inherited habit.'
-
-But however diverse are the methods of securing the safety of the pupæ
-in the different families of butterflies, they must all be referred
-back to a single root, if the butterfly pedigree can be traced back to
-a single ancestral group. The caterpillar of the Sphingidæ does not
-creep up walls and trees when it is ready to enter on the pupa stage,
-as so many of the caterpillars of the diurnal butterflies do, but
-instead its instinct compels it to run about on the ground until it has
-found a spot which seems to it suited for boring into the earth, or,
-to speak less metaphorically, until it comes to a place which, from
-its nature, acts as a liberating stimulus to the instinct to burrow.
-Then it penetrates more or less deeply, according to the species, and
-makes a small chamber, which it lines with silken threads to prevent
-it collapsing; this done, it moults, and enters on the pupa stage.
-The exactness with which the individual movements are prescribed
-by instinct is seen in the way in which the size of the chamber is
-regulated so as to be exactly as large as is necessary to give the
-pupa room enough without leaving any superfluous free space. This is
-not so simple as it seems, and is not directly conditioned by the
-size of the animal, for the caterpillar is longer and altogether of
-greater volume than the pupa. The same thing is seen in the stag-beetle
-(_Lucanus cervus_), the largest of our indigenous beetles, which gets
-its name from the powerful antler-like jaws which distinguish the male.
-It also undergoes its pupal metamorphosis in the earth, and makes a
-large hard ball of clay, hollow inside, and as smooth as if polished,
-and its cavity is exactly the size of the future pupa, or to speak more
-precisely, of the fully-formed beetle. For, as Rösel von Rosenhof in
-his day 'observed with amazement,' the balls in which the males lie
-have a much longer cavity than those built by the females, and for
-this reason, that when the fully-formed beetle emerges from the pupa
-it must, if it is a male, have room to stretch out its horns, which
-have till then lain upon the breast. 'For the beetles do not leave
-their dwelling-place until all their parts are sufficiently strong and
-properly hardened, and till the season has arrived in which they are
-wont to fly about.' The male larva thus makes a much longer pupa-house
-than the female larva, in anticipation, so to speak, of the enormous
-size of the jaws which will grow out later!
-
-Here the instinct has two modes of expression, according as the bodily
-parts are male or female. Here we have to do with an action which is
-performed once in a lifetime, and thus the possibility of any other
-explanation of the origin of this instinct than through natural
-selection is excluded.
-
-Not less significant is the case of the silk-cocoons. The cocoons spun
-by the silkworm are egg-shaped, and consist of a single thread many
-thousand yards in length, which is wound round the spinning caterpillar
-so that not a space is left uncovered. The web is firm, tough, and very
-difficult to tear; therefore we must grant that the pupa resting within
-will enjoy a very considerable degree of security against injury. But
-the moth must be able to get out, and that this may be possible the
-caterpillar is impelled by instinct to make its spinning movements such
-that the cocoon is eventually looser at the anterior end, so that the
-insect, when it is ready to emerge, can tear it asunder with its feet
-and make a way out for itself. For this very reason, because the silk
-must be torn and spoilt by the emerging insect, silk-breeders kill the
-pupating insect before it begins to make its way out.
-
-But there are species whose cocoons are provided from the very start
-with an outlet, for the caterpillar spins the silk round itself in
-such a way that a round opening is left. But this opening would be not
-only a convenient door for the butterfly to emerge by, but an equally
-convenient entrance for all its enemies. It is, therefore, closed up.
-In the case of the 'emperor moth' (_Saturnia carpini_) this is effected
-by means of a circle of stiff bristles of silk on the inside (Fig. 33),
-the points of which bend outwards like those of a weir-basket (_r_);
-from the inside the emerging moth can easily push aside the bristles,
-while the threatening enemy from without is scared off by the stiff
-points of the bristles.
-
-[Illustration: FIG. 33. Cocoon of the Emperor Moth (_Saturnia
-carpini_), after Rösel. _A_, enclosed pupa. _B_, emerging moth. _r_,
-hedge of bristles. _fl_, wings.]
-
-Such a cocoon is comparable to a work of art in which every part
-harmonizes with the rest, and all together are adapted as well as
-possible to their purpose. And yet it is all accomplished without the
-caterpillar having the remotest conception of what it is aiming at when
-it winds the endless silken thread about itself in the artistic and
-precisely prescribed coils. Nor has it any time for trying experiments
-or for learning; it must make all the complex bendings and turnings
-of the head which spins the thread, and of the anterior part of the
-body which guides the thread, quite exactly and correctly the first
-time if a good cocoon is to be produced. Here every possibility of
-interpreting this instinct as 'an inherited habit' is excluded, for
-each caterpillar becomes a pupa only once; and it is just as impossible
-to suppose that it can be directed by intelligence, since it can
-neither know that it is about to become a pupa, nor that, in the pupa
-stage, it will be in danger from enemies which will attempt to force
-their way into the cocoon, nor that the hedge of bristles will protect
-it from such enemies. Our only clue to an interpretation is in the slow
-process by which minute useful variations in the primitive instinct
-of spinning are accumulated through selection; and it is wonderful to
-see how exactly these spinning powers are adapted to the particular
-life-conditions of individual species.
-
-Thus there are several of the Saturnides whose enormous caterpillars
-live on large-leaved trees, and these make use of the large leaves to
-form a shelter for the pupa stage, spinning them together so that the
-cocoon is for the most part surrounded by leaf. But as the leaf might
-easily fall off with the weight of the pupa, they make the leaf-stalk
-fast to the twig from which it grows by binding the two firmly together
-with a broad, strong, closely-apposed silken band. Seitz relates of
-the largest of all these spinners, the Chinese _Attacus atlas_, that
-this silk sheath 'is continued to the nearest strong branch, so that
-it is impossible with the hand to detach the leaves that conceal an
-Atlas-pupa from the tree.' To be sure, this pupa weighs about eleven
-grammes!
-
-Since instincts vary, as well as the visible parts of an animal, a
-fulcrum is afforded by means of which selection can bring about all
-these very special adaptations to given conditions, since it always
-preserves for breeding the best suited variations of an already
-existing instinct. Any other interpretation is once more excluded.
-
-The same may be said of insects and their egg-laying. This, too,
-is in many cases only performed once in a lifetime, and the insect
-dies before it has seen the fruit of its labour. Yet egg-laying is
-performed in the most effective manner, and with the most perfect
-security of result. It seems as if the insect knew, so to speak,
-exactly where, in what numbers, and how it should lay its eggs. Many
-Mayflies (Ephemeridæ) let their eggs fall all at once into the water
-in which the larvæ live; many Lepidoptera, such as _Macroglossa
-stellatarum_, lay their eggs singly, and on definite plants--the
-humming-bird hawk-moth, just referred to, on _Galium mollugo_; others,
-like _Melitæa cinxia_, lay their eggs in heaps on the leaves of the
-way-bread (_Plantago media_), or, like _Aglia tau_, on the bark of a
-large beech-tree. Nothing in these different modes of egg-laying is
-due to chance or caprice; all is determined and regulated by instinct,
-and all, as far as we can see, is as well adapted to its purpose as
-possible. When, for instance, _Macroglossa stellatarum_ lays her eggs
-singly, or in twos or threes, on the green leaves of the food-plant, it
-thereby obviates the danger of scarcity of food for the comparatively
-large caterpillars, since not many of them could subsist together on
-a single plant of Galium, while _Aglia tau_ can place several hundred
-eggs on the same beech-tree trunk without having to fear that its
-caterpillars will not find abundant nourishment. The precision with
-which the egg-laying instinct works is even greater in other species
-in which there are more special requirements, e.g. when the eggs have
-to be laid on the under side of the leaves, as in _Vanessa prorsa_, or
-where they have to be cemented together in a little pillar, so that
-they bear a deceptive resemblance to the green flower-buds of the
-food-plant (the stinging-nettle).
-
-It is certainly astonishing how exactly the stimulus in these cases is
-specialized to the liberation of the instinct. In general the smell
-of the food-plant of the caterpillar is enough for most butterflies,
-and this attracts the female ready to deposit its eggs, but complete
-liberation of the instinct is only effected by the visual impression
-of the under side of the leaf. We cannot but be astonished that there
-is room for such finely graded nerve-mechanisms in the little brain
-of a butterfly, and yet it would be easy enough to adduce still more
-complex instincts connected with oviposition in insects. The large
-water-beetle, _Hydrophilus piceus_, lays its eggs on a floating raft
-made by itself; the gall-wasps must first pierce with their ovipositor
-into a particular part of a particular plant to be able to lay the
-eggs in the proper place, and this in no haphazard way but with great
-carefulness and in a perfectly definite manner. But there is no
-necessity to refer here to many or to the most complicated cases of
-egg-laying; I only wish to show that, even in the simple cases, such
-as that of the butterflies just referred to, there is a precisely
-regulated combination of actions which is executed mechanically, and
-which cannot be interpreted as inherited habit, because it never was a
-habit in any individual of any generation.
-
-It is thus placed beyond the possibility of doubt that very many
-instincts, at least, must depend on selection, and it would be useless
-to go further in this direction by extending our survey to other
-groups of instincts. I shall, however, return later on to the study of
-instincts, and, after we have become acquainted with the main features
-of the laws of inheritance, it will then be seen that, even among
-higher animals, instincts can never be interpreted in terms of the
-Lamarckian principle.
-
-
-
-
-LECTURE IX
-
-ORGANIC PARTNERSHIPS OR SYMBIOSIS
-
- Hermit-crabs and sea-anemones--Hermit-crabs and hydroid polyps--Fishes
- and sea-anemones--Green fresh-water polyps--Green Amœba--Sea-anemones
- and yellow Algæ--Cecropia trees and ants--Lichens--Root fungi--Origin
- of Symbiosis--Nostoc and Azolla apparently contradict the origin
- through natural selection.
-
-
-WE have already seen, by means of many examples, to what a great degree
-animals and plants are able to adapt themselves to new conditions
-of life; how animals imitate their surroundings in colour and form,
-how instincts have varied in all directions, how plants have made
-use of the chance of frequent contact with little animals to obtain
-nourishment from them, and have developed contrivances adapted for
-bringing as many of these as possible into their power and causing them
-to yield them the largest possible amount of food. A great many of
-these could only be interpreted in terms of natural selection, and in
-others it seemed at least very probable that selection was one of the
-factors in bringing them about.
-
-Particularly clear proof of the reality of natural selection is
-afforded by those cases where one form of life associates itself with a
-very different one so intimately that they are dependent on one another
-and cannot live without one another--at least in extreme cases--and
-that new organs, and, indeed, new dual organisms, are sometimes
-produced by this interdependence of life. This phenomenon--so-called
-'Symbiosis'--was discovered by two sharp-sighted botanists, Anton de
-Bary and Schwendener. But Symbiosis occurs not only between plants;
-it occurs also between plants and animals and between two species
-of animal, and we understand by it a life of partnership depending
-on mutual benefits, so that each of the two species affords some
-advantage to the other, and makes existence easier for it. In this
-respect Symbiosis differs from Parasitism, in which one species is
-simply preyed upon by another without receiving any benefit from it in
-return, and also from the more innocent Commensalism of Van Beneden,
-the table-companionship in which one species depends for its existence
-on the richly-spread table of another. Symbiosis is particularly
-interesting, because, in addition to extreme cases with marked
-adaptations, many occur which are of great simplicity, and which seem
-to have brought about almost no change in the two associated species.
-
-We shall take our first examples from the Animal Kingdom.
-
-The partnership between certain sea-anemones (Actiniæ) and hermit-crabs
-(Paguridæ) had been noticed long before any particular attention
-was devoted to it. Many species of hermit-crab frequently carry a
-large sea-anemone about with them on the mollusc shell which they
-use as a protecting-house; indeed, two or three of these beautiful
-many-tentacled polyps are often attached to them, and this is not at
-all a matter of chance, but depends upon instinct on the part of both
-animals; they have the feeling of belonging to each other. If the
-sea-anemone be taken away from the hermit-crab and put in a distant
-part of the aquarium, the crab seeks about till it finds it, then
-seizes it with its claws and sets it on its house again. The instinct
-to cover itself with Actiniæ is so strong within it that it loads
-itself with as many of these friends as it can procure, sometimes with
-more than there is room for on the shell. The sea-anemone on its part
-calmly submits to the crab's manipulations--a fact very surprising to
-any one who is aware of the anemone's ordinarily extreme sensitiveness
-to contact, and knows how it immediately draws itself together on any
-attempt to detach it from the ground, and will often let itself be
-torn in pieces rather than give way. The mutual instincts of the two
-creatures are thus adapted to each other; but it does not at first
-sight seem as if any structural changes had taken place in favour of
-the partnership. This is true, indeed, as regards the hermit-crab, but
-not as regards the sea-anemone, although the nature of the adaptation
-on the sea-anemone's part only becomes apparent when the two animals
-are closely observed in their life together.
-
-We owe our understanding of this adaptive change in the sea-anemone,
-and, indeed, our knowledge of this whole case of Symbiosis, to the
-beautiful observations of Eisig. Starting from the hypothesis that the
-mutual relations could only be the outcome of natural selection, Eisig
-pointed out that this partnership must offer some advantage not to one
-partner only, but to both; otherwise it could not have arisen through
-selection. The advantage to the sea-anemone is obvious enough; since of
-itself it can only move very slowly, and is usually firmly fixed in one
-place, it is easy to see that it would be useful to it to be carried
-about on the floor of the sea by the hermit-crab, and to get its share
-of the hermit-crab's food. But the service yielded to the hermit-crab
-by the sea-anemone in return is not nearly so apparent. Eisig made
-an observation in the Zoological Station at Naples which solved this
-riddle. He saw an octopus attack the hermit-crab and attempt to draw
-it out of its shell with the point of one of its eight arms. But before
-this had succeeded there sprang from the body of the sea-anemone a
-large number of thin worm-like threads which spread over the arm of
-the robber, who immediately let go his hold of the crustacean and
-troubled himself no further with it. These threads, called acontia,
-are thickly beset with stinging-cells, which must at least cause a
-violent smarting on the soft skin of the octopus. Thus we see that the
-Actinia instinctively defends its partner from attacks, and does it so
-effectively that we need not wonder how the instinct to provide itself
-with Actiniæ could have arisen in the hermit-crab. But the acontia seem
-to have been greatly strengthened in the course of the sea-anemone's
-association with hermit-crabs, for they do not occur in all forms, and
-they are most highly developed in those which live in Symbiosis with
-crustaceans.
-
-[Illustration: FIG. 34. Hermit-crab (_E_), within a Gasteropod shell,
-on which a colony of _Podocoryne carnea_ has established itself.
-From the common root-work (which is not clearly shown) there arise
-numerous nutritive polyps with tentacles (_np_), among which are
-smaller 'blastostyle' polyps with a circle of medusoid buds (_mk_),
-spine-like personæ (_stp_), and on the margin of the mollusc shell a
-row of defensive individuals (_wp_). _F_, antennæ. _Au_, eyes of the
-hermit-crab; slightly enlarged.]
-
-In this case the structural change, the transformation of the
-mesenteric filaments that occur in all Actiniæ into projectile
-acontia, is comparatively slight, but in another partnership between
-hermit-crabs and polyps the latter have undergone a much more marked
-adaptation. At Naples _Eupagurus prideauxii_ is one of the commonest
-hermit-crabs. It lives at a depth of about a hundred feet, and is
-often brought to the Zoological Station by the fishermen in large
-quantities. Its borrowed mollusc shell often bears a little polyp,
-_Podocoryne carnea_ (Fig. 34), which forms colonies of often several
-hundred individuals, arising from a common root-work of stolons which
-covers the shell. The polyp colony is composed of different kinds
-of individuals or personæ, illustrating the principle of division
-of labour: it includes (1) nutritive persons (_np_) which possess a
-proboscis, mouth, and tentacles on their club-shaped bodies; (2) much
-smaller blastostyles (_bl_), that is to say, polyps with degenerate
-mouth and tentacles, which are wholly given over to the production
-of buds (_mk_), which then develop into sexual animals, little
-free-swimming medusoids; and (3) protective personæ in the form of hard
-spines (_stp_), beneath the shelter of which the soft polyps withdraw
-when the mollusc shell is rocked about on the sea-floor by the rolling
-of the waves. In addition to these three different kinds of individuals
-or personæ there are also (4) defensive polyps (_wp_) of long,
-thread-like shape, thickly set with stinging-cells, but possessing
-neither mouth nor tentacles. It might at first be thought that these
-are for the defence of the colony, but this is not so; the fact is
-that they rather serve for the direct defence of the hermit-crab.
-This is indicated by the position they occupy in the colony; they are
-not regularly distributed over the surface, but are ranged round the
-edge, and, indeed, only on the edge which surrounds the opening of
-the mollusc shell. Here these defensive polyps stand in close array,
-sometimes spirally contracted, sometimes hanging loosely down over
-the hermit-crab like a fringe. Their function, like the acontia of
-Actiniæ, is to defend the crab when an enemy tries to follow it within
-the shelter of its domicile. This can easily be demonstrated by drawing
-out the hermit-crab from the Gasteropod shell, and, when the colony has
-settled down again, seizing the shell with the forceps and drawing it
-slowly through the water. The water-stream which then flows upon the
-shell mimics the attack of an enemy, and immediately all the defensive
-polyps, as at a given signal, strike from above downwards, and repeat
-this three or four times; they are scaring off the supposed enemy.
-
-In this species of polyp a special form of individual has developed
-with a quite definite position in the colony, and furnished with a
-special instinct or reflex mechanism which is directly useful only to
-the crab, and has therefore, in a sense, arisen for its advantage. This
-can quite well be explained through natural selection, for indirectly
-these polyps are also of use to the colony, inasmuch as they protect
-their valuable partner, and thus render it possible for the hydroid
-colony to make the partnership of use to the hermit-crab as well as to
-itself.
-
-This mutual arrangement thus satisfies the requirement which, from
-the selectionist point of view, must be made in regard to all that is
-new--that it must be useful to its possessor.
-
-If it be asked what service the hermit-crab renders to the polyp colony
-in return, the answer is that, as in the symbiosis with sea-anemones,
-the hermit-crab carries the polyps to their food, which is also its
-own. Hermit-crabs eat all sorts of animal food, living or dead, which
-they find on the sea-floor, and the remains of their meal fall to
-the share of the polyps. Once, without special intention, I laid a
-hermit-crab with its polyp colony in a flat vessel of sea-water beside
-a bright green living sponge. After some time the majority of the
-polyps had become bright green; they had filled themselves with the
-green cells of the sponge.
-
-I do not know how else we should picture to ourselves the origin
-of symbiotic instincts in such lowly animals except through the
-transmission and augmentation of variations in the instincts of the
-two partners--variations which made their possessors more capable of
-survival. Mollusc shells, ever since there were any, must have served
-as a foundation and point of attachment for polyp colonies; as a
-matter of fact, we find to-day on mollusc shells many kinds of polyp
-colonies which show no special adaptation to a life of partnership
-with hermit-crabs. From such indifferent associations a symbiotic
-one must gradually have been evolved in some instances, through the
-preservation and augmentation of every useful variation, both of
-instincts and reflex actions, as well as of form and structure. I
-shall not attempt to trace the course of this evolution in detail, but
-it is obvious that the development of defensive polyps, and of their
-instinct to defend the crustacean, can be interpreted neither as due
-to any direct influence nor as due to the effect of use, but only to
-the utility of this arrangement, the beginnings of which--polyps with
-stinging-cells--were already present. Their augmentation and perfecting
-must be referred entirely to natural selection. It is the same with
-adaptations which do not refer directly to the crustacean partner,
-but rather to the disposition of the polyps on the shell. The spinous
-personæ which protect the softer polyps from being crushed by being
-rolled about on the pebbles by the waves cannot possibly be regarded
-as the direct result of this crushing. But it is obvious that some
-such colonies must have had among their members some with a stronger
-external skeleton, and therefore less easily crushed than the rest, and
-this would lead to their more frequent survival.
-
-No adaptation seems to have taken place in the hermit-crab in this
-case, but that is probably only apparently the case; the probability
-is that it would not tolerate the presence of the polyp-colony on the
-shell unless its instinct compelled it thereto, just as its instinct
-impels it to cover itself with sea-anemones, and fearlessly to grasp
-the dangerous animal, which, however, only shows its partner its softer
-side. Truly, such transformations of instinct are wonderful enough, but
-that they should have come about through intelligence is here quite
-inconceivable; there remains nothing but natural selection.
-
-A case in which no apparent corporeal adaptations have occurred, but
-which depends altogether on slight modifications of the instincts, is
-afforded by the well-known relations between ants and aphides. These
-two groups of insects live in a kind of symbiosis, although they are
-by no means inseparably connected with each other. Wherever strong
-colonies of aphides cover the young shoots of a plant, such as a
-stinging-nettle, a rose, or an elder, we almost always find ants which
-walk cautiously about among the plant-lice, often in great numbers,
-stopping now and again to stroke them with their antennæ, and then
-licking up the sweet juice from the intestine which they now give
-forth. Darwin showed by experiment that the aphides retain this juice
-if no ants are on the spot, and only give it off when ants are put
-beside them. Herein lies the proof that we have again to do with a
-case of modification of instincts. This juice is, of course, not the
-secretion of special glands, as it was still believed to be in Darwin's
-time, and it does not come from the so-called 'honey-tubes' situated on
-the back of the abdomen of the aphides; it is simply their excrement,
-which is liquid like their food, and the voiding of it has become
-instinctively connected with the presence of the friendly ants.
-
-That the aphides are not in any way afraid of the ants implies, in
-itself, a modification of their instinct, for these poisonous insects,
-prone to biting, are otherwise much dreaded in the insect world.
-Moreover, the aphides, harmless as they seem, are not quite without
-means of defence, although these are never used against the ants.
-Other animals which approach them they bespatter with the sticky,
-oily secretion prepared in the so-called 'honey-tubes' already noted,
-squirting it especially into the eyes of an assailant, so that the
-attack is abandoned.
-
-Of course the aphides have no idea wherein the utility of their
-friendship for the ants consists, but it is not difficult for us to
-discover it, since the ants, by their mere presence in the aphid
-colony, frighten and keep off their enemies. We see, then, that the
-conditions for a process of natural selection are here afforded:
-the instinct to be friendly to the ants is thoroughly useful, and
-the instinct of the ants to seek out the aphides, and, instead of
-devouring them, to 'milk' them, is also advantageous; it must be an
-old acquisition, an instinct early developed, for in several species it
-has gone so far that the aphides are carried into the ants' nest, and
-are there (as one might say) kept and tended as domesticated animals.
-
-A pretty case of symbiosis between two animals is reported by
-Sluiter, and I mention it because it concerns a vertebrate animal,
-and intelligence has something to do with it. In the neighbourhood
-of Batavia there are frequently to be found on the coral reefs
-large yellow sea-anemones, with very numerous and comparatively
-long tentacles, and a little brightly-coloured fish, of the genus
-_Trachichthys_, makes use of these forests, beset with stinging-cells,
-to find security from its enemies. These appear to be numerous, for
-in an aquarium, at any rate, the little fish very soon falls a victim
-to one or other of them, unless he is supplied with the protective
-sea-anemones. When this is the case it swims blithely about among the
-tentacles, and the sea-anemone does not sting it; for there has been a
-modification of instinct on its part as well as on that of the fish.
-The advantage it gains from the fish is, that the latter brings large
-morsels of food--in the aquarium, pieces of meat--into the anemone's
-mouth. In doing so it tears away fibres for itself, and even if the
-Actinia has swallowed pieces too quickly, the fish pulls them half out
-of the gullet again, and only relinquishes them to be consumed by its
-partner when it has satisfied its own appetite. In this case, again,
-the modification of the instinct is the only adaptation which has been
-brought about by the symbiosis, and its origin seems difficult to
-understand. How can the fish have first formed the habit of putting
-its prey into the mouth of the anemone instead of eating it directly?
-Although in many cases it is difficult to guess at the beginnings of
-a process of selection, because they are scarcely discoverable in
-the subsequently accumulated variations, yet in this instance we may
-perhaps picture them to ourselves in this way: The fish was in the
-habit of letting fall pieces of food which could not be swallowed
-whole, and of diving down upon them repeatedly, to tear off a fragment
-each time. As the sea-floor in flat places is often covered with
-sea-anemones, these pieces would often sink down upon one, which would
-welcome it as a dainty, and set about swallowing it, slowly in its own
-fashion. The fish must then have found by experience that it could tear
-off little bits much more easily from a piece that was held firmly by
-the anemone than from one that was lying loose upon the ground, and
-this may have caused it to do intentionally what was at first done by
-chance. But the sea-anemone, suffering no harm from the fish--indeed,
-its association of ideas, if I may use the expression, must rather
-have been little fishes and unexpected food--had no cause to shoot
-its microscopic arrows at it, and did not do so even when the fish
-concealed itself among the tentacles. This latter habit on the part of
-the fish would be developed into an instinct through natural selection,
-since the individuals that most frequently exhibited it would be the
-best protected, and therefore, on an average, the most likely to
-survive. Whether the benevolent attitude of the anemone towards the
-fish is to be regarded as the expression of an instinct is open to
-dispute, for it is quite conceivable that each individual sea-anemone
-is disposed to gentleness by the behaviour of the fish, and so the
-development of a special hereditary instinct was unnecessary, because
-without it each anemone reacted in the manner most likely to secure its
-own advantage[7].
-
-[7] Since the above was written Plate has observed several similar
-cases in the Red Sea. A little fish lives along with the anemone,
-_Crambactis aurantiaca_, a foot in size, and not only conceals itself
-among its tentacles, but remains among them when the anemone draws them
-in. These fishes, therefore, must be immune against the stinging-cells
-of the sea-anemone; and in the same way another species of fish
-appears to be immune from the strong poison secreted by sea-urchins
-of the genus _Diadema_ from the points of their spines, among which
-the fishes live. This relation certainly seems more like a one-sided
-adaptation on the part of the fishes than a true symbiosis, but in the
-cases observed by Sluiter the return service of the fishes seems to
-be regularly rendered. Here, as everywhere else in nature, there are
-transition stages, and a one-sided protective relation may gradually,
-under favourable circumstances, be transformed into a symbiosis.
-
-The same may be true of the fish as far as laying its booty in the
-mouth of the anemone is concerned; there may be no inherited instinct
-in this; it may be an intelligent action, which is learnt anew in the
-lifetime of each individual.
-
-It might of course be objected to this interpretation that the
-beginning of the process, namely, the assumption that chance fragments
-from the food of the fish falling just on the anemone is very
-improbable; but I once observed that flat rocks washed over by the
-sea on the Mediterranean coast (not far from Ajaccio) were so thickly
-covered with green anemones that at first I took the green growth for
-some strange sea-grass new to me until I had pulled up a little tuft
-of the supposed plants and identified them as the soft tentacles of
-_Anthea cereus_. Anemones must be equally abundant in the tropical seas
-of Java, and a sinking fragment must often alight on the mouth of one
-of them.
-
-Much attention and keen discussion have in the last few decades been
-focussed on cases of symbiosis between unicellular Algæ and simple
-animals. A good example is our green fresh-water polyp, _Hydra viridis_
-(Fig. 35, _A_). Its beautiful colour is due to chlorophyll, and it was
-long a matter of surprise that animals should produce chlorophyll,
-which is a characteristic and fundamental important substance of
-assimilating plants, until Geza Entz and M. Braun demonstrated that the
-green did not belong to the animal at all, but to unicellular green
-Algæ, so-called Zoochlorellæ, which are embedded in the endoderm cells
-of the polyps in great numbers (Fig. 35, _zchl_). As these algoid
-cells assimilate, and thus liberate oxygen, their presence is of
-advantage to the polyp. That--as was at first believed--they also yield
-nourishment to the polyp I consider very probable, notwithstanding
-the apparently opposed results of the experiments of so acute an
-observer as von Graff, for I have seen a large number of these animals
-thrive for months, and multiply rapidly by budding in pure water which
-contained no food of any kind. In favour of this view, too, are some
-observations, to be cited presently, on unicellular animals, in regard
-to whose nourishment by the zoochlorellæ living within them there can
-be no doubt at all.
-
-[Illustration: FIG. 35. _Hydra viridis_, the Green Fresh-water Polyp.
-_A_, the entire animal, greatly enlarged. _M_, the mouth. _t_,
-tentacles. _sp_, testis. _ov_, ovary, both in the ectoderm. _Ei_, a
-ripe ovum, already green, in process of being extruded. After Leuckart
-and Nitsche.
-
-_B_, section of the body-wall, about the position of the ovary in _A_.
-_Eiz_, the ovum lying in the ectoderm (_ect_), in which zoochlorellæ
-(_zchl_), belonging to the endoderm (_ent_), have already migrated
-through the supporting middle lamella (_st_). _eik_, nucleus of ovum.
-After Hamann.]
-
-The little algæ on their part find a peaceful and relatively secure
-abode within the polyp, and they apparently do not occur outside of
-it, at least they do not now migrate from outside into the animal,
-but are carried over as a heritable possession of the polyps from
-one generation to another, and in a very interesting manner, namely,
-by means of the eggs, and by these alone. As Hamann has shown, the
-zoochlorellæ migrate at the time when an egg is formed in the outer
-layer of the body of the polyp (Fig. 35) from the inner layer outwards,
-piercing through the supporting layer between them (st) and penetrating
-into the egg (_B_, _Eiz_). They make their way only into the egg, not
-into the sperm-cells, which in any case are too small to include them.
-Thus they are absent from no young polyp of this species, and it is
-easy to understand why earlier experimental attempts to rear colourless
-polyps from eggs could never succeed even in the purest water.
-
-[Illustration: FIG. 36. _A_, _Amœba viridis_. _k_, the nucleus.
-_cv_, contractile vacuole. _zchl_, the zoochlorellæ. _B_, a single
-zoochlorella under high power. After A. Gruber.]
-
-Quite similar green algæ live in symbiosis with unicellular animals,
-as, for instance, with an amœba (Fig. 36) and with an Infusorian of the
-genus _Bursaria_. In the Zoological Institute in Freiburg there is a
-living colony of a green amœba and a green _Bursaria_, both of which
-came from America, sent to us some years ago by Professor Wilder, of
-Chicago, inside a letter with dried _Sphagnum_, or bog-moss. The plants
-came from stagnant water in the Connecticut valley in Massachusetts.
-That in this case the zoochlorellæ are of use to the animals within
-which they live, not only by giving off oxygen, but also by yielding
-food-stuff, has been proved by A. Gruber, who bred the two green
-species for seven years in pure water which contained no trace of any
-kind of organic food for them. Nevertheless, they multiplied rapidly,
-and still form a green scum on the walls of the glass in which they are
-kept. They only die away when they are kept in the dark, where the algæ
-are unable to assimilate; then one green cell after another wanes and
-disappears, and, in consequence, their hosts also die from the double
-cause of lack of oxygen and lack of food.
-
-Even in this case the symbiotically united organisms have not remained
-unaltered. The algæ at least differ from others of their kind in their
-power of resistance to living animal protoplasm. They are not digested
-by it, and we may infer from this that they possess some sort of
-protective adaptation against the dissolving power of animal digestive
-juices; they must, therefore, have undergone some variation, and
-adapted themselves to the new situation. Probably their cell-membrane
-has become impenetrable to the stuffs which would naturally digest
-them, an adaptation which could not be referred to direct effect or to
-use, but only to the accumulation of useful variations which cropped
-up--in other words, to natural selection. That any adaptive variation
-has taken place on the part of the host, whether polyp, amœba, or
-Infusorian, cannot be made out. None of these have altered their
-original mode of life; they do not depend on the nourishment afforded
-by the algæ, but feed on other animals, if these come in their way,
-and they live in water rich in oxygen like other species allied to
-them, and therefore are not altogether dependent on the algæ in this
-connexion; but they can no more help having their partners than the pig
-can help having Trichinæ in its muscles.
-
-Similar plant-cells, not green however, but yellow, called
-zooxanthellæ, live in great numbers in the endoderm of various
-sea-anemones and in the soft plasmic substance of many Radiolarians.
-In both these cases we must look for the benefit they confer on their
-host in the oxygen they give off, for, like the green zoochlorellæ,
-they break up carbonic acid gas in the light, and give off oxygen;
-they no longer occur, as far as is known, in a free state, but are
-always associated with the host, and they must therefore have altered
-in constitution, and have adapted themselves to the conditions of the
-symbiosis.
-
-Higher plants, too, sometimes have symbiotic relations with animals;
-the most remarkable and best-known example is the relation between ants
-and certain trees, in which the ants protect trees which afford them in
-return both a dwelling-place and food. We owe our knowledge of these
-cases to Thomas Belt and Fritz Müller, and more recently it has been
-materially increased by Schimper's researches.
-
-In the forests of South America there grow 'Imbauba,' or
-candelabra-trees, species of the genus _Cecropia_, which well deserve
-their name, for their bare branches stretch out like candelabra,
-and bear little bunches of leaves only at their tips. These leaves
-are menaced by the leaf-cutting ants of the genus _Œcodoma_, which
-attack numerous species of plants in these regions, often in tens of
-thousands, biting off the leaves, cutting them in pieces on the ground,
-and carrying them on their backs piece by piece to their nests. There
-they use them to make a kind of compost heap, on which fungi, to which
-the ants are very partial, readily grow. The candelabra-tree protects
-itself from these dangerous robbers, inasmuch as it has established an
-association with another ant (_Azteca instabilis_), which finds a safe
-dwelling-place in its hollow, chambered stem (Fig. 37, _A_), and feeds
-on a brown sap which oozes from the inside. On the stem there are even
-little pits regularly arranged in definite places (_E_), through which
-the female of _Azteca_ can easily bore her way into the interior. There
-she lays her eggs, and soon the whole interior of the trunk teems with
-ants, which come trooping out whenever the tree is shaken.
-
-[Illustration: FIG. 37. _A_, a piece of a twig of an Imbauba-tree
-(_Cecropia adenopus_), with the leaves cut off. At the leaf-bases
-are the hair-cushions (_P_). _E_, the opening for the associated ant
-(_Azteca instabilis_). _B_, a piece of the hair-cushion with the
-egg-shaped nutritive corpuscles (_nk_). After Schimper.]
-
-This alone would not suffice to protect the tree against the
-leaf-cutting ants, for how should the Aztec ants living inside notice
-the presence of the lightly climbing leaf-cutters? But that is
-provided for, for the Aztecs also frequent the outside of the trunk,
-and just where attack would be most disastrous, namely, at the stalks
-of the young leaves. At these places there is a peculiar velvet-like
-cushion of hair (_P_), from which grow little stalked white papillæ
-(Fig. 37, _B_), which are rich in nourishment, and are not only eaten
-by the ants, but are harvested by them, being carried off into the
-ants' dwellings, presumably to feed their larvæ. In this case, then,
-a particular organ, offering special attraction to ants, has been
-developed by the plant at the places more especially threatened; while,
-as regards the ants, it is probable that only the instincts of feeding
-and habitat require to be modified, since courage and thirst for battle
-are present in all ants, almost any species being ready at any time to
-throw itself on any other which intrudes into its domain.
-
-It should be noted that not all the candelabra-trees live in symbiosis
-with ants, and so secure a means of defence against the leaf-cutters.
-Schimper found in the primitive forests of South America several
-species of _Cecropia_ which never had ants in the chambers of their
-hollow stem. But these species did not exhibit the nutritive cushions
-at the base of the leaf-stalk; these contrivances for attracting and
-retaining the presence of partner ants were altogether absent. Indeed,
-only one species, _Cecropia peltata_, has produced these peculiar
-structures, and, as they are of no _direct_ use to the tree, we must
-say that it has produced them only for the ants. Here, again, natural
-selection must have gradually brought about the development of these
-nutritive cushions, though as yet we do not know what the beginnings of
-the process may have been. In no case can the origin of these cushions
-be referred to any direct influence of the environmental conditions.
-
-We may now pass to the association of two species of plants, of
-which the lichens furnish the best-known and probably most complete
-illustration. Till about twenty years ago the lichens, which in so many
-diverse forms clothe the bark of trees, the stones, and the rocks,
-were regarded as simple plants like the flowering plants, the ferns,
-or the mosses; and many lichenologists occupied themselves with the
-exact systematic distinction of about a thousand species, each of which
-could be as well and exactly classified, according to form, colour,
-habitat, and minute structure, as any other kind of plant. Then De Bary
-and Schwendener discovered that the lichens were made up of two kinds
-of plants, fungi and algæ, so intimately associated with and adapted
-to one another, that on coming together they always assume the same
-specific form.
-
-[Illustration: FIG. 38. A fragment of a Lichen (_Ephebe kerneri_),
-magnified 450 times. _a_, the green alga-cells. _P_, the fungoid
-filaments. After Kerner.]
-
-The framework, and therefore the largest part, and the one which
-determines the form of a lichen, is due to the fungus (Fig. 38).
-Colourless threads of fungus ramify in a definite manner according
-to the species of fungus, and in the network of spaces left by this
-ramification green alga-cells (_a_) lie singly, or in rows, or groups.
-The fungus is propagated by multitudes of minute spores, which it
-produces periodically, and these are disseminated in the air by the
-bursting of the sporangia and are carried away by the wind in the form
-of fine dust; the alga multiplies simply by continual division into
-two, but it also, like the whole lichen, can survive desiccation,
-and, after falling to pieces, is likewise carried through the air as
-microscopic dust.
-
-The partnership of the two plants rests on a basis of mutual benefit;
-the fungus, like all fungi, is without chlorophyll, and cannot
-therefore decompose carbonic acid gas or elaborate its own organic
-food-stuffs; it receives these from the alga. The alga has in the
-network of the fungus a safe shelter and basis of attachment, for the
-fungus is able to bore into the bark of trees and even into stones;
-besides which it absorbs water and salts, and supplies these to the
-partner alga. We here see the mutual advantage derived from the
-partnership, which is really an extremely intimate one. Fungus spores,
-sown by themselves, spring up and develop some branchings of fungoid
-hyphæ, a so-called mycelium, but without the requisite partner alga
-these remain weak and soon die away. The alga, on the other hand,
-can, in some cases, though not in all, survive without the fungus if
-the necessary conditions of its life be supplied to it, but it grows
-differently and more luxuriantly in association with the fungus.
-
-The same species of alga may be found associated with different species
-of fungi, and then each partnership forms a distinct species of lichen
-of definite and characteristic appearance; Stahl even succeeded in
-making new species of lichen artificially by bringing the spores of a
-lichen-forming fungus into contact with alga-cells, with which they had
-never been associated in free nature.
-
-The most remarkable feature of this remarkable association seems to me
-to be the formation of common reproductive bodies--an adaptation in
-face of which all doubt as to the theory of selection must disappear.
-Periodically there are developed in the substance of the lichen small
-corpuscles, the so-called soredia, each of which consists of one or
-more alga-cells surrounded and kept together by threads of the fungus.
-When they are developed in large numbers they form a floury dust over
-the maternal lichen, which 'breaks up' and leaves them, like the spores
-of the fungus, to be carried away by the wind. If these alight on
-favourable soil nothing more is needed than the external conditions
-of development, light, warmth, and water, to enable the lichen to
-spring up anew. The great advantage to the preservation of 'species' is
-obvious, for, when multiplication by the ordinary method occurs among
-lichens, the spores of the fungus, even if they have fallen on good
-ground, can only develop into a new lichen if chance bring to them the
-proper partner alga.
-
-Obviously there must be, in the formation of the soredia, great
-advantage for the species, or rather 'for the two species,' for the
-fungi as well as the algæ benefit by the arrangement, which ensures the
-continuance of the partnership. It was not without reason, however,
-that the dual organism was so long regarded as a simple species in the
-natural history sense, _for that is what it really is_, although it has
-arisen in a manner quite different from the usual origin of species. As
-we know species which consist only of single cells, and others which
-consist of many cells, differentiated in different ways, and forming
-a cell-community or 'person,' and, finally, others which consist of a
-community of diversely differentiated personæ, making up a 'stock'; so
-in the lichens we see that even different species may combine to form
-a new physiological whole, a vital unit, an individual of the highest
-order. When, at the outset of these lectures, I said that the theory
-of evolution was now no longer a mere hypothesis, and that its general
-truth could no longer be doubted by any one acquainted with the facts
-available, I had in my mind, among other facts, especially that of
-symbiosis, and above all the case of the lichens.
-
-There are many other interesting cases of symbiosis between two
-different kinds of plants, and one side of the partnership is
-represented by fungi in a relatively large number of instances. The
-reason is not far to seek: fungi must always be dependent on other
-plants for their food; they must be parasitic, because they cannot
-themselves produce the organic substances they require. They must
-therefore associate themselves in some way with other organisms, living
-or dead, and as a general rule they simply prey upon their associate,
-sucking up its juices and killing it. But in not a few cases they can
-render services in return, and, as we have seen in the case of the
-lichens, symbiosis may then occur. Fungi in general have the power of
-discovering and absorbing the least trace of water in the soil, and
-with it they absorb the salts necessary to the plant, and in this,
-apparently, consists the service which they are able to render even to
-large plants fixed deep in the earth, such as shrubs and trees. The
-roots of many of our forest trees, e.g. beech, oak, fir, silver poplar,
-and bushes like broom, heaths, and rhododendrons, are thickly wrapped
-round with a network of fungoid threads, and the mutual relations just
-indicated exist between these and the plants in question (Fig. 39,
-_A_ and _B_). The plants give to the fungi some contribution from the
-superfluity of their food-stuffs, and receive in return water and
-salts, which are of value especially in times of drought. Perhaps there
-is some connexion between this and the fact that limes wither and lose
-their leaves so quickly during great summer-heat; these and many other
-of our trees possess no root-fungi or mycorhizæ.
-
-It is easy to understand, therefore, that genuine 'symbiosis' may have
-arisen from parasitism. But that this is not the only path that leads
-to symbiosis is shown by the cases of animal symbiosis we have already
-discussed.
-
-[Illustration: FIG. 39. _A_, fragment of a Silver Poplar root, with an
-envelope of symbiotic fungoid filaments (mycelium); after Kerner. _B_,
-apex of a Beech root, with the closely enveloping mantle of mycelium;
-enlarged 480 times.]
-
-The partnership between polyps and hermit-crabs may have arisen from a
-one-sided commensalism, since polyps establishing themselves on mollusc
-shells which were often made use of by hermit-crabs would be better fed
-than those which settled down on stones. There are still species which
-make use of both modes of settlement. Then followed the adaptation
-of the crustacean to the polyp, for, first, those hermit-crabs would
-thrive best which tolerated the presence of the polyp; then those which
-sought its presence, that is to say, which gave a preference to shells
-covered with polyps; and, finally, those which would take no others,
-and even themselves fixed the sea-anemone upon it, if it chanced to be
-removed. Intelligence need not be taken into account in the matter at
-all, not even in the hermit-crab's case. We have only to recall the
-complex instincts, exercised only once in a lifetime, which compel the
-silkworm and the emperor moth to elaborate their effective cocoons.
-The elaboration of the spinning-instinct can only be due to natural
-selection, for the insect can have had no idea of the utility of its
-performance, and the same is true in the case of the sea-anemones
-or the hydroid polyps and the hermit-crab. The sea-anemone is quite
-unconscious that it is defending its partner, the hermit-crab, when it
-lashes out its stinging acontia on any disturbance, and the hermit-crab
-is equally unaware that the sea-anemone is contributing to its safety;
-both animals act quite unconsciously, purely instinctively, and the
-origin of these instincts, on which the symbiosis is based, must be
-due, not to intelligent activities which have become habitual, but only
-to the survival of the fittest.
-
-According to the principle of natural selection nothing can arise
-but that which is of use directly or indirectly to its possessor.
-Nevertheless, there are cases in which it appears as if something had
-arisen, which was of no use to the species in which the variation
-appeared, but only to the species protected by it. This is the case
-in the remarkable symbiosis between algæ of the family Nostocaceæ and
-the floating, moss-like water-fern _Azolla_. This plant, in external
-appearance almost like duckweed, has on the under surface of its leaves
-a minute opening, leading into a relatively roomy hair-lined cavity,
-and in this cavity there is always, enclosed in jelly, a bluish green
-unicellular alga, _Anabæna_. The cavity is present in every leaf, and
-the alga is present in every cavity, making its way in from a deposit
-of alga-cells which is found on the incurved tip of every young shoot.
-As soon as a young leaf of _Azolla_ unfolds from the bud it receives
-its _Anabæna_ cells from this deposit, and no one has yet found
-either twigs or leaves which were free from the algæ. But no one has
-succeeded in discovering any benefit derived by the _Azolla_ from this
-partnership.
-
-This looks like a contradiction of the theory of selection, but there
-remains the possibility that there is some benefit rendered to the
-_Azolla_ by the alga, though we cannot see it as yet. There is also
-the possibility that the cavity is an organ which was of use to the
-plant at an earlier time, perhaps as an insect-trap, but has now lost
-its significance, and is utilized by the alga as a dwelling-place.
-This, however, is contradicted by the remarkable distribution of the
-four known species of _Azolla_. Two of these are widely distributed in
-America; the third lives in Australia, Asia, and Africa; the fourth
-in the region of the Nile: all four have cavities in their leaves,
-and in all these forms the cavity is inhabited by the same species of
-_Anabæna_. This indicates that the leaf-cavity and the partnership
-with the alga must have originated in remote antiquity; the symbiosis
-must date from a time before the four modern species of _Azolla_ had
-split off from a single parent-species. But no rudimentary organ,
-that is to say, no organ not of use to the plant itself, would have
-been preserved through such a vast period of time, as we shall see
-later, for useless organs disappear in the course of ages. As the
-cavity has not yet disappeared, we may assume with some probability
-that it is useful to the plant, whether by means of the _Anabæna_, or
-in some other unknown way. To draw an argument against the reality of
-the processes of selection from our lack of knowledge of what this
-advantage may be would be as unreasonable as if, notwithstanding our
-experience that stones sink in the water, we were to assume of a
-particular stone which we did not see sink, because it was hidden from
-our sight by bushes, that perhaps it had not sunk, but was capable of
-floating.
-
-
-
-
-LECTURE X
-
-THE ORIGIN OF FLOWERS
-
- Introduction--Precursors of Darwin--Pollination
- by wind--Arrangements in flowers for securing
- cross-fertilization--Salvia, Pedicularis--Flowers visited by
- flies--Aristolochia--Pinguicula--Daphne--Orchids--Flowers are
- built up of adaptations--Mouth-parts of insects--Proboscis of
- butterflies--Mouth-parts of the cockroach--Of the bee--Pollen
- baskets of bees--Origin of flowers--Attraction of insects by
- colour--Limitation of the area visited--Nägeli's objection to
- the theory of selection--Other interpretations excluded--_Viola
- calcarata_--Only those changes which are useful to their possessors
- have persisted--Deceptive flowers--Cypripedium--Pollinia of
- Orchis--The case of the Yucca-moth--The relative imperfection of
- the adaptations tells in favour of their origin through natural
- selection--Honey thieves.
-
-
-WHEN one species is so intimately bound up with another that neither
-can live for any length of time except in partnership, that is
-certainly an example of far-reaching mutual adaptation, but there
-are innumerable cases of mutual adaptation, in which, although there
-is no common life in the same place, yet the first form of life is
-adjusted in relation to the peculiarities of the second, and the second
-to those of the first. One of the most beautiful, and, in regard to
-natural selection, the most instructive of these cases is illustrated
-by the relations between insects and the higher plants, relations which
-have grown out of the fact that many insects have formed the habit of
-visiting the flowers of the plants for the sake of the pollen. In this
-connexion the theory of selection has made the most unexpected and
-highly interesting disclosures, for it has informed us how the flowers
-have arisen.
-
-In earlier times the beauty, the splendour of colour, and the fragrance
-of flowers were regarded as phenomena created for the delight of
-mankind, or as an outcome of the infinite creative power of Mother
-Nature, who loves to run riot in form and colour. Without allowing our
-pleasure in all this manifold beauty to be spoilt, we must nowadays
-form quite a different conception of the way in which the flowers
-have been called into being. Although here, as everywhere else in
-Nature, we cannot go back to ultimate causes, yet we can show, on
-very satisfactory evidence, that the flowers illustrate the reaction
-of the plants to the visits of insects, and that they have been in
-large measure evoked by these visits. There might, indeed, have
-been blossoms, but there would have been no flowers--that is to say,
-blossoms with large, coloured, outer parts, with fragrance, and with
-nectar inside, unless the blossoms had been sought out by insects
-during the long ages. Flowers are adaptations of the higher flowering
-plants to the visits of insects. There can be no doubt about that now,
-for--thanks to the numerous and very detailed studies of a small number
-of prominent workers--we need not only suppose it, we can prove it
-with all the certainty that can be desired. The mutual adaptations of
-insects and flowers afford one of the clearest examples of the mode
-of operation and the power of natural selection, and the case cannot
-therefore be omitted from lectures on the theory of descent.
-
-That bees and many other insects visit flowers for the sake of the
-nectar and pollen has been known to men from very early times. But this
-fact by itself would only explain why adaptations to flower-visiting
-have taken place in these insects to enable them, for instance, to
-reach the nectar out of deep corolla-tubes, or to load themselves with
-a great quantity of pollen, and to carry it to their hives, as happens
-in the case of the bees. But what causes the plants to produce nectar,
-and offer it to the insects, since it is of no use to themselves? And
-further, what induces them to make the pillage easier to the insects,
-by making their blossoms visible from afar through their brilliant
-colours, or by sending forth a stream of fragrance that, even during
-the night, guides their visitors towards them?
-
-As far back as the end of the eighteenth century a thoughtful and
-clear-sighted Berlin naturalist, Christian Konrad Sprengel, took
-a great step towards answering this question. In the year 1793 he
-published a paper entitled 'The Newly Discovered Secret of Nature in
-the Structure and Fertilization of Flowers[8],' in which he quite
-correctly recognized and interpreted a great many of the remarkable
-adaptations of flowers to the visits of insects. Unfortunately, the
-value of these discoveries was not appreciated in Sprengel's own time,
-and his work had to wait more than half a century for recognition.
-
-[8] _Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung
-der Blumen_, Berlin, 1793.
-
-Sprengel was completely dominated by the idea of an all-wise Creator,
-who 'has not created even a single hair without intention,' and, guided
-by this idea, he endeavoured to penetrate into the significance of
-many little details in the structure of flowers. Thus he recognized
-that the hairs which cover the lower surface of the petals of the
-wood-cranesbill (_Geranium sylvaticum_) protect the nectar of the
-flower from being diluted with rain, and he drew the conclusion,
-correct enough, though far removed from our modern ideas as regards the
-directly efficient cause, that the nectar was there for the insects.
-
-He was also impressed by the fact that the sky-blue corolla of the
-forget-me-not (_Myosotis palustris_) has a beautiful yellow ring round
-the entrance to the corolla-tube, and he interpreted this as a means by
-which insects were shown the way to the nectar which is concealed in
-the depths of the tube.
-
-[Illustration: FIG. 40. _Potentilla verna_, after Hermann Müller. _A_,
-seen from above. _Kbl_, sepals. _Bl_, petals. _Nt_, nectaries near the
-base of the stamens. _B_, section through the flower. _Gr_, stigma.
-_St_, stamen. _Nt_, nectary.]
-
-We now know that such 'honey-guides' are present in most of the flowers
-visited by insects, in the form of spots, lines, or other marking,
-usually of conspicuous colour, that is, of a colour contrasting with
-the ground colour of the flower. Thus, in species of Iris, regular
-paths of short hairs lead the way to the place where the nectar lies.
-In the spring potentilla (_Potentilla verna_) (Fig. 40) the yellow
-petals (_A_, _Bl_) become bright orange-red towards their bases,
-and this shows the way to the nectaries, which lie at the bases
-of the stamens (_st_), and are protected by hairs, the so-called
-'nectar-covers' (_Saftdecke_) of Sprengel, from being washed by rain.
-
-The recognition of the honey-guides led Sprengel on to the idea that
-the general colouring of the flower effects on a large scale what the
-honey-guides do in a more detailed way--it attracts the attention of
-passing insects to where nectar is to be found; indeed, he went an
-important step further by recognizing that there are flowers which
-cannot fertilize themselves, in which the insect, in its search for
-honey, covers itself with pollen, which is then rubbed off on the
-stigma of the next flower visited, fertilization being thus effected.
-He demonstrated this not only for the Iris, but for many other
-flowers, and he drew the conclusion that 'Nature does not seem to have
-wished that any flower should be fertilized by its own pollen.' How
-near Sprengel was to reaching a complete solution of the problem is
-now plain to us, for he even discovered that many flowers, such as
-_Hemerocallis fulva_, remained infertile if they were dusted with their
-own pollen.
-
-Even the numerous experiments of that admirable German botanist, C. F.
-Gärtner, although they advanced matters further, did not suffice to
-make the relations between insects and flowers thoroughly clear; for
-this the basis of the theory of Descent and Selection was necessary.
-Here, again, it was reserved for Charles Darwin to lead the way where
-both contemporaries and predecessors had been blindly groping. He
-recognized that, _in general_, self-fertilization is disadvantageous
-to plants; that they produce fewer seeds, and that these produce
-feebler plants, than when they are cross-fertilized; that, therefore,
-those flowers which are arranged to secure cross-fertilization have an
-advantage over those which are self-fertilized. In many species, as
-Sprengel had already pointed out, self-fertilization leads to actual
-infertility; only a few plants are as fertile with their own pollen as
-with that of another plant; and Darwin believed that, in all flowering
-plants, crossing with others of the same kind, at least from time to
-time, is necessary if they are not to degenerate.
-
-Thus the advantage which the flowers derive from the visits of
-insects lies in the fact that insects are instrumental in the
-cross-fertilization of the flowers, and we can now understand how the
-plant was able to vary in a manner favourable to the insect-visits, and
-to exhibit adaptations which serve exclusively to make these visits
-easier; we understand how it was possible that there should develop
-among flowers an endless number of contrivances which served solely
-to attract insects, and even how, for the same end, the insignificant
-blossoms of the oldest Phanerogams must have been transformed into real
-flowers.
-
-We must not imagine, however, that the obviously important crossing of
-plant-individuals, usually called 'cross-pollination,' can be effected
-only by means of insects. There were numerous plants in earlier times,
-and there is still a whole series in which cross-fertilization is
-effected through the air by the wind; these are the anemophilous or
-wind-pollinated Angiosperms.
-
-To these belong most of the catkin-bearers, such as hazel and birch,
-and also the grasses and sedges, the hemp and the hop, and so forth.
-In these plants there is no real flower, but only an inconspicuous
-blossom, without brightly-coloured outer envelopes, without fragrance
-or nectar; all of them have smooth pollen grains, which easily
-separate into fine dust and are carried away by the wind until they
-fall, by chance, far from their place of origin, on the stigma of a
-female blossom.
-
-[Illustration: FIG. 41. Flower of Meadow Sage (_Salvia pratensis_),
-after H. Müller. _st´_, immature anthers concealed in the 'helmet' of
-the flower. _st´´_, mature anther lowered. _gr´_, immature stigma.
-_gr´´_, mature stigma. _U_, the lower lip of the corolla, the
-landing-stage for the bee.]
-
-By far the greater number of the phanerogams, however, especially
-all our indigenous 'flowers,' are, as a rule, fertilized by means of
-insects, and it is amazing to see in what diverse ways, often highly
-specialized, they have adapted themselves to the visits of insects.
-Thus there are flowers in which the nectar lies open to view, and
-these can be feasted on by all manner of insects; there are others in
-which the nectar is rather more concealed, but still easily found, and
-reached by insects with short mouth-parts, e.g. large flowers blooming
-by day and bearing much pollen, like the Magnolias. These have been
-called beetle-flowers, because they are visited especially by the
-honey-loving Longicorns.
-
-Other flowers blooming by day are especially adapted to fertilization
-by means of bees; they are always beautifully coloured, often blue;
-they are fragrant, and contain nectar deep down in the flower, where
-it can only be reached by the comparatively long proboscis of the
-bee. Different arrangements in the different flowers secure that
-the bee cannot enjoy the nectar without at the same time effecting
-the cross-pollination. Thus the stamens of the meadow sage (_Salvia
-pratensis_) are at first hidden within the helmet-shaped upper lip of
-the flower (Fig. 41, _st´_), but bear lower down on their stalk a short
-handle-like process, which turns the pollen-bearing anther downwards
-(_st´´_) as soon as it is pressed back by an intruding insect. The
-pollen-sacs then strike downwards on the back of the bee, and cover it
-with pollen. When the bee visits another more mature flower, the long
-style, which was at first hidden within the helmet, has bent downwards
-(_gr´´_), and now stands just in front of the entrance to the flower,
-so that the bee must rub off a part of the pollen covering its back on
-to the stigma, and fertilization is thus effected.
-
-There are other flowers which are specially disposed to suit
-the visits of the humble-bees, as, for instance, _Pedicularis
-asplenifolia_, the fern-leaved louse-wort, a plant of the high Alps
-(Fig. 42). The first thing that strikes us about this plant is the
-thickly tufted hair covering on the calyx (_k_), which serves to keep
-off little wingless insects from the flower; then there is the strange
-left-sided twisting of the individual flowers, whose under lip allows
-only a strong insect like the humble-bee to gain access, towards the
-left, to the corolla-tube (_kr_), in the depths of which the nectar is
-concealed. While the humble-bee is sucking up the nectar it becomes
-dusted over with pollen from the anthers, which falls to dust at a
-touch, and when it insinuates itself into a second flower its powdered
-back comes first into contact with the stigma of the pistil (_gr_)
-which projects from the elongated bill-shaped under lip, dusting it
-over with the pollen of the first visited flower. Butterflies and
-smaller bees cannot rob this flower; it is strictly a humble-bee's
-flower.
-
-[Illustration: FIG. 42. Alpine Lousewort (_Pedicularis asplenifolia_).
-_A_, flower seen from the left side, enlarged three times; the arrows
-show the path by which the humble-bee enters. _B_, the same flower,
-seen from the left, after removal of the calyx, the lower lip and the
-left half of the upper lip. _C_, ovary (_ov_), nectary (_n_), and base
-of style. _D_, tip of style, bearing the stigma. _E_, two anthers
-turned towards one another. _o_, upper lip. _u_, lower lip. _gr_,
-style. _st_, anthers. _kr_, corolla-tube. _k_, calyx.]
-
-There are not a few of such flowers adapted to a very restricted
-circle of visitors, and in all of them we find contrivances which
-close the entrance to all except what we may call the welcome insects;
-sometimes there are cushions of bristles which prevent little insects
-from creeping up from below, or it is the oblique position of the
-flower which prevents their getting in from the stem; sometimes it is
-the length and narrowness of the corolla-tube, or the deep and hidden
-situation of the nectar, which only allows intelligent insects to find
-the treasure.
-
-Very remarkable are those flowers which are adapted to the visits of
-flies, for they correspond in several respects to the peculiarities
-of these insects. In the first place, flies are fond of decaying
-substances and the odours given off by these, and so the flowers which
-depend for their cross-fertilization on flies have taken on the dull
-and ugly colours of decay, and give out a disagreeable smell. But flies
-are also shy and restless, turning now hither, now thither, and cannot
-be reckoned among the 'constant' insect visitors, that is to say, they
-do not persistently visit the same species; it is, therefore, evident
-that they might easily carry away the pollen without any useful result
-ensuing. Moreover, their intelligence is of a low order, and they do
-not seek nectar with the perseverance shown by bees and humble-bees. It
-is not surprising, therefore, to find that many of the flowers adapted
-for the visits of flies are so constructed that they detain their
-visitors until they have done their duty, that is to say, until they
-have effected, or at least begun, the process of cross-pollination.
-
-[Illustration: FIG. 43. Flower of Birthwort (_Aristolochia clematitis_)
-cut in half. _A_, before pollination by small flies. _b_, the bristles.
-_B_, after pollination. _P_, pollen mass. _N_, stigma, _b_, the
-bristles. _b´_, their remains. After H. Müller.]
-
-[Illustration: FIG. 44. Alpine Butterwort (_Pinguicula alpina_). _A_,
-section through the flower. _K_, calyx. _bh_, bristly prominences.
-_sp_, spur. _st_, stamen. _n_, stigma. _B_, stigma and stamen more
-magnified. After H. Müller.]
-
-Our birthwort (_Aristolochia clematitis_) and the Cuckoo-pint
-(_Arum maculatum_) are pit-fall flowers, whose long corolla-tubes
-have an enlargement at the base, in which both pistil and stamens
-are contained. In the birthwort (Fig. 43) the narrow entrance-tube
-is thickly beset with stiff hairs (_A_, _b_), whose points are
-all directed towards the base. Little flies can creep down quite
-comfortably into the basal expansion, but once there they are kept
-imprisoned until the flower, in consequence of the pollination of
-the stigma, begins to wither, the first parts to go being these very
-bristles (_B_, _b´_), whose points, like a fish-weir, prevented the
-flies from creeping out. Other 'fly-flowers,' as for instance the
-Alpine butterwort (_Pinguicula alpina_) (Fig. 44), securely imprison
-the plump fly as soon as it has succeeded in forcing itself in far
-enough to reach, with its short proboscis, the nectar contained in the
-spur (_sp_) of the corolla. The backward-directed bristles hold it fast
-for some time, and it is only by hard pressing with the back against
-the anthers (_st_) lying above it, and against the stigma (_n_), that
-it ultimately succeeds in getting free, but it never does so without
-having either loaded itself with pollen, or rubbed off on the stigma
-the pollen it brought with it from another similar flower. The Alpine
-butterwort is protogynous, that is to say, the pistil ripens first,
-the pollen later, so that the possibility of self-fertilization is
-altogether excluded.
-
-It would be impossible to give even an approximate idea of the
-diversity of the contrivances for securing fertilization in flowers
-without spending many hours over them, for they are different in
-almost every flower, often widely so, and even in species of the same
-genus they are by no means always alike; for not infrequently one
-species is adapted to one circle of visitors, and its near relative
-to another. Thus the flower of the common Daphne (_Daphne mezereum_)
-(Fig. 45, _A_ and _C_) is adapted to the visits of butterflies, bees,
-and hover-flies, while its nearest relative (_Daphne striata_) (Fig.
-45, _B_ and _D_) has a somewhat narrower and longer corolla-tube, so
-that only butterflies can feast upon it. This example shows that there
-are exclusively 'butterfly flowers,' but specialization goes further,
-for there are flowers adapted to diurnal and others to nocturnal
-Lepidoptera. The former have usually bright, often red colours, and
-a pleasant aromatic fragrance, and in all of them the nectar lies at
-the bottom of a very narrow corolla-tube. To this class belong, for
-instance, the species of pink, many orchids, such as _Orchis ustulata_,
-and _Nigritella angustifolia_ of the Alps, which smells strongly of
-vanilla; also the beautiful campion (_Lychnis diurna_) and the Alpine
-primrose (_Primula farinosa_). The flowers adapted to nocturnal
-Lepidoptera are characterized by pale, often white colour, and a strong
-and pleasant smell, which only begins to stream out after sunset, and
-indeed many of these flowers are quite closed by day. This is the case
-with the large, white, scentless bindweed (_Convolvulus sepium_), which
-is chiefly visited and fertilized by the largest of our hawk-moths
-(_Sphinx convolvuli_). The pale soapwort (_Saponaria officinalis_)
-exhales a delicate fragrance which attracts the Sphingidæ from afar,
-and the sweet smell of the honeysuckle (_Lonicera periclymenum_) is
-well known, and has the same effect; an arbour of honeysuckle often
-attracts whole companies of our most beautiful Sphingidæ and Noctuidæ
-on warm June nights, to the great delight of the moth-collecting youth.
-
-[Illustration: FIG. 45. _Daphne mezereum_ (_A_ and _C_) and Daphne
-striata (_B_ and _D_). The former visited by butterflies, bees, and
-flies, the latter by butterflies only. _A_ and _B_, vertical sections
-of the flowers. _St_, stamens. _Gr_, style. _n_, nectary. _C_ and _D_,
-flowers seen from above. After H. Müller.]
-
-I cannot conclude this account of flower-adaptations without
-considering the orchids somewhat more in detail, for it is among
-them that we find the most far-reaching adaptations to the visits of
-insects. Among them, too, great diversity prevails, as is evident
-from the fact that Darwin devoted a whole book to the arrangements
-for fertilization in orchids, but the main features are very much the
-same in the majority. Figure 46 gives a representation of one of our
-commonest species (_Orchis mascula_), A shows the flower in side view,
-_B_ as it appears from in front. The flower seems as it were to float
-on the end of the stalk (_st_), stretching out horizontally the spur
-(_sp_) which contains the nectar. Between the large, broad under lip
-(_U_), marked with a honey-guide (_sm_), and offering a convenient
-alighting surface, and the broad, cushion-like stigma (_n_) lies the
-entrance to the spur. Fertilization occurs in the following way:--The
-fly or bee, when it is in the act of pushing its proboscis into the
-nectar-containing spur, knocks with its head against the so-called
-rostellum (_r_), a little beak-like process at the base of the stamens
-(_p_). The pollen masses are of very peculiar construction, not falling
-to dust, but forming little stalked clubs, with the pollen grains glued
-together, and so arranged that they spring off when the rostellum is
-touched and attach themselves to the head of the insect, as at _D_ on
-the pencil (Fig. 46). When the bee has sucked up the nectar out of the
-spur, and then proceeds to penetrate into another flower of the same
-species, the pollinia have bent downwards on its forehead (_E_), and
-must unfailingly come in contact with the stigma of the second flower,
-to which they now remain attached, and effect its fertilization. What
-a long chain of purposeful arrangements in a single flower, and no
-interpretation of them is available except through natural selection!
-
-[Illustration: FIG. 46. Common Orchis (_Orchis mascula_). _A_, flower
-in side view. _st_, stalk. _sp_, spur with the nectary (_n_). _ei_,
-entrance to the spur. _U_, lower lip. _B_, flower from in front. _p_,
-pollinia. _Sm_, honey-guide. _ei_, entrance to the nectar. _na_,
-stigma. _r_, rostellum. _U_, lower lip. _C_, vertical section through
-the rostellum (_r_), pollinium (_p_). _ei_, entrance. _D_, the pollinia
-removed and standing erect on the tip of a lead-pencil. _E_, the same,
-somewhat later, curved downwards.]
-
-And how diversely are these again modified in the different genera
-and species of orchids, of which one is adapted to the visits of
-butterflies exclusively, as _Orchis ustulata_, another to those of
-bees, as _Orchis morio_, and a third to those of flies, as _Ophrys
-muscifera_. These flowers are adapted to insect visits in the minutest
-details of the form of the petals, which are smooth, as if polished
-with wax, where insects are not intended to creep, but velvety or hairy
-where the path leads to the nectar, and at the same time to the pollen
-and the stigma. And then there is the diversity in the form and colour
-of the 'honey-guides' on the 'alighting surface,' that is, the under
-lip of the flower, upon which the insect sits and holds fast, while it
-pushes its head as far as possible into the spur, so that its proboscis
-may reach the nectar lying deep within it! Even though we cannot
-pretend to guess at the significance of every curve and colour-spot in
-one of the great tropical orchids, such as _Stanhopea tigrina_, yet we
-may believe, with Sprengel, that all this has its significance, or has
-had it for the ancestors of the plant in question, and in fact that
-the flower is made up of nothing but adaptations, either actual or
-inherited from its ancestors, although sometimes perhaps no longer of
-functional importance.
-
-So far, then, we have illustrated the fact that there are hundreds and
-thousands of contrivances in flowers adapted solely to the visits of
-insects and to securing cross-fertilization, and these adaptations go
-so far that we might almost believe them to be the outcome of the most
-exact calculation and the most ingenious reflection. But they all admit
-of interpretation through natural selection, for all these details,
-which used to be looked upon as merely ornamental, are directly or
-indirectly of use to the species; directly, when, for instance, they
-concern the dusting of the insect with the pollen; indirectly, when
-they are a means of attracting visits.
-
-Moreover, the evidence of the operation of the processes of selection
-becomes absolutely convincing when we consider that, as in symbiosis,
-there are always two sets of adaptations taking place independently
-of one another--those of the flowers to the visits of the insects,
-and those of the insects to the habit of visiting the flowers. To
-understand this clearly we must turn our attention to the insects, and
-try to see in what way they have been changed by adapting themselves to
-the diet which the flowers afford.
-
-As is well known, several orders of insects possess mouth-parts which
-are suited for sucking up fluids, and these have evolved, through
-adaptation to a fluid diet, from the biting mouth-parts of the
-primitive insects which we see still surviving in several orders.
-Thus the Diptera may have gradually acquired the sucking proboscis
-which occurs in many of them by licking up decaying vegetable and
-animal matter, and by piercing into and sucking living animals. But
-even among the Diptera several families have more recently adapted
-themselves quite specially to a flower diet, to honey-sucking, like the
-hover-flies, the Syrphidæ, and the Bombyliidæ, whose long thin proboscis
-penetrates deep into narrow corolla-tubes, and is able to suck up the
-nectar from the very bottom. The transformation was not so important
-in this case, since the already existing sucking apparatus only
-required to be a little altered.
-
-Again, in the order Hemiptera (Bugs) the suctorial proboscis does not
-owe its origin to a diet of flowers, for no member of the group is now
-adapted to that mode of obtaining food.
-
-[Illustration: FIG. 47. Head of a Butterfly. _A_, seen from in front.
-_au_, eyes. _la_, upper lip. _md_, rudiments of the mandibles. _pm_,
-rudimentary maxillary palps. _mx´_, the first maxillæ modified into the
-suctorial proboscis. _pl_, palps of labium or second maxillæ, cut off
-at the root, remaining in _B_--which is a side view. _at_, antennæ.
-Adapted from Savigny.]
-
-The proboscis of the Lepidoptera, on the other hand, depends entirely
-on adaptation to honey-sucking, and we may go the length of saying that
-the order of Lepidoptera would not exist if there were no flowers.
-This large and diverse insect-group is probably descended from the
-ancestors of the modern caddis-flies or Phryganidæ, whose weakly
-developed jaws were chiefly used for licking up the sugary juices of
-plants. But as flowering plants evolved the licking apparatus of the
-primitive butterflies developed more and more into a sucking organ, and
-was ultimately transformed into the long, spirally coiled suctorial
-proboscis as we see it in the modern butterflies (Fig. 47). It has
-taken some pains to trace this organ back to the biting mouth-parts of
-the primitive insects, for nearly everything about it has degenerated
-and become stunted except the maxillæ (_mx´_). Even the palps (_pm_)
-of these have become so small and inconspicuous in most of the
-Lepidoptera that it is only quite recently that remains of them have
-been recognized in a minute protuberance among the hairs. The mandibles
-(_md_) have quite degenerated, and even the under lip has disappeared,
-and only its palps are well developed (_B_, _pl_). But the first
-maxillæ (_mx´_), although very strong and long, are so extraordinarily
-altered in shape and structure that they diverge from the maxillæ of
-all other insects. They have become hollow, probe-like half-tubes,
-which fit together exactly, and thus form a closed sucking-tube of most
-complex construction, composed of many very small joints, after the
-fashion of a chain-saw, which are all moved by little muscles, and are
-subject to the will through nerves, and are also furnished with tactile
-and taste papillæ. Except this remarkable sucking proboscis there are
-no peculiarities in the body of the butterfly which might be regarded
-as adaptations to flower-visiting, with a few isolated exceptions, of
-which one will be mentioned later. This is intelligible enough, for
-the butterfly has nothing more to seek from the flower beyond food for
-itself; it does not carry stores for offspring.
-
-The bees, however, do this, and accordingly we find that in them the
-adaptations to flower-visiting are not confined to the mouth-parts.
-
-As far as we can judge now, the flower-visiting bees are descended
-from insects which resembled the modern burrowing-wasps. Among these
-the females themselves live on nectar and pollen, and build cells in
-holes in the ground, and feed their brood. They do not feed them on
-honey, however, but on animals--on caterpillars, grasshoppers, and
-other insects, which they kill by a sting in the abdomen, or often only
-paralyse, so that the victim is brought into the cells of the nest
-alive but defenceless, and remains alive until the young larva of the
-wasp, which emerges from the egg, sets to work to devour it.
-
-[Illustration: FIG. 48. Mouth-parts of the Cockroach (_Periplaneta
-orientalis_), after R. Hertwig. _la_, upper lip or labrum. _md_,
-mandibles. _mx_^1, first maxillæ, with _c_, cardo, _st_, stipes, _li_,
-internal lobe or lacinia, _le_, external lobe or galea, and _pm_, the
-maxillary palp. _mx_^2, the labium or second maxillæ, with similar
-detailed parts.]
-
-Before I go on to explain the origin of the sucking proboscis of the
-bee from the biting mouth-parts of the primitive insects I must first
-briefly consider the latter.
-
-The biting mouth-parts of beetles, Neuroptera, and Orthoptera (Fig.
-48), consist of three pairs of jaws, of which the first, the mandibles
-(_md_), are simply powerful pincers for seizing and tearing or chewing
-the food. They have no part in the development of the suctorial
-apparatus either in bees or in butterflies, so they may be left out
-of account. The two other pairs of jaws, the first and second maxillæ
-(_mx_^1 and _mx_^2), are constructed exactly on the same type, having
-a jointed basal portion (_st_) bearing two lobes, an external (_le_)
-and an internal (_li_), and a feeler or palp, usually with several
-joints, directed outwards from the lobes (_pm_ and _pl_). The second
-pair of maxillæ (_mx_^2) differs from the first chiefly in this, that
-the components of the pair meet in the median line of the body, and
-fuse more or less to form the so-called 'under lip' or labium. In the
-example given, the cockroach (_Periplaneta orientalis_), this fusion
-is only partial, the lobes having remained separate (_le_ and _li_);
-and the same is true of the bee, but in this case the inner lobes have
-grown into a long worm-like process which is thrust into the nectar in
-the act of sucking.
-
-[Illustration: FIG. 49. Head of the Bee. _Au_, compound eyes. _au_,
-ocelli. _at_, antennæ. _la_, upper lip. _md_, mandibles. _mx_^1, first
-maxillæ, with _pm_, the rudimentary maxillary palp. _mx_^2, second
-maxillæ with the internal lobes (_li_) fused to form the 'tongue.'
-_le_, the external lobes of the second maxillæ, known as 'paraglossæ.'
-_pl_, labial palp.]
-
-Even the burrowing-wasps exhibit the beginnings of variation in this
-direction, for the under lip is somewhat lengthened and modified into
-a licking organ. The adaptation has not gone much further than this,
-even in one of the true flower-bees, _Prosopis_, which feeds its larvæ
-with pollen and honey, and it is only in the true honey-bee that the
-adaptation is complete (Fig. 49). Here the so-called 'inner lobe' of
-the under lip (_li_) has elongated into the worm-shaped process already
-mentioned; it is thickly covered with short bristles, and is called
-the 'tongue' of the bee (_li_). The outer lobes of the under lip have
-degenerated into little leaf-like organs, the so-called accessory
-tongue or paraglossa (_le_), while the palps of the under lip (_pl_)
-have elongated to correspond with the tongue, and serve as a sensitive
-and probably also as a smelling organ, in contrast to the palps of the
-first maxillæ, which have shrunk to minute stumps (_pm_). The whole
-of the under lip, which has elongated even in its basal portions,
-forms, with the equally long first maxillæ, the proboscis of the bee.
-The first maxillæ are sheath-like half-tubes, closely apposed around
-the tongue, and form along with it the suctorial tube, through which
-the nectar is sucked up. Thus, of the three pairs of jaws in insects,
-only the first pair, the mandibles, have remained unaltered, obviously
-because the bee requires a biting-organ for eating pollen, for kneading
-wax, and for building cells.
-
-But bees do not only feast on nectar and pollen themselves, they carry
-these home as food for their larvæ. The form already mentioned,
-_Prosopis_, takes up pollen and nectar in its mouth, and afterwards
-disgorges the pulp as food for its larvæ, but the rest of the true
-bees have special and much more effective collecting-organs, either a
-thick covering of hair on the abdomen, or along the whole length of the
-posterior legs, or finally, a highly developed collecting apparatus,
-such as that possessed by the honey-bee--the basket and brush on the
-hind leg. The former is a hollow on the outer surface of the tibia, the
-latter a considerable enlargement of the basal tarsal joint, which, at
-the same time, is covered on the inner surface with short bristles,
-arranged in transverse rows like a brush. The bee kneads the pollen
-into the basket, and one can often see bees flying back to the hive
-with a thick yellow ball of pollen on the hind leg. In those bees which
-collect on the abdomen, like _Osmia_ and _Megachile_, the pollen mass
-forms a thick clump on the belly, and in the case of _Andrena_ Sprengel
-observed long ago that it sometimes flew with a packet of pollen bigger
-than its own body on the hind leg.
-
-All these are contrivances which have gradually originated through the
-habit of carrying home pollen for the helpless larvæ shut up in the
-cells. They have developed differently in the various groups of bees,
-probably because the primary variations with which the process of
-selection began were different in the various ancestral forms.
-
-In the ancestors of those which carry pollen on the abdomen there
-was probably a thick covering of hair on the ventral surface of the
-body, which served as a starting-point for the selection, and, in
-consequence, the further course of the adaptation would be concerned
-solely with this hair-covered surface, while variations in other less
-hairy spots would remain un-utilized.
-
-After all this it will no longer seem a paradoxical statement that the
-existence of gaily coloured, diversely formed, and fragrant flowers is
-due to the visits of insects, and that, on the other hand, many insects
-have undergone essential transformations in their mouth-parts and
-otherwise as an adaptation to a flower diet, and that an entire order
-of insects with thousands of species--the Lepidoptera--would not be
-in existence at all if there had been no flowers. We must now attempt
-to show, in a more detailed way, how, by what steps, and under what
-conditions, our modern flowers have arisen from the earlier flowering
-plants. In this I follow closely the classic exposition which we owe to
-Hermann Müller.
-
-The ancestral forms of the modern higher plants, the so-called
-'primitive seed plants' or 'Archisperms,' were all anemophilous, as the
-Conifers and Cycads are still. Their smooth pollen-grains, produced
-in enormous quantities, fell like clouds of dust into the air, were
-carried by the wind hither and thither, and some occasionally alighted
-on the stigma of a female flower. In these plants the sexes often occur
-separately on different trees or individuals, and there must be a
-certain advantage in this when the pollination is effected by the wind.
-
-The male flowers of the Archisperms would be visited by insects in
-remote ages, just as they are now; but the visitors came to feed upon
-the pollen, and did not render any service to the plant in return;
-they rather did it harm by reducing its store of pollen. If it was
-possible to cause the insect to benefit the plant at the same time
-as it was pillaging the pollen, by carrying some of it to female
-blossoms and thereby securing cross-fertilization, it would be of
-great advantage, for the plant would no longer require to produce such
-enormous quantities of pollen, and the fertilization would be much
-more certain than when it depended on the wind. It is obvious that the
-successful pollination of anemophilous plants implies good weather and
-a favourable wind.
-
-[Illustration: FIG. 50. Flowers of the Willow (_Salix cinerea_); after
-H. Müller. _A_, the male. _B_, the female catkin. _C_, individual male
-flower; _n_, nectary. _D_, individual female flower; _n_, nectary. _E_,
-Poplar, an exceptional hermaphrodite flower.]
-
-It is plain that the utilization of the insect-visitors in
-fertilization might be secured in either of two ways; the female
-blossoms might also offer something attractive to the insects, or
-hermaphrodite flowers might be formed. As a matter of fact, both ways
-have been followed by Nature. An example of the former is the willow,
-the cross-fertilization of which was forced upon the insects by the
-development in both female and male blossoms of a nectary (Fig. 50,
-_C_ and _D_), a little pit or basin in which nectar was secreted. The
-insects flew now to male and now to female willow-catkins, and in doing
-so they carried to the stigma of the female blossom the pollen, which
-in this case was not dusty but sticky, so that it readily adhered to
-their bodies.
-
-The securing of cross-fertilization by the development of hermaphrodite
-flowers has, however, occurred much more frequently, and we can
-understand that this method secured the advantageous crossing much more
-perfectly, for the pollen had necessarily to be carried from blossom
-to blossom, while, in cases like that of the willow, countless male
-blossoms might be visited for nectar one after the other before the
-insect made up its mind to fly to a female blossom of the same species.
-The beginnings of the modification of the unisexual flowers in this
-direction may be seen in variations which occur even now, for we not
-infrequently find, in a male catkin, individual blossoms, which, in
-addition to the stamens, possess also a pistil with a stigma. (Fig. 50
-_E_ shows such an abnormal hermaphrodite flower from a poplar.)
-
-As soon as hermaphrodite flowers came into existence the struggle
-to attract insects began in a more intense degree. Every little
-improvement in this direction would form the starting-point of a
-process of selection, and would be carried on and increased to the
-highest possible pitch of perfection.
-
-It was probably the outer envelopes of the blossoms which first changed
-their original green into other colours, usually those which contrasted
-strongly with the green, and thus directed the attention of the insects
-to the flowers. Variations in the colour of ordinary leaves are
-always cropping up from time to time, whether it be that the green is
-transformed into yellow or that the chlorophyll disappears more or less
-completely and red or blue coloured juices take its place. Many insects
-can undoubtedly see colour, and are attracted by the size of coloured
-flowers, as Hermann Müller found by counting the visits of insects to
-two nearly related species of mallow, one of which, _Malva silvestris_,
-has very large bright rose-red flowers visible from afar, while the
-other, _Malva rotundifolia_, has very inconspicuous small pale-red
-flowers. To the former there were thirty-one different visitors, to the
-latter he could only make sure of four. The second species, as is to be
-expected, depends chiefly on self-fertilization.
-
-It has recently been disputed from various quarters that insects are
-attracted by the colours of the flowers, and these objections are
-based chiefly on experiments with artificial flowers. But when, for
-instance, Plateau, in the course of such experiments saw bees and
-butterflies first fly towards the artificial flowers, and then turn
-away and concern themselves no more about them, that only proves that
-their sight is sharper than we have given them credit for; for though
-they may be deceived at a distance, they are not so when they are near;
-it is possible, too, that the sense of smell turns the scale[9]. I
-have myself made similar experiments with diurnal butterflies, before
-which I placed a single artificial chrysanthemum midst a mass of
-natural flowers. It rarely happened indeed that a butterfly settled on
-the artificial flower; they usually flew first above it, but did not
-alight. Twice, however, I saw them alight on the artificial flower,
-and eagerly grope about with the proboscis for a few moments, then fly
-quickly away. They had visited the real chrysanthemums or horse-daisies
-with evident delight, and eagerly sucked up the honey from the many
-individual florets of every flower, and they now endeavoured to do
-the same in the artificial flower, and only desisted when the attempt
-proved unsuccessful. In this experiment the colours were of course
-only white and yellow; with red and blue it is probably more difficult
-to give the exact impression of the natural flower-colours; and in
-addition there is the absence of the delicate fragrance exhaled by the
-flower.
-
-[9] The experiments of Plateau have since been criticized by
-Kienitz-Gerloff, who altogether denies their value (1903).
-
-It must be allowed that the colour is certainly not the sole attraction
-to the flower; the fragrance helps in most cases, and even this is
-not the object of the insect's visits. The real object is the nectar,
-to which colour and fragrance only show the way. The development of
-fragrance and nectar must, like that of the colour, have been carried
-on and increased by processes of selection, which had their basis in
-the necessity for securing insect-visits, and as soon as these main
-qualities of the flower were established greater refinements would
-begin, and flower-forms would be evolved, which would diverge farther
-and farther, especially in shape, from the originally simple and
-regular form of the blossom.
-
-The reason for this must have lain chiefly in the fact that, after
-insect-visits in general were secured by a flower, it would be
-advantageous to exclude all insects which would pillage the nectar
-without rendering in return the service of cross-fertilization--all
-those, therefore, which were unsuited either because of their minute
-size or because of the inconstancy of their visits. Before the
-butterflies and the bees existed, the regularly formed flat flower with
-unconcealed nectar would be visited by a mixed company of caddis-flies,
-saw-flies, and ichneumon-flies. But as the nectar changed its place to
-the deeper recesses of the flower it was withdrawn from all but the
-more intelligent insects, and thus the circle of visitors was already
-narrowed to some extent. But when in a particular species the petals
-fused into a short tube, all visitors were excluded whose mouth-parts
-were too short to reach the nectar; while among those which could
-reach it the process of proboscis-formation began; the under lip, or
-the first maxillæ, or both parts together, lengthened step for step
-with the corolla-tube of the flower, and thus from the caddis-flies
-came the butterflies, and from the ichneumon-flies the burrowing-wasps
-(_Sphegidæ_) and the bees.
-
-At first sight one might perhaps imagine that it would have been more
-advantageous to the flowers to attract a great many visitors, but
-this is obviously not the case. On the contrary, specialized flowers,
-accessible only to a few visitors, have a much greater certainty of
-being pollinated by them, because insects which only fly to a few
-species are more certain to visit these, and above all to visit many
-flowers of the same species one after another. Hermann Müller observed
-that, in four minutes, one of the humming-bird hawk-moths (_Macroglossa
-stellatarum_) visited 108 different flowers of the same species, the
-beautiful Alpine violet (_Viola calcarata_), one after the other, and
-it may have effected an equal number of pollinations in that short time.
-
-It was, therefore, a real advantage to the flowers to narrow their
-circle of visitors more and more by varying so that only the useful
-visitors could gain access to their nectar, and that the rest should
-be excluded. Thus there arose 'bee-flowers,' 'butterfly-flowers,'
-'hawk-moth flowers,' and, indeed, in many cases, a species of flower
-has become so highly specialized that its fertilization can only
-be brought about by a single species of insect. This explains the
-remarkable adaptations of the orchids and the enormous length of the
-proboscis in certain butterflies. Even our own hawk-moths _Macroglossa
-stellatarum_ and _Sphinx convolvuli_ show an astonishing length of
-proboscis, which measures 8 cm. in the latter species. In _Macrosilia
-cluentius_, in Brazil, the proboscis is 20 cm. in length; and in
-Madagascar there grows an orchid with nectaries 30 cm. in length,
-filled with nectar to a depth of 2 cm., but the fertilizing hawk-moth
-is not yet known.
-
-Thus we may say that the flowers, by varying in one direction or
-another, have selected a definite circle of visitors, and, conversely,
-that particular insect-groups have selected particular flowers for
-themselves, for those transformations of the flowers were always most
-advantageous which secured to them the exclusive visits of their best
-crossing agents, and these transformations were, on the one hand,
-such as kept off unwelcome visitors, and, on the other hand, such as
-attracted the most suitable ones.
-
-From the botanical point of view the assumption that flowers and
-flower-visiting insects have been adapted to each other by means
-of processes of selection has been regarded as untenable, because
-every variation in the flower presupposes a corresponding one in the
-insect. I should not have mentioned this objection had it not come
-from such a famous naturalist as Nägeli, and if it were not both
-interesting and useful in our present discussion. Nägeli maintained
-that selection could not, for instance, have effected a lengthening
-of the corolla-tube of a flower, because the proboscis of the insects
-must have lengthened _simultaneously_ with it. If the corolla-tube
-had lengthened alone, without the proboscis of the butterfly being
-at the same time elongated, the flower would no longer be fertilized
-at all, and if the lengthening of the proboscis preceded that of the
-corolla-tube it would have no value for the butterfly, and could not
-therefore have been the object of a process of selection.
-
-This objection overlooks the facts that a species of plant and of
-butterfly consists not of one individual but of thousands or millions,
-and that these are not absolutely uniform, but in fact heterogeneous.
-It is precisely in this that the struggle for existence consists--that
-the individuals of every species differ from one another, and that
-some are better, others less well constituted. The elimination of the
-latter and the preferring of the former constitutes the process of
-selection, which always secures the fitter by continually rejecting the
-less fit. In the case we are considering, then, there would be, among
-the individuals of the plant-species concerned, flowers with a longer
-and flowers with a shorter corolla-tube, and among the butterflies
-some with a longer and some with a shorter proboscis. If among the
-flowers the longer ones were more certain to be cross-fertilized than
-the shorter ones, because hurtful visitors were better excluded, the
-longer ones would produce more and better seeds, and would transmit
-their character to more descendants; and if, among the butterflies,
-those with the longer proboscis had an advantage, because the nectar
-in the longer tubes would, so to speak, be reserved for them, and they
-would thus be better nourished than those with the shorter proboscis,
-the number of individuals with long proboscis must have increased from
-generation to generation. Thus the length of the corolla-tube and the
-length of the proboscis would go on increasing as long as there was
-any advantage in it for the flower, and both parties must of necessity
-have varied _pari passu_, since every lengthening of the corolla
-was accompanied by a preferring of the longest proboscis variation.
-The augmentation of the characters depended on, and could only have
-depended on, a guiding of the variations in the direction of utility.
-But this is exactly what we call, after Darwin and Wallace, Natural
-Selection.
-
-We have, however, in the history of flowers, a means of demonstrating
-the reality of the processes of selection in two other ways. In the
-first place, it is obvious that no other interpretation can be given
-of such simultaneous mutual adaptations of two different kinds of
-organisms. If we were to postulate, as Nägeli, for instance, did, an
-intrinsic Power of Development in organisms, which produces and guides
-their variations, we should, as I have already said, be compelled
-also to take for granted a kind of pre-established harmony, such as
-Leibnitz assumed to account for the correlation of body and mind:
-plant and insect must always have been correspondingly altered so that
-they bore the same relation to each other as two clocks which were
-so exactly fashioned that they always kept time, though they did not
-influence each other. But the case would be more complicated than that
-of the clocks, because the changes which must have taken place on both
-sides were quite different, and yet at the same time such that they
-corresponded as exactly as Will and Action. The whole history of the
-earth and of the forms of life must, therefore, have been foreseen
-down to the smallest details, and embodied in the postulated Power of
-Development.
-
-But such an assumption could hardly lay claim to the rank of a
-scientific hypothesis. Although every grain of sand blown about by
-the wind on this earth could certainly only have fallen where it
-actually did fall, yet it is in the power of any of us to throw a
-handful of sand wherever it pleases us, and although even this act of
-throwing must have had its sufficient reason in us, yet no one could
-maintain that its direction and the places where the grains fell were
-predestined in the history of the earth. In other words: That which we
-call chance plays a part also in the evolution of organisms, and the
-assumption of a Power of Development, predestinating even in detail,
-is contradicted by the fact that species are transformed in accordance
-with the chance conditions of their life.
-
-This can be clearly demonstrated in the case of flowers. That the wild
-pansy (_Viola tricolor_), which lives in the plains and on mountains of
-moderate elevation, is fertilized by bees, and the nearly allied _Viola
-calcarata_ of the High Alps by Lepidoptera, is readily intelligible,
-since bees are very abundant in the lower region, and make the
-fertilization of the species a certainty, while this is not so in the
-High Alps. There the Lepidoptera are greatly in the majority, as every
-one knows who has traversed the flower-decked meads of the High Alps in
-July, and has seen the hundreds and thousands of butterflies and moths
-which fly from flower to flower. Thus the viola of the High Alps has
-become a 'butterfly-flower' by the development of its nectaries into a
-long spur, accessible only to the proboscis of a moth or butterfly. The
-chance which led certain individuals of the ancestral species to climb
-the Alps must also have supplied the incentive to the production of the
-changes adapted to the visits of the prevalent insect. The hypothesis
-of a predestinating Power of Development suffers utter shipwreck in
-face of facts like these.
-
-We have, furthermore, an excellent touchstone for the reality of the
-processes of selection in the _quality_ of the variations in flowers
-and insects. Natural selection can only bring about those changes
-which are of use to the possessors themselves; we should therefore
-expect to find among flowers only such arrangements as are, directly or
-indirectly, of use to them, and, conversely, among insects only such as
-are useful to the insect.
-
-And this is what we actually do find. All the arrangements of the
-flowers--their colour, their form, their honey-guides, their hairy
-honey-paths (Iris), their fragrance, and their honey itself--are all
-indirectly useful to the plant itself, because they all co-operate in
-compelling the honey-seeking insect to effect the fertilization of
-the flower. This is most clearly seen in the case of the so-called
-'Deceptive' flowers, which attract insects by their size and beauty,
-their fragrance, and their resemblance to other flowers, and force
-their visitors to be the means of their cross-fertilization, although
-they contain no nectar at all. This is the case, according to Hermann
-Müller, with the most beautiful of our indigenous orchids, the lady's
-slipper (_Cypripedium calceolaris_). This flower is visited by bees
-of the genus _Andrena_, which creep into the large wooden-shoe-shaped
-under lip in the search for honey, only to find themselves prisoners,
-for they cannot get out, at least by the way they came in, because of
-the steep and smoothly polished walls of the flower. There is only one
-way for the bee; it must force itself under the stigma, which it can
-only do with great exertion, and not without being smeared with pollen,
-which it carries to the next flower into which it creeps. It can only
-leave this one in the same way, and thus the pollen is transferred to
-the stigma by a mechanical necessity.
-
-Such remarkable cases remind us in some ways of those cases of mimicry
-in which the deceptions have to be used with caution or they lose their
-effect. One might be disposed to imagine that such an intelligent
-insect as a bee would not be deceived by the lady's slipper more than
-once, and would not creep into a second flower after discovering that
-there was no nectar in the first. But this conclusion is not correct,
-for the bees are well accustomed in many flowers to find that the
-nectar has already been taken by other bees; they could therefore not
-conclude from one unsuccessful visit that the _Cypripedium_ did not
-produce nectar at all, but would try again in a second, a third, and a
-fourth flower. If these orchids had abundantly covered flower-spikes
-like many species of _Orchis_, and if the species were common, the bees
-would probably soon learn not to visit them, but the reverse is the
-case. There is usually only one or, at most, two open flowers on the
-lady's slipper, and the plant is rare, and probably occurs nowhere in
-large numbers.
-
-If we could find a flower in which the nectar lay open and accessible
-to all insects, and which did not require any service from them
-in return, the case could not be interpreted in terms of natural
-selection; but we do not know of any such case.
-
-[Illustration: FIG. 51. The Yucca-moth (_Pronuba yuccasella_). _M_,
-laying eggs in the ovary of the Yucca flower. _n_, the stigma. After
-Riley.]
-
-Conversely, too, there are no adaptations in the insects which are
-useful only to the flowers, and which are not of some use, directly or
-indirectly, to the insect itself. Bees and butterflies certainly carry
-the pollen from one flower to the stigma of another, but they are not
-impelled to do this by a special instinct; they are forced to do it
-by the structure of the flower, which has its stamens so placed and
-arranged that they must shake their pollen over the visitor, or it may
-be that the anthers are modified into stalked, viscid pollinia which
-spring off at a touch, and fix themselves, so to speak, on the insect's
-head. And even this is not all in the case of the orchis, for the
-insect would never of its own accord transfer these pollinia on to the
-stigma of the next flower; this is effected by the physical peculiarity
-which causes the pollinia, after a short time, to bend forwards on the
-insect's head.
-
-All this fits in as well as possible with the hypothesis: how could an
-instinct to carry pollen from one flower to the stigma of another have
-been developed in an insect through natural selection, since the insect
-itself has nothing to gain from this proceeding? Accordingly, we never
-find in the insect any pincers or any kind of grasping organ adapted
-for seizing and transmitting the pollen.
-
-There is, however, one very remarkable case in which this appears to
-be so, indeed really is so, and nevertheless it is not contradictory
-to, but is corroborative of, the theory of selection. The excellent
-American entomologist, Riley, established by means of careful
-observations that the large white flowers of the Yucca are fertilized
-by a little moth which behaves in a manner otherwise unheard of among
-insects. Only the females visit the flowers, and they at once busy
-themselves collecting a large ball of pollen. To this end they have
-on the maxillary palps (Fig. 52, _C_, _mxp_) a long process (_si_),
-curved in the form of a sickle, and covered with hairs, which probably
-no other Lepidopteron possesses, with the help of which the moth
-very quickly sweeps together a ball of pollen, it may be three times
-the size of her own head. With this ball the insect flies to the
-next flower, and there she lays her egg, by means of an ovipositor
-otherwise unknown among Lepidoptera (Fig. 52, _A_, _op_), in the pods
-of the flower. Finally, she pushes the ball of pollen deep into the
-funnel-shaped stigmatic opening on the pistil (Fig. 51, _n_), and so
-effects the cross-fertilization. The ovules develop, and when the
-caterpillars emerge from the egg four to five days later they feed on
-these until they are ready to enter on the pupa stage. Each little
-caterpillar requires about eighteen or twenty seeds for its nourishment
-(Fig. 52, _B_, _r_).
-
-[Illustration: FIG. 52. The fertilization of the Yucca. _A_, ovipositor
-of the Yucca-moth. _op_, its sheath. _sp_, its apex. _op_^1, the
-protruded oviduct. _B_, two ovaries of the Yucca, showing the holes by
-which the young moths escape, and (_r_) a caterpillar in the interior.
-_C_, head of the female moth, with the sickle-shaped process (_si_) on
-the maxillary palps for sweeping off the pollen and rolling it into a
-ball. _mx_^1, the proboscis. _au_, eye. _p_^1 base of first leg. _D_,
-longitudinal section through an ovary of the Yucca, soon after the
-laying of two eggs (_ei_). _stk_, the canal made by the ovipositor.]
-
-Here, then, we find an adaptation of certain parts of the moth's body
-in relation to the fertilization of the flower, but in this case it is
-as much in the interest of the moth as of the plant. By carrying the
-pollen to the stigma the moths secure the development of the ovules,
-which serve their offspring as food, so that we have here to do with
-a peculiar form of care for offspring, which is not more remarkable
-than many other kinds of brood-care in insects, such as ants, bees,
-Sphex-wasps, ichneumon-flies, and gall-flies.
-
-It might be objected that this case of the Yucca is not so much one
-of effecting fertilization as of parasitism; but the eggs, which are
-laid in the seed-pods, are very few, and the caterpillars which emerge
-from them only devour a very small proportion of the seeds, of which
-there may be about 200 (Fig. 52, _B_). Thus the plants also derive
-an advantage from the moth's procedure, for quite enough seeds are
-left. The form and position of the stamens and of the stigma seem to
-be as exactly adapted to the visits of the moth as the moth is to the
-transference of the pollen, for the Yucca can only be fertilized by
-this one moth, and sets no seed if the moth be absent. For this reason
-the species of Yucca cultivated in Europe remain sterile.
-
-Thus the apparent contradiction is explained, and the facts everywhere
-support the hypothesis that the adaptations between flowers and insects
-depend upon processes of selection.
-
-This origin is incontrovertibly proved, it seems to me, in another way,
-namely, by the merely _relative_ perfection of the adaptations, or
-rather, by their relative imperfection.
-
-I have already pointed out that all adaptations which depend upon
-natural selection can only be relatively perfect, as follows from the
-nature of their efficient causes, for natural selection only operates
-as long as a further increase of the character concerned would be of
-advantage to the existence of the species. It cannot be operative
-beyond this point, because the existence of the species cannot be more
-perfectly secured in this direction, or, to speak more precisely,
-because further variations in the direction hitherto followed would no
-longer be improvements, even though they might appear so to us.
-
-Thus the corolla of many flowers is suited to the thick, hairy head
-and thorax of the bee, for to these only does the pollen adhere in
-sufficient quantity to fertilize the next flower; yet the same flowers
-are frequently visited by butterflies, and in many of them there
-has been no adaptation to prevent these useless visits. Obviously
-this is because preventive arrangements could only begin, according
-to our theory, when they were necessary to the preservation of the
-species; in this case, therefore, only when the pillaging visits
-of the butterflies withdrew so many flowers from the influence of
-the effective pollinating visitor, the bee, that too few seeds were
-formed, and the survival of the species was threatened by the continual
-dwindling of the normal number. As long as the bees visit the flowers
-frequently enough to ensure the formation of the necessary number of
-seeds a process of selection could not set in; but should the bees
-find, for instance, that nearly all the flowers had been robbed of
-their nectar, and should therefore visit them less diligently, then
-every variation of the flower which made honey less accessible to the
-butterflies would become the objective of a process of selection.
-
-Everywhere we find similar imperfections of adaptation which indicate
-that they must depend on processes of selection. Thus numerous flowers
-are visited by insects other than those which pollinate them, and these
-bring them no advantage, but merely rob them of nectar and pollen;
-the most beautiful contrivances of many flowers, such as _Glycinia_,
-which are directed towards cross-fertilization by bees, are rendered
-of no effect because wood-bees and humble-bees bite holes into the
-nectaries from the outside, and so reach the nectar by the shortest
-way. I do not know whether bees in the native land of the _Glycinia_
-do the same thing, but in any case they can do no sensible injury to
-the species, since otherwise processes of selection would have set in
-which would have prevented the damage in some way or other, whether by
-the production of stinging-hairs, or hairs with a burning secretion, or
-in some other way. If the actual constitution of the plant made this
-impossible, the species would become less abundant and would gradually
-die out.
-
-Thus the relative imperfection of the flower-adaptations, which in
-general are so worthy of admiration, affords a further indication that
-their origin is due to processes of selection.
-
-
-ADDITIONAL NOTE TO CHAPTER X.
-
-It has been remarked that the chapter on the Origin of Flowers in the
-German Edition contains no discussion and refutation of the objections
-which have up till recently been urged against the theory of flowers
-propounded by Darwin and Hermann Müller. I admit that this chapter
-seemed to be so harmonious and so well rounded, and at the same time so
-convincing as to the reality of the processes of selection, that the
-feeble objections to it, and the attempts of opponents to find another
-explanation of the phenomena, might well be disregarded in this book.
-
-However, the most important of these objections and counter-theories
-may here be briefly mentioned.
-
-Plateau in Ghent was the first to collect _facts_ which appeared to
-contradict the Darwinian theory of flowers; he observed that insects
-avoided _artificial_ flowers, even when they were indistinguishable
-in colour from natural ones as far as our eyes could perceive, and he
-concluded from this that it is not the colour which guides the insects
-to the flowers, that they find the blossoms less by their sense of
-sight than by their sense of smell. But great caution is required
-in drawing conclusions from experiments of this kind. I once placed
-artificial marguerites (_Chrysanthemum leucanthemum_) among natural
-ones in a roomy frame in the open air, and for a considerable time I
-was unable to see any of the numerous butterflies (_Vanessa urticæ_),
-which were flying about the real chrysanthemums, settle on one of the
-artificial flowers. The insects often flew quite close to them without
-paying them the least attention, and I was inclined to conclude that
-they either perceived the difference at sight, or that they missed the
-odour of the natural flowers in the artificial ones. But in the course
-of a few days it happened twice in my presence that a butterfly settled
-on one of the artificial blooms and _persistently groped about with
-fully outstretched tube to find the entrance to the honey_. It was only
-after prolonged futile attempts that it desisted and flew away. That
-bees are guided by the eye in their visits to flowers has been shown by
-A. Forel, who cut off the whole proboscis, together with the antennæ,
-from humble-bees which were swarming eagerly about the flowers. He thus
-robbed them of the whole apparatus of smell, and nevertheless they flew
-down from a considerable height direct to the same flowers. An English
-observer, Mr. G. N. Bulman, has been led to believe, with Plateau, that
-it is a matter of entire indifference to the bees whether the flowers
-are blue, or red, or simply green in colour, if only they contain
-honey, and that therefore the bees could have played no part in the
-development of blue flowers, as Hermann Müller assumed they had, and
-that they could have no preference for blue or any other colour, as Sir
-John Lubbock and others had concluded from their experiments. This is
-correct in so far that bees feed as eagerly on the greenish blossoms of
-the lime-tree as they do on the deep-blue gentian of the Alpine meadows
-or the red blossoms of the Weigelia, the dog-roses of our gardens or
-the yellow buttercups (_Ranunculus_) of our meadows; they despise
-nothing that yields them honey. But it certainly does not follow
-from this that the bees may not, under certain circumstances, have
-exercised a selecting influence upon the fixation and intensification
-of a new colour-variety of a flower. This is less a question of a
-_colour-preference_, in the human sense, on the part of the bees
-than of the _greater visibility_ of the colour in question in the
-environment peculiar to the flower, and of the amount of rivalry the
-bees meet with from other insects in regard to the same flower. In
-individual cases this would be difficult to demonstrate, especially
-since we can form only an approximate idea of the insect's power of
-seeing colour, and cannot judge what the colours of the individual
-blossoms count for in the mosaic picture of a flowery meadow. Yet this
-is the important point, for, as soon as the bees perceive one colour
-more readily than another, the preponderance of this colour-variety
-over other variations is assured, since it will be more frequently
-visited. In the same way we cannot guess in individual cases why one
-species of flower should exhale perfume while a nearly related species
-does not. But when we remember that many flowers adapted for the visits
-of dipterous insects possess a nauseous carrion-like smell, by means
-of which they not only attract flies but scare off other insects, we
-can readily imagine cases in which it was of importance to a flower
-to be able to be easily found by bees without betraying itself by its
-pleasant fragrance to other less desirable visitors.
-
-Thus, therefore, we can understand the odourless but intensely blue
-species of gentian, if we may assume that its blue colour is more
-visible to bees than to other insects. If I were to elaborate in
-detail all the principles which here suggest themselves to me I should
-require to write a complete section, and I am unwilling to do this
-until I can bring forward a much larger number of new observations
-than I am at present in a position to do. All I wish to do here is
-to exhort doubters to modesty, and to remind them that these matters
-are exceedingly complex, and that we should be glad and grateful that
-expert observers like Darwin and Hermann Müller have given us some
-insight into the principles interconnecting the facts, instead of
-imagining whenever we meet with some little apparently contradictory
-fact, which may indeed be quite correct in itself, that the whole
-theory of the development of flowers through insects has been
-overthrown. Let us rather endeavour to understand such facts, and to
-arrange them in their places as stones of the new building.
-
-Often the contradiction is merely the result of the imperfect
-theoretical conceptions of its discoverer, as we have already shown in
-regard to Nägeli. Bulman, too, fancies he has proved that bees do not
-distinguish between the different varieties of a flower, but visit them
-indiscriminately with the same eagerness, thus causing intercrossing
-of all the varieties, and preventing any one from becoming dominant.
-But are the varieties which we plant side by side in our gardens of the
-kind that are evolved by bees? That is to say, are their _differences
-such as will turn the scale for or against the visits of the bees_?
-If one were less, another more easily seen by the bees; or if one
-were more fragrant, or had a fragrance more agreeable to bees than
-the other, the result of the experiment would probably have been very
-different.
-
-One more objection has been made. It is said that the bees, although
-exclusively restricted, both themselves and their descendants, to
-a diet of flowers, are not so constant _to a particular flower_ as
-the theory requires. They do indeed exhibit a 'considerable amount
-of constancy,' and often visit a large number of flowers of the same
-species in succession, but the theory requires that they should
-not only confine themselves to this one species, but to a _single
-variety_ of this species. These views show that their authors have not
-penetrated far towards an understanding of the nature of selection.
-Nature does not operate with individual flowers, but with millions and
-myriads of them, and not with the flowers of a single spring, but with
-those of hundreds and thousands of years. How often a particular bee
-may carry pollen uselessly to a strange flower without thereby lowering
-the aggregate of seeds so far that the existence of the species seems
-imperilled, or how often she may fertilize the pistil of a useful
-variation with the pollen of the parent species, without interrupting
-or hindering the process of the evolution of the variety, no mortal
-can calculate, and what the theory requires can only be formulated in
-this way: The constancy of the bees in their visits to the flowers must
-be so great that, on an average, the quantity of seeds will be formed
-which suffices for the preservation of the species. And in regard
-to the transformation of a species, the attraction which the useful
-variety has for the bees must, on an average, be _somewhat stronger_
-than that of the parent species. As soon as this is the case the seeds
-of the variety will be formed in preponderant numbers, although they
-may not all be quite pure from the first, and by degrees, in the course
-of generations, the plants of the new variety will preponderate more
-and more over those of the parent form, and finally will alone remain.
-In the first case we have before our eyes the proof that, in spite of
-the imperfect constancy of the bees, a sufficient number of seeds is
-produced to secure the existence of the species. Or does Mr. Bulman
-conclude from the fact that the bees are _not absolutely constant_ that
-flowers are not fertilized by bees at all?
-
-I cannot conclude this note without touching briefly upon what the
-opponents of the flower theory have contributed, and what explanation
-of the facts they are prepared to offer.
-
-In his important work, _Mechanische-physiologische Theorie der
-Abstammungslehre_, published in 1884, Nägeli, as a convinced opponent
-of the theory of selection, attempted an explanation. He was quite
-aware that his assumption of an inward 'perfecting principle' would not
-suffice to explain the mutual adaptations of flowers and insects, and
-he refers the transformation of the first inconspicuous blossoms into
-flowers to the mechanical stimulus which the visiting insects exerted
-upon the parts of the blossom. By the pressure of their footsteps,
-the pushing and probing with their proboscis, they have, he says,
-transformed gradually, for instance, the little covering leaves at the
-base of a pollen vessel into large flower petals, caused the conversion
-of short flower-tubes into long ones, and of the pollen, once dry and
-dusty, into the firmly adhesive mass formed in the anther lobes of
-our modern flowers. The colour of the flowers depends, according to
-him, upon the influence of light, which certainly no more explains the
-yellow ring on a blue ground in the forget-me-not than it does the many
-other nectar-guides which show the insect the way to the honey. Nägeli
-works with the Lamarckian principle in the most daring way, and with
-the same _naïveté_ as Lamarck himself in his time, that is, without
-offering any sort of explanation as to how the minute impression made,
-say by the foot or by the proboscis of an insect, upon a flower, is to
-be handed on to the flowers of succeeding generations. He treats the
-unending chain of generations as if it were a single individual, and
-operates with his 'secular' stimulus, and with 'weak stimuli, lasting
-through countless generations,' as though they were a proved fact. But
-I have not even touched upon the question as to whether these 'stimuli'
-could produce the changes he ascribes to them, even if they were
-continually affecting the flower. How the scale-like covering leaves
-of the pollen vessels could become larger and petal-like through the
-treading of an insect's foot is as difficult to see as why a honey-tube
-should _become longer_ because of the butterfly's honey-sucking: might
-it not just as well become _wider_, _narrower_, or even _shorter_? I
-see no convincing reason why it should become _longer_! And even if
-it did so, it would necessarily continue to lengthen as time went on,
-and this is not the case, for we find corolla-tubes of all possible
-lengths, but, _it is to be noted, always in harmony with the length of
-the proboscis of the visiting insect_. In a similar way Henslow has
-recently attempted to refer the origin of flowers to the mechanical
-stimulus exercised upon it by the visiting insects. 'An insect hanging
-to the lower petal of a flower elongates the same by its weight, and
-the lengthened petal is transmitted by heredity.'...'The irritation
-caused by its feet in walking along the flower causes the appearance
-of colouring matter, and the colour is likewise transmitted.'...'As it
-probes for honey it causes a flow of sweet sap to that part, and this
-also becomes hereditary!'
-
-In this case, also, it is simply taken for granted that every little
-passing irritation not only produces a perceptible effect, but that
-this effect is transmissible. In a later lecture we shall have to
-discuss in detail the question of the inheritance of functional
-modifications. It is enough to say here that, if this kind of
-transmission really took place even in the case of such minute and
-transitory changes, there could be no dispute as to the correctness
-of the 'Lamarckian principle,' since every fairly strong and lasting
-irritation could be demonstrated with certainty to produce an effect.
-When a butterfly, floating freely in the air, sucks honey from a tube,
-the irritation must be almost analogous to that caused by a comb
-lightly drawn by some one through our hair, and this is supposed to
-effect the gradual lengthening of the corolla-tube of the flower!
-
-The secretion of honey, too, depends upon the persistent irritation
-of the proboscis! Then 'deceptive flowers,' like the Cypripedium we
-have mentioned, could not exist at all, for they contain no honey,
-although the proboscis of the bee must cause the same irritation in
-them as in other orchids which do contain honey. This whole 'theory' of
-direct effect is, moreover, only a crude and apparent interpretation,
-which explains the conditions only in so far as they can be seen from
-a distance; it fails as soon as they are more exactly examined; all
-the great differences in the position of the honey, its concealment
-from intelligent insects, its protection from rain by means of hairs,
-and against unwelcome guests by a sticky secretion, the development
-of a corolla-tube which corresponds in length to the length of the
-visiting insect's proboscis, the development of spurs on the flower,
-in short, all the numerous contrivances which have reference to
-cross-fertilization by insects remain quite unintelligible in the light
-of this theory--it is a mere _pis aller_ explanation for those who
-continue to struggle against accepting the theory of selection.
-
-
-
-
-LECTURE XI
-
-SEXUAL SELECTION
-
- Decorative colouring of male butterflies and birds--Wallace's
- interpretation--Preponderance of males--Choice of the females--Sense
- by sight in butterflies--Attractive odours--Scent-scales--Fragrance of
- the females--The limits of natural and sexual selection not clearly
- defined--Odours of particular species--Odours of other animals at
- the breeding season--Song of the Cicadas, and of birds--Diversity
- of decoration successively acquired--Humming-birds--Substitution
- of other aids to wooing in place of personal decoration--Smelling
- organs of male insects and crabs--Contrivances for seizing and
- holding the female--Small size of certain males--Weapons of males
- used in struggle for the females--Turban eyes of Ephemerids--Hoods
- that can be inflated on the head of birds--Absence of secondary
- sexual characters in lower animals--Transference of male characters
- to the females--Lycæna--Parrots--Fashion operative in the phyletic
- modifications of colour--Pattern of markings on the upper surface of a
- butterfly's wing simpler than on the under side--Conclusion.
-
-
-WE found in the process of Natural Selection an explanation of
-numerous effective adaptations in plants and animals, as regards form,
-colouring, and metabolism, of the most diverse weapons and protective
-devices, of the existence of those forms of blossoms which we call
-flowers, of instincts, and so on. The origin of the most characteristic
-parts of whole orders of insects can only be understood as adaptations
-to the environment brought about by means of natural selection.
-Impressed by this, we have now to ask whether _all_ the transformations
-of organisms may not be referred to adaptation to the continually
-changing conditions of life? We shall return to this question later,
-but in the meantime we are far from being able to answer it in the
-affirmative, for there are undoubtedly a great many characters, at
-least in animals, which cannot have owed their origin to natural
-selection in the form in which we have studied it so far.
-
-How could the splendid plumage of the humming-birds, of the pheasants,
-of the parrots, the wonderful colour-patterns of so many diurnal
-butterflies, be referred to the process of natural selection, since all
-these characters can have no significance for their possessors in the
-struggle for existence? Or of what use in the struggle for existence
-could the possession of its gorgeous dress of feathers be to the bird
-of Paradise; or of what service is the azure blue iridescence of the
-_Morpho_ of Brazil, which makes it conspicuous from a distance when
-it plays about the crowns of the palm-trees? We might indeed suppose
-that they are warning signs of unpalatableness, like those of the
-Heliconiides or of the gaily coloured caterpillars, but, in the first
-place, these gay creatures are by no means inedible, and are indeed
-much persecuted; and, secondly, the females have quite different and
-very much darker and simpler colours. The gleaming splendour of all
-these birds of Paradise and humming-birds, as well as that of many
-butterflies, is found in the male sex only. The females of the birds
-just mentioned are dark in colour and without the sparkling decorative
-feathers of the males; they are plain--just like the females of many
-butterflies. Alfred Russel Wallace has suggested that the explanation
-of this lies in the greater need of the females for protection, since,
-as is well known, they usually perform the labours of brooding, and are
-thus frequently exposed to the attacks of enemies. It is undoubtedly
-true that the dark and inconspicuous colouring of many birds and
-butterflies depends on this need for protection, but this does not
-explain the brilliant colours of the males of these species. Or can it
-be that these require no explanation further than that they are, so to
-speak, a chance secondary outcome of the structural relations of the
-feathers and wing-scales respectively, which brought with it some other
-advantage not known to us? Perhaps something in the same way as the red
-colour of the blood in all vertebrates, from fishes upwards, cannot be
-useful on the ground that it appears red to us, but because it is the
-expression of the chemical constitution of the hæmoglobin, a body which
-is indispensable to the metabolism, which here has the secondary and
-intrinsically quite unimportant peculiarity of reflecting the red rays
-of light.
-
-No one can seriously believe this in regard to butterflies who knows
-that their colours are dependent on the scales which thickly cover the
-wings, and the significance of which, in part at least, is just to give
-this or that colour to the wing. They are degenerate or colourless
-among the transparent-winged butterflies, and their colour depends
-partly on pigment, partly on fluorescence and interference conditioned
-by the fine microscopical structure of a system of intercrossing lines
-on faintly coloured scales. The scales of our male 'blue' butterflies
-(_Lycæna_) only appear blue because of their structure, while the brown
-scales of their mates are due to a brown pigment. If the pigment be
-removed from the scales of the female by boiling with caustic potash,
-and they be then dried, they do not look blue like those of the male;
-the scales of the male, therefore, must possess something which those
-of the female do not.
-
-Still less will any one be disposed to regard the marvellous splendour
-of the plumage of the male bird of Paradise, with its erectile
-collars--glistening like burnished metal--on the neck, breast or
-shoulders, with its tufts, with its specially decorative feathers
-standing singly out from the rest of the plumage, on head, wings, or
-tail, with its mane-like bunch of loose, pendulous feathers on the
-belly and on the sides, in short, with its extraordinary, diverse, and
-unique equipment of feathers, as a mere unintentional accessory effect
-of a feather dress designed for flight and protective warmth. Such
-conspicuous, diverse, and unusual specializations of plumage must have
-some other significance than that just indicated.
-
-Alfred Russel Wallace regards these distinctive features of the male
-as an expression of the greater vigour, and the more active metabolism
-of the males, but it is unproved that the vigour of the male birds
-is greater than that of the females, and it is not easy to see why
-a more active metabolism should be necessary for the production of
-strikingly bright colours than for that of a dark or protective colour.
-Moreover, there are brilliantly coloured females, both among birds
-and butterflies, and in nearly allied species the males may be either
-gorgeous or quite plain like the females.
-
-Darwin refers the origin of these secondary sexual characters to
-processes of selection quite analogous to those of ordinary natural
-selection, only that in this case it is not the maintenance of the
-species which is aimed at, but the attainment of reproduction by the
-single individual. The males are to some extent obliged to struggle
-for the possession of the females, and every little variation which
-enables a male to gain possession of a female more readily than his
-neighbour has for this reason a greater likelihood of being transmitted
-to descendants. Thus, attractive variations which once crop up will be
-transmitted to more and more numerous males of the species, and among
-these it will always be those possessing the character in question
-in the highest degree which will have the best chance of securing a
-mate, and so the character will continue to be augmented as long as
-variations in this direction appear.
-
-Two kinds of preliminary conditions, however, must be assumed. As the
-ordinary natural selection could never have operated but for the fact
-that in every generation a great many individuals, indeed the majority
-of them, perish before they have had time to reproduce, so the process
-of sexual selection could never have come into operation if every
-male were able ultimately to secure a mate, no matter what degree of
-attractiveness to the latter he possessed. If the numbers of males and
-females were equal, so that there was always one female to one male,
-there could be no choice exercised either by male or female, for there
-would always remain individuals enough of both sexes, so that no male
-need remain unmated.
-
-But this is not the case: the proportions of the sexes are very rarely
-as 1 : 1; there is usually a preponderating number of males, more
-rarely of females. Among birds the males are usually in the majority,
-still more so among fishes; and among diurnal butterflies there are
-often a hundred males to one female (Bates), although there seem to
-be a few tropical Papilionidæ among which the females have rather the
-preponderance. Darwin called attention to the fact that one could
-infer the greater rarity of the females even from the pricelists of
-butterflies issued by the late Dr. Staudinger in connexion with his
-business, for the females in most species, except the very common ones,
-are priced much higher than the males, often twice as high. In the
-whole list of many thousands of species there are only eleven species
-of nocturnal Lepidoptera in which the males are dearer than the females.
-
-Among the Mayflies or Ephemerides, too, the males are in the majority;
-in many of them there are sixty males to one female: but there are
-other kinds of insects, such as the dragon-flies (Libellulidæ), in
-which the females are three or four times as numerous. There are also,
-it may be remembered, some kinds of insects, such as Aphides, which
-have become capable of parthenogenetic reproduction, and in which the
-males are becoming extinct, e.g. in the case of _Cerataphis_ in British
-orchid-houses.
-
-The first postulate implied in 'sexual selection,' namely, that there
-be an unequal number of individuals in the two sexes, is therefore
-fulfilled in Nature; we have now to inquire whether the second
-condition postulated--the power of choice--may also be regarded as a
-reality.
-
-This point has been disputed from many sides, and even by one of the
-founders of the whole selection theory, Alfred Russel Wallace. This
-naturalist doubts whether a choice is exercised among birds by either
-sex in regard to pairing, and maintains that, even if there could be
-a choice, this could not have produced such differences in colour and
-character of the plumage, since that would presuppose the existence of
-similar taste in the females through many generations. In a similar way
-it has been doubted whether butterflies can be said to exercise any
-real power of sexual choice, whether a more beautiful male is as such
-preferred to a less beautiful suitor.
-
-It must be admitted that direct observation of choosing is difficult,
-and that as yet there is very little that can be said with certainty
-on this point. But there are, after all, some precise observations on
-mammals and birds which prove that the female shows active inclination
-to, or disinclination for, a particular male. If we hold fast to
-this fact, and add to it that the distinctive markings of the males
-are wonderfully developed during the period of courtship, and are
-displayed before the females, and that they only appear in mammals,
-birds, amphibians, and fishes at the time of sexual maturity, it seems
-to me that there can be no doubt that they are intended to fascinate
-the females, and to induce them to yield themselves to the males. The
-opponents of the theory of sexual selection attach too much importance
-to isolated cases; they imagine that each female must make a choice
-between several males. But the theory of sexual selection does not
-demand this, any more than the theory of natural selection requires the
-assumption that every individual of a species which is better equipped
-for the struggle for existence must necessarily survive and attain
-to reproduction, or, conversely, that the less well equipped must
-necessarily perish.
-
-All that the theory requires is, that the selective and eliminative
-processes do, _on an average_, secure their ends, and in the same way
-the theory of sexual selection does not need the assumption that every
-female is in a position to exercise a scrupulous choice from among a
-troop of males, but only that, on an average, the males more agreeable
-to the females are selected, and those less agreeable rejected. If this
-is the case, it must result in the male characters most attractive to
-the females gaining preponderance, and becoming more and more firmly
-established in the species, increasing in intensity, and finally
-becoming a stable possession of all the males.
-
-When we go more into details we shall see that the _particular
-qualities_ of the distinctive masculine characters are exactly such as
-they would be if they owed their existence to processes of selection;
-in other words, from this point of view the phenomena of the decorative
-sexual characters can be understood up to a certain point. It seems
-to me that we are bound to accept the process of sexual selection as
-really operative, and instead of throwing doubt upon it, because the
-choice of the females can rarely be directly established, we should
-rather deduce from the numerous sexual characters of the males, which
-have a significance only in relation to courtship, that the females of
-the species are sensitive to these distinguishing characters, and are
-really capable of exercising a choice.
-
-In my mind at least there remains no doubt that the 'sexual selection'
-of Darwin is an important factor in the transformation of species, even
-if I only take into consideration those secondary sexual characters
-which are related to wooing. We shall see, however, that there are
-others in regard to whose origin through processes of selection doubt
-is still less legitimate, and from which, on this account, we can argue
-back to the courtship characters.
-
-The first beginning of transformation is not, even in ordinary natural
-selection, to be understood as due to selection, but is to be regarded
-as _a given variation_ (the causes of which we shall discuss later on);
-it is only the increase of such incipient variations in a definite
-direction that can depend on natural selection, and they _must_ depend
-on it in so far as the transformations are purposeful. Now, all
-secondary sexual characters can be recognized as useful, save only the
-decorative distinctions, although these also undoubtedly represent
-intensifications of originally unimportant variations. Are we then
-to regard these alone as the mere outcome of the internal impulsive
-forces of the organism, while in the case of the analogous sexual
-characters for tracking, catching, and holding the female, and so
-forth, the augmentation and the directing must be referred to processes
-of selection? But if there be any utility at all in the decorative
-sexual characters it can only lie in their greater attractiveness to
-the females, and it can only be of any account if the females have,
-in a certain sense, the power of choice. Independently, therefore, of
-direct observations as to the actual occurrence of choosing, we should
-be compelled by our chain of reasoning to assume that there was such a
-power of choice--and I shall immediately discuss it more precisely.
-
-If we consider the decorative, distinctive characters of the males more
-closely, we find that they are of very diverse kinds. The males of many
-animals are distinguished from the females chiefly by greater beauty
-of form, and especially of colour. This is the case in many birds,
-some amphibians, like the water-salamander, many fishes, many insects,
-and above all, in diurnal Lepidoptera. Especially among birds the
-dimorphism between the sexes is in obvious relation to the excess in
-the number of male individuals, or--what practically comes to the same
-thing--to polygamy. For when a male attaches to himself four or ten
-females the result is the same as if the number of female individuals
-were divided by four or by ten. Thus the fowls and pheasants, which
-are polygamous, are adorned by magnificent colours in the male sex,
-while the monogamous partridges and quails exhibit the same colouring
-in both sexes. Of course 'beautiful' is a relative term, and we must
-not simply assume that what seems beautiful to us appears so to all
-animals; yet when we see that all the male birds which are beautifully
-decorative according to our taste--whether humming-birds, pheasants,
-birds of Paradise, or rock-cocks (_Rupicola crocea_)--unfold their
-'feather-wheels, 'fans,' 'collars,' and so forth, before the eyes of
-the females in the breeding season, and display them in all their
-brilliance, we must conclude that, in these instances at least, human
-taste accords with that of the animals. That birds have sharp vision
-and distinguish colours is well known; it is not for nothing that the
-service berries and many other berries suitable for birds are red, the
-mistletoe berries white, in contrast to the evergreen foliage of this
-plant, the juniper berries black so that they stand out amid the snows
-of winter; in this direction, then, there is no difficulty in the way
-of sexual selection.
-
-Even among much lower animals, like the butterflies, there seems to me
-no reason for the assumption that they do not see the gorgeous colours
-and often very complicated markings, the bars and eye-spots, on the
-wings of their fellows of the same species. Of course if each facet of
-the insect eye contributed only a single visual impression, as Johannes
-Müller supposed, then even an eye with 12,000 facets would give but a
-rough and ill-defined picture of objects more than a few feet away, and
-I confess that for a long time I regarded this as an obstacle in the
-way of referring the sexual dimorphism of butterflies to processes of
-selection. But we now know, through Exner, that this is not the case;
-we know that each facet gives a little picture, and not an 'inverted'
-but an 'upright' one, and experiment with the excised insect eye has
-directly shown that it throws on a photographic plate a tolerably clear
-image of even distant objects, such as the frame of a window, a large
-letter painted on the window, or even a church tower visible through it.
-
-Furthermore, the structure of the eye allows of incomparably clearer
-vision of near objects, for in that case the eyes act like lenses, and
-reveal much more minute details than we ourselves are able to make out.
-Here again, therefore, there is no obstacle to the Darwinian hypothesis
-of a choice on the part of the females, for although it cannot be
-demonstrated from the structure of the eye itself that insects see
-colour, and that colours have a specially exciting influence on them,
-yet we can deduce this with certainty from the phenomena of their life.
-The butterflies fly to gaily coloured flowers, and as they find in them
-their food, the nectar of the flowers, we may take for granted that the
-sight of the colour of their food-providing plants is associated with
-an agreeable sensation, and this is an indication that similar colours
-in their fellows may awaken similar agreeable sensations.
-
-[Illustration: FIG. 53. Scent-scales of diurnal butterflies. _a_, of
-Pieris. _b_, of Argynnis paphia. _c_, of a Satyrid. _d_, of Lycæna. All
-highly magnified.]
-
-This conclusion is furthermore confirmed by the fact that, in the male
-sex, numerous species of butterfly possess another means of exciting
-the females, namely, by pleasant odours. Volatile ethereal oils are
-secreted by certain cells of the skin, and exhale into the air through
-specially constructed scales. Usually the apparatus for dispersing
-fragrance occurs on the wing in the form of the so-called scent-scales
-(_Duftschuppen_), peculiar modifications of the ordinary colour-scales
-of the wing, but sometimes they take the form of brush-like hair-tufts
-on the abdomen, and they are in all cases so arranged that the volatile
-perfume from the cells of the skin penetrates into them, and then
-evaporates through very thin spots on the surface of the scale, or
-through brush-like, expanded fringes on their tips. Many of these have
-long been known to entomologists, because their divergence in form
-from the ordinary scales attracted attention; and it was also observed
-that they never occurred on the females, but only on the males. Their
-significance, however, remained obscure until, by a happy chance, Fritz
-Müller, in his Brazilian garden, discovered the fact that there are
-butterflies which give off fragrance like a flower, and then close
-investigation revealed to him the connexion between this delicate
-odour and the so-called 'male scales.' One can convince oneself of the
-correctness of the observation even in some of our own butterflies by
-brushing the finger over the wing of a newly caught male Garden White
-(_Pieris napi_). The finger will be found covered with a white dust,
-the rubbed-off wing-scales, and it will have a delicate perfume of
-lemon or balsam, thus proving that the fragrance adheres to the scales.
-
-[Illustration: FIG. 54. A portion of the upper surface of the wing of
-a male 'blue' (_Lycæna menalcas_); after Dr. F. Köhler. _bl_, ordinary
-blue scales. _d_, scent-scales. Highly magnified.]
-
-[Illustration: FIG. 55. _Zeuxidia wallacei_, male, showing four tufts
-of long, bristle-like, bright yellow scent-scales (_d_) on the upper
-surface of the posterior wing.]
-
-In the last case, that is, among the Whites (Pieridæ) (Fig. 53, _a_),
-the scent-scales are distributed fairly regularly over the upper
-surface of the wing, and the same is true of our blue butterflies, the
-Lycænnidæ whose minute lute-shaped scales are shown singly in Fig.
-53, _d_, but in their natural position among the ordinary scales in
-Fig. 54. In many other diurnal, and also in nocturnal Lepidoptera, the
-fragrant scales are united into tufts and localized in definite areas.
-They then often form fairly large spots, stripes, or brushes, which
-are easily visible to the naked eye. Thus the males of our various
-species of grass-butterflies (Satyridæ) have velvet-like black spots
-on the anterior wings, while the fritillary, _Argynnis paphia_, has
-coal-black stripes on four longitudinal ribs of the anterior wing
-which are absent in the females, and which are composed of hundreds of
-odoriferous scales. Certain large forest butterflies of South America,
-resembling our _Apatura_, bear in the middle of the gorgeous green
-shimmering posterior wing a thick expansible brush of long, bright
-yellow scent-scales, and a similar arrangement obtains in the beautiful
-violet butterfly of the Malay Islands, the _Zeuxidia wallacei_ depicted
-in Fig. 55. In many of the Danaides, which we have already considered
-in relation to mimicry, the scent apparatus is even more perfect, for
-it is sunk in a fairly deep pocket on the posterior wings, and in this
-the scent-producing, hair-like scales lie concealed until the butterfly
-wishes to allow the fragrance to stream forth. In many South American
-and Indian species of _Papilio_ the fragrant hairs are disposed in a
-sort of mane on a fold of the edge of the posterior wing, and so on.
-The diversity of these arrangements is extreme, and they are widely
-distributed among both diurnal and nocturnal Lepidoptera, in the
-latter sometimes in the form of a thick, glistening, white felt which
-fills a folded-over portion of the edge of the posterior wing. In many
-cases the perfume can be retained, and then, by a sudden turning out
-of the wing-fold, be allowed to stream forth. But there are a great
-many species of butterfly which do not possess odoriferous scales, and
-they are often wanting in near relatives of fragrant species; they are
-obviously of very late origin, and arose only after the majority of
-our modern species were already differentiated. It often seems as if
-they bore a compensatory relation to beauty of colour, somewhat in the
-same way as many modestly coloured flowers develop a strong perfume,
-while, conversely, many magnificently coloured flowers have no scent at
-all. Although among butterflies, as among flowers, there are species
-which possess both beauty and fragrance, yet our most beautiful diurnal
-butterflies, the Vanessas, the Apaturas, and Limenitis, possess no
-scent-scales; and many inconspicuous, that is, protectively coloured
-nocturnal Lepidoptera, are strongly fragrant, like most night-flowers:
-I need only mention the convolvulus hawk-moth (_Sphinx convolvuli_),
-whose musk-like odour was known to entomologists long before the
-discovery of scent-scales.
-
-It is, however, always only in the males that this odoriferous
-apparatus is present. It must not be believed on this account that this
-fragrance has the significance of a means of attraction comparable to
-the perfume of the flowers which induces butterflies to visit them;
-indeed, we cannot assume that the odour carries to a distance, for,
-as far as we can make out, it is perceptible only within a very short
-radius, and this is indicated also by the manifold arrangements of the
-odoriferous organs, which are all calculated to retain the fragrance,
-and then--in the immediate neighbourhood of the female--to let it
-suddenly stream forth. Obviously, this arrangement can have no other
-significance than that of a sexual excitant; its use is to incline the
-female to the male, to fascinate her, just as do the beautiful colours,
-in regard to which we must draw the same inference. It is in this
-direction that the already mentioned relation of compensation between
-beautiful colours and pleasant odours is particularly interesting, for
-it confirms our interpretation of the decorative colours as a means of
-sexual excitement. The most delicately fragrant or the most beautifully
-coloured males were those which most excited the females, and thus
-most easily attained to reproduction. The expression used by Darwin,
-that the females 'choose,' must be taken metaphorically; they do not
-exercise a conscious choice, but they follow the male which excites
-them most strongly. Thus there arises a process of selection among
-these distinctively male characteristics.
-
-If the odoriferous organs we have been discussing had merely been a
-means of attraction, serving to announce the proximity of a member of
-the species, then they should have occurred, not in the males but in
-the females, for these are sought out by the males, not conversely.
-The males are able to track their desired mates from great distances,
-and many remarkable examples of this are known, some of them indeed
-sounding almost fabulous. The females must therefore also exhale a
-fragrance, and perhaps continually, but it is much more delicate,
-carries extraordinarily far, and is quite imperceptible to our weak
-sense of smell. It is possible that it streams out from all the scales
-covering the wings and body, for, as I long ago pointed out, all the
-scales retain a connexion with the living cells of the skin, however
-minute these may be, and it is therefore quite possible that the cells
-produce scent imperceptible by us, and let it exhale through the
-ordinary scales, since the male scent-scales owe their ethereal oil to
-the large gland-like cells of the hypodermis on which they are placed.
-
-Here we see very clearly the difference between ordinary natural
-selection and sexual selection. The male odoriferous organs depend on
-the latter, for they do not serve for the maintenance of the species,
-but are of advantage in the courting competitions among the males for
-the possession of the females, while the assumed fragrant cells of the
-females must depend on natural selection, since they are of general
-importance for the mutual discovery of the sexes, which would otherwise
-be in most cases impossible. This hypothetical 'species scent,' as
-we may call it, is first of all useful in securing the existence of
-the species, and must therefore be referred to natural selection. The
-other, the 'male scent,' might be, and actually is, wanting in many
-species, although it may be necessary to reproduction in cases where it
-has become a male specific character, and could not be absent from any
-male without dooming him to sterility.
-
-That the 'species scent' really exists admits of no doubt, although
-we may be unable to perceive it. Entomologists have long been in the
-habit of catching the males of the rarer Lepidoptera, especially of the
-nocturnal forms, by freely exposing a captive female. Some years ago
-I kept for some time in my study, with a view to certain experiments,
-females of the eyed hawk-moth (_Smerinthus ocellatus_), and placed them
-at first, without any special intention, in a gauze-covered vessel near
-the open window. The very next morning several males had gathered and
-were sitting on the window-sill, or on the wall of the room close to
-the vessel, and by continuing the experiment I caught, in the course
-of nine nights, no fewer than forty-two males of this species, which
-I had never believed to be so numerous in the gardens of the town. The
-males of the nocturnal Lepidoptera obviously possess an incredibly
-delicate organ of smell, and its bearers, the antennæ, are usually
-larger and more complex in structure in the male sex than in the female.
-
-Butterflies are by no means the only creatures that produce a peculiar
-odour at the breeding season; many other animals do the same, though
-in their case it does not seem so pleasant to our sense of smell.
-It is true that the scent of the musk-deer and that of the beaver
-(_Castoreum_), when much diluted, are agreeable to man, but others,
-like the odours exhaled by stags or by beasts of prey, are very
-disagreeable to us, though they have for the species that produce them
-the same significance as the others, and are therefore to be referred
-to sexual selection.
-
-Darwin referred all the different _mechanisms for the production of
-sounds_, up to the song of birds, to sexual selection, but it is
-probable that natural selection has also to do with this in many ways.
-It is certainly only the males which produce the well-known song of the
-Cicadas, crickets, grasshoppers and birds, and I do not see any reason
-to doubt that this 'music' affects the females by arousing sexual
-excitement. To some extent, then, the rivalry among the males for the
-possession of the females--that is to say, sexual selection--must have
-produced these mechanisms of song; and how long-continued and gradual
-the accumulations must have been which produced the song of the thrush
-or of the nightingale from the chirping of the sparrow we may learn
-from the innumerable species which, as regards beauty of song, may be
-ranged between these two extremes.
-
-My assumption that natural selection has also been operative in the
-case of the song of insects and birds is based on the fact that many
-of our songsters live widely scattered, and that the characteristic
-note must be a means by which the two sexes find each other. That
-they should find each other is an indispensable condition for the
-maintenance of the species. Thus it is well known that each species has
-a characteristic 'note' or love-call, which the male utters during the
-breeding season, and which is answered by the female. From this simple
-love-call the modern song of many species must have developed by means
-of sexual selection.
-
-It is remarkable that here again the various distinguishing characters
-of the male seem to be often mutually restrictive or mutually
-exclusive. The best singers among our birds are inconspicuously
-coloured, grey or brown-grey, and this can hardly be regarded as due
-to chance, but as the outcome of a greater sensitiveness on the part
-of the females either to the song or to the beauty of their mates. And
-since, according to the theory, only those characters of the males
-could be increased which decided the choice, it therefore seems to
-me that this mutual exclusiveness of the two kinds of distinguishing
-characters is another indication of the reality of sexual selection.
-It proves--so at least I am inclined to believe--that the excitement
-of the female has been essentially affected by _only one_ of the
-characters of the male, that in the bird of Paradise it was mainly
-the brilliance of the plumage which roused excitement, while in the
-nightingale it was mainly the song.
-
-It might be objected to this that there are brilliant butterflies which
-also possess scent-scales. This is really the case; thus a magnificent
-blue iridescent _Apatura_ from Brazil has on the posterior wings a
-large yellow brush of scent-hairs, and even the beautiful blue males of
-our Lycænids have scent-scales in addition to their beautiful colour.
-But this can hardly be considered as a contradiction, but is rather
-an exception, which is the easier to explain since the odoriferous
-apparatus is a relatively simple arrangement, which did not require
-such a long series of generations for its evolution as the complicated
-song-box and brain-mechanism of the singing-birds.
-
-Moreover, it may also be that the scent-scales have arisen later
-than the decorative colouring, and they would do so the more easily
-since the brilliant blue, when once it was perfectly developed, and
-was common to all the males of the species in an equal degree, was
-no longer distinctive, and would have no specially exciting effect,
-while a novel preferential character in the male would have a much
-stronger effect. In the same way, the different parts of the body would
-be furnished in succession with decorative and, therefore, exciting
-distinctive characters. To understand this effect on the opposite
-sex we need only think of analogous phenomena in human kind, and of
-the strongly exciting effect that the sight of the secondary sexual
-characters of the woman has upon the man.
-
-By the successive additions of new decorative characters after the
-older ones became general and reached a climax, the origin of the
-extraordinary diversity of the decorative plumage in one and the same
-species of bird, can be readily understood, and the same is true of
-the complicated decorative coloration of the butterflies in so far as
-it depends on sexual selection, and not on other factors. The details
-did not arise all at once, but one after the other, and every character
-went on increasing till it had reached its limit of increase, but
-whenever it was common in its highest development to all the males
-it was no longer an object of preference or the cause of specially
-violent excitement, so that a new process of selection would begin in
-reference to some other part of the body. We thus understand how, among
-male birds of Paradise and humming-birds, such a marvellous diversity
-of colours and of decorative feathers is found combined in one and the
-same species.
-
-Whoever has seen the Gould Collection of humming-birds in London must
-have observed with amazement that among the 130 or so species of these
-beautiful little birds nearly every group of feathers in the body has
-been affected by the decorative colouring. In one species the little
-feathers on the region of the throat are emerald green, metallic blue,
-or rose; in another the feathers of the neck have been transformed into
-an erectile collar of rose-coloured feathers with a metallic sheen; or,
-again, it is the little feathers round the ear that stand erect and are
-brilliantly coloured. Sometimes we find that the feathers of the tail
-are lengthened, it may be only two of them, or the various lengths may
-be graduated like steps; sometimes the tail has assumed the form of a
-wedge, or is fan-like, or is shaped like the tail of a swallow, and all
-this in combination with the most diverse colours and patterns, black
-and white, ultramarine blue, and so forth. Or it may be the outermost
-tail-feathers which are the longest, the inner ones the shortest, or
-the four outer feathers are broad, pointed, directed outwards, and only
-half as long as the other two, which are very long and straight. Some
-species exhibit a sort of fine swan's down on the legs, others have a
-gorgeous metallic red cap on the head--in short, the variety is beyond
-description, just as we should expect it to be if now this and now that
-chance variation attracted the favourable regard of the selecting sex,
-and thus attained to its highest pitch of development.
-
-The decorative colouring of male birds may be replaced, not only by
-the power of song, but in other ways also. Not all the male birds
-of Paradise possess the familiar feather ornaments. The Italian
-traveller Beccari has called attention to a species, the males of
-which are simply coloured brown, like the females of other species.
-This _Amblyornis inornata_ entices its mate to itself in the pairing
-time in a very peculiar manner, for it arranges in the midst of the
-primitive forests of New Guinea a little 'love garden' or bower, a spot
-several feet in extent, strewn with white sand, on which it places
-shining stones and shells, and brightly coloured berries. In this case
-a special instinct has developed, which has replaced the personal
-charm of the bird in the eyes of the female. For this very reason the
-case seems to me to have some theoretical importance, for it serves
-indirectly to show that the personal excellences do actually function
-as a means of exciting and attracting, if any one should still doubt it.
-
-All the distinguishing characters of the male which we have hitherto
-considered have had reference to gaining the favour of the female, but
-there are many other secondary sexual characters which are employed in
-quite a different manner to secure possession of the female. I have
-already mentioned that in many butterflies the males possess a much
-larger organ of smell. The antennæ of the males of numerous beetles,
-such as the cockchafer and its relatives, are also much larger, and
-furnished with much broader accessory branches, than those of the
-female, and the same is the case in many of the lower crustaceans, like
-the large transparent Daphnid of our lakes, _Leptodora hyalina_. Here
-the anterior antenna bears (Fig. 56, _A_ and _B_, _at´_) olfactory
-filaments; in the female this appendage is small and stump-like, while
-in the male (_A_) it grows to a long, somewhat curved rod, which
-is extended obliquely into the water, and in addition to the nine
-olfactory filaments of the female (_ri_) bears from sixty to ninety
-more (_ri´_).
-
-[Illustration: FIG. 56. _Leptodora hyalina._ _A_, head of the male.
-_B_, head of the female. _Au_, eye. _g. opt_, optic ganglion. _gh_,
-brain. _at´_, first antenna with olfactory filaments _ri_ and _ri´_.
-_sr_, œsophageal nerve-ring. _n_, nerve. _m_, muscles.]
-
-In this and many other such cases it is not the struggle of the
-species for existence which has so markedly augmented this distinctive
-characteristic of the male; it is undoubtedly the struggle of the males
-among themselves, their competition for the possession of the females.
-In regard to decorative distinctions, the reality of a rivalry in
-wooing and the ultimate victory of the most decorative may perhaps be
-still doubted; but it is quite certain that, on an average, the male
-which can smell and track best will also gain possession of the females
-more easily than one less well equipped. Exactly the same is also true
-of those cases in which the male distinguishing character does not
-refer merely to finding the female, but to holding her fast, or, as we
-may say, to capturing her.
-
-Thus the males of the Copepods possess on their anterior antennæ an
-arrangement which enables them to throw a long whiplike structure
-like a lasso round the head of the female as she rapidly swims away.
-The antennæ of the male Daphnids, too, are in one genus (_Moina_)
-developed into a grasping apparatus, instead of into smelling organs as
-in _Leptodora_. Fig. 57 shows the male, Fig. 58 the female of _Moina
-paradoxa_; the first antennæ of the male are not only much longer and
-stronger than those of the female (_at_^1), but they are also armed
-with claws at the end, so that the males can catch their mates as with
-a fork, and hold them fast. And even that was not enough, for, in
-addition, the males of most Daphnids possess a large sickle-shaped but
-blunt claw on the first pair of legs (Fig. 57, _fkr_), which enables
-them to cling to the smooth shell of the female, and to clamber up on
-it to get into the proper position for copulation.
-
-[Illustration: FIG. 57. _Moina paradoxa_, male. _at^1_, first antennæ,
-with claws at the tip for capturing the female. _at^2_, second antennæ.
-_fkr_, claws on the first pair of legs for clambering. _gh_, brain.
-_lbr_, upper lip. _md_, mandible. _md_, mid-gut, with the liver lobes
-(_lh_). _h_, heart. _sp_, testis. _aft_, anus. _sb_, caudal setæ.
-_skr_, caudal claws. _sch_, shell. _schr_, cavity of the shell. _kie_,
-gill-plates. Magnified 100 times.]
-
-If we inquire into the manner of the origin of secondary sexual
-characters of this kind, we shall find that both may have been
-increased by sexual selection, for a male with a better sickle will
-succeed more quickly in getting into the proper position for copulation
-than one with a less perfect mechanism. This assumption does not rest
-on mere theory, for I was once able, by a happy chance, to observe for
-a considerable time, under the microscope, a female to whose shell two
-males were clinging, each trying to push the other off. Nevertheless it
-seems to me very questionable whether the origin of this sickle-claw
-can be referred to sexual selection, for without this clamping-organ
-copulation in most Daphnids would not be possible. It was thus not as
-an advantage which one male had over another that the clamping-sickle
-evolved, but rather as a necessary acquisition of the whole family,
-which must have developed in all the species at the same time as the
-other peculiarities, and notably those of the shell. The competition of
-the males among themselves is thus in this case simply an expression
-of the struggle for existence on the part of the species as such, and
-it is not a question merely of a character which makes it easier for
-the males to gain possession of the females, but of one which had
-necessarily to arise lest the species should become extinct. In other
-words, in this case natural selection and sexual selection coincide.
-
-[Illustration: FIG. 58. _Moina paradoxa_, female. The letters of Fig.
-57 apply _mutatis mutandis_. _brr_, brood-pouch. _ov_, ovary. _sr_,
-margin of shell.]
-
-The case of the antennæ of _Moina_, which have been modified into
-grasping organs, is quite different; these owe their origin not to
-natural selection, but to sexual selection, for antennæ of that kind
-are not indispensable to the existence of the species, as we can
-see from the closely related genera, _Daphnia_ and _Simocephalus_,
-where the males have quite short stump-like antennæ, furnished with
-olfactory filaments not much more numerous than the females possess.
-Just as these supernumerary olfactory filaments were produced by
-sexual selection, and not by the ordinary natural selection, because
-those males with the more acute sense of smell had an advantage over
-those in which it was blunter, so the males of the genus _Moina_
-which could grasp most securely had an advantage over those that
-gripped less firmly, and thus arose these two different kinds of male
-characteristics. Neither of them is of advantage to the species as
-such, but only to the males in their competition for the possession of
-the females.
-
-But, where the production of a novel character in the male is
-concerned, natural selection cannot proceed in a different manner
-from sexual selection; the process of selection is exactly the same:
-the better equipped males survive, the less well-equipped die without
-begetting offspring; the difference lies only in the fact that in the
-one case the improvement is in the species as such, in the other case
-only in one sex without the existence of the species being thereby
-made more secure. Such cases are instructive, because they make a
-denial of the process of sexual selection quite impossible if that of
-species-selection is admitted. If processes of selection are operative
-at all as factors in transformation, they must act even where the
-advantage is not to the species but only 'intra-sexual,' and the one
-process must often run into the other, so that it is often quite
-impossible to draw an exact line of demarcation between them.
-
-Numerous secondary sexual differences probably depend purely on species
-selection, that is to say, they include an improvement of the species
-in relation to the struggle for existence. We may find a case in point
-in the dwarf-like smallness of the males in many parasitic crustaceans,
-in some worms, in many Rotifers, and in the Cirripedes. It can hardly
-have been of advantage for the individual male to be smaller than
-his fellows, but it was of advantage for the species to produce as
-many males as possible in order to ensure a meeting with the females,
-and, as the enormous production of males made it advantageous for the
-species that as little material as possible should be used in their
-individual production, we can readily understand the minuteness of the
-males, and in some cases, as in the Rotifers and _Bonellia_, their
-poor equipment, lack of nutritive organs, and ephemeral existence. The
-marine worm, _Bonellia viridis_, whose female may be a foot in length,
-is not the only case in which a microscopically small male lives like
-a parasite inside the female. Among the round-worms, too, there is a
-species called _Trichosomum crassicauda_, discovered by Leuckart in
-the rat, the dwarf males of which live in the reproductive organs of
-the female. All these are arrangements for securing the propagation of
-the species, which might have been endangered if the males had had to
-seek out the females, which, in the case of _Bonellia_, live in holes
-in the rocks on the sea-floor, and, in the case of _Trichosomum_, are
-concealed in the urinary bladder of the rat. Obviously, this is the
-reason which, in addition to the one already mentioned, has conditioned
-and produced, or helped to produce, the remarkable minuteness of
-certain males.
-
-From another category of sexual differences we see in how many ways
-species-selection and sexual selection play into each other's hands.
-In many species of animals the males are eager for combat, and they
-are equipped with special weapons, or excel the females in general
-strength of body. As these males struggle, in the literal sense of the
-word, for the possession of the females, Darwin referred to sexual
-selection those distinguishing characters which gave the stronger male
-the victory over the weaker, and thus raised the victorious characters
-to the rank of general characters of the species. And it certainly
-cannot be doubted that, for instance, the strength and the antlers of
-the stag must have been increased through the combats which recurred
-every year at the breeding season, for the stronger always win in these
-battles. The case is the same with the strength and the weapons of many
-other male animals. The lion is effectively protected by his mane from
-the bite of a rival, and the same protective arrangement occurs in
-quite a different family of mammals--in an eared seal, which is called
-the 'sea-lion' for this very reason. Among the seals the secondary
-sexual characters are often very strongly developed, at least in all
-the polygamous species, for in these the struggle for the females is
-very keen. In the 'sea-lions' and 'sea-elephants' there are often fifty
-females to one male, and the latter are 'enormously larger' than the
-females, while in monogamous species of seal the two sexes are alike in
-size.
-
-Darwin has shown that actual combat for the females takes place among
-most mammals, not only among stags, lions, and seals, but even among
-the moles and the timid hares. Even among birds such combats occur, and
-this is sometimes particularly noteworthy in those species in which the
-males possess the most decorative colouring, like the humming-birds. In
-some cases among birds there has also been a development of weapons.
-Witness the spur of the cock, whose merciless combats with his rivals
-Man has, as is well known, made positively atrocious for his own
-amusement, by preventing the flight of the vanquished.
-
-In Darwin's great work on sexual selection a considerable number of
-cases are cited from among lower vertebrates, such as crocodiles
-and fishes, and even from insects, in which the males fight for
-the possession of the females, and exhibit distinctive masculine
-characters adapted to such combats. But I do not propose to enter upon
-a discussion of such cases, since my aim is rather to elucidate the
-relation between sexual selection and species-selection than to discuss
-all the phenomena of the former in detail. But the combats of males
-illustrate with particular clearness the relation of sexual selection
-and species-selection, since many of the weapons or protective
-arrangements which may have arisen through sexual selection imply at
-the same time an improvement to the species in relation to the struggle
-for existence. Thus greater strength or sharper and larger teeth in the
-males mean a gain to the species, and it is indifferent to the species
-whether the weaker males succumb to a strange enemy (species-selection)
-or to their stronger rivals (sexual selection), provided only that the
-better equipped survive and leave descendants similarly endowed.
-
-I have intentionally begun the consideration of sexual selection with
-the cases most difficult to interpret on this theory, with those which
-have called forth the greatest divergence of opinion--the decorative
-colours and forms, the song of birds and of insects, the alluring
-odours--in short, all the courtship-adaptations of the males; these are
-the most difficult to deal with, because it is not easy to demonstrate
-directly that the females _do_ choose. But if we revise them briefly
-in reverse order, I believe that all doubt as to the reality of choice
-on the part of the females will disappear. Thus the last-mentioned
-sexual characters of greater strength and greater perfection of weapons
-and defence in the males have been evolved by sexual selection in
-close co-operation with species-selection. We should have to deny
-species-selection altogether if we were to dispute this form of sexual
-selection, which is closely connected with pure species-selection,
-such, for instance, as is revealed in the production of dwarf males,
-where there does not seem to be any aid from sexual selection at all.
-
-Then came the cases in which the tracking and grasping organs of the
-males were strengthened or were increased in number, and here too
-species-selection may have had its share, for instance, in evolving
-the sickle-claws of the Daphnids, which were inevitably advanced and
-perfected through sexual selection, which must in this case have
-operated independently of any choice on the part of the female. In
-other cases the result may be referred to pure sexual selection,
-as in the grasping antennæ of the male _Moina_, or in the highly
-developed olfactory antennæ of the male _Leptodora_. That new organs,
-too, can arise in this way is shown by the 'turban eyes'--to which
-little attention has hitherto been paid--of some Ephemerids of the
-genera _Cloë_ and _Potamanthus_, which were long ago described by
-Pictet, the monographer of this family. These are large turban-shaped
-compound eyes, occurring beside the ordinary eyes in the males alone,
-which in these genera are in a majority of sixty to one. Whole swarms
-of these males fly about over the water on the search for females,
-and their highly developed organ of vision seems to decide matters
-for them just as the organ of smell does for _Leptodora_. Neither of
-these sense-organs can have any other advantage than that of making
-their possessors aware of the female, for the whole activity of the
-short-lived adult Ephemerides is limited to reproduction; they take no
-food, and have nothing whatever to do except to reproduce.
-
-Finally, when in an enormous number of cases we find in addition to one
-or the other of the already mentioned male distinguishing characters
-some which do not directly lead to gaining possession of the female,
-but do so only by sexually exciting her, can we doubt that the same
-principle has been operative, that here too processes of selection
-are fundamental, depending on the fact that in the wooing of the
-female the successful male is the one who most strongly excites her?
-There is no question of æsthetic pleasure in this, as the opponents
-of the theory of sexual selection have often urged, but only of
-sexual excitement, which may be aroused by very different means, by
-colours and shapes, but also by love-calls, songs, or odours. There
-are a few tropical birds (_Chasmorhynchus_) which have as the only
-distinguishing character of the male sex a hollow and soft appendage
-several inches long borne on the head. Usually it hangs down limply at
-the side of the head, but during the breeding season it is inflated
-from the mouth-cavity, and then stands erect like a spur. One species
-of this genus has as many as three of these horns, one of which is
-upright, while the other two stand out laterally from the head. Can it
-be supposed that these remarkable horns satisfy the female's 'sense
-of beauty'? To human beings they appear rather ugly than beautiful,
-both when limp and when inflated, but at any rate they are striking,
-and will be regarded by the female bird as something out of the
-common, and, since they are only fully displayed during the breeding
-season, that is, when the male is sexually excited, they will have an
-exciting effect on the female too. These inflated horns are symptoms
-of excitement, and they arouse it in the female. In exactly the same
-way the decorative feathers, the ruby-red and emerald-green feather
-collars of the humming-birds and birds of Paradise, are only erected
-and displayed when the males are wooing, and they, too, act as signs
-of excitement. This is not to say that the gorgeousness of colour,
-the eye-spots on the train of the peacock and the Argus pheasant, and
-the hundreds of different kinds of beautiful feathers, do not also
-exercise a fascinating influence; on the contrary, we cannot avoid
-assuming this, since otherwise we could find no sufficient reason for
-their origin. But the primary effect in wooing is not due to the mere
-pleasure in the sight, or in the odour, or in the song, but to the
-contagious excitement which these express. The females do not behave as
-dispassionate judges, but as excitable persons which fall to the lot of
-the male who is able to excite them most strongly. It may be, however,
-that a sense of æsthetic satisfaction in perceiving such symptoms of
-excitement may also have been evolved as an accessory effect, at least
-in the higher and more intelligent animals.
-
-In the lower animals, which are lacking not only in intelligence but
-also in the higher and more complex differentiation of the sensory
-system, the development of such secondary sex characters is rare or
-altogether absent. Animals which have no sense of hearing can develop
-no song, and animals which do not see cannot acquire gorgeous colours
-as a means of exciting one sex through the other. But distinctive sex
-coloration may arise even in lowly animals, though there can be no
-question of æsthetic pleasure associated therewith; if the animals are
-able to see the colours at all, sexual excitement may be associated
-with these.
-
-We need not wonder, therefore, that in the somewhat stupid fishes, in
-the butterflies, and in the lower crustaceans, like the Daphnids, we
-still find brilliant colours, which we can hardly interpret otherwise
-than as the results of sexual selection. On the other hand, the absence
-of such characters in animals of a still lower order, with still
-simpler sense-organs, like the Polyps, Medusæ, Echinoderms, most Worms,
-and the Sponges, affords an indirect confirmation of the correctness
-of our view as to the reality of a sexual selection in the more highly
-organized animals.
-
-We see, then, that numerous peculiarities which distinguish the
-males of a species from the females depend on the process of sexual
-selection. This may be said of ornamental outgrowths, colours,
-remarkable feathers and feather-groups, peculiar odoriferous organs,
-vocal organs, artistic instincts, and also weapons, like antlers,
-tusks, and spurs, notable size and strength of body, and protective
-devices like manes; and again, the various organs for catching and
-holding the females, or for finding them out by sight or smell, must
-also be referred, at least in part, to sexual selection. The diversity
-of the male sexual characters is so great that I cannot give more than
-a faint idea of them without entering on a long catalogue; whoever
-wishes a complete survey has only to consult Darwin's _Descent of Man_.
-
-But the significance of sexual selection is by no means exhausted with
-the production of the male sexual characters, for these characters are
-often more or less completely transferred to the females, and thus give
-rise to a transformation of the whole species, and not only of the male
-section of it. This is obviously a very important consequence of sexual
-selection, one which, as we shall see, materially deepens our insight
-into the mode of origin of new species.
-
-First let us try to determine the facts. Many male characters are not
-represented in the female in any degree, and therefore have never
-been transmitted to them at all. Such are the mane of the lion, the
-grasping antennæ of _Moina_, the turban eyes of the Ephemerides, the
-intensification of the sense of smell in _Leptodora_, the lasso-like
-antennæ of the Copepods, the scent-scales of the butterflies, and the
-musk glands of the alligators and stags. But in other cases there has
-been transmission, though only to a slight extent. Thus many female
-humming-birds have a faint indication of the magnificent metallic
-colouring of the males; many female blue butterflies have a tinge of
-the beautiful blue of their mates; the females of the stag-beetle
-(_Lucanus cervus_) possess a diminutive suggestion of the antler-like
-jaws of the male, and the female crickets, although they do not chirp,
-have a slight indication of the 'musical' mechanism of the male on the
-wing-coverts, and some of them even produce feeble notes at certain
-times.
-
-It can be proved, however, that such transmissions may, in the
-course of many successive generations, become intensified until the
-characters are exhibited by the females in the same degree as in the
-males. I know no better example of this than that afforded by the
-beautiful butterflies of the genus _Lycæna_. In this genus, which is
-rich in species and widely distributed over the whole earth, and must
-therefore be an old one, the upper surface of the wing is blue in by
-far the greater number of species, at least in the male sex. But there
-are three or four species which are dark-brown, and quite or nearly
-alike in the two sexes; such are the species _Lycæna agestis_, _L.
-eumedon_, _L. admetus_, and others. Everything indicates that this is
-the primitive colour of the genus. Moreover, there are some species
-with brown females, in which the males are not completely blue, but
-which have a slight bluish tinge, like _L. alsus_, the smallest of
-our indigenous Blues. Then follows a host of beautiful species, like
-_L. alexis_, _L. adonis_, _L. damon_, _L. corydon_, and many others,
-with brown females, and among these there occasionally occur females
-more or less tinged with blue. These lead on to _L. meleager_, which
-has two forms of female, a common brown and a rarer blue; and thus
-we reach _L. tiresias_, _L. optilete_, and _L. argiolus_, in which
-all the females are blue, although less intensely and completely so
-than their mates. The climax of this evolutionary series is reached
-by some species like _L. beatica_, belonging to tropical or at least
-warm countries, in which both sexes are of an equally intense blue. As
-we know that, in species with an excess of males, sexual characters
-always begin in the males, there can be no doubt as to the direction
-of evolution--from brown to blue--in this series. Furthermore, the
-entire absence of scent-scales in most of the species with brown males
-indicates the great age of these species, for, as far as I have been
-able to investigate, all the males of the blue species possess them.
-
-Darwin regarded this transferring of the male characters to the females
-as due to inheritance, and it really seems as if it were simply a case
-of transmission by inheritance to one sex of what has been acquired
-by the other. Yet we have to ask whether we can continue to regard
-the facts in this light. In any case this 'transmission' is not an
-inevitable physiological process, necessarily resulting from the
-intrinsic conditions of inheritance, for we see that it often does
-not occur, even in many cases in which we can see no external reasons
-why it should not do so, though in other cases the failure may be
-presumably correlated with the external conditions of life. Thus, for
-instance, the persistent retention of the brown colour in the majority
-of our female Lycænidæ has probably its reason in the greater need
-of protection on the part of the much rarer females, and this must
-be so also in the case of many birds in which the brilliant colours
-of the males have not been transferred to the females. Wallace first
-pointed out that all birds whose females brood in exposed nests are
-inconspicuously coloured in the female sex, even if the males are
-brightly coloured, while those whose nests are concealed in holes of
-trees or the like, or which build domes over them, not rarely exhibit
-brilliant colouring in both sexes. This is the case in woodpeckers and
-parrots, while the gallinaceous birds, which brood in the open, have
-usually inconspicuously coloured females, for the most part very well
-adapted to their surroundings.
-
-If we grasp the fact that a transference of the characters which have
-arisen through sexual selection can take place, we have a valuable
-aid in the interpretation of many phenomena which would otherwise
-remain quite inexplicable. What is the meaning of the gay colours of
-the parrots, which occur in such incredibly diverse combinations
-in this large and widely distributed family? Or of the marvellously
-complex markings and colour-patterns of the butterflies? In some cases
-they may be protective, as is the green of many parrots; in others,
-warning signs of unpalatability, like the bright colours and contrasted
-markings of many Heliconiidæ and Eusemiidæ and other butterflies with
-a nauseous taste; but there remain a great many cases to which neither
-of these explanations applies, which could only be regarded as pure
-freaks of nature if we did not know that male sexual characters can
-be transferred to the females, and that thus all the individuals of a
-species can be totally altered in their colouring.
-
-Thus the occurrence not only of conspicuous, but of complicated,
-coloration is explained.
-
-Darwin has shown that, in the equipment developed by the males in their
-competition for the possession of the females, it is by no means only
-those characters which may be considered 'beautiful' in themselves that
-have to be considered; it is rather the striking characteristics which
-mark their possessor and distinguish it from others that are primarily
-important. In fact, it is the principle of 'mode' or 'fashion' which is
-operative; something new is demanded, and as far as possible something
-quite different from that which was previously considered beautiful.
-Thus the starting-point for such processes of selection may have been
-afforded by white spots on a black ground, or, indeed, by any light
-spots on a dark ground, which may have been the primitive colour in
-most cases. If in the course of a long series of generations these
-spots became the common property of all the males, a possibility of
-further change was opened up as soon as a new contrast cropped up as
-a chance variation, which would then, under favourable conditions, be
-the starting-point of a new process of selection. Darwin has cited
-some cases in which, from a comparison of the dress of the young bird
-with that of the adult, we may conclude that a transformation of the
-colouring of the whole plumage must have taken place in the course of
-the phylogenetic history.
-
-In other cases the course of the process of selection has been such
-that, though the general colouring has not been changed, variations
-have appeared in particular regions of the body--spots or stripes which
-accumulated through the ages and co-operated to form an increasingly
-diverse and complex colour-scheme, such a 'marking' of the animal as we
-may observe to-day, especially in butterflies, but also in birds.
-
-It is a fine corroboration of the origin of bright colours through
-sexual selection that, even in those groups of the animal kingdom which
-are in general sexually monomorphic, there always occur some species
-in which male and female are quite different, and a host of species in
-which both sexes are alike in the main, yet with differences in certain
-minor points. Among the parrots similarity of colouring prevails as
-a general rule, but in New Guinea there lives a parrot the female of
-which is a gorgeous blood-red and the male a beautiful light-green;
-minor differences occur in many species, for instance, the female
-of the horned parrot (_Cyanorhamphus cornutus_ Gm.) lacks the two
-long black and red feathers on the head, that of the grass-parakeet
-(_Melopsittacus undulatus_) is a slightly paler green and has not
-the beautiful blue spots on the cheeks which the male possesses.
-Innumerable similar instances might be cited, serving to show that all
-these distinguishing characters of the males have been acquired step by
-step and piece by piece, and are slowly and independently transferred
-to the females--if, indeed, at all.
-
-In yet another way the correctness of the Darwinian theory of sexual
-selection may be deduced from the markings and coloration of birds and
-butterflies.
-
-It has frequently struck me, during the long period in which I have
-been studying brightly coloured birds and butterflies, that those
-colour-patterns which are referable to sexual selection are much
-simpler than those which must be referred to species-selection,
-especially in the case of what we call 'sympathetic coloration.' How
-crude is the decorative pattern of most parrots, notwithstanding all
-the brilliance of their colour. Large tracts of the body are red,
-others green, yellow, blue, and occasionally one finds a red and
-blue striped feather collar, a head which is red above and yellow
-underneath, but it is seldom that the colours vary enough in a small
-space to give rise to a delicate decorative pattern. The gayest of
-parrots are the Brush Tongues (_Trichoglossus_), and even among them
-subtlety of coloration does not go further than the combination of
-three colours on one of the long tail-feathers, or the production of
-a double band round the neck, and so forth. If we compare with this
-the complex markings of the inconspicuously coloured females of the
-pheasants, of the partridges, or that of the upper surface of the
-many birds in mingled grey, blackish-brown and white, which resemble
-the ground or the dried leaves when they crouch, we find that the
-colour-pattern in these cases is infinitely finer and more complex.
-
-This seems to me quite intelligible when we remember, on the one hand,
-that species-selection must operate far more intensively than sexual
-selection, and that in the production of a protective colouring it is
-a question of deceiving the eye of a sharp-sighted enemy, while the
-aim of sexual selection is to secure the approval of others of the
-same species. As long as the enemy on the search for prey perceives
-the difference between the markings of its victim and those of the
-surroundings, so long will the gradual and steady improvement of the
-protective coloration continue, so long will new shades and new lines
-be added. We can thus understand how there would be gradually reached
-a complexity of marking to which sexual selection can never attain,
-or at least only in regard to a few specially favourable points.
-The eye-spots on the train feathers of the Argus pheasant and the
-peacock are such points, and these occur among polygamous birds in
-which sexual selection must be very intense; they are placed, too,
-on a part of the body, the wheel-shaped train, which is peculiarly
-suited for communicating the excitement of the male to the female, and
-must therefore be especially influenced by the latter. In general,
-however, we may say on _a priori_ grounds that the intensity of
-species-selection is greater than that of sexual selection, because the
-former ceaselessly and pitilessly eliminates the less perfect, while
-the claims of the latter are in any case less imperative, and are also
-often mollified by a variety of chance circumstances.
-
-But in the case of insects, in particular, we have to add that the
-protective colours and the decorative colours have been, so to speak,
-painted by different artists--the former by birds, lizards, and other
-persecutors endowed with well-developed eyes, the latter by the insects
-themselves, whose eyes can hardly possess, for objects not quite near,
-that acuteness of vision which the bird's eye has. Thus we find that
-the protective coloration of butterflies has often a very complex
-marking, while the same butterfly may exhibit only a rather crude
-though brilliant pattern on its upper surface, where the coloration is
-due to sexual selection. Thus the famous _Kallima_ has on its under
-surface the likeness of a dry or decayed leaf composed of a number
-of colour-tones--quite a complex painting. But if we look at the
-upper surface we see a deep brown with a shimmer of steel blue as the
-ground-colour of the wings, and on it a broad yellow band and a white
-spot: that is the whole pattern. We find a similar state of things
-among many of the forest butterflies of Brazil, and also among our
-indigenous butterflies. The pattern of our gayest diurnal butterflies,
-the red Admiral and the tortoiseshell butterfly (_Vanessa atalanta_ and
-_Vanessa cardui_), is somewhat crude on the upper surface, and very
-simple compared with the protective colouring of the under surface,
-which is made up of hundreds of points, spots, strokes, and lines of
-every shape and colour. On the other hand, the upper surface of the
-anterior wings in the hawk-moths and the Noctuidæ exhibits protective
-coloration, and is made up of curious zigzag complex lines, strokes,
-and spots, so that it resembles the bark of a tree or a bit of an old
-wooden fence--a painting, like the modern impressionist work, which,
-with an apparently meaningless confusion of colour splashes, conveys a
-perfect impression even of the details of a landscape. In the owl-moths
-(Noctuidæ) the wing surfaces, which are brightly coloured, are simple,
-almost crude, in pattern, as in the moths of the genus _Catocala_, with
-their red, blue, or yellow posterior wings, traversed by a large black
-band; while in the Geometer-moths, whose wings are spread out flat when
-at rest, the protective upper surface of all four wings is covered
-with a complex pattern of lines, spots, and streaks in different
-shades of grey, yellow, white, and black, so that it bears a deceptive
-resemblance to the bark of a tree or the side of a wall. For a long
-time I could not understand how such a definite and constant pattern
-could arise through natural selection if it was a case of mimicking the
-impression of bark or of any other irregularly covered surface, the
-colours of which are not mingled in exactly the same way everywhere.
-But now I think I understand it; for in the apparently meaningless
-colour-splashes of an 'impressionist' landscape the different splashes
-must be exactly where they are, otherwise on stepping back from the
-picture one would see, not a Haarlem hyacinth-field, or an avenue of
-poplars with their golden autumn leaves, but a mere unintelligible
-daub. It is the _type_ of the colour-pattern that must be attained,
-and in nature this is attained very slowly, step by step, spot after
-spot, and therefore, obviously, no correct stroke once attained will
-be given up again, since, in combination with the rest, it secures
-the proper type of colour-pattern. Only thus, it seems to me, can we
-understand how apparently meaningless lines, like the figures 1840 on
-the under surface of _Vanessa atalanta_, could have become a constant
-characteristic of the species.
-
-To sum up briefly, we may say that sexual selection is a much more
-powerful factor in transformation than we should at first be inclined
-to believe. It cannot, of course, have been operative in the case of
-plants, nor can it be taken into consideration in regard to the lower
-animals, for these, like the plants, do not pair, or, at any rate,
-do so without any possibility of choice. Animals which live on the
-sea-floor, or which are attached there, must simply liberate their
-reproductive cells into the water, and cannot secure that they unite
-with those of this or that individual. This is the case among sponges,
-corals, and Hydroid polyps. In some other classes the sense organs are
-too poorly developed, and the eyes in particular too imperfect to be
-excited in different degrees by any peculiarities in the appearance
-or behaviour of the males. This is what Darwin meant when he ascribed
-to these animals 'too imperfect senses and much too low intelligence'
-'to estimate the beauty or other attractive points of the opposite
-sex, or to feel anything like rivalry.' Accordingly, in the Protozoa,
-Echinoderms, Medusæ, and Ctenophores, secondary sexual characters are
-entirely absent, as pairing also is.
-
-In those worms that pair we first meet with secondary sexual
-characters, and from this level upwards they are never quite absent
-from any large group, and gradually play an increasingly important rôle.
-
-But the significance of sexual selection lies, as we have seen,
-not only in the fact that one sex of a species, usually the male,
-is modified, but in the possibility of the transference of this
-modification to the females, and further, in the fact that the process
-of variation may start afresh at any time, and thus one variation may
-be developed upon or alongside of another. In this way we can explain
-certain complex and often fantastic forms and colourings which we could
-not otherwise understand; thus the extraordinary number of nearly
-related species in some animal groups, such as butterflies and birds,
-in which the differences mainly concern the colour-patterns.
-
-Darwin has shown convincingly that a surprising number of characters
-in animals, from worms upwards, have their roots in sexual selection,
-and has pointed out the probability that this process has played an
-important part in the evolution of the human race also, though, in this
-case, all is not yet so clearly and certainly known as among animals.
-
-To conclude this section, I should like once again to call attention
-to the deficiency which is necessarily involved in the assumption
-of any selection, sexual selection included, namely, that the first
-beginning of the character which has been intensified by selection
-remains obscure. Darwin attached importance to the occurrence of
-ordinary individual variation, but it is open to question whether the
-insignificant variations thus produced could give an adequate advantage
-in the competition for the possession of the females; and, further,
-whether we have not grounds for the assumption that larger variations
-also occur. This question may also be asked in regard to ordinary
-natural selection, although in that case we can imagine the beginnings
-to be smaller, since here the advantage of a variation lies only in
-the fact that it is useful, not in its being appreciated by others.
-As a matter of fact, this very difficulty as to the first beginnings
-of variations has been frequently urged against both hypotheses of
-selection, and rightly so, inasmuch as this must be above all else
-the point of attack for further investigations. But it is a mistake
-to deny the whole processes of selection simply because this point is
-not yet clear. Later on we shall attempt to gain some insight into
-the causes of variation, and then we shall return to this question
-of the beginnings of the selective processes. In the meantime let it
-suffice to say that Darwin was very well aware that, in addition to the
-ordinary individual variations, there were also larger deviations which
-occurred discontinuously in single forms. He believed, however, that
-such occurrences were very rare, and, on the whole, he was not inclined
-to ascribe to them any particular importance in the transformation of
-species. He rather referred the organic transformations which have
-taken place in the course of the earth's history, in the main, to the
-intensification of the ordinary individual variations, and I believe
-that he was right in so doing, since adaptations from their very nature
-cannot have been brought about by sudden chance leaps in organization,
-but can only have become exactly suited to chance conditions of life
-through a gradual accumulation of minute variations in the direction
-of utility. Whether, however, purely sexual distinctions may not have
-had their primary roots in discontinuous variations must be inquired
-into later. Theoretically, there is nothing against this assumption,
-when such characters are not adaptations like the lasso antennæ of
-the Copepods, or the turban eyes of the Ephemerids; mere distinctive
-markings, decorative coloration, peculiar outgrowths, and the like,
-may, if they arose discontinuously, very well have formed the basis for
-further sexual selection, as long as they were not prejudicial to the
-existence of the species.
-
-
-
-
-LECTURE XII
-
-INTRA-SELECTION OR SELECTION AMONG TISSUES
-
- Does the Lamarckian principle really play a part in the
- transformations of species?--Darwin's position in regard to this
- question--Doubts expressed by Galton and others--Neo-Lamarckians
- and Neo-Darwinians--Results of exercise and practice: functional
- adaptation--Wilhelm Roux, _Kampf der Theile_.
-
-
-WE have devoted a whole series of lectures to studying the
-Darwin-Wallace principle of Natural Selection and the range of its
-operation. It seemed to us to make innumerable adaptations intelligible
-up to a certain point. We now understand how the purposefulness, which
-we meet with everywhere among organisms, can have arisen without
-the direct interference of a Power working intentionally towards an
-end--simply as the outcome and result of the survival of the fittest.
-The two forms of the processes of selection, 'natural selection' in
-the narrower sense, and 'sexual selection,' dominate, so to speak, all
-parts and all functions of the organism, and are striving to adapt
-these as well as possible to the conditions of their life. And although
-the range of operation of Natural Selection is incomparably greater,
-because it actually affects every part, yet we must attribute to sexual
-selection also, at least among animals, a range of influence by no
-means unimportant, since through it, as far as we can see at present,
-not only do the secondary sexual characters in all their diversity
-arise, but by the transference of these to the other sex that too is
-modified, and thus the whole species may be influenced, and may indeed
-be started afresh on an unlimited series of further transformations.
-
-But although the processes of selection play such an important part in
-the transformations of the forms of life, we have to inquire whether
-they are the _sole_ factors in these transformations, whether the
-accumulation of chance variations in the direction of utility has been
-the sole factor in bringing about the evolution of the animate world,
-or whether other factors have not also co-operated with it.
-
-We are all aware that Lamarck regarded the direct influence of use
-and disuse as the most essential factor in transformation, and that
-Darwin, though hesitatingly and cautiously, recognized and accepted
-this factor, which he believed to be indispensable. Indeed, it seems
-at first sight to be so. There is a whole range of facts which seem to
-be intelligible only in terms of the Lamarckian theory; in particular,
-the existence of numberless vestigial or rudimentary organs which
-have degenerated through disuse, the remains of eyes in animals which
-live in darkness, of wings in running birds, of hind legs in swimming
-mammals (whales), and of ear muscles in Man, who no longer points his
-ears, and so forth through a long list.
-
-According to Wiedersheim, there are in Man alone about two hundred of
-these vestigial or rudimentary organs, and there is no higher animal
-which does not possess some. In all, therefore, a piece of the past
-history of the species is embodied in the actually existing organism,
-and bears witness to the fact that much of what the ancestors possessed
-is now superfluous, and is either transformed, or is gradually set
-aside, or is still in process of being set aside. It seems obvious that
-this gradual dwindling and degeneration of an organ no longer needed
-cannot be explained through natural selection in the Darwin-Wallace
-sense, for the process goes on so exceedingly slowly that the minute
-differences in the size of an organ, which may occur among individuals
-of the species at any given time during the retrogressive process,
-cannot possibly have a selection value. Whether the degenerate and
-now functionless hind leg of the whale is a little larger or a little
-smaller can have no importance in the struggle for existence; the
-smaller organ cannot be considered either as a lesser hindrance in
-swimming or as a greater economy of material, and the case is the same
-in regard to most other instances of degeneration through disuse. We
-therefore require another interpretation, and at first sight this seems
-to be supplied by the Lamarckian principle.
-
-But the reverse process, the strengthening, the enlarging, and the more
-perfect development of a part, very often goes on proportionately to
-its more frequent use, and here again the Lamarckian principle seems to
-afford a simple explanation. For we know that exercise strengthens a
-part, as disuse weakens it, and if we could assume that these results
-of use and disuse were transmitted from the individual who brought them
-about or 'acquired' them in the course of his life to his offspring,
-then there would be nothing to object to in the Lamarckian principle.
-But it is precisely here that the difficulty lies. Can we assume such
-a transmission of 'acquired' characters? Does it exist? Can it be
-demonstrated?
-
-That Lamarck did not even put this question to himself, but assumed
-such transmission as a matter of course, is readily intelligible
-when we consider the time at which he lived. He was himself one of
-the first to grasp the idea of the transmutation-hypothesis, and he
-was only too glad to have any sort of principle of interpretation
-ready to work with. But Charles Darwin, too, attributed a not
-inconsiderable influence to this principle, although the transmission
-of 'acquired' characters which it took for granted was not accepted
-without reflective hesitation. He even directed his own particular
-theory of heredity, as we shall see, especially to the explanation of
-this supposed form of inheritance, and we can very well understand
-this, after what I have said as to the impossibility of explaining
-the disappearance of organs which have become superfluous by the
-Darwin-Wallace theory of Natural Selection. Darwin needed the
-Lamarckian principle for the explanation of these phenomena, and it
-was this that decided him to assume the transmission of 'acquired'
-characters, although the proofs of it can hardly have satisfied him.
-For when we are confronted with facts which we see no possibility
-of understanding save on a single hypothesis, even though it be an
-undemonstrable one, we are naturally led to accept the hypothesis, at
-least until a better one can be found. It is in this way, obviously,
-that we are to understand Darwin's attitude to the Lamarckian
-principle; he did not reject it, because it seemed to him to offer the
-only possible explanation of the disappearance of characters which have
-become useless; he adhered to it, although the transmission of acquired
-characters which it assumed must have seemed, and, in point of fact,
-did seem to him doubtful, or at least not definitely proved. Doubts,
-some faint, some stronger, as to this assumed form of inheritance were
-hardly expressed till somewhat late in the day--almost twenty years
-after the appearance of the _Origin of Species_--first by Francis
-Galton (1875), then by His, who definitely declared himself at least
-against any inheritance of mutilations, and by Du Bois-Reymond, who,
-in his address _Ueber die Uebung_ in 1881, said: 'If we are to be
-honest, we must admit that the inheritance of acquired characters is a
-hypothesis inferred solely from the facts which have to be explained,
-and that it is in itself quite obscure.'
-
-This is how it must appear to every one who examines it simply in
-respect of its theoretical possibility, its conceivability. This is how
-it appeared to me when I attempted, in 1883, to arrive at clearness on
-the subject, and I then expressed my conviction that such a form of
-inheritance was not only unproved, but that it was even theoretically
-unthinkable, and that we ought to try to explain the fact of the
-disappearance of disused parts in some other way, and I attempted to
-give an explanation, as will be seen later.
-
-Thus war was declared against the Lamarckian principle of the direct
-effect of use and disuse, and there arose a strife which has continued
-down to the present day, the strife between the Neo-Lamarckians and the
-Neo-Darwinians, as the two disputing parties have been called.
-
-In order to form an independent opinion in regard to this famous
-dispute, it is, first of all, necessary to examine what actually takes
-place when an organ is exercised or is left inactive, and further,
-whether we can assume that the results of this exercise or inaction can
-be transmitted to descendants.
-
-That exercise in general has a strengthening, and neglect of it a
-weakening influence on the relevant organ has long been known and is
-familiar to all; gymnastics make the muscles stronger, the thickness of
-the exercised muscle and the number of its fibres increases; the right
-arm, which is much more used than the left, is capable of performing
-twenty per cent. more work. Similarly, the activity of glands is
-increased by exercise, and the glands themselves are increased in
-size, as are the milk-glands of the cow through frequent milking; and
-that even the nerve-elements can be favourably influenced by exercise
-is proved by actors and professors of mnemonics, who have by practice
-increased their powers of memory to an almost incredible degree. I have
-heard of a singer who had learned by heart 160 operas; and which of us
-has not experienced how quickly the capacity for learning by rote can
-be again increased by practice, even after it has been neglected or
-left unexercised for a long time?
-
-I have always been particularly struck with the practising of a piece
-of music, with its long succession of periods of different phrase,
-with its changes in melody, rhythm, and harmony, which nevertheless
-becomes so firmly stamped on the memory that it can be played, not
-only consciously, but quite unconsciously, when the player is thinking
-intensely of other things. It is in this case not the memory alone, but
-the whole complicated mechanism of successive muscle-impulses, with
-all the details of fast and slow, loud and soft, that is engraved on
-the brain elements, just like a long series of reflex movements which
-set one another a-going. Though in this case we cannot demonstrate
-the material changes which have taken place in the nervous elements,
-there can be no doubt that changes have taken place, and that these
-consist in a strengthening of definite elements and parts of elements.
-The strengthening causes certain ganglion-cells to give a stronger
-impulse in a particular direction, and this impulse acquires increasing
-transmissive power, and so on.
-
-Our first theoretical insight into these relations came through Wilhelm
-Roux, who, in 1881, gave expression to what had previously been
-an open, if not quite conscious, secret, that 'functional stimulus
-strengthens the organ,' that is to say, that an organ increases through
-its own specific activity. Up till that time it had been believed that
-it was merely the increased flow of blood that caused the increase in
-the size of a much-used part. Roux showed that there is a 'quantitative
-self-regulation of the organ according to the strength of the stimulus
-supplied to it'; that the stimulated organ, that is, the organ which
-is performing its normal function, may, in spite of the increased
-breaking down or combustion (dissimilation), assimilate all the more
-rapidly; that its used-up material is 'over-compensated,' and that
-therefore it grows. He called this the 'trophic' or nutritive effect of
-the stimulus, and in terms of this he explained the increase and the
-heightened functional capacity of the much-used organ. Conversely, he
-referred the decrease of a disused organ to 'functional atrophy,' which
-sets in when there is a deficient compensation for the substance used
-up in the metabolism.
-
-But if we press for deeper analysis, we must ask: 'On what does this
-trophic effect of functional stimulus depend?' Roux could not answer
-this question when he wrote, nor can we do so now, as Plate has justly
-emphasized. We are here face to face with the fundamental phenomenon
-of life, metabolism; and, since we do not understand the causes of
-this, we are not in a position to say why it varies in this way or in
-that according to the 'stimulus.' But the fact itself is certain that
-the organs respond up to a certain point to the claims made upon them;
-they increase in proportion as they function more frequently or more
-vigorously, they are able to respond to increased functional demands,
-and this Roux has called 'functional adaptation.' As an animal adapts
-itself to the claims of the conditions of its life, for instance, by
-taking on a green or a brown protective colour according as it lives
-on green or brown parts of plants, so the individual organ adapts
-itself to the strength of the stimulus which impels it to function,
-and increases or decreases in proportion to it. If _one_ kidney in Man
-degenerate, or be surgically removed, the other begins to grow, and
-goes on increasing until it has reached nearly twice its former size.
-The specific stimulus which is brought to bear upon it by the urea
-contained in the blood, and which forces it to grow, is twice as great
-in the absence of the other kidney, and therefore the remaining kidney
-grows in response to the increased stimulus and its 'trophic effect'
-until its increase in size has reduced the functional intensity to the
-normal proportion.
-
-Adaptation of an organ in the opposite direction takes place when the
-function diminishes or ceases. If a nerve supplying a muscle or a
-gland be cut through, the organ concerned begins to degenerate and to
-lose its normal structure to a greater or less degree. Sensory nerves
-also degenerate in their peripheral part when they are cut through. In
-such cases there may be no alteration either in the nutritive mechanism
-or in the blood-vessels, &c., but the functional stimulus--in the case
-of the muscle, the stimulus from the will--no longer affects the organ,
-and its metabolism is so much lowered in consequence that it begins to
-degenerate.
-
-When we admit that the fit adaptation of the organism, as far as we
-understand it, must depend upon processes of selection, we may refer
-this 'functional adaptation' also to primitive processes of selection,
-which prevailed at the very beginning of life upon our earth, and
-represented, so to speak, the first adaptation that was established,
-but we can say nothing with certainty in regard to this matter as long
-as we do not understand the essence of assimilation. It is conceivable,
-however, that a _primary_ adaptiveness may have arisen, so to speak,
-abruptly, through a concurrence of favourable circumstances, as we
-shall endeavour to show later on when we discuss the beginnings of life.
-
-Even although we cannot lay bare the primary roots of 'functional
-adaptation' we can gain from the fact itself very valuable insight
-into phenomena which would otherwise be unintelligible and mysterious:
-_the perfectly adapted structure of many tissues and their power of
-adaptation to changed conditions_. In this lies, in the main, the
-advance in our knowledge which is due to Roux's _Kampf der Theile_.
-
-If a number of embryonic cells of different capacity, say _A_, _B_, and
-_C_, be affected by different kinds of functional stimuli, _a_, _b_,
-and _c_, those cells will grow most rapidly which are most frequently
-affected by the stimulus appropriate to them. The proportion in which
-the cells _A_, _B_, and _C_ will ultimately be present in the tissues
-will depend upon the frequency with which the stimuli _a_, _b_, and
-_c_ act upon the tissue. But the tissue will be still more precisely
-determined as to its structure if the three kinds of stimuli affect the
-cell-mass, not uniformly all over, but only at certain spots, or along
-particular paths, one in this, the other in that. Thus the cells _A_
-will predominate over the cells _B_ and _C_ at all the places which
-are most frequently affected by the stimulus _a_, the cells _B_ in the
-sphere of the stimulus _b_, and the cells _C_ in that of the stimulus
-_c_; there they will increase most rapidly and so crowd out the other
-kinds of cells, and thus a spatial arrangement will be established
-within the tissue, a 'structure' which corresponds and is well adapted
-to its end. This is what Roux deduced from his _Struggle of the
-Parts_, and I subsequently defined the process as histonal or tissue
-selection.
-
-Let us first take an example. The anatomist Hermann Meyer showed in
-1869 that the so-called 'spongiosa,' that is, the bony tissue of spongy
-structure within the terminal portions of the long bones in Man and
-Mammals, has a minute structure conspicuously well adapted to its
-office. The thin bone lamellæ of this 'spongiosa' lie precisely in the
-direction of the strongest strain or pressure which is exerted upon the
-bone at the particular area. Arch-like in form, they are kept apart by
-means of buttresses, and no architect could have done better if he had
-been entrusted with the task of making a complicated system of arches
-with the greatest possible carrying and resisting power combined with
-the greatest possible economy of material.
-
-This well-adapted structure is now interpreted through the _Struggle of
-the Parts_ as a self-differentiation, for if there be in the rudiments
-or primordia of the bone differently endowed elements[10], that is,
-cells which respond in diverse ways to different stimuli, these must
-arrange themselves locally, owing to the struggle for space and food,
-in a manner corresponding to the distribution of the different stimuli
-in the bone. The largest amount of bone substance will be formed in the
-directions of the strongest strain and the greatest pressure, because
-the bone-forming cells are excited by this, their functional stimulus,
-to growth and multiplication. Thus the buttress and arch structure
-comes about, and between the delicate bone lamellæ spaces will remain
-free, and these, being relieved from the burden of strain and pressure
-by the aforesaid bony lamellæ, will offer suitable conditions of life
-to cells with other functional properties, such as connective tissue
-cells or vascular cells.
-
-[10] I do not here enter into the question whether we have not in
-this case to do with similar elements, which have the power of
-differentiating into one or another kind of cell according to the
-nature of the external stimuli by which they are influenced.
-
-The structure of the bone spongiosa is not everywhere the same,
-and it is demonstrably related with precision to the conditions of
-strain and pressure at each particular region. Thus, just below the
-soft cartilaginous covering of the joints there are no long pillars
-with short arches, but only rounded meshes, because the pressure is
-here almost equally strong from all sides. The long parallel pillars
-only occur further down in the bone, and they lie in two directions
-which intersect each other obliquely, corresponding to the two main
-directions of pressure. But it is only under the functional stimulus
-of pressure that the bone-forming cells have an advantage over the
-others, and multiply more quickly, thus crowding out those that are not
-attuned to the appropriate functional stimulus.
-
-In a similar manner Roux interprets, in the light of the struggle of
-the parts, the striking adaptations in the course, the branching, and
-the lumen-formation of the blood-vessels, in the direction of the
-intersecting connective tissue strands in the tail-fin of the dolphin,
-in the direction of the fibres in the tympanum, and in many other
-adaptations in the histological structure of complex tissues.
-
-In this there is manifestly an important step of progress, for it is
-obvious that the direction of the bone-lamellæ and such like could not
-have been determined by individual selection, and the same is true
-in regard to many other histological details. It cannot be disputed,
-however, that there is a kind of selection-process here also, similar
-to that which we think of, with Darwin and Wallace, as occurring
-between individual organisms. Just as in the latter, which we shall
-henceforward call _personal selection_, variability and inheritance
-lead, in the struggle for existence, to the survival of the fittest,
-so, in histonal differentiation, the same three factors lead to the
-victory of what is best suited to the parts of the body in question.
-The tissues and the parts of the tissues have to distribute and
-arrange themselves so that each comes to fill the place in which it
-is most effectively and frequently affected by its specific stimulus,
-that is, the stimulus in regard to which it is superior to other
-parts; but these places are also those the occupation of which by
-the best re-acting parts makes the whole tissue capable of more
-effective function, and therefore makes its structure the fittest.
-Variability--in this case that of embryonic cells, with different
-primary constituents--must be assumed; inheritance is implied in
-the multiplication of the cells by division; and the 'struggle for
-existence' here assumes its frequent form of a competition for food
-and space; the cells which assimilate more rapidly because of the
-more frequent functional stimulus increase more rapidly, draw away
-nourishment from the more slowly-multiplying cells around them, and
-thus crowd these out to a greater or less extent.
-
-We might even speak of histonal selection among unicellulars, for it
-is conceivable that in primitive living substance, such as that of a
-moneron, there may be minute differences among the vital particles,
-involving also functional distinctions, which, under the influence of
-diverse stimuli, may gradually give rise to an increasingly complex
-differentiation. For the variations in the primary living substance
-most strongly affected by a particular stimulus would tend to
-accumulate at the places most frequently reached by that stimulus, and
-would crowd out other variations at that spot, just as the body and its
-individual parts may be said to have taken their architectural form
-in exact response to the demands made upon them by function. In this
-case, of course, personal selection and histonal selection co-operate,
-for every improvement in the organization of the fundamental living
-substance means at the same time a lasting improvement in the whole
-individual.
-
-In many-celled organisms, however, we must admit that there is an
-essential difference between personal and histonal selection, inasmuch
-as the latter can give rise to adaptive structural modifications
-corresponding to the needs of the tissue at the moment, but not to
-permanent and cumulative changes in the individual elements of the
-tissue. If a broken bone heals crookedly, the spongy substance within
-the healed portion does not remain as it was before, for the pillars
-and arches, which now no longer run in the direction best suited to
-their function, break up, and a new system of arches is formed, not
-in line with the earlier one, but adapted to the new conditions of
-pressure. This is certainly an adaptation through selection, but the
-elements, that is the cells which form the bone substance in response
-to strain and pressure, or those which in response to the stimulus of
-the blood flowing into the spaces form the blood-vessels, or those
-which being quite freed from one-sided pressure develop into connective
-tissue, must be presupposed. These kinds of cells must be virtually
-implied in the germ-rudiment; they are themselves adaptations of the
-organism, and can therefore only be referred to _personal selection_.
-And this is true of all adaptations of the _elements_ of multicellular
-organisms, and thus of the _cells_. Their adaptation according to the
-principle of division of labour, their differentiation into muscle,
-nerve, and gland cells can only be referred to natural selection in the
-Darwin-Wallace sense, and cannot depend upon histonal selection. In
-the spongy substance of the bone a better bone-cell does not struggle
-with an inferior one and leave behind it by its survival a host of
-descendants which are, if possible, better than itself; the struggle
-for existence and for descendants, in this case, is between two kinds
-of cell which were different from the beginning, and of which one
-has the advantage at one spot, another at another. The case may be
-compared to that of a flock of nearly allied species of bird, of which
-one species thrives best in the plains, another among the hills, and a
-third among the mountain forests, all mingled together in a vast new
-territory to which they had migrated, and in which all three kinds of
-conditions were represented. A struggle would arise among the different
-species, in which in every case the particular species would be
-victorious which was best adapted to the local conditions. But each
-would thrive best in the region in which it was superior to the others,
-and very soon the three species would be distributed as they were in
-the land from which they came--in the plains, the high lands, and the
-mountain forests. This would be the result of a struggle between the
-three species, _not between individuals within each species_, and it
-could not therefore bring about an improvement of a single species,
-but only the local prevalence of one or another. The characters which
-made one species adapted for the plain, another for the mountain forest
-were _already there_; they can only be referred to personal selection,
-which brought about the adaptation of their ancestors in the course
-of ages to the conditions of their life. Something similar is true of
-the adaptations of the tissues; the differentiation of the individual
-kinds of cells is an ancient inheritance, and depends upon personal
-selection, but their distribution and arrangement into specially
-adapted tissues, so far as there is any plasticity at all, depends
-upon histonal selection. Obviously, however, only as far as the tissue
-is plastic, that is, with the power of adjusting itself to particular
-local conditions. Only adaptations of this kind can be referred to
-histonal selection; the ground-plan, even of the most complicated
-tissue, such as the large glands of mammals, the kidneys, the liver,
-and so on, must have been implicit in the germ, and must therefore be
-referred to personal selection. A precise limitation of the respective
-spheres of action of personal selection and histonal selection is not
-possible as yet, since hardly any investigations on the subject are
-available.
-
-Roux undoubtedly over-estimated the influence of his 'struggle
-of parts' when he believed that the most delicate adaptations of
-the different kinds of cells depended on it. I admit that, for a
-considerable time, I made the same mistake, until it became clear to
-me, as it did first in regard to the sex-cells, that this is not, and
-cannot be the case. How, for instance, could the diverse and minutely
-detailed adaptations of the sex-cells--which we are to discuss in a
-subsequent lecture--have arisen in this way? As far as the individual
-sperm-cell is concerned, it is a matter of indifference whether its
-head is a little thinner or thicker, its point a little sharper
-or blunter, its tail a little stronger or weaker. This does not
-decide whether the cell is to thrive better, or to occur in greater
-numbers than some other variety. But it does decide whether it is to
-be able to penetrate through the minute micropyle, or through the
-firm egg-envelope, into the egg, there to effect fertilization. An
-individual with less well formed sperm-cells will be able to fertilize
-fewer eggs, and therefore to leave fewer descendants which might
-inherit its tendency to produce inferior sperm-cells, and conversely.
-Thus it is not the sperm-cells of any one individual which are selected
-according to their fitness, it is the individuals themselves which
-compete with one another in the production of germ-cells which shall
-fertilize best, that is, most certainly. The struggle is thus not
-intercellular, but a struggle between persons.
-
-The same is true of all cells differentiated for particular functions;
-every new kind of glandular, muscular, or nerve cell, such as have
-arisen a thousandfold in the course of phylogeny, can only have
-resulted from a struggle between individuals which turned on the
-possession of the best cells of a particular kind, _not from a struggle
-between the cells themselves_, since these would gain no advantage
-from serving the organism, as a whole, better than others of their
-kind. In regard to the sex-cells we might admit, in addition to
-personal selection, the possibility of an internal struggle between the
-sperm-cells or egg-cells of the same individual, inasmuch as each of
-these cells is the primordium of a new individual, and as those better
-adapted for reproduction might transmit their better quality to these
-new individuals. I will not here enter into my reasons for regarding
-this idea as erroneous, for in any case this interpretation would not
-apply to any other kind of cells. If, for instance, it were a question
-of the transformation of an ordinary mucus or salivary gland into a
-poison gland, it would not matter in the least to the individual cell
-whether it yielded a harmless or a poisonous secretion; but individuals
-with many poisonous cells would have an advantage in the struggle for
-existence.
-
-I agree so far with Plate when he refers the differentiation of
-the tissues entirely to personal selection, but not in his further
-conclusion that histonal selection does not exist. The ground-plan
-of the architectural structure of the organ depends upon personal
-selection, but the realization of the plan in particular cases is
-not predetermined down to the minutest details, but is regulated by
-histonal selection, and is thus to a certain extent an adaptation to
-local conditions of stimulus. The direction, strength, and size of
-every single bone lamella is not predetermined from the germ, but only
-the occurrence and nature of bone-cells and bone lamellæ in general.
-The direction and the strength which these bone lamellæ may assume
-depends on the local conditions of strain and pressure which affect the
-cell-mass, as is shown very clearly by the spongiosa of an obliquely
-healed bone, which we have already described.
-
-But let us now turn to the question which is here most important for
-us: _whether functional adaptations can be transmitted_. We must
-admit that the insight we have so far gained into the causes of these
-adaptations does not make it much easier to answer the question.
-Histonal selection is a purely _local_ process of adaptation to the
-conditions of stimuli prevailing at the moment, and no one will be
-likely to suppose that the distorted position of the spongiosa of
-a badly healed fracture could reappear in the straight bone of a
-descendant; this would be quite contrary to the principle, for the
-crooked lamellæ would in that case no longer be the best adapted. Even
-the question _whether the strengthening of a muscle through use can be
-transmitted_ cannot be answered in the light of the knowledge we have
-hitherto gained. The 'trophic effect of the functional stimulus' is
-brought into activity through entirely local influences, through which
-only the parts most strongly affected by the stimulus can be caused to
-vary. Thus the problem remains unaltered, How can purely local changes,
-not based in the germ, but called forth by the chances of life, be
-transmitted to descendants?
-
-If all species, even in the highest groups, reproduced by dividing
-into two, we might imagine that a direct transmission of the changes
-acquired in the course of the individual life through use or disuse
-took place, though this would presuppose a much more complicated
-mechanism than is apparent at first sight. But it is well known that
-multiplication by fission is for the most part restricted to simple
-organisms, and that the great majority of modern plants and animals
-reproduce by means of germ-cells, which develop within the organism in
-regions often very remote from the parts, the results of the exercise
-of which are said to be transmitted. Moreover, the germ-cells are of
-very simple structure, at least as far as our eyes can discern; for
-we see in a germ-cell neither muscles nor bones nor ligaments, glands
-nor nerves, but only a cell-body consisting of that semifluid living
-matter to which the general name of protoplasm has been given, and of
-a nucleus, in regard to which we cannot say that it differs in any
-essential or definite way from the nucleus of any other cell. How then
-could the changes which take place in a muscle through exercise, or
-in the degeneration of a joint in consequence of disuse, communicate
-themselves to a germ-cell lying inside the body, and do so in such
-a fashion that this germ-cell is able, when it grows into a new
-organism, to produce of itself, in the relevant muscle and joint, a
-change the same as that which had arisen in the parent through use and
-disuse? That is the question which forced itself upon me very early,
-and in following it up I have been led to an absolute denial of the
-transmission of this kind of 'acquired characters.'
-
-In order to explain how I reached this result, and what it is
-based upon, it is indispensable that we should first make ourselves
-acquainted with the phenomena of heredity in general, and the
-inseparably associated phenomena of reproduction, so that we may form
-some sort of theoretic conception of the process of inheritance--a
-picture, necessarily provisional and imperfect, of the mechanism
-which enables the germ-cell to reproduce the whole organism, and not
-merely, like other cells, others like itself. We are thus led to an
-investigation of reproduction and heredity, at the conclusion of which
-we shall feel justified in returning to the question of the inheritance
-of acquired characters, in order to give a verdict as to the retention
-or dismissal of the Lamarckian principle.
-
-
-
-
-LECTURE XIII
-
-REPRODUCTION IN UNICELLULAR ORGANISMS
-
- Reproduction by division--In Amœbæ--In Infusorians--Divisions
- following one another in immediate succession--Formation of germ-cells
- in the Metazoa--Contrast between germ-cells and body-cells--Potential
- immortality of unicellular organisms--Beginning of natural
- death--Budding and division in the Metazoa.
-
-
-WE wish to consider the reproduction of organisms with special
-reference to the problem of heredity, and it is most instructive to
-begin with the lowest forms of life--the unicellulars--because their
-structure, as far as we can see with the instruments at our command,
-is very simple, and, what is even more important, is relatively
-homogeneous.
-
-[Illustration: FIG. 59. An Amœba: the process of division. _A_, before
-the beginning of the division. _B_, the nucleus divided into two. _C_,
-the two daughter-Amœbæ. Magnified about 400 times.]
-
-Suppose that there are bacteria-like organisms of quite homogeneous
-structure, and that these multiply by simply dividing into two, each
-rod-like creature dividing transversely in the middle of its length,
-the two halves would represent independent daughter-organisms, whose
-structure would correspond exactly with that of the mother-organism,
-could not indeed in any way deviate from it, and consequently would
-take over all its characters, that is, would inherit them. The size
-of body is the only feature which is not obviously inherited, but
-in reality it is potentially heritable, since the structure of the
-divided portions involves the capacity and the limits of their possible
-growth. Moreover, the size of body is not invariable in any species;
-a particular size is only reproduced under similar conditions of
-development. Inheritance here consists simply in a continuation of the
-mother-organism into its two daughter-cells.
-
-Even in an Amœba (Fig. 59) we might picture the process of inheritance
-as equally simple, though in so doing we should probably be making a
-fallacious inference, for the structure of these lowest unicellular
-animals probably seems to us simpler and more homogeneous than it
-really is. Among Infusorians it is quite obvious that inheritance
-implies more than the mere halving of the mother-animal into the two
-daughter-cells; something more must be involved. For among these
-unicellular animals the differentiation of the body is not only
-great, but it is unsymmetrical. The posterior and the anterior ends
-are different, and the transverse division of the animal, in which
-the process of reproduction here consists, does not produce two
-halves, but two very unequal portions. In the division of _Stentor_,
-the so-called trumpet-animalcule (Fig. 60), the anterior portion
-contains the funnel-shaped mouth and gullet with its complicated
-nutritive apparatus, the circular peristome with its spirally curved
-rows of composite ciliated plates, the so-called membranellæ, and so
-forth; the posterior half contains nothing of all this, but possesses
-the foot of the mother-Stentor with its attaching organ, which the
-anterior half lacks. But each of the two portions possesses the
-power of 'regeneration,' that is, it is able to develop anew the
-missing parts, mouth or foot, and so on. So that here there is no
-longer merely a simple continuance of the maternal organization in
-the daughter-animals, there is something new added, something which
-requires explanation; we are confronted with the first riddle of
-heredity. Simple growth does not explain the phenomenon, for what has
-to be added to complete the halved portions has a different structure,
-a different form, different accessory apparatus from any that the
-halves themselves possess. It in no way affects this state of matters
-that in the normal process of division in Infusorians the formation
-of the new mouth and peristome-region begins before the halves have
-actually separated, for even if a Stentor be cut in two artificially
-the cut halves form complete animals. And, indeed, a Stentor may be cut
-into three or four pieces, and in certain conditions each piece will
-develop into a complete animal. These pieces must therefore possess
-something more than the mere power of growth. We shall try later on to
-discover whether this marvellous invisible transmission of characters,
-this implication of the whole in each of the parts, can be in any
-way theoretically expressed and included in our scheme of conceptual
-formulation.
-
-[Illustration: FIG. 60. _Stentor rœselii_, trumpet-animalcule.
-Process of division. _wsp_, ciliated spiral leading to the mouth
-(_m_); _cv_, contractile vacuole. _A_, in preparation for division,
-the nucleus (_k_) has coalesced into a long twisted band. _B_, a
-second ciliated spiral (_wsp´_) has begun to be formed; the nucleus
-(_k_) is contracted. _C_, just before the constricting off of the two
-daughter-Stentors. Magnified about 400 times. After Stein.]
-
-Now that we have become familiar with these facts it will no longer
-surprise us to learn that the reproduction of unicellular animals does
-not always depend on _equal_ division, but that unequal spontaneous
-divisions are also possible, so that one or several smaller portions of
-the cell-body, containing a portion of the cell-nucleus, can separate
-off from the mother-animal. This occurs especially among the suctorial
-Infusorians or Acinetæ. In relation to the phenomena of inheritance the
-problem raised by the equal division of the Infusorians repeats itself,
-and it is in no way affected by the fact that equal division can take
-place several times, or many times in succession, so that from one
-animal a large number of small pieces of the same size may be produced
-in rapid succession. The characteristic marks of the mother-animal are
-not infrequently lost sight of, wholly or partially, when this occurs,
-and the divided portions seem to consist of a homogeneous cell-body
-and nucleus; but they possess the power of regenerating themselves, or
-of developing, if the expression be preferred, into animals similar
-to the maternal-organism. Such divided portions might very well be
-called germs, only it must not be forgotten that the relation of the
-mother-animal to these germs is a different one from that of a higher
-animal or plant to its germ-cells; the unicellular animal breaks up by
-continued division into these 'germs,' while the Metazoon continues its
-individual existence unimpaired by the production of its germ-cells.
-
-The beginning of a so-called 'spore-formation' is to be found in many
-Infusorians. Thus the holotrichous species, _Holophrya multifiliis_
-(Fig. 61), reproduces by first becoming enclosed in a cyst or capsule,
-and then dividing many times in rapid succession, so that 2, 4, 8,
-16, &c. individuals arise consecutively, and subsequently burst forth
-from the cyst (Fig. 61, _B_). In the Gregarines and other Sporozoa the
-period of division lasts much longer, and the encysted animal divides
-into 128, 256, or even more portions; but in this case also each part
-or 'spore' receives a piece of the maternal cell-body and cell-nucleus,
-so that there is no difference in principle between this and the simple
-division into two exhibited by _Stentor_; as in that case, so here,
-it is not the fully differentiated structure of the animal which is
-handed on to the divided parts; it is only the power to redevelop this
-anew on their own account. Thus here again we are face to face with the
-fundamental problem of heredity: How is it possible that the power of
-reproducing the complex whole can be inherent in the simple parts?
-
-[Illustration: FIG. 61. _Holophrya multifiliis_, an Infusorian
-parasitic on the skin of fishes. _A_, in its usual condition; _ma_,
-macronucleus; _mi_, micronucleus; _cv_, contractile vacuole; _m_,
-mouth. _B_, after binary fission has been several times repeated within
-the cyst (_cy_); _tt_, results of the division. _C_, one of these units
-much enlarged.]
-
-In contrast to the unicellular organisms, the great majority of the
-multicellulars, the Metazoa and Metaphyta, many-celled animals and
-plants, differ not only in the multitude of their cells, but even
-more in the manifold differentiation of these cells according to the
-principle of division of labour, so that the various functions of
-the animal are not performed by all the cells uniformly, but each
-function is relegated to a particular set of cells specially organized
-with reference to it. Thus there is differentiation between motile,
-nutritive, and reproductive cells, and there may also be glandular,
-nerve, muscle, and skin cells, and we know how this differentiation
-into a great number of different kinds of cells with highly specialized
-functions has arisen, especially among the higher animals, in a
-multiplicity which cannot easily be overlooked. Thus we find a large
-number of the most diverse kinds of cells, all of which serve for
-the maintenance of the body, in contrast to the simply reproductive
-cells or germ-cells. These alone possess the power of reproducing,
-under certain conditions, a new individual of the same species. We can
-contrast with these germ-cells, which serve, not for the maintenance of
-the individual, but only for that of the species, all the other kinds
-of cells under the name of somatic or body-cells. The problem which we
-have to solve now lies before us in the question, How comes it that
-the germ-cell is able to bring forth from itself all the other cells
-in definite sequence and arrangement, and is thus able to build up the
-body of a new individual?
-
-[Illustration: FIG. 62. _Pandorina morum_; after Pringsheim. I, A young
-colony, consisting of 16 cells. II, Another colony, whose cells have
-reproduced daughter-colonies; all the cells uniformly alike. III, A
-young Volvox-colony; _sz_, somatic cells; _kz_, germ-cells.]
-
-The similarity of this problem to that formulated in regard to
-unicellular organisms is at once obvious, but it becomes still more
-emphatic when we remember that the gulf between unicellular organisms
-and the higher animals and plants is bridged over by certain transition
-forms which are of the greatest interest, especially in relation to the
-problems of inheritance.
-
-Among the lower Algæ there is a family, the Volvocineæ, in which the
-differentiation of the many-celled body on the principle of division of
-labour has just set in; in some genera it has been actually effected,
-though in the simplest way imaginable, and in others it has not yet
-begun. Thus in the genus _Pandorina_ the individual consists of sixteen
-green cells, united into a ball (Fig. 62, I), each one exactly like the
-other, and all functioning alike. They are all united into a spherical
-body, a whole, by a gelatinous matrix which they all secrete, and thus
-they form a cell-colony, a cell-stock, a many-celled individual; but
-each of these cells has not only all the typical parts--cell-body,
-nucleus, and contractile vacuole--but each possesses a pair of flagella
-or motor organs, an eye-spot, and a chlorophyll body which enables
-them to assimilate nourishment from the water and the air. Each one
-of these cells thus performs all the somatic functions, that is, all
-that are necessary to the maintenance of the individual life. But each
-also possesses the power of reproducing the whole colony from itself,
-that is, it also performs the function of reproduction necessary to
-the maintenance of the species. When such a colony, whose sixteen
-cells are continually growing, has led for some time a free-swimming
-life in the water, the cells retract their flagella, and each begins
-to multiply by dividing into 2, 4, 8, finally into 16 cells of the
-same kind, which remain together, forming a spherical mass enclosed in
-a gelatinous secretion (Fig. 62, II). Thus there are now, instead of
-sixteen cells in the mother-colony, sixteen daughter-colonies, each
-with sixteen cells which soon acquire flagella and eye-spots, and are
-then ready to burst forth from the dissolving jelly of the maternal
-stock as independent individuals. This _Pandorina_ shows no trace of
-a differentiation of its component cells to particular and different
-functions, but a nearly allied genus of the same family, the genus
-_Volvox_ (Fig. 62, III), consists of two kinds of cells--on the one
-hand of small cells (_sz_) which occur in large numbers and compose the
-wall of the hollow gelatinous mass, forming, so to speak, the skeleton
-of the _Volvox_; and, on the other hand, of a much smaller number of
-cells which are very much larger (_kz_). The former, the 'body' or
-'somatic' cells, are green, and have a red 'eye-spot' and two flagella;
-they are connected with each other by processes from their cell-bodies,
-and are able, by means of the co-ordinated action of their flagella,
-to propel the whole colony with a slow rotatory movement through the
-water. Many of my readers are doubtless familiar with these light green
-spheres, which are quite recognizable with the naked eye, and people
-our marsh pools and ponds in Spring in such abundance that it is only
-necessary to draw a glass of water to procure a large number of them.
-
-The little flagellated cells just described serve not only for the
-locomotion of the colony, but also for nutrition, for the secretion
-of the jelly, and for the excretion of waste products; in short, they
-perform all the functions necessary to the maintenance of life, but
-not that of reproduction. They can, indeed, multiply by dividing when
-the colony is young, like the cells of _Pandorina_, but they cannot
-reproduce the whole colony but only cells like themselves, that is,
-other somatic cells. In _Volvox_ the maintenance of the species, the
-production of a daughter-colony, is the function of the second and
-larger kind of cells, the reproductive cells, which are contained in
-the interior (filled with a watery fluid) of the gelatinous sphere.
-They possess no flagella (_kz_), and so take no share in the swimming
-movements of the somatic cells. For the present we need not allude to
-the fact that there are several kinds of these cells, and need only
-state that the simplest among them, the so-called 'Parthenogonidia,'
-after they have reached a considerable size, begin a process of
-division which results in the formation of a daughter-colony. Usually
-there are several of these large reproductive cells in a _Volvox_
-colony, and as soon as these have developed into a similar number of
-daughter-colonies they burst out through a rupture in the now flaccid
-jelly of the maternal sphere and begin to lead an independent life. The
-mother-sphere, which now consists only of somatic cells, is unable to
-produce new reproductive cells; it gradually loses its spherical form,
-sinks to the ground, and dies.
-
-In _Volvox_ we have, for the first time, a cell-colony in which a
-distinction has been established between body or somatic cells and
-reproductive or germ-cells. In contrast to _Pandorina_, a large number,
-indeed the majority of the cells of the colony, have lost the power
-of reproducing the whole by division, and only the few reproductive
-cells possess this, while they, in turn, have lost other functions,
-notably that of locomotion. Their power of reproducing the whole, that
-is to say, their hereditary capacity, gives them a greater theoretical
-interest than the cells of _Pandorina_, for the latter require only to
-produce others like themselves, because there is only one kind of cell
-in the colony, while in _Volvox_ the reproductive cell can not only
-produce others like itself, by division, but can produce the body-cells
-as well. The problem is quite analogous to the one which we have had
-to face in regard to the unicellular animals of complex structure, the
-Infusorians. The question, How can the part of the trumpet-animalcule
-which is mouthless develop from itself a new mouth and ciliated
-apparatus? here transforms itself into the question, How can a cell
-by division give rise not only to others like itself, but also to the
-body-cells, which are of quite different structure? This is, in its
-simplest form, the fundamental problem of all reproduction through
-germ-cells, to which we must now pass on. But first a short digression.
-
-We have already noted that unicellular organisms multiply by division,
-and originally, as well as in the great majority of cases to-day, by
-division into two. It follows, therefore, that there is no _natural_
-death among them, for, if there were, the species would die out as
-the individuals grew old; but this does not happen. The two daughter
-organisms which arise from the binary fission of an Infusorian are
-in no way different in regard to their power of life; each of them
-possesses an equal power of doubling itself again by division, and
-so it goes on, as far as we can see, for an unlimited time. Thus the
-unicellular organisms are not subject to natural death; their body is
-indeed used up in the course of ordinary life so that the formation of
-new cilia and so on is necessary, but it is not worn away in the same
-sense in which our body is and that of all Metazoa and Metaphytes,
-where, through functioning, the organs are gradually worn away until
-they become incapable of function. Our body grows old, and can at last
-no longer continue to live; but among unicellular organisms there is
-no growing old, and no death in the normal course of the development
-of the individual. The unicellulars are, as we may say, immortal; that
-is, while individuals may be annihilated, by external agencies, boiling
-heat, poisons, being crushed, or eaten, and so on, at every period
-some individuals escape such a fate, and perpetuate themselves through
-succeeding ages. For, strictly speaking, the daughter-individual
-is only a continuation of the mother-individual; it contains not
-only half of the substance, but also the organization, and life is
-continued directly from mother to daughter. The daughter is simply
-half of the mother, which is subsequently regenerated; and the other
-half of the mother lives on in the other daughter, so that nothing
-dies in this multiplication. It may be said that the daughter has to
-develop the other half of its body anew, and that therefore it is a
-new individuality, and not merely a continuation of the old, and that
-therefore the unicellular animals are not immortal. The 'immortality'
-of the Protozoa may be scoffed at; the idea may seem absurd that the
-'immortal' Protozoa are still the same individuals which lived upon
-the earth millions of years ago, but all such objections mean no more
-than doctrinaire quibbling with the concepts of 'individual' and
-'immortality,' which do not exist in nature at all, but are mere human
-abstractions, and therefore only of relative value. My thesis as to the
-potential immortality of the Unicellulars aims at nothing more than
-impressing on Science the fact that the occurrence of physiological,
-that is, natural, death is causally associated with the transition
-from single-celled to many-celled organisms; and this is a truth which
-will not be overthrown by any sophisms. It is the Volvocineæ which
-show us, so to speak, the exact point at which natural death set in,
-at which it was introduced into the world of life. In _Pandorina_ the
-state of things is still the same as in single-celled organisms, for
-each cell is still all in all, each can bring forth the whole, none
-dies from physiological causes involved in the course of development,
-and they are therefore 'immortal' in the sense stated. But in _Volvox_
-the 'individual' dies when it has given off its reproductive cells,
-because here the contrast between germ-cells and body has developed.
-Only the body is mortal in the sense of being subject to natural death;
-the germ-cells possess the potential immortality of the single-celled
-animals, and it is necessary that they should possess it if the species
-is to continue to exist.
-
-From this alone it does not seem quite clear why the body or soma
-should be subject to death, and when I first endeavoured to arrive at
-clearness in regard to these matters I tried to find out why a natural
-death of the body was necessitated by the course of evolution. I did
-not at once discover the true explanation, but without delaying to
-discuss my mistakes I shall proceed to expound what I believe to be the
-true reason. It lies simply in the fact, which we shall inquire into
-later on in more detail, that every function and every organ disappears
-as soon as it becomes superfluous for the maintenance of the particular
-form of life in question. The power of being able to live on without
-limit is useless for the somatic cells, and thus also for the body,
-since these cannot produce new reproductive cells after those that had
-been present are liberated; and with this the individual ceases to be
-of any value for the preservation of the species. What advantage would
-it be to the species if the _Volvox_ balls were to continue living for
-an unlimited time after the reproductive cells were developed and had
-been liberated? Obviously their further fate can have no influence
-whatever in determining or preserving the characters of the species,
-and it is quite indifferent to the continuance of the species whether
-and how long they go on living. Therefore the soma has lost the
-capacity which conditions endless continuance of life and continued
-renewal of body-cells.
-
-In regard to these views it has been asked jeeringly, how
-'immortality,' if it were really a property of the Unicellulars and of
-undifferentiated cell-colonies, could be lost, as if the world, which
-we believe to be everlasting, should give up its everlastingness.
-But the jeer recoils on the superficial outlook which is unable to
-distinguish between the immortality dreamed of by the poets, religious
-and secular, and the real power that certain forms of life have to
-resist being permanently exhausted by their own metabolism. That
-we should call this 'immortality' does not seem to me to require
-any apology, for the right has always been conceded to science to
-transfer popular words and ideas in a restricted and somewhat altered
-sense to scientific conceptions when it seems necessary. That the
-word 'immortality' in this case expresses the state of matters more
-precisely and better than any other cannot be doubted, any more than
-we can doubt that there exists in regard to natural death a real
-difference, which we must take account of, between the Unicellulars
-and the higher organisms. What enables the species in the case of the
-higher organisms, like ourselves for instance, to last through ages is
-not the immortality of the individual, of the person, but only that
-of the germ-cells; these alone, among the cells of the whole body,
-have retained the primæval power. A small piece of the individual is
-still immortal, but only a minute part, which cannot be considered as
-equivalent to the whole, either morphologically or from the point of
-view of the conception of individuality. Can anyone consider himself
-identical with his children? If any one should imagine this, it would
-still not be the case, for he himself would in the course of time
-suffer natural death, and his children would continue to live on until
-they too had brought forth children, and in their turn also came to
-die. It is quite different with an Infusorian, which never lies down to
-die, but simply splits itself afresh into two halves which continue to
-live.
-
-It is hardly credible that such a simple and clear truth should have
-remained so long undiscovered, and it is even more incredible that
-since it was enunciated it should have been until quite recently
-laughed at as false, as a piece of pseudo-science, and as valueless.
-But it is the fate of all knowledge which rests on an intelligent and
-comprehensive working up of facts to be attacked, until it gradually
-bears down antagonism by the weight of its truth, and compels at least
-a silent recognition.
-
-The fact that natural death made its appearance with the appearance of
-a 'body,' a soma, as distinguished from the germ-cells, will sooner or
-later compel recognition. When I pointed out above that the explanation
-of natural death lay in the fact that it would be superfluous for the
-soma to continue to live on unlimitedly, after it had discharged its
-germ-cells, and so fulfilled its duty to the species, I only intended
-to say that this was the general reason for the introduction of natural
-death. I have no doubt that the actual beginning of this phenomenon
-could have, and probably did come about in other ways. Many kinds of
-cells in higher animals perish as a result of their function; it is,
-so to speak, their business to perish, to break up; this is the case
-with many glandular and epithelial cells. It may very well be that, in
-many of the highly differentiated tissue-cells, such as nerve, muscle,
-and glandular cells, the high differentiation in itself excludes the
-possibility of unlimited length of life and multiplication. Through
-this alone, therefore, the exhaustion of the body and an ultimate
-death may be explicable from internal causes. But the deeper cause
-remains what I have already indicated, for it is obvious that if the
-continued life, that is, the immortality of the soma, were necessary
-to the preservation of the species it would have survived through
-natural selection; that is to say, had it been so, then histological
-differentiations incompatible with immortality would not have made
-their appearance; they would always have been eliminated on their way
-to development, since only that which is adapted to its end survives.
-Only if the immortality of the soma were indifferent for the species
-could the soma have become so highly organized that it became subject
-to death.
-
-Thus the old song of the transitoriness of life does not apply to
-all the forms of life: natural death is a phenomenon which made its
-appearance comparatively late in the development of the organic world,
-a phenomenon which, up to a certain point, we can quite well understand
-from the standpoint of purposefulness.
-
-It would take me too far from the goal towards which we are at present
-making if I were now to attempt to show, in connexion with natural
-death, that the durability of the soma, or what we usually call
-the normal duration of life, is also exactly regulated by natural
-selection, so that each species possesses exactly that duration of life
-which is most favourable to it, according to its physical constitution,
-its physiological capacity, and the conditions of life to which it has
-to adapt itself[11]. But, interesting as this subject is, I must not
-digress further, but return to our proper subject of study, namely,
-reproduction in its relation to inheritance.
-
-[11] See Weismann, _Ueber die Dauer des Lebens_, Jena, 1882. Translated
-in _Essays on Heredity_.
-
-We digressed from this study after having seen that all, even the most
-complex, multicellular plants and animals, in which the differentiation
-of the cells into a number of cell-groups with the most diverse
-functions has attained the highest degree of complexity, are able
-to produce special cells, the germ-cells, which have the power of
-reproducing from themselves another organism of the same species, and
-with the same complex structure. It might be thought that such cells
-must necessarily be very complex in their own structure, but in most
-cases nothing of the kind is to be seen, and the germ-cells often
-appear simpler in organization than many of the tissue-cells, such as
-the glandular-cells; and where there is an unusual size or complexity
-of structure in the germ-cell it usually bears no relation to the
-grade of organization of the young creature that is to arise from it,
-but is due solely to the special conditions imposed on the particular
-germ-cell, if a young organism is to be evolved from it. We shall soon
-see what is meant by this.
-
-I must note here that plants and animals do not multiply by means of
-germ-cells alone, but that many species--the majority of plants and
-the simpler forms of animals--also exhibit multiplication by budding
-or division. All animals and plants which do not stop short at the
-stage of the individual, the 'person,' but rise to the higher stage
-of the 'stock' (or corm), illustrate this. The first person from
-which the formation of the stock proceeds gives rise by budding or
-division to new persons which remain attached to it, and in turn by
-repeated production of buds give rise to a third, fourth, or _n_^{th}
-generation of persons, all remaining in connexion with the first, and
-together forming the composite individuality of the animal-colony or
-plant-stock. Such colonies or stocks are seen in polyps and corals,
-Siphonophoræ and Bryozoa, and among plants, according to Alexander
-Braun, in all phanerogams which do not consist only of a single shoot.
-In these cases we find that definite, or perhaps indefinite groups
-of cells in the stock may give rise to a new person, and we have to
-inquire how this power may be theoretically interpreted.
-
-New stocks may also have their origin from such buds, or from single
-persons of the stock. The fresh-water polyp (_Hydra_) gives rise by
-budding to a small stock of at most three or four persons; but the
-young animals budded off only remain attached to the parent hydra
-until they have attained their full development; then they detach
-themselves and settle down independently, and begin to bud off in turn
-a similar and transitory stock. Among plants there are many which, like
-_Dentaria bulbifera_ and _Marchantia polymorpha_, multiply by so-called
-'brood-buds,' that is, buds which fall from the stock and grow into new
-plants. The whole horticultural propagation of plants by cuttings also
-depends on the process of budding, for what is cut off from the parent
-plant and stuck into the earth is a single shoot, that is, a 'person'
-which possesses the power of sending down roots into the earth, and by
-continual budding giving rise to new shoots or persons which together
-make up a new plant-stock.
-
-I must not, however, spend much time over this so-called 'asexual'
-reproduction by budding and division, because it does not suggest
-any way by which we may penetrate more deeply into the processes of
-inheritance, and we may be content if we can bring them into harmony
-with other theoretical views which we deduce from other phenomena.
-These forms of reproduction were long regarded as the oldest and
-the simplest, and it is only since the time of Francis Balfour that
-the conviction has gradually gained ground that this cannot be so,
-but that they are rather secondary methods of multiplication in the
-Metazoa and Metaphyta, which therefore rest on a very complex basis.
-We have seen that the germ-cells made their appearance along with
-the multicellular body, and the step from _Pandorina_ to _Volvox_ is
-as small a step as can be well imagined. It is thus proved that the
-oldest mode of multiplication among multicellular organisms was that
-through germ-cells, at least along this line of evolution. _Volvox_
-does not reproduce by dividing, or by the development of buds from
-any part of the spherical colony of cells. What is known as budding
-among single-celled organisms is only an unequal cell-division, and
-has nothing but its external appearance in common with the budding of
-higher plants and animals. The latter, therefore, is something new, of
-later and independent origin; _the primitive mode is reproduction by
-unicellular germs_.
-
-
-
-
-LECTURE XIV
-
-REPRODUCTION BY GERM-CELLS.
-
- Historical--Differentiation of germ-cells into male and
- female--Pandorina--Volvox--Sperm-cells and ova in Algæ--Zoosperm form
- of the male germ-cells--Zoosperms of the Barnacles--Adaptation of the
- sperm-cells to the conditions of fertilization--Daphnids--Spermatozoa
- in different animal groups--Their minute structure--Form and structure
- of the egg-cell--Adaptation of the ovum to given conditions--Dimorphic
- ova in the same species--Nutritive cells associated with
- egg-cells--Complex structure of the bird's egg.
-
-
-IF we now turn to the reproduction of the Metazoa and Metaphyta by
-means of germ-cells we find that a great number of lowly plants
-produce germ-cells which require nothing more for the development
-of a new plant beyond certain favourable external conditions, above
-all, moisture and warmth. Such, for instance, are the 'spores' of the
-ferns, which are formed on the under surface of the fronds in little
-clusters of a brown or yellow colour, easily visible to the naked
-eye. These spores are individually very small, so that thousands
-go to form one spore-cluster or sporangium, and millions of spores
-are given off annually by a single fern. Each spore is a germ-cell
-enclosed in a protective capsule, and may, if carried by the wind to
-a spot favourable to germination, become a young plant, the so-called
-prothallium, from which the fern-plant proper subsequently develops.
-
-This reproduction by spores has been regarded as a form of 'asexual
-reproduction' so-called, and has been classed along with budding and
-fission under this head. But it has nothing in common with these
-forms of multiplication except the negative character that the act
-of fertilization, which we shall inquire into later on, does not in
-this case occur. This mode of classification has no longer any more
-justification than the division of the animal kingdom into backboned
-and backboneless animals, in which the negative character of the
-absence of vertebræ has led to the slumping of quite heterogeneous
-forms in one group. I do not mean to dispute that both these
-classifications were fully justified in their own time; indeed
-they expressed a step of progress. Nowadays, however, the division
-'Invertebrata' or 'backboneless animals' as a scientific conception has
-been abandoned, and the same should be done with the category 'asexual
-reproduction,' since it groups together quite different things, such
-as multiplication by single-celled and many-celled 'germs,' and is
-moreover based on a quite erroneous idea of what 'fertilization'
-really is. Both terms may very well be retained as a mere matter of
-convenience, but it is much to be desired that the two apt designations
-proposed by Haeckel--Monogony for asexual, and Amphigony for sexual
-reproduction--should come into general use.
-
-Meanwhile it is enough to say that reproduction by 'spores' occurs
-normally in Algæ, fungi, mosses, and fern-like plants, and that
-there are also animals in which the germ-cells possess the power of
-giving rise of themselves to a new individual. But the cases which
-I am chiefly thinking of are those of so-called virgin birth or
-parthenogenesis, which are not to be compared with multiplication by
-spores in regard to their mode of origin; there is a peculiarity in
-the origin of this mode of multiplication which I can only make clear
-after we have studied the normal forms of what is called 'sexual
-reproduction.'
-
-We shall therefore pass on to this mode of reproduction. It is well
-known that, in all higher animals, just as in Man, an individual cannot
-reproduce by itself; the co-operation of two individuals is necessary,
-and these--the male and the female--differ essentially from each other
-in many particulars. Their union in the act of procreation induces the
-development of a new individual, whether this matures within the mother
-in a special receptacle, or whether it is deposited as a 'fertilized
-egg,' as in birds, the lower vertebrates, and most 'invertebrates.'
-
-As long as Man has lived he has regarded this process of procreation as
-the essential factor in the origin of new individuals, and as he had no
-insight into the essence of the process he had necessarily to regard
-reproduction as something entirely mysterious, and the co-operation of
-the two sexes as a _conditio sine qua non_ of reproduction in general;
-thus copulation and reproduction seemed identical.
-
-This was in the main the state of opinion at the time of the discovery
-of innumerable minute filaments, the so-called 'spermatozoa' in the
-'fertilizing' spermatic fluid of the male. The discovery was made in
-1677 by Leeuwenhoek in the case of birds, mammals, and many other
-animals. Albrecht von Haller (1708-77) was at first inclined to regard
-these spermatozoa as the rudiments of the embryo, but later on, in the
-course of his long life, he withdrew this theory, and declared them
-to be a kind of parasite in the spermatic fluid without anything to
-do with fertilization. The same opinion was expressed in 1835 by K.
-E. von Baer, in opposition to the opinion of Prevost and Dumas, who
-had rightly interpreted the spermatozoa as the essential elements of
-the spermatic fluid. When one follows the matter out in detail, one
-finds it almost incredible that such a number of mistakes should have
-been made, and so many circuitous paths traversed, before even the
-limited knowledge current in the middle of the nineteenth century was
-attained--that is to say, enough to give ground for the assertion that
-fertilization depends upon the contact of the spermatozoon with the
-body of the egg. In 1843 Martin Barry had found the spermatozoa within
-the egg-envelope of the rabbit ovum, but it was some time later (1852)
-that the investigations of Meissner, Bischoff, and Newport established
-the fact that the zoosperm penetrates through the egg-envelope. All
-else remained quite obscure, and could not be cleared up as long as it
-was believed, on the strength of observations which were in themselves
-correct enough, that _several_ zoosperms were always necessary to
-fertilize one ovum.
-
-To an understanding of the process even in its most general outlines
-there was lacking, apart from technical methods, an appreciation of the
-morphological value of the ovum and the spermatozoon. It was necessary
-to recognize both ovum and spermatozoon as _cells_ before their union
-in fertilization could be regarded as the fusion of two cells, as a
-copulation or conjugation of two minute elementary organisms. But this
-knowledge only gained ground very gradually, and even in the sixties
-opinions on the subject were very much divided. Moreover, there was an
-entire absence of knowledge in regard to 'sexual' reproduction among
-the lower plants, the Algæ, Fungi, Mosses, and Ferns, as well as of
-any detailed acquaintance with the processes of fertilization among
-flowering plants. All this had to be elucidated by the labours of many
-distinguished observers before it was possible to say so much even as
-this, that the process of fertilization depends in general on the union
-of two cells.
-
-I need not discuss the whole of this long process of scientific
-development, and have only touched upon it because I wished to
-emphasize that the conception of the process of fertilization was
-for a long time quite erroneous, and has only attained to clearness
-in recent times. Pairing as it is seen in the higher animals was for
-long regarded as the essential part of the process, and a mysterious
-life-awakening influence was assumed in regard to it; and even when it
-was understood that not the copulation, but the union of two living
-units which was always brought about thereby--the union of the male
-and the female germ-cells--was the essence of 'fertilization,' this
-was still regarded as a life-awakening process, and the way to a true
-understanding of the facts was thus once more blocked.
-
-The simplest form of sexual reproduction in many-celled animals
-is found, among others, in the Volvocineæ, those green, spherical,
-freshwater cell-colonies which we have already studied in relation to
-reproduction by asexual germ-cells. Among them it is the rule that,
-after a long series of generations producing only 'asexual' germ-cells,
-colonies occur in which each germ-cell is no longer able to develop a
-new colony alone, but can do so only after it has united with another
-germ-cell.
-
-Now, as we have seen, there are Volvocineæ in which the differentiation
-of cells into those of the body (soma) and those concerned with
-reproduction has not been established, and all the cells are therefore
-alike. In these, as for instance in the genus _Pandorina_ (Fig. 62,
-p. 257), when sexual reproduction is to occur the whole colony breaks
-up into sixteen cells; these burst forth from the gelatinous matrix
-in which they have been hitherto enclosed, swim about in the water
-with the help of their two flagella, meet other similar free-swimming
-cells and conjugate with these. The two swimming cells come close to
-each other, draw in their flagella, sink to the ground in consequence,
-and fuse completely both as to the cell-body and the nucleus. They
-assume a spherical form, lose the eye-spot, become surrounded with a
-tough cell-skin or cyst, and so remain for a longer or shorter time as
-so-called 'zygotes' or lasting spores. Then they develop by repeated
-cell-division into one of the sixteen-celled _Pandorina_ colonies with
-which we are already familiar; this bursts forth from the capsule and
-swims freely about in the water again.
-
-Here, therefore, the so-called sexual reproduction depends on the
-fusion of two cells similar in appearance, and when this phenomenon
-was first known it was regarded as something quite different from the
-corresponding reproduction in other multicellular organisms. But we
-now know that quite nearly related Volvocineæ belonging to the genus
-_Volvox_ and to other genera, which exhibit a differentiation into
-body-cells and reproductive cells, may reproduce sexually by means of
-two _different_ kinds of germ-cells; and we have also learned through
-Goebel and others that even genera like _Pandorina_, which consist of
-only one kind of cells, may yet produce male and female reproductive
-cells differing essentially in form from one another. In _Eudorina_,
-for instance, a gelatinous sphere containing sixteen or thirty-two
-individual cells, asexual reproduction occurs in exactly the same way
-as in _Pandorina_, that is, each of these cells divides four or five
-times in rapid succession, and thus forms a new colony, which then
-bursts forth; but when the time for sexual reproduction comes the
-colonies behave differently, for some become female and some male.
-In the former the cells remain as they were before, but in the male
-colonies the sixteen or thirty-two cells undergo a peculiar process
-of division, which ends in each becoming a mass (16-32) of so-called
-'zoosperms,' that is, minute, narrow, longitudinally elongated cells
-with two flagella (Fig. 63 at _D_ shows those of _Volvox_). In
-_Eudorina_ they differ from the female germ-cells or ova externally
-in form and size, as well as by being much more actively motile, and
-they contain green and subsequently yellow colouring matter, and a
-red eye-spot. We here find, for the first time among multicellular
-organisms, the differentiation of male and female germ-cells; and we
-learn from this that the essence of fertilization does not lie in this
-differentiation, since it may be absent, but that this distinction
-of female and male cells is only of secondary moment. From the fact
-that the egg-cells are larger and less active, the 'sperm-cells'
-or zoosperms smaller and livelier, we can already anticipate what
-will be more definitely established as our knowledge of the facts
-increases--that a differentiation according to the principle of
-division of labour has taken place even in the germ-cells, and that the
-first effect of this is to render the meeting of the cells destined for
-conjugation easier and more certain. The much smaller and more slender
-zoosperms swim about in the water in clusters until they come in
-contact with a female colony; then they separate from each other, bore
-their way into the soft jelly of the female colony, and 'fertilize' the
-egg-cell, that is to say, each male cell fuses with a female cell and
-forms a 'lasting spore,' exactly as in _Pandorina_.
-
-[Illustration: FIG. 63. _Volvox aureus_, after Klein and Schenck.
-_A_, besides the small flagellate somatic cells of the colony there
-are five large egg-cells (_t_) which are capable of parthenogenetic
-development, three recently fertilized egg-cells (_o_) and a number of
-male germ-cells (_a_) in process of multiplication. From each of these,
-by continued division, a bundle of spermatozoa arises. _B_, a bundle
-of thirty-two sperm-cells in process of development, seen from above.
-_C_, the same seen from the side. Magnified 687 times. _D_ individual
-spermatozoa, magnified 824 times.]
-
-In _Volvox_ the state of matters is similar to that in _Eudorina_;
-here again, in addition to the 'asexual' reproduction through the
-'Parthenogonidia' which are like egg-cells in appearance (Fig. 63,
-_A_, _t_), there are also male and female germ-cells which are usually
-produced alternately with the former, but sometimes at the same time,
-as in Fig. 63. The egg-cells are large and have no flagella, the
-sperm-cells lie together in clusters, and after they are mature (_D_)
-they swim freely in the water and then bore into another colony, where
-each unites with an egg-cell. The difference between the two kinds of
-cells consists therefore in the much greater number, the much smaller
-size, and the greater activity of the male cells, and in the smaller
-number but much larger size of the female cells--a differentiation
-in accordance with the principle of division of labour, depending on
-the fact that the two kinds of cells must reach each other, and yet
-must contain a certain mass of living protoplasm. While the small size
-but large number of male cells, combined with their motility, gives
-them an advantage in seeking out and boring into the female cells,
-the large size of the latter, on the other hand, makes up for the
-loss in mass which the fertilized egg would otherwise suffer from the
-diminution in size of the male cell. This difference in size may be
-greatly accentuated; thus in one of the brown sea-wracks, for instance,
-the spermatozoa are only 5 micro-millimetres in length, while the ova
-are spherical and have a diameter of 80-100 micro-millimetres, thus
-containing a mass 30-60,000 times greater (Möbius). Fig. 64 shows an
-ovum of this species surrounded by spermatozoa.
-
-In the course of the evolution of species this contrast between female
-and male germ-cells became more and more marked, not always in the same
-direction, however, but in one or another according to the conditions
-of fertilization. It would be erroneous to suppose that, with the
-higher differentiation of the organism as a whole, the differentiation
-of the germ-cells became increasingly complex. On the contrary we find
-even among Algæ, as the case of _Fucus_ shows, a marked difference
-between the sex-cells, which rather decreases than increases among many
-of the higher plants. It is not on the more or less complex structure
-of the organism itself that the nature and degree of the dimorphism
-of the germ-cells depends, but on the special conditions which are
-involved in each case, both in the union of the two kinds of sex-cells
-and in the subsequent development of the product of this union, the
-'fertilized ovum.'
-
-[Illustration: FIG. 64. _Fucus platycarpus_, brown sea-wrack. _Ei_,
-ovum, surrounded by swarming sperm-cells (_sp_). After Schenck.]
-
-Thus it comes about that the male or 'sperm-cells' of the lower plants,
-of the lower animals, and, again, of the highest animals are similar
-in structure. In all these organisms the male germ-cells exhibit the
-minuteness, the form, and the activity of the so-called 'zoosperms'
-or 'spermatozoa,' that is to say, they are thread-like, very minute
-corpuscles, which move rapidly forwards in water or other fluid with
-undulatory movements, and penetrate into the ovum with similar boring
-movements when they have been fortunate enough to reach their goal. At
-the anterior end they possess a more or less conspicuous thickening,
-the so-called 'head' in which the nucleus lies, and this is followed
-by the 'tail,' a thread-like structure consisting of cytoplasm which
-effects undulatory movements comparable to those of the flagella
-of Infusorians and Volvocineæ. The whole spermatozoon is thus a
-specialized 'flagellate cell.'
-
-When these 'zoosperms' were recognized as the 'fertilizing elements' in
-higher animals, and when 'sperm-threads' had been found, not only in
-all mammals and birds, reptiles, amphibians, and fishes, but even in
-many 'invertebrates,' the conclusion was suggested that the function
-of fertilization might be discharged by this lively motile substance;
-for until the eighth decade of the nineteenth century fertilization was
-still regarded by many as an 'awakening of life' in the egg. Since life
-depends on movement, in truth on infinitely fine molecular movements,
-of which the movement of the whole germ-cell from place to place is
-only a visible outcome, fertilization was pictured, by a not very
-luminous process of reasoning, as the awakening of life in the ovum--in
-itself incapable of further life--through the transference to it of
-movement through the agency of the zoosperm. Some investigators even
-went the length of regarding the ovum as 'dead organic material.'
-
-I mention this at this point, though I do not propose in the meantime
-to inquire further into the significance of the conjugation of the
-sex-cells. But the view just referred to is so completely refuted even
-by the external form of the male germ-cells in many groups of plants
-and animals, that I cannot discuss these differences in form without at
-the same time indicating the conclusions which they directly suggest.
-
-The great majority of plants and animals exhibit the zoosperm form of
-male germ-cells, and this must obviously be interpreted in the light of
-the fact that the ova to be fertilized are not generally to be found in
-direct proximity to the sperms shed by the male organism, but are at
-some distance from them. Among Medusæ and Polyps both male and female
-germ-cells are liberated into the water, simultaneously it may be,
-but separated from each other by distances of some feet or yards. The
-spermatozoa then swim about seeking the ova, which are also floating
-freely in the sea, guided by a power of attraction on the part of the
-latter--an attraction of whose nature we know nothing, though in the
-case of certain fern-ova it has been traced to the secretion of malic
-acid (Pfeffer).
-
-The same conditions obtain among Sponges. Here, again, the persons or
-stocks are either male or female; the latter produce large delicate
-ova, which lie in the interior of the sponge and there await the
-fertilizing sperms; the former give off the ripe sperms into the water
-in such abundance that thousands and millions of zoosperms burst
-forth simultaneously in all directions; these seek about for a female
-sponge, penetrate into its canal system, and so ultimately reach the
-ova. Of course only a very few of them reach their goal; the greater
-number are lost in the water and become the prey of Infusorians,
-Radiolarians, or other lowly animals. The fact that enormous numbers
-thus miss their true aim shows us why these zoosperms must be produced
-in such quantities; it is simply an adaptation to the extraordinarily
-high ratio of elimination in these cells, just as the number of young
-annually produced by an animal, or of seeds by a plant, is regulated
-by natural selection according to the ratio of elimination of the
-particular species. The more numerous the descendants which succumb
-each time to unfavourable circumstances, to enemies, or to lack of
-food, the more prolific must the species be. The same holds true of
-the regulation of the number of male germ-cells to be produced by an
-individual; there must be so many developed that, in spite of the
-unavoidable enormous loss, on an average the number of mature ova
-necessary to the maintenance of the species always receive spermatozoa.
-
-Also associated with the prodigal production of zoosperms is their
-minuteness, for the more zoosperms that can be developed out of a
-given mass of organic substance the smaller they are. Each species
-is restricted within definite limits of production by its size and
-the volume of its body, and there is thus an advantage in having the
-zoosperms of the smallest possible size whenever the chance of the
-individual sperm successfully reaching an ovum is very small. In all
-such cases nature has abstained from burdening the male germ-cell with
-an appreciable contribution of material to the result of conjugation,
-that is, to the foundation of the new organism; the passive ovum
-contains in itself alone almost all that is necessary to the building
-up of the embryo. Fertilization of the ovum by the liberation of the
-sperm-cells into the water occurs not only in animals of low degree,
-such as Sponges, Medusæ, Star-fishes, Sea-urchins and their relatives,
-but also in much higher animals, such as many Fishes and Amphibians,
-and in these the male cells have the form of motile threads. But the
-spermatozoon-form of male cell does not occur only in animals and
-plants which live in the water, or in those which, like mosses and many
-vascular plants, are at least occasionally covered by a thin layer of
-rain or dew, in which the zoosperms can swim to the ova, it occurs also
-in a very large number of animals in which the sperms pass directly
-into the body of the female, in those, therefore, in which copulation
-takes place.
-
-But even where copulation occurs we find that in most cases, as, for
-instance, in Vertebrates, Molluscs, and Insects, the zoosperm-form
-is retained. The reason for this is obviously twofold: in the first
-place, in many cases the sperms do not directly reach the ovum as a
-consequence of copulation, but may have to go a long way within the
-body of the female, as in mammals; or even when the way is short and
-certain, the ovum may be encased in a firm covering or shell difficult
-to penetrate, and the thread-like zoosperm has to face the task of
-boring its way through this shell, or slipping in through a minute
-opening, the so-called micropyle. In either case it would be difficult
-to imagine a form of sperm-cell better adapted to the fulfilment of
-this task than that of a thread with a thin, pointed head-portion and
-a long motile tail, which enables the zoosperm to twist itself like a
-screw through a narrow opening in the egg-envelope, whether the opening
-was previously present or not.
-
-We can thus understand why, among insects for instance, the male cells
-should always occur in the form of zoosperms, although in this case
-they reach a special receptacle in the female reproductive organs,
-the 'receptaculum seminis,' and are stored up in this. When a mature
-ovum gliding downwards through the oviduct comes to the place where
-this receptacle opens into it, the liberation of a few sperm-cells
-suffices to fertilize it with certainty, provided that they possess the
-thread-like form, which allows them to slip in through the very minute
-opening in the egg-envelope. It might be inferred from the certainty
-with which the ovum must in this case be found by the spermatozoon
-that only a small number of the latter would require to be produced,
-and yet even here the number is very large, though not so enormous
-as in the sea-urchins and other marine animals, which simply allow
-the sperm-cells to escape into the water. The large number in insects
-is due to the fact that many of the sperms may miss the micropyle;
-and also that in many insects a very large number of eggs have to be
-fertilized in succession. In the course of a life lasting three or
-four years the queen bee lays many thousand of eggs, most of which are
-fertilized, and that from a seminal receptacle which has been filled
-only once.
-
-There are, however, other sperm-cells of thread-like form which are
-not produced in such enormous multitudes, but in a much more moderate
-number, perhaps a few hundreds in the testicle. This is so in the
-little Crustaceans, known as Ostracods, all the freshwater species of
-which possess zoosperms only moderately numerous and of quite unusual
-size.
-
-The comparatively small number is explained by the certainty with which
-each of them reaches the ovum, and the large size may be accounted
-for in part by the small number which suffices, and which, therefore,
-admits of the male cell also carrying a considerable portion of the
-material for the building up of the embryo. Probably, however, the
-thickness and firmness of the covering of the ovum has something to
-do with it, for it has no opening for the entrance of the male cell,
-and it is fully hardened by the time fertilization takes place.
-Perhaps nowhere can we see more clearly how every little detail of the
-structure of the organism is dominated by the principle of adaptation
-than in the arrangements for fertilization, and notably in those which
-obtain in the Ostracods. I pass by the complicated apparatus for
-copulation, since we do not yet understand it fully in all particulars.
-According to my own investigations and those of my former students,
-Dr. Stuhlmann and Dr. Schwarz, the essential point seems to be that
-the colossally large zoosperms, which show no activity within the
-body of the male, leave it one at a time, so to speak, in single
-file. In copulation they are pressed out singly, one after the other,
-through a very fine tube, and then they enter, still singly, through
-the reproductive aperture of the female into an equally fine passage
-with spiral windings, through which they ultimately reach a roomy
-pear-shaped receptacle, the 'receptaculum seminis' of the female. There
-they lie in a long band composed of several hundreds, and only now
-attain their full maturity by throwing off an outer cuticle--moulting,
-so to speak. It is only when they get into a fluid medium that they
-show the power of undulatory movement, feeble at first, but gradually
-more energetic and more violent. And these movements enable them to
-penetrate like gimlets into the calcareous egg-shell. In the normal
-course it happens that when a mature ovum is deposited from the opening
-of the oviduct, one of the giant zoosperms at the same time, or shortly
-afterwards, leaves the 'receptaculum seminis' of the female by way of
-the spiral passage, and reaches the exterior just behind the ovum. The
-actual process of penetration has not been observed as yet, but the
-zoosperm has been seen at a slightly later stage spirally coiled inside
-the ovum.
-
-[Illustration: FIG. 65. Copulation in a Daphnid (Lyncæid). Emptying of
-the sperm (_sp_) into the brood-chamber of the female (♀). _abd_ ♂, the
-abdomen of the male. Magnified 100 times.]
-
-In these Ostracods the sperms are often visible with the naked eye, and
-in some species they are twice the length of the animal; they are thus
-emphatically giant cells, which can develop a very considerable boring
-power.
-
-In respect to the various adaptations of the sperm-cells to the
-conditions of fertilization there is hardly any group more interesting
-than the water-fleas or Daphnids.
-
-It is amazing how greatly the size of the sperms varies among the
-Daphnids, and how it stands in inverse proportion to their number,
-and how both are obviously regulated in relation to the difficulties
-which stand in the way of each sperm-cell before it can reach the ovum.
-In some species the sperm-cells are very large, in others extremely
-small. In the genera _Daphnia_, _Lynceus_, and others, copulation
-occurs as shown in Fig. 65; the sperm-cells (_sp_) are liberated by
-the male into the capacious brood-cavity of the female, which at the
-moment is closed to some extent by the abdomen of the male, in reality
-closed only partially at the posterior end and at the sides. It seems
-inevitable that a large proportion of the male elements should stream
-out again and be lost because of the violent movements of both animals.
-Accordingly, we find that the sperm-cells are only about the hundredth
-part of a millimetre in length and of round or rod-like form, and are
-voided in multitudes into the brood-cavity. Fig. 66, _f_, _g_, and
-_h_, show such cells in different species, as they occur in the testes
-to the number of many thousands. But in all the species in which the
-brood-cavity is _closed_, and in which therefore there is not such a
-serious loss of sperm-cells, the elements are much larger, and at the
-same time less numerous. They are largest and least numerous in species
-of genera like _Daphnella_, _Polyphemus_, and _Bythotrephes_, in which
-the males have a copulatory organ, so that the possibility of loss
-of the male cells is excluded. Thus the round, delicate, and viscid
-sperm-cells of _Bythotrephes_ (Fig. 66, _b_) are more than a tenth of a
-millimetre in length, but they are developed in proportionately smaller
-numbers, so that more than twenty are never found in the testis, and
-often only six or eight, while in copulation only from three to five
-are ejected. But as there are only two eggs to be fertilized at a time,
-and as the male cells are expressed into the brood-cavity directly upon
-the eggs, so that they immediately adhere to them, this small number is
-amply sufficient.
-
-[Illustration: FIG. 66. Spermatozoa of various Daphnids. _a, Sida._
-_b, Bythotrephes._ _c, Daphnella._ _d, Moina paradoxa._ _e, Moina
-rectirostris._ _f, Eurycercus lamellatus._ _g, Alonella pygmæa._ _h,
-Peracantha truncata._ All magnified 300 times.]
-
-It is remarkable how different the sperm-cells sometimes are in quite
-nearly related species of Daphnids, as a glance at Fig. 66 will show;
-and, on the other hand, how similar they may be in two species which
-belong to different families, like _Bythotrephes longimanus_ (_b_),
-and _Daphnella hyalina_ (_c_). The last fact may be explained as an
-adaptation to similar conditions of fertilization. Both species have
-effective copulatory organs, and their large delicate sperm-cells must
-immediately adhere when they come into contact with the shell-less
-ovum, and penetrate into it by means of amœboid processes. Conversely,
-the difference between sperm-cells of allied species like _Sida
-crystallina_ (_a_), _Moina rectirostris_ (_e_) and _M. paradoxa_ (_d_)
-is related to different adaptations to nearly the same conditions of
-fertilization. In _Sida_ (Fig. 66 _a_) the large flat sperm-cells, with
-their fringed ends and their large soft surface, adhere easily to the
-ova, and the same end is attained in _Moina rectirostris_ by means of
-stiff radiating processes, while in the nearly related species, _Moina
-paradoxa_, the male cell (_d_) resembles an Australian boomerang and
-presses in like a wedge between the ova and the wall of the brood-sac.
-
-[Illustration: FIG. 67. Spermatozoa of various animals, after
-Ballowitz, Kölliker, and vom Rath. 1, man. 2, bat (_Vesperugo_). 3,
-pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (_Raja_). 8, beetle. 9,
-mole-cricket (_Gryllotalpa_). 10, freshwater snail (_Paludina_). 11,
-sea-urchin. Much magnified.]
-
-In Fig. 67 a small selection of animal male cells is figured, all of
-which take the form of sperm-threads or spermatozoa, and yet they
-differ very much from one another in detail. It would undoubtedly
-be of great interest to follow out these minute adaptations of the
-sperm-cells to the conditions of fertilization, and to demonstrate
-that their size, and especially their form, in the different species
-of animals are adjusted to the special constitution of the ovum, its
-envelope, and its micropyles, and to the ease or difficulty with which
-it can be reached; but much information must be forthcoming before we
-can even suggest, for instance, why the sperm-cell of the salamander
-is so enormously long, large, and pointed at the head, while that of
-Man (Fig. 67, 1) is comparatively short, with broad, flat head and a
-recently discovered minute apex; or why those of Man and many fishes
-(such as _Cobitis_) should be so much alike, and so on. From many
-sides, however, we are led to conclude that even down to the minutest
-details nothing is in vain, and that everything depends on adaptation.
-
-In general, even the peculiarities of form already indicate this; thus
-the spirally coiled structure of the head, which is especially well
-developed in the spermatozoa of birds (Fig. 67, 5), in those of the
-skate (7), and of the freshwater snail (_Paludina_) (10), works like a
-corkscrew, and makes it possible for the spermatozoon to pierce through
-the resistant envelope of the ovum. Similarly, the sharply pointed head
-of the insect spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping
-through the minute pre-formed micropyle in the hard egg-shell.
-
-Of the detailed and complicated structure of spermatozoa we have only
-recently been made aware through the increasing perfection of the
-microscope and of technical methods of investigation. Fig. 68 shows
-one after a diagrammatic figure by Wilson. We see the apical point
-(_sp_) for boring into the ovum, the nucleus (_n_) surrounded by a thin
-layer of protoplasm, which together form the head, then the middle
-portion (_m_) which contains the 'centrosome' (_c_), and the 'tail' or
-'flagellum' which effects the movement of the whole and which itself
-possesses a complex structure with an 'axial filament' (_ax_) and an
-enveloping layer, the latter often drawn out into a spirally twisted,
-undulating membrane of the most extreme delicacy, as is most clearly
-seen in the newt (Fig. 67, 6).
-
-[Illustration: FIG. 68. Diagram of a spermatozoon, after Wilson. _sp_,
-apical point. _n_, nucleus. _c_, centrospere. _m_, middle piece. _ax_,
-axial filament. _e_, terminal filament.]
-
-Not in the Daphnids alone, but in other groups of Crustaceans as
-well, sperm-cells of quite peculiar form occur, as, for instance, in
-the crayfish and its marine relatives, the crabs and the long-tailed
-Decapods. In these cases the spermatozoa bear long and stiff thorn-like
-processes, which, as in the sperm-cells of _Moina_, make them adhesive,
-and, according to Brandes, render it possible for them to cling among
-the bristles on the abdomen of the female until one of the many eggs
-leaving the oviduct comes within reach. For among these Crustacea
-there is no true copulation, but the masses of sperm-cells are packed
-together into sperm-packets or 'spermatophores,' and are affixed by the
-male near the opening of the oviduct, where they burst and pour forth
-their contents between the appendages of the female.
-
-All these remarkable and widely divergent structures and arrangements
-depend not upon chance or on the fantastic expression of a 'formative
-power,' as an earlier generation was wont to phrase it; they are
-undoubtedly without exception adaptations to the most intimate
-conditions of fertilization in each individual case. I lay particular
-stress upon a recognition of this, because it permits us to infer with
-certainty that even the variations of the single cell, if they are
-sufficiently important for the species, may be controlled by natural
-selection. It is obvious that the adaptations of the sex-cells must
-depend not on histonal selection, but only upon personal selection,
-since it is indifferent for the individual sperm-cells whether
-fertilization is accomplished successfully or not, while it is by
-no means indifferent for the species. The organism dies without
-descendants if its sperm-cells do not fertilize, and the carrying on
-of the species must be left to those of its fellows which produced
-sperm-cells which fertilize with more certainty; thus it is not
-the sperm-cells themselves, but the individual organisms which are
-selected, and that in relation to the quality of the sex-cells they
-produce.
-
-In contrast with the great diversity of form exhibited by the
-spermatozoa, the differentiation of the ovum appears very uniform,
-at least in regard to form and activity. The main form is spherical,
-but it is subject to many variations in the way of elongation or
-flattening. In the lower forms of life, as, for instance, among the
-sponges, and also in the polyps and Medusæ the egg-cells possess, until
-they are mature, the locomotor capacity of unicellular organisms;
-they creep about after the manner of amœbæ, and indeed, as I showed
-years ago, this movement from place to place in many polyps is exactly
-regulated; thus at a definite time they may leave the place where they
-originated and may, for instance, creep from the outer layer of cells
-(ectoderm) of the animal into the inner layer (endoderm) by boring
-through the so-called 'supporting lamella,' then they may creep further
-in the endoderm, and finally return to quite definite and often remote
-spots in the ectoderm (_Eudendrium_, Fig. 95). In another hydroid polyp
-(_Corydendrium parasiticum_) the mature egg-cells leave their former
-position within the endoderm and creep entirely outside of the animal
-which produced them, establishing themselves in a definite spot on its
-external surface, where they await the fertilizing zoosperms. Many ova
-can accomplish slight amœboid movements, but in most animals these
-do not suffice for movement from place to place, and the ova remain
-quietly in the spot where they were developed, or are passively pushed
-to another. Cases such as that of the polyp I have cited, in which the
-ovum actually comes to meet the male element, are quite exceptional,
-for in general the ovum is the passive and the spermatozoon the active
-or exploring element in fertilization. The female cell is entrusted
-with procuring and storing the material necessary to the building up of
-the embryo; and its peculiarities depend chiefly on this.
-
-[Illustration: FIG. 69. Ovum of the Sea-urchin, _Toxopneustes lividus_,
-after Wilson. _zk_, cell-body. _k_, nucleus or so-called 'germinal
-vesicle,' _n_, nucleolus or so-called 'germinal spot.' Below there is
-a spermatozoon of the same animal, with the same magnification (750
-times).]
-
-It is true that in plants this stored material is seldom considerable,
-and that is because the ovum so frequently remains even after
-fertilization within the living tissues of the plant, and is thence
-supplied, often very abundantly, with food-stuffs; and, moreover,
-because the young plant that springs from the fertilized ovum maybe
-very small and simple, and yet capable of immediately procuring its
-own nourishment. But there are exceptions to this; thus the ova of the
-brown sea-wracks, or Fucaceæ, for instance, are quite twenty times
-as large as the ordinary cells of the algæ (Fig. 64), and contain a
-quantity of food-stuff within themselves. In this case the ova are
-liberated into the water even before fertilization, and the nutrition
-of the embryo from the mother-plant is excluded.
-
-In these Algæ we meet, for the first time, with a special organ in
-which the ova arise. In animals this is much more generally the case,
-and from sponges upwards there are always quite definite parts and
-tissues of the body which are alone able to develop eggs, and these
-are usually well-defined organs of special structure, the ovaries.
-Similarly, in male animals the spermatozoa arise in special places, and
-usually in special organs, the spermaries or testes.
-
-Animal ova often consist of more than the simple cell-body, the
-protoplasm and its nucleus; they almost always contain in the cell-body
-a so-called 'Deutoplasm,' as Van Beneden has fittingly named the
-yolk-substance. This consists of fats, carbohydrates, or albuminoids,
-which often lie in the cell-body in the form of spherules, flakes, or
-grains--a nutritive material that is often surrounded and enclosed
-by a small quantity of living matter or formative protoplasm. Apart
-from these stores of yolk it would be impossible for a young animal
-to develop from the ovum of a snake or a bird, for such highly
-differentiated animals could not be formed from an egg of microscopic
-dimensions if this remained without some supply of food from outside of
-itself during the period of development. There is obviously need for a
-considerable amount of building material, so that all the organs and
-parts, which are composed of thousands and millions of cells, may be
-developed.
-
-Thus the size of the animal-ovum depends essentially on the quantity of
-yolk that has to be supplied to the egg, and this depends in the main
-on whether the egg is still drawing nourishment from the mother during
-the development of the young animal. Therefore, as a general rule,
-eggs which are laid, and are surrounded and protected by a shell, are
-usually much larger than the eggs of animals which go through their
-development within the body of the mother. The best known illustration
-of this proposition is offered by mammals and birds, animals of equally
-high organization and comparable in bodily size. While the eggs of
-birds may be as much as 15 centimetres in length, and may weigh 1½
-kilogrammes, those of most mammals remain microscopically minute, and
-scarcely exceed a length of 0.3 millimetres. The same principle is
-often illustrated within one and the same small group of animals, and
-even in the same species. Here, again, the Daphnids or water-fleas may
-serve as an example.
-
-Among these there are two kinds of eggs, summer and winter eggs, of
-which the former go through their development into a young animal
-within a brood-cavity on the back of the female, while the others are
-liberated into the water, and are surrounded by a hard shell. The
-summer eggs receive more or less nourishment from the mother by the
-extravasation of the nutritive constituents of the blood into the
-brood-cavity, and they thus require a smaller provision of yolk than
-the winter eggs, which are thrown entirely upon their own resources.
-Accordingly we find that in all Daphnids the summer eggs are at least
-a little smaller and have less yolk than the winter eggs, as in the
-genus _Daphnella_ (Fig. 70, _A_ and _B_), while in some species, e.g.
-of _Bythotrephes_, this difference increases so much that the summer
-eggs are almost without yolk, and therefore very minute (Fig. 71, _B_).
-The reason of this lies in the fact that in this case the brood-sac
-is filled with a nutritive fluid rich in albuminoid substances, so
-that the embryo during its development is continually supplied with
-concentrated nourishment. This is not the case with the winter eggs,
-because these are liberated into the water, and we therefore find that
-they are of enormous size and quite filled with yolk (Fig. 71, _A_).
-
-[Illustration: FIG. 70. _Daphnella._ _A_, summer egg. _B_, winter egg.
-_Oe_, 'oil-globules' of the summer egg.]
-
-[Illustration: FIG. 71. _Bythotrephes longimanus._ _A_, the brood-sac
-(_Br_) of the female containing two winter-ova (_Wei_), on which
-five large sperm-cells (_sp_) are lying. _R_, dorsal surface of the
-animal. _Dr_, glandular layer which secretes the shell-substance. _BK_,
-copulatory canal. _B_, the brood-sac (_Br_) containing two summer-ova
-(_Sei_). Both figures under the same magnification (100).]
-
-In this instance, as in all the simpler eggs, the yolk constituents
-are secretions of the cell-body of the ovum; but nature employs many
-devices, if I may so speak, to bring up the mass of the egg, and
-especially of the yolk, to the highest attainable point. Thus in many
-orders of Crustaceans, for instance in the water-fleas just mentioned,
-there are special egg-nourishing cells, that is, young ovum-cells
-which do not differ from the rest either in origin or in appearance,
-only they do not become mature eggs, but at a definite time cease to
-make progress, and then slowly break up, so that their substance may
-be absorbed as food by the true ova. Thus there is a much greater and
-at the same time more rapid growth than could be attained through
-nourishment from the blood alone. In the Daphnids the ovaries consist
-of groups of four cells each, only one of which becomes an ovum (Fig.
-72, _Ei_), while the other three (1, 2, and 4) form nutritive cells
-which break up. This is so in all summer eggs; but in the winter eggs
-a much larger number of nutritive cells may take part in equipping a
-single ovum, and in the genus _Moina_ over forty do so. But here the
-difference in size between the two kinds of eggs is very marked, the
-winter eggs being twice the diameter of the summer eggs.
-
-[Illustration: FIG. 72. _Sida crystallina_, a Daphnid: a fragment of
-the ovary showing one of the groups of four cells, of which 1, 2, and 4
-are nutritive cells, and only 3 becomes an ovum. Magnified 300 times.]
-
-In many insects also, e.g. in beetles and bees, similar nutritive cells
-occur, but there is in these forms a different arrangement which serves
-at the same time for the formation of the shell, and the supplying to
-the ovum of the necessary yolk-stuffs--the ovum is surrounded with a
-dense layer of epithelial cells, a so-called 'follicle.' In mammals and
-birds also these 'follicle cells' certainly play an important part in
-the nutrition of the ovum, though it is not yet quite clear how they
-act--whether they produce within themselves grains of yolk and other
-nutritive substances and convey these to the ovum by means of fine
-radiating processes, or whether they themselves ultimately migrate into
-the ovum and there break up. In any case it is worthy of note that
-all these follicular cells in insects and vertebrates have the same
-origin as the egg-cells, that is, they are modified germ-cells. The
-case is therefore essentially the same as in the nutritive cells of the
-Daphnids; nature sacrifices the greater number of the germ-cells in
-order to be able to provide more abundantly for the minority. She thus
-succeeds in raising the egg beyond itself, so to speak, and provides
-the means for a growth which could obviously not be attained by means
-of the ordinary nourishment supplied by the blood.
-
-[Illustration: FIG. 73. Diagrammatic longitudinal section of a hen's
-egg before incubation, after Allen Thomson. _Bl_, germinal disk. _GD_,
-yellow yolk. _WD_, white yolk. _DM_, vitelline membrane. _EW_, albumen.
-_Ch_, chalaza. _S_, shell membrane. _KS_, shell. _LR_, air chamber.]
-
-We now understand why the eggs of many animals should be of such
-enormous size and often of such complex structure. The eggs of birds
-are especially remarkable in this respect, and it has till recently
-been disputed whether they are really morphologically equivalent to
-a single cell. But this is undoubtedly the case, and though only the
-small thin germinal disk (Fig. 73, _Bl_) with its nucleus is the active
-part of this cell--the cell-body proper--yet all the rest--the enormous
-sphere of yolk with its regular layers of yellow (_GD_) and white
-(_WD_) yolk, the concentric layers of fluid albumen (_EW_) round about
-this, the chalazæ (_Ch_), and finally, the delicate shell membrane
-(_S_) and the limy shell (_KS_)--belong to this cell, and have arisen
-in connexion with it (Fig. 73).
-
-
-
-
-LECTURE XV
-
-THE PROCESS OF FERTILIZATION
-
- Cell-division and nuclear division--The chromatin as the material
- basis of inheritance--The rôle of the centrosphere in the mechanism
- of division--The Chromosomes--Fertilization of the egg of the
- sea-urchin according to Hertwig--Of the egg of Ascaris according
- to Van Beneden--The directive divisions, or the extrusion of the
- polar bodies--Halving of the number of chromosomes--The same in
- the sperm-cell--Reducing division in parthenogenetic eggs--In
- the bee--Exceptional and artificial parthenogenesis--Rôle of the
- centrosphere in fertilization and in parthenogenesis.
-
-
-NOW that we have made ourselves acquainted with the two kinds of
-germ-cells on the union of which 'sexual reproduction' depends, we may
-proceed to a more detailed discussion of the process of fertilization
-itself. But it is indispensable that we should take account of the
-processes of nuclear and cell-division, as these have been gradually
-recognized and understood in the course of the last decade. It may
-appear strange that the processes of division should throw light on the
-apparently opposite processes of cell-union, but it is the case, and
-no understanding of the latter is possible without a knowledge of the
-former.
-
-From the time of the discovery of the cell until well on in the sixties
-the process of cell-division was looked on as a perfectly simple
-process, as a mere constriction in the middle of the cell. It was
-observed that a cell in the act of dividing (Fig. 59, _A_) stretched
-itself out, that its nucleus also became longer, became thinner in the
-middle, assumed a dumb-bell form, and was then gradually constricted,
-giving rise to two nuclei (_B_), whereupon the body of the cell also
-constricted and the two daughter-cells were formed (_C_). In certain
-worn-out or highly differentiated cells a cell-division of this kind
-really seems to occur--the so-called 'direct' division--but in young
-cells, and indeed in all vigorous cells, the process, which looks
-simple, is, in reality, exceedingly complex. Not only is the structure
-of the nucleus incomparably more complex than was recognized a quarter
-of a century ago, but nature has placed within the cell a special and
-marvellously intricate apparatus, by means of which the component parts
-of the nucleus are divided between the two daughter-nuclei.
-
-For a long time all that was distinguished in the cell-nucleus was
-the nuclear membrane and a fluid content in which one or more nuclear
-bodies or nucleoli float. But this does not by any means exhaust
-what can now be recognized in the structure of the nucleus, and the
-most important constituents are not even among these, for recent
-researches, especially those of Häcker, have shown that the nucleolus
-or the nucleoli, to which there was formerly an inclination to attach
-a very high importance, must be regarded as only transient formations
-and not living elements--in fact, as mere collections of organic
-substance--'bye-products of the metabolism,' which at a definite time,
-that is just before the division of the nucleus, disappear from the
-nuclear space and are used up. We now know that in the resting cell,
-that is, in the cell which is not in the act of dividing (Fig. 74,
-_A_), a very fine network of pale threads, often very difficult to
-make visible, fills the whole nuclear cavity, like a spider's web or
-the finest soap bubbles, and that in this so-called nuclear framework
-there are embedded granules of rounded or angular form (_A_, _chr_)
-which consist of a substance which stains deeply with such pigments as
-carmine, hæmatoxylin, all aniline dyes, &c., and which has therefore
-received the name of chromatin. Often, indeed generally, these granules
-are exceedingly small, but sometimes they are bigger, and in that case
-they are less numerous and more easily made visible; in all cases,
-however, they are in a certain sense the most important part of the
-nucleus, for we must assume that it is their influence which determines
-the nature of the cell, which, so to speak, impresses it with the
-specific stamp, and makes the young cell a muscle-cell or a nerve-cell,
-which even gives the germ-cell the power of producing, by continued
-multiplication through division, a whole multicellular organism of a
-particular structure and definite differentiation, in short, a new
-individual of the particular species to which the parents belong. We
-call the substance of which these chromatin granules consist by the
-name first introduced into science by Nägeli, though only to designate
-a postulated substance which had not at that time been observed,
-but which he imagined to be contained within the cell-body--by the
-name _Idioplasm_, that is to say, a living substance determining the
-individual nature (εἶδος = form). I am anticipating here, and I reserve
-a more detailed explanation until I can gradually bring together all
-the facts which justify the conception I have just indicated of the
-'chromatin grains' as an 'idioplasm,' or, as we may also call it, a
-'hereditary substance.'
-
-That this chromatin must be something quite special we see from the
-processes of cell and nuclear division, which I shall now briefly
-describe.
-
-[Illustration: FIG. 74. Diagram of nuclear division, adapted from E.
-B. Wilson. _A_, resting cell with cell-substance (_zk_), centrosphere
-(_csph_) which contains two centrosomes, nucleolus (_kk_); and
-chromosomes (_chr_), the last distributed in the nuclear reticulum.
-_B_, the chromatin united in a coiled thread; the centrosphere divided
-into two and giving off rays which unite the halves. _C_, the nuclear
-spindle (_ksp_) formed, the rays more strongly developed, the nuclear
-membrane (_km_) in process of dissolution, the chromatin thread
-divided into eight similar pieces (_chrs_), the rays are attaching
-themselves to the chromosomes. _D_, perfected nuclear spindle with
-the two centrospheres at the poles (_csph_) and the eight chromosomes
-(_chrs_) in the equator of the spindle, all now longitudinally split.
-_E_, daughter-chromosomes diverging from one another, but still united
-by filaments, the centrosomes (_cs_) are already doubled for the next
-division. _F_, daughter-chromosomes, quite separated from one another,
-are already beginning to give off processes; the cell-substance is
-beginning to be constricted. _G_, end of the process of division:
-two daughter-cells (_tz_) with similar nuclear reticulum (_tk_) and
-centrospheres (_csph_), as in _A_.]
-
-When a cell is on the eve of dividing we observe first that the
-chromatin grains, which have till then been scattered throughout the
-network of the nucleus, approach each other and arrange themselves
-into a long thin thread which, irregularly intertwined, forms a loose
-skein, the so-called coil-stage (Fig. 74, _B_). The thread then begins
-to thicken, and somewhat later it can be seen to have broken up into
-a number of pieces of equal length, as if it had been cut into equal
-pieces with scissors (_C_).
-
-These pieces or chromosomes become shorter by slowly contracting,
-and thus each takes the form of an angular loop, a straight rod, or
-a roundish, oval, or spherical body (Fig. 74, _C_, _chrs_). While
-this is happening, we can see at the side of the nucleus, and closely
-apposed to it, a pale longitudinally striped figure with a swelling,
-similar to a handle, at both ends--the so-called nuclear spindle or
-central spindle (_ksp_). This is the apparatus for the division of the
-nucleus, and it was previously represented by a small body susceptible
-to certain stains--the centrosome, which was surrounded by a halo-like
-zone, the centrosphere or 'sphere.' This body was long overlooked,
-but now the majority of investigators assume that, though it is often
-inconspicuous and very difficult to make visible, it is nevertheless
-present in every cell which is capable of division, and that it is
-therefore a permanent and indispensable constituent of the cell (Fig.
-74, _A_ and _B_, _csph_).
-
-When a cell is on the point of dividing, this remarkable cell-organ,
-which has hitherto seemed no more than an insignificant, pale, little
-sphere, now becomes active. First of all, often before the formation
-of the chromatin coil, it doubles by division (_A_ and _B_ _csph_), at
-first only as regards the centrosome, and then as regards the sphere
-also (_B_); and while division is going on fine protoplasmic filaments
-issue from the dividing sphere and radiate like rays from a sun into
-the cell-substance. As they only retain their connexion with each other
-at the surfaces of the dividing halves of the sphere which are turned
-towards each other, we might almost say that fine threads are drawn out
-between the two halves as they separate, and these become longer the
-further apart the halves diverge. In this manner the much-talked-of
-'spindle figure' arises, which was first described in the seventies
-through the researches of A. Schneider, Auerbach, and Bütschli, but the
-significance and origin of which have claimed the labours of many later
-investigators down to our own day.
-
-The processes now to be described do not always take place in exactly
-the same manner, but the gist of the business is everywhere the same,
-and it consists in this, that the two ends or 'poles' of the spindle
-diverge further and further apart, and between them lies the nucleus
-whose membrane now disappears (_C_, _km_) while the spindle threads
-traverse its interior. Sometimes the membrane is retained, but
-nevertheless the spindle threads penetrate into the interior of the
-nucleus. But the chromosomes always range themselves quite regularly
-in the 'equatorial plane' of the spindle (_D_, _aeq_)--a process the
-precise mechanism of which is by no means clearly understood, and
-indeed the play of the forces in the whole process of nuclear division
-is still very imperfectly revealed to our intelligence.
-
-Thus we have now before us a pale, spindle-shaped figure, which takes
-only a faint stain, with the 'suns' (_cs_) at its 'poles,' and in its
-equatorial plane the loop- or rod-shaped, or spherical chromosomes
-(_chrs_). The whole is designated the 'karyokinetic,' the 'mitotic,' or
-the 'nuclear division figure.'
-
-The meaning and importance of this, at first sight, puzzling figure
-will at once become clear from what follows. It may be observed at this
-stage, if not even long before, that each of the chromatin rods or
-loops has split along its whole length like a log of wood, and that the
-split halves are beginning slowly and hardly noticeably to move away
-from each other, one half towards one, the other towards the other pole
-of the spindle (Fig. _D_ and _F_). Directly in front of the centrosome
-they make a halt, and now the material for the two daughter-nuclei
-is in its proper place (_F_, _chrs_). These develop quickly, each
-chromosome group surrounding itself with a nuclear membrane (Fig.
-_G_) within which the chromosomes gradually become transformed again
-into a nuclear network. Within the chromatin substance proper this is
-scattered about in small roundish or angular granules, lying especially
-at the intersecting points of the network. It may be stated at once,
-though the full significance of the statement can only be appreciated
-later, that we may assume with probability that this breaking up of the
-chromosomes is only apparent, and that these rods or spheres really
-continue to exist in the nuclear network, only in a different form,
-greatly spread out, somewhat after the manner of a Rhizopod which
-stretches out fine processes in all directions. These processes branch
-and anastomose, so that the body, which previously seemed compact,
-now appears as a fine network. In point of fact, it can be directly
-observed that the chromosomes, after the nucleus is completely divided
-into two daughter-nuclei, send out pointed processes (_F_ and _G_)
-which gradually increase in length and branch, while the body of the
-chromosome itself becomes gradually smaller. It is thus probable
-that, when such a daughter-nucleus is on the point of dividing anew,
-it may, by a drawing together of the processes or pseudopodia of the
-chromosomes, produce the same rods or spheres as those which previously
-gave rise to the network. More definite reasons for this interpretation
-will be adduced later on. In any case, the chromosomes, even in
-their compact rod-like state, consist of two kinds of substance,
-the chromatin proper, which stains deeply, and the linin, which is
-difficult to stain; and it is the latter which, by breaking up, forms
-the pale part of the nuclear network.
-
-Thus we can understand that the number of chromosomes remains the same
-in every cell-generation throughout development, as it is the same
-in all the individuals of a species. The numbers are known for many
-species: in some worms there are only two or four chromosomes, while
-in other related worms there are eight; in the grasshopper there are
-twelve, and in a marine worm, _Sagitta_, eighteen; in the mouse, the
-trout, and the lily there are twenty-four; in some snails thirty-two;
-in the sharks thirty-six, and in _Artemia_, a little salt-water
-crustacean, 168 chromosomes. In Man the chromosomes are so small that
-their normal number is not certain--sixteen have been counted. This
-counting can only be done during the process of nuclear division, for
-afterwards the chromosomes flow indistinguishably together, or rather
-apart, only to reappear, however, in the old form and number whenever
-the nucleus again begins to divide.
-
-It remains to be told what becomes of the centrosphere in
-cell-division. As soon as the formation of the daughter-nuclei has
-been brought about by the divergence of the split halves of the loops,
-the spindle figure begins to retrograde, its threads become pale and
-gradually disappear, as does the whole radiate halo of the centrosphere
-(Fig. _F_ and _G_). The cell-body has by this time also divided in the
-equatorial plane of the nuclear spindle, and the centrosome remains
-usually as a very inconspicuous pale body lying in the cytoplasm close
-to the nucleus, reawakening to renewed activity when cell-division is
-about to recommence (_G_, _csph_).
-
-These, briefly, are the remarkable processes of nuclear division. Their
-net result is obvious; the chromatin substance is divided between the
-daughter-nuclei with the greatest conceivable accuracy.
-
-It is not so easy to understand the mechanism of this partition, and
-there are various divergent theories on this point. According to the
-older idea of Van Beneden, the spindle fibres work like muscles, and
-by contracting draw the halves of the chromosomes which adhere to them
-towards the pole, while the rest of the fibres radiating out from
-the polar corpuscles act as resisting and supporting elements. This
-view, with many modifications however, has still its champions, and M.
-Heidenhain in particular has made a notable attempt to establish it
-and to work it out in detail. Opposed to it stand the views of those
-who, like O. Hertwig, Bütschli, Häcker, and others regard the rays not
-as specific elements which were pre-formed in the cell, but as the
-expression of the orientation of certain protoplasmic particles--an
-orientation evoked by forces which have their seat within the central
-corpuscles, and act in the manner of magnetic or electric forces. That
-the central corpuscles are centres of attraction seems to me hardly
-open to doubt, and I cannot regard the regular arrangement of the
-chromosomes in the equatorial plane of the spindle as due to a mere
-adhesion to contractile threads. Some still unknown forces--chemotactic
-or otherwise--must be at work here. Later on we shall study the
-phenomenon of the migration of the sperm-nucleus into the ovum, when it
-is accompanied by its central body and its halo of rays. Häcker seems
-to me justified in inferring from this phenomenon alone that the sudden
-origin of the rays is due to forces resident in the central corpuscle.
-But undoubtedly even this 'dynamic' explanation of karyokinesis is
-still only at the stage of hypothesis and reasoning from analogy, and
-is far removed from a definite knowledge of the forces at work.
-
-For the problems with which we are here chiefly concerned, the problems
-of heredity, it is enough to know that the cells of multicellular
-organisms possess an extremely complex apparatus for division, whose
-chief importance lies in the fact that through it the chromatin units
-of the nucleus are divided into precisely equal parts, and so separated
-from each other that one half forms one daughter-nucleus, the other
-half the other. It is not merely that there is an exact division of the
-whole chromatin in the mass, which could have been effected much more
-simply, but that there is _a regulated distribution of the different
-qualities of the chromatin_, as we shall see later.
-
-It must here be emphasized that the splitting of the chromosomes does
-not depend on external forces, but on internal ones involved in their
-organization, and in the definite attractions and repulsions of their
-component particles which come about in the course of growth. The
-chromosomes do not split like a trunk that has been broken open with
-an axe, but rather like a tree burst apart by the frost, that is, by
-the freezing of the water within itself. I consider it very important
-that we should recognize this, even though we do not yet know what
-the forces are that have control in this case, because it leads us to
-conclude that the structure of the chromosomes is extremely complex,
-that they are, so to speak, a world in themselves, that they possess an
-infinitely complex and delicate though invisible organization, in which
-intrinsic chemico-physical forces produce the regulated succession
-of changes which we observe. We shall afterwards see that we are led
-to the same conclusion from another direction--that is, from the
-phenomena of inheritance. We shall then recognize that the rod- or
-loop-shaped chromosomes cannot be simple elements, but are composed
-of linear series of 10, 20, or more globular single-chromosomes, each
-of which represents a particular kind of chromatin or hereditary
-substance. If we consider this carefully, we shall see that it would
-hardly be possible to think out a mode of nuclear division which would
-so exactly and securely fulfil the purpose of conveying these many
-kinds of chromatin to the two daughter-nuclei in like proportions as
-does the mechanism of distribution actually brought about by nature.
-The longitudinal splitting of the rods halves the chromosomes, and the
-spindle apparatus secures the proper distribution of the halves between
-the two daughter-nuclei.
-
-So much, at least, is certain, that no such complicated mechanism for
-'mitotic' division would have arisen if the very precise division of
-a substance _of the highest importance_ had not been concerned, and
-in this conclusion lies the first hint of the interpretation of the
-chromatin substance as the bearer of the hereditary qualities.
-
-We are now familiar with the cell-nucleus and the apparatus for
-its division, and we are thus fully prepared to begin the study of
-the phenomena of 'fertilization.' Here also the processes depend
-essentially on the behaviour of the cell-nuclei, for even the first
-observations made by O. Hertwig on the behaviour of the spermatozoon
-after it has penetrated into the ovum led to the suggestion that the
-essential fact is the union of two nuclei; and numerous later, more and
-more deeply penetrating researches have furnished abundant evidence
-that the so-called 'fertilization' _is essentially a nuclear fusion_.
-
-Let us begin with O. Hertwig's observations on the ovum of the
-sea-urchin. Eggs of this animal, which have been taken out of the ovary
-of the female, may easily be fertilized artificially by pouring over
-them spermatic fluid taken from a male, and diluted with sea-water.
-Before this is done only one nucleus can be observed in the ovum, but
-shortly afterwards two nucleus-like structures of unequal size can
-be seen within the ovum, and the smaller is surrounded by a circle
-of rays. Hertwig rightly interpreted this smaller nucleus as the
-modified remains of the penetrating spermatozoon, which then slowly
-approaches the nucleus of the egg, and ultimately fuses with it to form
-a 'segmentation nucleus.' From this starts the so-called 'segmentation'
-of the ovum, that is, the series of repeated divisions resulting in the
-formation of an ordered mass of cells, which by continued division of
-cells builds up the embryo.
-
-Simple as this process of nuclear conjugation may seem, it was by no
-means so easy to recognize, and several investigators, especially
-Auerbach, Schneider, and Bütschli, had seen stages of the process at
-an earlier date without arriving at the true interpretation of the
-phenomena. This was chiefly due to the fact that, in addition to the
-phenomena of fertilization proper, which we have briefly sketched,
-other nuclear changes take place in the maturing ovum, and these are
-not very easy to distinguish from the former; we refer to the phenomena
-of the so-called 'maturation of the ovum.' When the ovum-cell has
-attained its full size within the ovary it is not yet capable of being
-fertilized, but must first undergo two processes of division, to the
-right understanding of which Hertwig's investigations, and afterwards
-those of Fol, have contributed much.
-
-For a long time it had been a familiar observation that small
-refractive corpuscles were extruded from one pole of the ovum shortly
-before the beginning of embryonic development. These were called 'polar
-bodies,' because it was believed that they marked the place which would
-afterwards be intersected by the first plane of division; it was only
-known at that time that they had to be extruded from the egg, but no
-one had the remotest idea of their real nature.
-
-We now know that they are cells, and that their origin depends on a
-twice repeated division of the egg-cell; but it is a very unequal
-division, for these 'directive cells' or 'polar bodies' are always much
-smaller than the ovum, and indeed are usually so small that it is easy
-to understand why their cellular nature was for so long overlooked.
-Yet they have always a cell-body, and in many ova, for instance those
-of certain marine Nudibranchs, this is quite considerable; and they
-have likewise always a nucleus, which, notwithstanding the smallness of
-the cell-body, is in all cases exactly of the same size as the sister
-nucleus which remains behind in the ovum after division--a fact which
-is in itself enough to indicate that we have here to do essentially
-with readjustments and changes in the nucleus of the ovum.
-
-Long before the polar or directive divisions were recognized as
-divisions of the egg-cell it was known that the nucleus of the ovum
-disappeared as soon as the latter attained to its full size within
-the ovary. It was also known that this nucleus--the large so-called
-'germinal vesicle' lying in the middle of the ovum--left its central
-position and moved to the upper surface of the ovum, there to become
-paler and paler, and ultimately to disappear altogether from the
-sight of the observer. By many it was believed that it broke up, and
-that the 'segmentation nucleus,' which is afterwards obvious, is a
-new formation. The truth is that the germinal vesicle, at the time
-of its disappearance, is transformed into a division figure which
-is invisible without the aid of artificial staining. The nuclear
-membrane breaks up; the centrosome of the ovum, which, although hardly
-visible, had previously lain beside the germinal vesicle, divides
-into two centrosomes and their centrospheres, and these now form the
-'mitotic figure' by moving away from each other and sending out their
-protoplasmic rays. This nuclear spindle soon ranges itself at right
-angles to the surface of the egg, which at the same time arches itself
-into a protuberance, and soon two daughter-nuclei are formed, one of
-them lying within the protuberance (Fig. 75, _A_, _Rk1_). This soon
-separates itself off from the ovum, surrounded by a small quantity
-of cell-substance. The other daughter-nucleus remains within the
-ovum, but neither of them remains in a state of rest; both are again
-transformed into a spindle and divide once more; the minute first
-'polar body' dividing into two 'secondary polar bodies' of half the
-size (_B_, _Rk1_), while the nuclear spindle within the egg brings
-about a second division of the ovum (_B_, _Rk2_) whose unequal products
-are the second polar cell and the mature ovum--that is, the ovum ready
-for fertilization. The process is now complete; the egg-cell, which
-has lost very little plasmic material through the 'polar bodies' and
-has not become visibly smaller, has now a nucleus (_B_, _Eik_) which
-has become considerably smaller through the two rapidly successive
-divisions, and, as we shall see later, has also undergone internal
-changes. In this state it is 'ripe,' that is, it is ready to enter into
-conjugation with the nucleus of a male cell, and this we have already
-recognized as the essential element in the process of fertilization.
-
-These processes of 'maturation of the ovum' are common to all animal
-ova which require fertilization, and they follow almost the same
-course, only that in many cases the second division of the first
-polar body does not take place, so that only two polar bodies in all
-are formed. All these processes have nothing directly to do with
-fertilization, but it is only through them that the ovum becomes
-capable of fertilization. This does not prevent the spermatozoon from
-previously making its way into the ovum, for this is usually the case
-(Fig. 75, _A_, _sp_); there it waits until the second 'directive
-division' of the ovum has been accomplished, utilizing the time to
-become transformed in the manner necessary for the conjugation of the
-two nuclei. Only in a few species, for example in the sea-urchin, does
-the egg complete its polar divisions within the ovary, therefore before
-it has come into contact with the sperm at all.
-
-[Illustration: FIG. 75. Process of fertilization in _Ascaris
-megalocephala_, the thread-worm of the horse, adapted from Boveri and
-Van Beneden. _A_, ovum in process of the first directive division;
-_Rk_ 1, first polar body; _sp_, spermatozoon with two chromosomes in
-its nucleus, attaching itself to the ovum, and about to penetrate
-into it; a protrusion of the egg-protoplasm is meeting it. _B_, the
-second directive division has been completed; _Rk2_, the second polar
-body; _Eik_, the reduced nucleus of the ovum. The first polar body
-(_Rk_ 1) has divided into two daughter-cells, _spk_; the nucleus of
-the spermatozoon remains visible with its two centrospheres (_csph_).
-_C_, the sperm nucleus (♂_k_) and the ovum nucleus (♀_k_) have grown,
-each has two loop-like chromosomes; only the male nucleus has a
-centrosphere, which has already divided into two (_csph_). _D_, the two
-nuclei lie apposed between the poles of the nuclear spindle. _E_, the
-four chromosomes have split longitudinally; the spindle for the first
-division of the ovum (the segmentation spindle, _fsp_) has been formed.
-_F_, divergence of the daughter-chromosomes towards the two poles;
-division of the ovum into the first two cleavage cells or embryonic
-cells.]
-
-That we may be able to penetrate still more deeply into the processes
-of fertilization, the best illustration to take seems to me to
-be, as yet, the ovum of the thread-worm of the horse (_Ascaris_
-_megalocephala_), which has become famous through the classical
-observations of Ed. van Beneden. Many favourable circumstances
-unite in this case to make the essentials of the process clearly
-recognizable. Fertilization takes place within the body of the female,
-in an enlarged portion of the oviduct, within which a number of the
-remarkable sperm-cells are always found in a mature female. They are
-remarkable in being not thread-like, but rather spheroidal cells,
-bearing, however, a small protuberance something like a pointed horn
-(Fig. 75, _A_, _sp_). When such a sperm-cell comes in contact with the
-upper surface of an ovum a swelling forms at the place touched, and the
-sperm-cell attaches itself firmly to this, and is drawn by it into the
-ovum. Without doubt, amœboid movements on the part of the sperm-cell
-itself play some part in this, as can be most plainly seen in the
-large sperm-cells of many Daphnids which we have already discussed. In
-the egg of the thread-worm the whole sperm-cell with its nucleus can
-soon be detected within the substance of the ovum, and it then changes
-rapidly. Its main body fades more and more completely, until at last
-it disappears altogether, while the nucleus becomes vesicle-like and
-soon attains a considerable size (Fig. 75, _B_, _spk_). Meanwhile the
-residue of the germinal vesicle which remained behind in the ovum after
-the second directive division (_B_, _Eik_) has changed into a large
-vesicle-like nucleus (_C_, ♀ _k_), which in the ovum of _Ascaris_, as
-well as in the spermatozoon, at first contains a nuclear reticulum with
-irregular fragments of chromatin. Later on, these form a spiral coil in
-the manner we have already described, and finally this breaks up into
-two large and relatively thick angular loops or chromosomes (Fig. 75,
-_C_ and _D_, _chr_).
-
-At the same time a nuclear division apparatus has formed in the space
-between the two nuclei--the so-called male and female 'pronuclei'
-(♂ _k_, ♀ _k_)--two centrospheres (_csph_) become visible, at first
-lying close together, but afterwards moving apart (_D_) to form the
-poles of a nuclear spindle, in the equatorial plane of which the four
-chromosomes of the male and female pronuclei are now arranged. The
-nuclear membranes disappear, and the two nuclei now unite to form one,
-the segmentation nucleus (_D_). A dividing spindle then develops and
-brings about the first embryonic cell-division (_E_), and thus the
-beginning of the 'segmentation' of the ovum; each of the four chromatin
-loops splits longitudinally, and each of the split halves migrates, one
-to one, the other to the other daughter-nucleus (_F_). As this same
-method of distribution of the chromatin substance is repeated at every
-successive cell-division throughout embryogenesis, and indeed through
-the whole of development, it follows that the result of fertilization
-is, that all the cells of the body of the new animal which develops
-from the ovum contain an equal quantity of paternal and of maternal
-chromatin. If we are right in regarding the chromatin substance as the
-hereditary substance, it becomes immediately apparent that this equal
-division is of the most far-reaching importance, for it shows us that
-the so-called process of fertilization is the union of equal quantities
-of hereditary substance of paternal and maternal origin.
-
-The process of fertilization is now known in all its details in a great
-number of animals in the most diverse groups; it is everywhere the same
-in its essential features; there is always only one sperm-cell which
-normally enters into conjugation with the ovum-nucleus, and in every
-case the sperm-cell, however minute it may be to begin with, forms a
-nucleus nearly or exactly as large as the nucleus of the ovum, and in
-all cases it contains the same number of chromosomes as the nucleus of
-the ovum. Of special interest, however, is the fact that this number
-is always half the number of the chromosomes exhibited by the somatic
-cells of the particular animal in question, and that the reduction of
-the number of chromosomes to half the normal is effected in both male
-and female germ-cells by the last divisions of these cells, which take
-place before they have attained to a state of maturity. In the ovum the
-reduction occurs in the directive divisions, to which we must therefore
-turn our attention once more, with special reference to the number of
-chromosomes.
-
-We saw that, in the full-grown ovarian egg, the germinal vesicle rises
-to the surface and there becomes transformed into the first polar
-spindle. Now this shows, in its equatorial plane, double the number of
-chromosomes normal to the species. This duplication comes about, not
-directly before the nuclear division, but much earlier in the young
-mother-egg-cell; it is only the change in the time of the splitting
-of the chromosomes that is unusual. The first maturation division
-takes place nevertheless in accordance with the usual plan of nuclear
-division; it is, as I have called it, an 'equation division,' that is,
-both daughter-nuclei again receive the same number of chromosomes as
-the young mother-egg-cell had to start with, namely, the normal number
-of the species. Thus, if the young mother-egg-cell had four chromosomes
-(Fig. 76, _A_), this number would double to eight at an early stage
-(_B_), but the first maturing division would give each daughter-nucleus
-four (_C_ and _D_). In the second maturation division the case is
-different, for here no splitting and duplicating of the number of
-chromosomes takes place, but the existing number, by being distributed
-between the two daughter-nuclei, is reduced to half in each (_E_ and
-_F_). For this reason I have called it a 'reducing division.' In our
-example, therefore, the ovum, as well as the second polar body, would
-contain only two chromosomes (Fig. 76, _F_).
-
-[Illustration: FIG. 76. Diagram of the maturation divisions of the
-ovum. _A_, primitive germ-cell. _B_, mother-egg-cell, which has grown
-and has doubled the number of its chromosomes. _C_, first maturation
-division. _D_, immediately thereafter; _Rk1_, the first directive cell
-or polar body. _E_, the second maturation spindle has been formed; the
-first polar body has divided into two (2 and 3); the four chromosomes
-remaining in the ovum lie in the second directive spindle. _F_,
-immediately after the second maturation division; 1, the mature ovum;
-2, 3, and 4, the three polar cells, each of these four cells containing
-two chromosomes.]
-
-I cannot enter into the details of the process here, for we are
-dealing with essentials and not with isolated and, so to speak,
-chance details, but I must emphasize the fact that the same process
-of reduction of the number of chromosomes takes place in this or an
-analogous manner in all animal ova, and can be demonstrated also
-in most of the chief groups of plants. Whether it be, as many have
-maintained, that the reduction is not always first effected by the
-'maturation divisions,' but in some cases takes place earlier in the
-primitive egg-cell[12], so much is certain, that the nuclei which come
-together for 'fertilization' only contain half the normal number of
-chromosomes, and this is true not only of the ovum but also of the
-sperm-nucleus.
-
-[12] See the discussion of this point in chapter xxii.
-
-Arguing from general considerations, but especially from the theory
-which regards the chromosomes as the bearers of the hereditary
-substance, I had come to the conclusion, before there was any full
-knowledge of the phenomena of the maturation of the ovum, that a
-reduction of the chromosomes by half _must_ take place, and had
-postulated a similar 'reducing division' for the sperm-cell, and
-further, for plants as well as animals--indeed, for all sexually
-reproducing forms of life. The two divisions in the sperm-cell
-corresponding to the polar divisions of the ovum with their reduction
-of chromosomes were demonstrated by Oscar Hertwig in the case of the
-thread-worm of the horse (_Ascaris megalocephala_)--a form which
-has proved so very important in relation to the whole theory of
-fertilization. It is true that in this case the course of the phenomena
-of reduction is less convincing than in some other forms which have
-been investigated more recently, as, for instance, the mole-cricket and
-the bugs. In these instances, at any rate, a 'reducing division' in
-spermatogenesis, quite corresponding to that of the egg-cell, has been
-demonstrated, and this demonstration is of particular value owing to
-the fact that the development of the sperm-cell, as we shall presently
-see, throws an entirely new light on that of the ovum, and especially
-on the phyletic significance of the polar bodies.
-
-We began our consideration of the processes of reduction with the
-full-grown egg-cell, but now let us go back to the earliest rudiments
-of the ovary of the embryo, and we find that it consists of a single
-primitive egg-cell, from which, by division, all the other egg-cells
-arise. In the same way the first rudiment in the testis or spermary
-is formed by a primitive sperm-cell, which does not differ visibly
-from the primitive egg-cell. Both now multiply by division for a
-considerable time, and in the ovary this is followed by a period of
-growth, during which multiplication ceases, and each cell increases
-considerably in size and lays in a store of yolk. Each cell thus
-ultimately reaches the condition with which we started previously, that
-of the full-grown _mother-egg-cell_.
-
-Although the primitive sperm-cells do not exhibit such pronounced
-growth as the ova, they have likewise their period of growth, during
-which multiplication by division ceases, and the cells increase only in
-size (Fig. 77, _A_). When they have attained their maximum of growth
-the number of chromosomes is seen to have been doubled by longitudinal
-splitting (as in the diagram, Fig. 77, _B_, from four to eight).
-From this _mother-sperm-cell_ there now arise by two divisions in
-rapid succession (_C_-_F_) four sperm-cells, and the same reduction
-of the number of chromosomes to half is effected as in the polar
-divisions of the egg-cell. In the first division, four chromosomes
-go to each daughter-cell (_D_), in the second, two (_F_). The only
-essential difference between the corresponding processes in the
-egg-cell and the sperm-cell lies in the fact that the divisions of the
-so-called 'spermatocyte' or mother-sperm-cell are equal, so that four
-granddaughter-cells of equal size arise, while in the mother-egg-cell
-or 'ovocyte' the divisions are very unequal. In the former the result
-of the divisions is _four_ cells capable of fertilizing, in the latter
-_one_ cell capable of being fertilized and three minute 'polar cells'
-which are incapable of conjugating with a sperm-cell and giving rise to
-a new individual.
-
-[Illustration: FIG. 77. Diagram of the maturation-divisions of the
-sperm-cell, adapted from O. Hertwig. _A_, primitive sperm-cell. _B_,
-mother-sperm-cell. _C_, first maturation division. _D_ 1 and 2, the two
-daughter-cells. _E_, the second maturation division, by which the four
-cells of _F_ arise, each with half the number of chromosomes.]
-
-There can thus be no doubt that the polar cells, as Mark and Bütschli
-long ago suggested, are abortive ova, that is, that, at a remote period
-in the evolution of animal life, each of these four descendants of a
-mother-egg-cell became a germ-cell capable of development. It is not
-difficult to infer that the unequal division, which now leads to an
-insufficient size in three of these descendants, has gone on _pari
-passu_ with the continually increasing size of the mature ovum, and had
-its reason in the fact that it was important above all things to store
-in the ovum as much protoplasm and yolk as possible. We have already
-seen that even the dissolution of a number of the sister-cells of the
-ovum is sometimes demanded, so that the ovum may be surrounded by
-nutritive follicular cells. In short, the greatest possible quantity of
-nourishment is conveyed to the ovum in every conceivable way, and it
-is thus stimulated to a growth which no single cell could attain to if
-it were dependent on the ordinary nutrition supplied by the blood. And
-we can understand that nature--to speak metaphorically--did not wish
-to destroy her own work by finally distributing among four ova all the
-nourishment she had succeeded in heaping up in all sorts of ways within
-the mother-egg-cell.
-
-But it may be asked, Why have all these unnecessary divisions been
-maintained up till the present day? Why have they not long ago been
-given up, since they can and do only lead to the production of three
-abortive ova, which are foredoomed to perish? Are they mere vestiges,
-processes which are in themselves meaningless, but have, so to speak,
-been maintained by the principle of inertia? This principle is
-certainly operative in some sense and to some extent even in living
-nature; a process which has been regularly repeated through a long
-series of generations does not at once cease to be performed when it is
-no longer of use to the organism concerned. The eyes of animals which
-have migrated to lightless depths do not disappear all at once and
-leave no trace; they degenerate very gradually and only in the course
-of many generations; and it would thus be quite possible to defend the
-position that these polar or 'maturation divisions' of the ovum are
-purely _phyletic reminiscences_ without actual significance.
-
-But I cannot agree with this opinion. If it were actually so we should
-expect that the formation of the polar bodies would not still take
-place in all cases in almost the same manner, for all rudimentary parts
-and processes vary greatly; we should expect that in many animal groups
-the polar divisions would not occur, or perhaps that only half the
-number would occur. But this is not so; in all multicellular organisms,
-from the lowest to the highest, two reducing divisions take place,
-and always in almost the same manner, with the exception of a single
-category of ova, of which I shall presently have to speak. We shall see
-later that even in unicellular organisms analogous processes may be
-observed.
-
-But it is also intelligible that this twice repeated division of
-the mother-egg-cell is necessary if the reduction in the number of
-chromosomes to half is only possible in this way, since _this reduction
-is indispensable_. If each of the two conjugating germ-cells contained
-the full normal number of chromosomes, the segmentation-nucleus
-would contain a double number, and if that went on, the number of
-chromosomes would increase in arithmetical proportion from generation
-to generation, and would soon become enormous. Even though we were not
-otherwise certain that these chromosomes are units of a permanent
-nature, which only apparently break up in the nuclear reticulum,
-but in reality persist, the fact of reduction would point in this
-direction. For if they were not permanent structures and distinct from
-one another, and if their number depended solely on the quantity of
-chromatin which the nucleus contains, the reduction in number might be
-secured if the chromosomes in the growing egg and sperm-cells increased
-in size more slowly than the cell-body and the other parts of the cell.
-But from the fact that the reduction takes place not in this simple
-way, but, in sperm-cells and in ova which require to be fertilized,
-only through cell-division and a specific mode of nuclear division, we
-may conclude that it cannot happen otherwise, that chromosomes are not
-mere aggregates of organic substance, but organs whose number can only
-be reduced by the extrusion of some of them from the cell.
-
-It is true that there are ova in which the process of reduction does
-not follow the course we have described, but the exceptions only
-serve to confirm our view of the reducing significance of the polar
-divisions, and of their persistence because of the necessity for
-reduction.
-
-As far back as the middle of the nineteenth century it was known
-that in various animals the eggs develop without fertilization. This
-reproduction by 'parthenogenesis' was first established with certainty
-by the German bee-keeper Dzierzon in 1845, and then scientifically
-corroborated by Rudolph Leuckart and C. Th. von Siebold. Although
-parthenogenesis was at first observed only in a few groups of the
-animal kingdom, in bees and some nocturnal Lepidoptera (Psychidæ
-and Tineidæ), it has become more and more apparent in the course of
-years that this 'virgin reproduction' is by no means a rare form
-of reproduction, and that it occurs regularly and normally in many
-cases, especially in the very diverse groups of the great series of
-Arthropoda. Thus among insects it is found in certain saw-flies,
-gall-flies, ichneumon-flies, in the honey bee, and in common wasps, and
-it is particularly widespread among plant-lice (Aphides) such as the
-vine-aphis (_Phylloxera_), whose prodigious multiplication in a short
-time depends partly on the fact that all the generations, with the
-exception of one, consist only of females with a parthenogenetic mode
-of reproduction.
-
-Among the lower Crustaceans also parthenogenesis plays a large rôle,
-and in many species it even occurs as the sole mode of reproduction,
-but more often--as is also the case among insects--it occurs
-alternately with bi-sexual reproduction. For parthenogenesis must not
-be regarded as asexual reproduction, but rather as _unisexual_, that
-is, as arising from sexually differentiated individuals (females),
-and from germ-cells (true ova), but brought about by the agency of
-individuals of only one sex, the female. These parthenogenetic eggs
-emancipate themselves, so to speak, from the law that was previously
-regarded as without exception, that all ova require fertilization to
-enable them to develop. That this law admits of many exceptions is
-now universally admitted; thus in the small family of water-fleas
-(Daphnids) there are even two kinds of eggs, the summer and winter eggs
-we have already mentioned, which are produced by the same female, and
-yet the former kind develop without fertilization, while the latter
-require to be fertilized before they can develop.
-
-It was obviously important to learn the state of affairs in regard to
-reducing divisions in parthenogenetic ova, to find out whether here
-also, three, or, in some circumstances, two polar bodies were formed,
-and whether the second polar division reduced the number of chromosomes
-to half. If the theory previously advanced as to the importance of the
-chromatin, and especially of the reducing effect of the second maturing
-division be correct, we should expect the second division to be wanting
-in parthenogenetic eggs, since otherwise the number of chromosomes
-would be reduced to half in each generation, and would thus gradually
-disappear or sink to one.
-
-Having directed my attention to this problem, I succeeded in
-establishing for a Daphnid, _Polyphemus_, that the second polar
-division does not occur, and that only one polar body is formed.
-Blochmann found the same in the parthenogenetic eggs of plant-lice or
-Aphides, in which, moreover, the eggs requiring fertilization exhibit,
-like the winter eggs of Daphnids, two polar divisions. It was thus
-established that at least those eggs of Aphides and Daphnids which
-are wholly parthenogenetic retain the full number of chromosomes
-of their species, as is represented in the diagram, Fig. 78. When
-parthenogenesis set in the polar divisions were limited to one, and
-that this could happen justifies us in concluding _a posteriori_
-that it could have happened also in the case of ova which required
-fertilization if that had been necessary or even merely indifferent.
-The polar divisions are thus not mere 'vestigial' processes; they have
-an immediate significance, and it lies in the reduction of the number
-of chromosomes.
-
-But I must make a reservation here; it is not universally true of
-parthenogenetic eggs that maturation takes place without the second
-polar division. The first exception was observed in the salt-water
-crustacean, _Artemia salina_. In this case only one polar body is
-actually extruded and the number of chromosomes remains normal, as I
-was able to demonstrate with the small number of ova at my disposal;
-but according to the investigations of Brauer on more abundant material
-it appears that, while the second polar division is suppressed in the
-majority of the ova, and the external extrusion of a second polar body
-never occurs, the second polar division does nevertheless sometimes
-take place. The two daughter-nuclei arising from this division unite
-again immediately afterwards to form a single nucleus, and this now
-functions as a segmentation nucleus. Of course it again contains the
-full number of chromosomes, namely, twice 84=168.
-
-In _Artemia_, therefore, the adaptation of the ova to parthenogenetic
-development is not yet fully established, and the complete abandonment
-of the second polar division seems to be phyletically striven for,
-since, although the division still takes place, its effect is
-neutralized immediately afterwards.
-
-[Illustration: FIG. 78. Diagram of the maturation of a parthenogenetic
-ovum. The number of chromosomes normal to the species has been assumed
-to be four. _Uei_, a primitive germ-cell. _M Eiz_, a mother-egg-cell,
-with twice the normal number of chromosomes. _Eiz_, mature ovum after
-the separation of the first and only polar body. _Rk_^1.]
-
-Among bees the state of affairs is again exceptional. Here the female,
-the so-called queen bee, possesses a capacious sperm-sac, in which
-the spermatozoa received in copulation remain living for years,
-and the fertilization of an ovum is effected in the usual way from
-this sac while the egg from the ovary is passing down the oviduct.
-The queen bee has the power of releasing some spermatozoa from the
-receptacle, or of not doing so, and thus of fertilizing the egg, or
-of not fertilizing it. Since the notable observations of Dzierzon and
-the investigations of von Siebold and Leuckart which followed them, it
-has been assumed that only those eggs were fertilized which were laid
-in the cells destined for rearing females (workers or queens), while
-those which were to give rise to 'drones' or males remained normally
-unfertilized. Only in the last decade of the past century did the
-bee-keepers begin to cast doubt on this so-called 'Dzierzon theory';
-various violent and obstinate attacks were made upon it, and these
-were supported by new and apparently convincing experiments. Dickel, a
-teacher in Darmstadt, has been particularly strenuous in attempting to
-overthrow the old theory, by emphasizing the fact that von Siebold's
-old investigations on bee eggs afforded no convincing proof. Von
-Siebold made his investigations on eggs freshly taken from the hive,
-and was never able to find spermatozoa in 'drone eggs' (that is, eggs
-laid in drone cells and therefore destined to develop into drones),
-while he was often able to demonstrate the presence of from one to
-four spermatozoa in 'worker eggs.' But he only examined 'drone eggs'
-which were already twelve hours old, and in these, as we now know, he
-would not have found spermatozoa in any case, even if they had been
-fertilized, because in ova at that stage the development of the embryo
-has already fully begun, and nothing remains of the spermatozoa. In
-the bee, according to Buttel-Reepen, the fertilizing spermatozoon is
-transformed in twenty minutes after it has penetrated into the egg into
-a minute 'sperm-nucleus' which is almost invisible even in sections,
-and certainly nothing whatever could be seen of it by the old method of
-squeezing the fresh ovum.
-
-It had therefore to be admitted that Dzierzon's theory rested on an
-insecure foundation, and I accordingly set two of my students at that
-time, Dr. Paulcke and Dr. Petrunkewitsch, to examine the eggs of the
-bee anew with regard to the point in question, using the greatly
-improved methods at their disposal. These investigations have been
-carried out in the Freiburg Zoological Institute during the last three
-years, and have resulted in establishing the absolute correctness of
-Dzierzon's theory: the 'drone eggs' do remain unfertilized, while the
-eggs from which females are to develop are fertilized without exception.
-
-In this case, therefore, we have, in the same animal, eggs which
-can be fertilized and eggs which, without fertilization, develop
-parthenogenetically, and it is therefore of the greatest possible
-interest to know the state of matters in them in regard to the
-directive divisions and the reduction of the chromosomes.
-
-Dr. Petrunkewitsch's investigations have shown that in both cases,
-that is, whether a spermatozoon penetrates into the ovum or does
-not, a twice-repeated division of the nuclear material in the ovum
-takes place. Moreover, the two daughter-nuclei which result from the
-second division do not, as Brauer showed was sometimes the case in
-_Artemia_, unite again afterwards; they remain separate, and the
-number of chromosomes--there are sixteen of them--is thereby reduced
-to half in the segmentation nucleus. But this is not all, for before
-embryonic development has begun the normal number can be again seen in
-the segmentation nucleus; the chromosomes must therefore have _doubled
-their number by division within the nucleus_.
-
-It is probable that something similar takes place in the cases of
-exceptional parthenogenesis which have long been known, but this point
-has not yet been sufficiently investigated. Nevertheless I cannot pass
-them over, as they are instructive from another point of view.
-
-[Illustration: FIG. 79. The two maturation divisions of the 'drone
-eggs' (unfertilized eggs) of the Bee, after Petrunkewitsch. _Rsp 1_,
-the first directive spindle. _k 1_ and _k 2_, the two daughter-nuclei
-of the same. _Rsp 2_, the second directive spindle. _k 3_ and _k 4_,
-the two daughter-nuclei. In the next stage _k 2_ and _k 3_ unite to
-form the primitive sex-nucleus. Highly magnified.]
-
-In some silk-moths (Bombycidæ) and hawk-moths (Sphingidæ), especially
-in the silk-moth proper (_Bombyx mori_), in _Liparis dispar_, and
-in quite a number of other Lepidoptera, it sometimes happens that,
-out of a large number of unfertilized eggs, a few will develop and
-produce caterpillars. This is interesting enough, but it gains
-increased importance through the investigations of the Russian
-naturalist, Tichomiroff, who succeeded in considerably increasing
-the number of unfertilized eggs that developed by gently rubbing
-them with a paint-brush, or by dipping them for a little in dilute
-sulphuric acid. It is thus possible to make eggs, which would not
-ordinarily develop without being fertilized, capable of parthenogenetic
-development by means of mechanical or chemical stimulus. This sounds
-almost incredible, but it is beyond a doubt, and it is still further
-corroborated by the fact that Prof. Jacques Loeb has succeeded in
-inciting the eggs of a sea-urchin to parthenogenetic development by
-means of a chemical stimulus. When he added to the sea-water in which
-the eggs were laid a certain quantity of chloride of magnesium the
-ova developed, and not only went through the process of segmentation,
-but even reached the stage of the quaint easel-like Pluteus larva.
-Quite recently Hans Winkler has made the interesting observation
-that from sea-urchin sperms which have been killed by heat it is
-possible to extract in aqueous solution a substance capable of exciting
-unfertilized sea-urchin eggs to development, although they only go as
-far as to the sixteen-cell stage.
-
-From all these results we can at least infer so much, that chemical
-changes and influences may determine whether the ripe ovum shall go on
-to embryonic development or not, and that these influences, may be very
-diverse in nature in different cases. I shall return later to these
-important facts.
-
-When we sum up the facts we have cited with reference to the reduction
-of the number of chromosomes, it appears that nature is, as it were,
-striving to keep the number constant for each species; that in
-germ-cells which are destined for amphimixis they are reduced to half
-the normal number, but that this halving of the number is suppressed
-where fertilization is always absent, or that the reduction to half is
-compensated for again in various ways, whether by subsequent fusion
-of the two daughter-nuclei, which have arisen from the process of
-reduction, or by an independent duplicating of the chromosomes in the
-segmentation nucleus.
-
-We might perhaps be inclined to conclude from all this that the
-occurrence of development depended on the presence of the normal
-number of chromosomes; and I used to regard this as possible. But
-facts which have been more recently brought to light have excluded
-this view. Above all, we now know that every nuclear division depends
-on the presence of a dividing apparatus, a centrosphere, but that this
-organ degenerates in the ova of most animals and is completely lost
-after the second polar division has been effected. The mature ovum is
-therefore in itself incapable of entering on its embryonic development,
-no matter how many chromosomes its nucleus contains; it is only capable
-of further division when the fertilizing sperm-cell brings with it
-its dividing apparatus or centrosphere. In thread-like sperms this
-lies in the median portion (Fig. 68_C_), and after the tail-piece has
-been dissolved, which happens soon after the sperm enters the egg, the
-central corpuscle, at first very small, can be recognized in front of
-the sperm-nucleus, where it is soon transformed into an 'aster' and
-divides into two. Then both spheres move apart (Fig. 75_D_, p. 296) and
-form the nuclear spindle between them by the confluence of their rays.
-
-From this the division of the ovum into the two first embryonic cells
-proceeds. The two pronuclei in the ovum, the male and the female, are
-thus exactly alike as to number of the chromosomes, and frequently
-at least as to size and appearance (Fig. 75_C_). But they differ
-in the possession or absence of a dividing apparatus, and in the
-great majority of cases it is the male nucleus that brings with it
-the central corpuscle which seems to be indispensable to embryonic
-development (_B_, _cspt_). Hitherto, at least, only two exceptions to
-this are known. In the little segmented worm, _Myzostoma_, which is
-parasitic on sea-lilies or Crinoids, Wheeler observed that the ovum
-retained its central corpuscle even after the polar divisions, while
-the sperm-cell which penetrated into the egg had none. More recently
-Conklin made the interesting discovery that in the egg of a marine
-Gasteropod (_Crepidula_) both the egg-nucleus and the sperm-nucleus
-retain their centrosphere and together form the segmentation spindle,
-one lying at one pole and the other at the opposite.
-
-All these observations confirm the view that the sperm and the
-egg-cell are alike in this respect also. Each of them can, in certain
-circumstances, bring with it the dividing apparatus indispensable to
-development, though it is usually the sperm-cell that does so.
-
-I should indeed assume that the sperm-cell and the egg-cell were
-essentially alike, even although there were no exception to this
-rule, that is, although the centrosome of the ovum perished in
-all eggs which were fertilized. For this is obviously a secondary
-arrangement, an adaptation to fertilization, that the ovum should be
-incapable of development without fertilization, and it is made so by
-the disappearance of its centrosome. In all other cells, as far as is
-known, the central corpuscle persists after division, so that this
-remarkable cell-organ is transmitted from cell to cell just like the
-nucleus, and like it, never rises _de novo_. It is only in the egg-cell
-that it disappears, though even there often very late, for it may be
-present, as an aster, even after the sperm has penetrated into the
-ovum and disclosed its own central body, or even brought it the length
-of dividing into two (Fig. 80, _A_ and _B_). But the ovum-centrosome
-disappears as soon as the second polar division is accomplished.
-
-That this disappearance is really a secondary arrangement, which may be
-again departed from, is proved by the case of those eggs which are able
-to develop parthenogenetically, for in them the central body does not
-disappear, but persists in the ovum after the first polar division, as
-Brauer showed in _Artemia_. It then behaves exactly like the sphere of
-the sperm-nucleus in the fertilized ovum, that is, it duplicates itself
-and forms the segmentation spindle.
-
-[Illustration: FIG. 80. Fertilization of the ovum of a Gasteropod
-(_Physa_), after Kostanecki and Wierzejski. _A_, the whole spermatozoon
-lies in the ovum. _sp_, its already divided centrosphere. _Rk 1_,
-the first polar body. _Rsp 2_, the second directive spindle. _B_,
-_spk_, the sperm-nucleus, the second directive spindle still has its
-centrosphere, which afterwards disappears. The first polar body (_Rk
-1_) has divided into two. Highly magnified.]
-
-Thus the beginning of embryonic development in the ovum depends not on
-a definite number of chromosomes, but on the presence of an apparatus
-for division. Upon what the awakening of this to activity just at that
-time depends cannot as yet be exactly stated; we can only indicate
-that all parts of the cell have interrelations with each other, and
-that, therefore, the division mechanism is dependent on the condition
-of the rest of the cell-parts at the moment, and on the substances
-which they contain or produce. From what we know experimentally in
-regard to artificial parthenogenesis it is not difficult to imagine
-that some sort of chemical substances are necessary to stimulate the
-central corpuscle to activity. In any case, the whole nutrition of
-the central corpuscle depends on the cell in which it lies, as is
-shown by the fact that the sperm-nucleus, whose centrosome before
-the entrance of the sperm into the ovum was inactive and scarcely
-recognizable, grows rapidly after entrance and forms a large aster
-round itself--is, in short, in the highest degree active (Fig. 80). As
-the chromosomes certainly play an important part in the life of the
-cell, and materially help to determine its various phases, it cannot
-be disputed that they also may share in awakening the activity of the
-central corpuscle. But this influence is only indirect; it is not the
-mere number of chromosomes that decides whether the central corpuscle
-is to become active or remain inactive. This cannot be assumed, because
-we have in the maturation divisions a proof that division may take
-place with a double number of chromosomes as well as with the undoubled
-number; while in the divisions of the mother-egg-cells and the
-mother-sperm-cells we have proof that a doubled number of chromosomes
-does not in itself compel to division.
-
-The exceptional and artificially produced cases of parthenogenesis
-which we have discussed above are probably to be interpreted thus:
-through slight differences in the constitution of the ovum, or through
-certain mechanical or chemical stimuli, the metabolic processes in
-the ovum are so altered that the centrosome of the ovum, instead of
-breaking up, is stimulated to growth, and thus produces the active
-dividing apparatus which is otherwise only brought into it by the
-sperm. This is a more exact definition of the interpretation I gave
-earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which
-was then the only case known, when I said 'the nucleoplasm of some ova
-must possess the power of growth in a greater degree than the majority.'
-
-But we are not yet in a position to go further, or to define more
-exactly the nature of the processes of metabolism which are involved.
-
-
-
-
-LECTURE XVI
-
-FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS, AND ITS IMMEDIATE
-SIGNIFICANCE
-
- Fertilization in a lichen, Basidiobolus--In Phanerogams--Here too
- there is reduction of the number of chromosomes by a half--'Polar
- cells' in lower and higher plants--Conjugation among unicellular
- organisms--Noctiluca--The maternal and paternal chromosomes
- remain apart--Actinophrys--Infusoria--Sexual differentiation
- of the two conjugates in Vorticella--Importance of the process
- of Amphimixis--Not a 'life-awakening' process--May occur
- independently of multiplication--The Rejuvenescence hypothesis--Pure
- parthenogenesis--The cycle idea--Does Amphimixis prevent natural
- death?--Maupas' experiments with Infusorians--Bütschli's
- view--Potential immortality of unicellular organisms--The
- immortality of unicellular organisms and of the germ-cells depends
- on the fact that there is no time-limit to the multiplication
- of the smallest living particles--Parthenogenesis is not
- self-fertilization--Petrunkewitsch's observations on the ova of
- bees--Is the chromatin really the 'hereditary substance'?--Nägeli's
- conclusion from the difference in size between ovum and
- spermatozoon--Artificial division of Infusorians--Boveri's
- experiments with the fertilization of pieces of ova not containing
- a nucleus--Fertilization gives an impulse to development even to
- non-nucleated pieces of ova--Merogony--The female and male nuclear
- substances are essentially alike--Summary.
-
-
-I NOW turn to the consideration of the process of fertilization in
-plants and unicellular organisms.
-
-With regard to plants, it can now be definitely asserted that in them,
-too, fertilization is essentially a conjugation of nuclei; it depends
-on the union of the nuclei of the two 'sex-cells.' These sex-cells are
-usually very small among lower plants, indeed up to the phanerogams;
-this is especially true of the zoosperm-like male germ-cells, but it
-usually holds also true of the ovum, which is but seldom burdened
-with an abundant supply of yolk. In spite of the many difficulties
-which this smallness of size puts in the way of observation, the
-untiring exertions of a host of excellent investigators have succeeded
-in following the process of fertilization in all the larger groups
-of plants--in algæ, fungi, mosses, ferns, and horse-tails among
-cryptogams, and in phanerogams.
-
-I shall first give an example from among the lower plants (Fig. 81).
-In one of the lichens, _Basidiobolus ranarum_, each of two adjacent
-cells in the fungus-thread gives off a bill-like process, and the two
-processes become closely apposed (Fig. 81, _a_). The nucleus of each
-cell moves into the bill-shaped process, is there transformed into a
-nuclear spindle (_B_, _ksp_) and divides, so that one daughter-nucleus
-comes to lie in the apex point of the bill, the other at the base.
-The cell-body also divides, though very unequally, and the final
-outcome of the process is two cells in each, of which one is small
-and occupies the apex of the bill, while the other is large and fills
-all the rest of the cell-space. The former do not play any further
-part of importance, but break up, the latter are the sex-cells, the
-cytoplasm of which now coalesces through a gap in the cell-walls, while
-their nuclei become closely apposed and ultimately unite (_C_, ♂ and
-♀ _k_). From this union arises the fertilized spore, the so-called
-'zygote' (_D_). The two small abortive cells so greatly resemble in
-their origin the polar cells of the animal ovum that it is difficult to
-resist the supposition that they bring about a reduction in the number
-of chromosomes. But the number of the chromosomes has not yet been
-determined either in them or in the sex-nuclei.
-
-[Illustration: FIG. 81. Formation of polar bodies in a lichen,
-_Basidiobolus ranarum_. _A_, the two conjugating cells with the
-bill-like processes in which the nuclei lie. _B_, the nuclei dividing.
-_ksp_, the nuclear spindle. _C_, after the division into a polar body
-(_rk_) and a sex-nucleus (♂ _k_ and ♀ _k_). _D_, after the union of
-the nuclei to form a conjugation nucleus (_copk_); the fertilized ovum
-is surrounded by envelopes and modified into a lasting spore. After
-Fairchild.]
-
-We have come to know the processes of fertilization among phanerogams
-chiefly through Strasburger, Guignard, and more recently through the
-Japanese botanist Hirase. The agreement with the animal process is
-surprisingly great, notwithstanding the notable differences in the
-external conditions of fertilization.
-
-As is well known, the male cells in the highest flowering plants are
-not zoosperms but roundish cells, each of which, enclosed, together
-with a sister-cell--the so-called 'vegetative' cell--in a thick
-cellulose capsule constitutes a pollen-grain. The pollen-grains reach
-the stigma, under which, buried deep within the 'ovule,' the female
-sex-cell rests, enclosed in a long, sac-like structure called the
-'embryo-sac' (Fig. 82, _A_). Beside it (_eiz_) there lie several other
-cells, usually seven in number, two of which, the so-called 'synergidæ'
-(_sy_), have their place at one end of the embryo-sac, just in front of
-the ovum (_eiz_). Probably these give off a secretion which exercises
-an attractive (chemotactic) influence on the male fertilizing body
-('the pollen-tube'), and thus, so to speak, show it the way to the ovum.
-
-[Illustration: FIG. 82. Fertilization in the Lily, _Lilium martagon_,
-after Guignard. _A_, the embryo-sac before fertilization; _sy_,
-synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei';
-_ap_, antipodal cells. _B_, the upper part of the embryo-sac,
-into which the pollen-tube (_pschl_) has penetrated with the male
-sex-nucleus (♂_k_) and its centrosphere; below that is the ovum
-with its (also doubled) centrosphere (_csph_). _C_, remains of the
-pollen-tube (_pschl_); the two sex-nuclei are closely apposed. Highly
-magnified.]
-
-When a pollen-grain has reached the stigma it sends out a tube, usually
-after a few hours, which penetrates into the soft tissue of the
-style, and grows deep down into the interior of the ovule, ultimately
-penetrating as far as the embryo-sac through a special little opening
-in the covering of the ovule, the so-called 'micropyle' (Fig. 82_B_,
-_pschl_). Its blunt end is now closely apposed to this, so that the
-true sperm-nucleus (_B_, ♂_k_), surrounded by some protoplasm, can
-leave the pollen-tube and wander in among the cells of the embryo-sac.
-Later on we shall see that two generative nuclei migrate from the
-pollen-tube, but in the meantime we shall devote our attention only to
-one of them, the fertilizing nucleus, which immediately moves towards
-the ovum-nucleus and apposes itself closely to it. Then follows the
-fusion or conjugation of the two nuclei, which are alike in size and
-appearance, just as in the fertilization of the animal ovum (_C_, ♂ _k_
-and ♀ _k_). Whether in this case, too, the sperm-nucleus brings with it
-a central corpuscle, or whether, as Guignard believed he observed, the
-ovum retains its central corpuscle (_C_, _csph_), or finally, whether
-both modes occur, is not yet known with certainty. The fact that, as
-a rule, seeds capable of reproduction only form in an ovule when the
-stigma has been previously dusted with pollen, leads us to suppose
-that, in this case, as among animals, the ovum lacks something that is
-necessary to induce embryonic development, only retaining this power
-in very exceptional cases, namely, when adapted for parthenogenesis.
-And this something may very well be the dividing apparatus of the cell,
-the centrosome with the centrosphere. But whether this supposition
-prove correct or not, a nuclear spindle always forms simultaneously
-with the fusion of the two sex-nuclei into a segmentation nucleus, and
-this spindle is the starting-point of the young plant, thus exactly
-corresponding to the first segmentation of the animal ovum. It agrees
-with it also in the important respect that it again contains the full
-number of chromosomes--twenty-four in the lily--while the two nuclei,
-male and female, only exhibit half the number each, that is, twelve.
-
-Thus a reduction in the number of chromosomes to half takes place in
-plants also, but it is not yet known with certainty whether this is
-brought about in the same way as among animals, namely, by reducing
-divisions. Without entering more fully into this still unsolved and
-very complex problem, I should like to state that I consider this very
-probable; indeed, I agree with the view of V. Häcker[13], that the
-reducing divisions of plants are only more difficult to recognize as
-such, and, furthermore, are often disguised by the fact that they often
-occur alongside of, or between divisions which are not reducing. If it
-were possible to reduce the number of chromosomes in a cell to half
-without the aid of cell-division, if, for instance, only half were to
-integrate again from the chromatin-network, this must have been quite
-as possible in the case of animal cells, and then, moreover, the single
-chromosome would not have had the significance of an individuality,
-and no special form of nuclear division would have been introduced to
-reduce their number. That it has been introduced seems to me to prove
-that it was necessary, and since it was so among animals, it could not
-have been dispensed with among plants either.
-
-[13] See V. Häcker, _Praxis und Theorie der Zellen- und
-Befruchtungslehre_, Jena, 1899, pp. 144-5.
-
-Moreover, throughout the vegetable kingdom divisions often occur in
-connexion with the origin of the sex-cells which can be compared,
-in occurrence and result, with the maturation divisions of animal
-germ-cells. In the lichen, _Basidiobolus_, we have already seen that an
-abortive cell separates itself off from the sex-cell before the latter
-becomes capable of reproduction (Fig. 81, _C_). Similar cell-divisions
-occur in many if not in all groups of plants. In the marine algæ of
-the genus _Fucus_ it has even been proved that the division of the
-first primordial cell of the ovary into the so-called 'stalk-cell' and
-the primitive egg-cell is a reducing division, and brings down the
-number of chromosomes from thirty-two to sixteen. In vascular plants
-the reduction is not postponed until the formation of the sex-cells,
-but occurs earlier in the formation of the spores, as Calkins has
-demonstrated for ferns; in the Conifers and other Gymnosperms
-several so-called 'preparatory' divisions precede the formation of
-the germ-cells, and we know by comparison with the alternation of
-generations in vascular plants that these are related to the gradual
-waning of the strictly sexual generation. As the 'polar bodies' or
-'directive corpuscles' of the animal ovum are rudimentary egg-cells,
-so the cells which, in the pollen-grains, separate themselves from
-the sex-cells proper are rudimentary Prothallium-cells, and, like the
-animal cells, they perish without playing any further physiological
-rôle. I will not assert that it is precisely in these divisions
-that the reducing divisions are concealed, for the analogy with the
-spore-formation of ferns leads us rather to suppose that it may lie
-further back; but in any case there is no lack of opportunity in the
-ontogeny of phanerogamic plants for the interpolation of a reducing
-division, and as long as it remains unproved that a reduction of the
-chromosomes can take place directly, that is, without the help of
-nuclear division, we shall continue to expect with confidence that the
-reducing divisions of phanerogams will be discovered in the future.
-Processes of a similar kind are known among unicellular organisms, and
-there, too, they are associated with nuclear divisions.
-
-In passing to the so-called 'sexual reproduction' of unicellular
-organisms, I should like first to call attention to the fact that
-the expression 'reproduction' is not very suitable in this case, for
-the process in question does not always effect an increase in the
-number of individuals as reproduction ought to do, but leads, in
-fact, in many cases, even to a decrease, when two individuals unite
-to form one. Even if the phenomena of sexual 'reproduction' among
-higher organisms, which we have already studied, had not made it
-clear to us that there are two associated processes, quite different
-in nature, the conjugation of unicellular organisms would have led
-us to that conclusion. It has long been known that two unicellular
-plants or animals occasionally become closely apposed and fuse;
-and this process of 'conjugation' was many years ago regarded as
-an analogue to 'fertilization,' although it is only through the
-laborious investigations of the last two or three decades that
-this supposition has been proved to be correct. We now know that
-a process quite analogous to that which we have learnt to know as
-'fertilization' takes place among unicellulars, only in this case it is
-not directly connected with reproduction and multiplication, but occurs
-independently of them, and, in its most primitive form, it results, not
-in an increase but--for a short time at least--in a diminution of the
-number of individuals. This occurrence of the process independently
-of reproduction appears to me of inestimable value theoretically, for
-it frees us completely from the old deep-rooted preconceptions in the
-interpretation of fertilization.
-
-[Illustration: FIG. 83. Conjugation of Noctiluca, after Ischikawa.
-_A_, two Noctilucas beginning to coalesce; _pr_, the protoplasm drawn
-out into processes which traverse the gelatinous substance of the
-cell; _k_, the nucleus. _B_, the cells and their gelatinous substance
-have fused; the nuclei, in which the chromosomes are visible, are
-closely apposed; _CK_, centrospheres. _C_, the two nuclei are united
-in one nuclear spindle; beginning of division. _D_, completion of the
-division. Highly magnified.]
-
-First let us briefly sketch the process itself in the main forms of its
-occurrence.
-
-The most primitive form of conjugation is undoubtedly the complete
-fusion of two unicellular organisms of the same species, as we see it
-to-day in unicellular plants, and also among the lowest unicellular
-animals, such as the flagellate Infusorians, Gregarines, and Rhizopods.
-It is well seen, for instance, in the Noctilucæ, those unicellular
-flagellate organisms which cause the familiar marine phosphorescence
-extending uniformly over wide surfaces of water (Fig. 83). In these
-forms Prof. Ischikawa of Tokio was able to trace the whole process
-of conjugation. To begin with, two Noctilucas range themselves side
-by side (Fig. 83) and coalesce at the surfaces in contact, both as
-to the spherical gelatinous envelope (_A_, _G_) and the protoplasm
-(_pr_) itself, which branches in amœboid fashion into the jelly. The
-union becomes gradually complete, and the two animals form a single
-sphere (_B_) with one cell-body. But the two nuclei (_K_) also place
-themselves side by side (_B_), and though they do not actually fuse,
-they form together, under the guidance of two centrospheres (_C_), a
-single nuclear division-figure, which is obviously analogous to the
-segmentation spindle of the fertilized egg. Then follows a division,
-by means of which the chromatin substance of the nuclei of both
-animals is divided between the two daughter-nuclei, and after this
-has been accomplished the united individual again separates into two
-independent Noctilucas (_D_). Although I have spoken here--that is, in
-referring to the Protozoa--of chromosomes, I must immediately add that
-these have not yet been seen with full clearness in Noctiluca itself;
-nothing more has been recognized than deeply staining thickenings
-of the spindle fibrils, which move from the equator of the nuclear
-spindle towards the pole. Since, however, in other Protozoa, as, for
-instance, in the beautiful freshwater Rhizopod (_Euglypha alveolata_),
-these thickenings of the nuclear spindle fibrils have been clearly
-recognized as chromosomes, doubt on this point is hardly justifiable.
-Apart from this, the assumption that each of the two daughter-nuclei
-receives half the chromosomes of each of the conjugated nuclei rests on
-a secure basis, not only because otherwise the whole process would have
-no meaning, but because the position of the mitotic figure conditions
-this. Even the fact that the two conjugation-nuclei lying side by side
-remain apart during nuclear division is not without parallel; Häcker
-and Rückert observed it also in the segmentation-nucleus of much higher
-animals, the Copepods, and it has no effect in altering the process of
-division, but only proves that the chromosomes of maternal and those of
-paternal origin in the combination-nucleus remain independent--a fact
-the significance of which I shall discuss later on.
-
-The process of conjugation occurs, in the same manner as in
-_Noctiluca_, in a freshwater Rhizopod, the well-known Sun-animalcule,
-_Actinophrys sol_ (Fig. 84), but in this case complete fusion of the
-two nuclei takes place (Fig. 84, _V_) before the formation of the
-division-spindle (_VI_, _sp_), which, with the simultaneous division
-of the cell-body, gives rise to two new individuals. The process in
-this case is especially interesting, because Schaudinn has succeeded in
-observing a maturation division (_III_, _Rsp_, directive spindle) as
-well as in demonstrating polar bodies (_IV_, _Rk_). Thus the analogy
-with the process of fertilization in the Metazoa and the Metaphyta is
-almost complete.
-
-But that the conjugation of unicellular organisms, like the
-fertilization of multicellular organisms, is essentially a matter of
-nuclear conjugation is shown more distinctly still by the ciliated
-Infusorians, the most highly organized of the Protozoa.
-
-[Illustration: FIG. 84. Conjugation and polar body formation in
-the Sun-animalcule, _Actinophrys sol_, after Schaudinn. _I_, two
-free-swimming conjugated individuals, which in _II_ have become
-surrounded by a transparent gelatinous cyst. _III_, formation of
-the directive spindles (_RSp_). _IV_, the polar bodies are formed
-(_RK_); _K_, the two sex-nuclei. _V_, these are fused to form the
-conjugation-nucleus (_K_). _VI_, the conjugation-nucleus is transformed
-into the division-spindle; the polar bodies (_RK_) have penetrated the
-internal cyst-wall, and are in process of degeneration.]
-
-Here there is usually no complete union of the cell-bodies of the
-two animals, but only an adhering of the apposed surfaces. In the
-relatively large _Paramœcium caudatum_ the process of conjugation is
-very exactly known through the beautiful investigations of Maupas
-and R. Hertwig. In this case the mouth-surfaces of the two animals
-come together and unite over a short area, and then the two animals
-swim about together in this conjugated state. During this time very
-remarkable changes take place in their nuclei.
-
-It is well known that these Infusorians have a double nucleus, a
-large one, the macronucleus (Fig. 85, _ma_), and one which is usually
-very small, the micronucleus (_mi_). We may ascribe to the former
-of these the guidance and regulation of the everyday processes of
-life, that is, briefly, of metabolism, and the preservation of the
-integrity of the whole animal. The small nucleus has often been
-designated the 'reproductive nucleus,' but as it plays no other
-part in reproduction, as far as can be recognized, than that of
-dividing into two daughter-nuclei, I cannot regard this designation
-as suitable; it obviously originated in the mistaken interpretation,
-prevalent till very lately, of conjugation as a 'kind of reproduction,'
-and this in its turn depends on the conception, transferred from
-multicellular organisms, of fertilization as a 'sexual reproduction.'
-We shall immediately see that the micronucleus plays the main part in
-conjugation, and from this we may suppose that it otherwise fills no
-rôle in the life of the animal, and therefore it may best be designated
-the 'supplementary' or reserve nucleus. In every conjugation the
-macronucleus, which has hitherto been active, breaks up and becomes
-completely absorbed, very much like a ball of food. This of course
-takes place slowly; the large nucleus elongates, becomes indented,
-falls into several pieces, and these are so gradually absorbed that,
-even after the act of conjugation has been accomplished, irregular
-fragments of the macronucleus often lie about in the animal (Fig. 85,
-9).
-
-But while the macronucleus falls to pieces the previously minute
-micronucleus grows enormously and forms a distinct longitudinally
-striated spindle (1, _mi_). About the same time these divide in both
-animals, and each of the daughter-nuclei immediately divides again,
-so that after these two divisions four spindle-shaped descendants
-of the micronucleus are to be seen in each animal (Fig. 85, 4). We
-have previously noted that the apparatus for nuclear division in
-unicellular organisms was similar to that in multicellular organisms,
-and yet was different from it. In these ciliated infusorians we see
-an essential difference, for the striated spindle, after the division
-into daughter-chromosomes has taken place, lengthens out enormously,
-and becomes so thin in the middle of its length (2) that the two
-daughter-nuclei at the ends of this long stalk suggest the appearance
-of a very long and thin dumb-bell, or of a long silk purse. Of asters
-(centrospheres) there is nothing to be seen, and the mechanism of
-division is still very obscure; it almost seems as if a rapidly growing
-substance forced the two groups of chromosomes apart.
-
-Hardly have these four descendants of the micronucleus arisen when
-three of them begin to break up and very shortly disappear; only the
-fourth is of any further importance, and it divides once more (5),
-and so gives rise to the two nuclei which play the chief part in the
-process of conjugation--the copulation-nuclei, exactly analogous to
-the male and female pronuclei in the fertilized ovum (5, _mi_^4). But
-in this case each of the two animals functions doubly, that is, both
-as male and female, for each sends one of the two copulation-nuclei
-across the bridge formed by the union of the apposed surfaces into
-the other animal (6, _mi_ ♂), so that it may form, by union with the
-nucleus which has remained there, a double nucleus (7), a structure
-which corresponds to the segmentation nucleus of the ovum (_copk_).
-From it there then arises by division a new macronucleus and a new
-micronucleus, not usually directly, however, that is, not by a single
-division, but through several successive nuclear divisions, into the
-meaning of which I cannot here enter. Immediately after the union of
-the two sex-nuclei the two animals sever their connexion with each
-other; each begins again to feed, and is subject to multiplication by
-division just as it was before conjugation took place (8 and 9).
-
-[Illustration: FIG. 85. Diagram of the conjugation of an Infusorian,
-_Paramœcium_, after R. Hertwig and Maupas. 1, two animals with the
-mouth-openings apposed; _ma_, the macronucleus beginning to degenerate;
-_mi_^1, the micronucleus has already increased considerably in
-size and is beginning to divide. 2. each micronucleus has divided
-into two daughter-nuclei (_mi_^2), which are connected only by the
-division-strand (_ts_). 3, to the left each of the daughter-micronuclei
-(_mi_^2) is beginning to divide; to the right this division is already
-completed and the grand-daughter-nuclei of the original micronucleus
-hang together by their division-strands (_ts_). 4, in each of the
-animals there are now four grand-daughter-micronuclei (_mi_^3). 5,
-three of these are in process of dissolution, the fourth is dividing
-into two great-grand-daughter-nuclei (_mi_^4), which are the two
-sex-nuclei. 6, one (the male) sex-nucleus (_mi_ ♂) migrates into the
-other animal, and there unites with the remaining (female) sex-nucleus.
-7, the conjugation-nucleus (_copk_) being formed. 8, the animals
-have separated; the conjugation-nucleus divides into (9) the new
-macronucleus (_n ma_) and the new micronucleus (_n mi_).]
-
-Although the course of this remarkable process exhibits all manner of
-differences in detail in different species, it is everywhere the same
-in its essential feature, and this essential feature is undoubtedly the
-union of an equal quantity of the nuclear substance of two animals to
-form a new nucleus. It is thus essentially the same process which we
-have already recognized among higher animals as 'fertilization.' The
-differences are of minor importance, and they arise partly from the
-fact that the sex-cells of multicellular animals are not independent
-self-supporting units, and partly from their differentiation into
-'male' and 'female' cells. The minuteness of the sperm-cell, for
-instance, conditions its penetration of the ovum, which is always much
-larger and passive, and also the thorough fusion of its cell-body with
-the cell-body of the ovum. That this difference has very little deep
-significance is best seen from the fact that, even among Infusorians,
-there are forms in which the two conjugating individuals are quite
-different, especially in size, and in which the much smaller 'male'
-animal fuses completely with the much larger 'female,' and indeed bores
-its way into it after the manner of a sperm-cell. This is the case
-among the bell-animalcules (Vorticellinæ) (Fig. 86), the conjugating
-pairs of which had been observed long before our present insight
-into these processes had been attained. Indeed, the facts had been
-interpreted as a kind of 'budding process,' the minute and differently
-shaped 'male' animal (_mi_), which at the time of conjugation is
-attached to the larger 'female' (_ma_), was regarded as its bud. This
-supposed bud, however, does not grow out from the animal, but into it!
-
-Thus we see here again that a differentiation of individuals as males
-and females may occur among unicellular organisms, just as in the
-sex-cells of higher animals and plants, and this proves to us once
-more that all these differences of sex, whether in reproductive cells
-of multicellular organisms, or in the entire multicellular animal or
-plant, or finally, in unicellular organisms, are not of essential,
-but only of secondary significance, however important they may be for
-securing fertilization or conjugation in each special case. They are
-always only adaptations to the special conditions, and only occur where
-they are necessary to ensure the union, and always in such a manner
-that the union of the two cells is facilitated. In most Infusorians
-such a differentiation into male and female animals was not necessary,
-because these organisms are very motile, and are thus readily able
-to meet and unite; it was therefore sufficient for them to remain
-hermaphrodite. The bell-animalcules, however, are sedentary, and for
-them it was obviously an advantage that, at the time of conjugation,
-smaller, free-swimming, and also more simply organized individuals
-should arise, which were able to seek out the larger sedentary forms.
-Here, then, as in many other unicellular animals, these little male
-individuals only occur when they are necessary, that is, at the time of
-conjugation. Similarly, in the green alga, _Volvox_, male and female
-cells arise only at the time of conjugation, reproduction being at
-other times effected by means of parthenogonidia, that is, by elements
-which require no fertilization.
-
-[Illustration: FIG. 86. Conjugation of an Infusorian. _Vorticella
-nebulifera_, showing sexual differentiation of the whole organism.
-After Greef. _I_, the 'microgonidium' or male individual (_mi_)
-attaches itself to the 'macrogonidium' or female individual (_ma_);
-_cv_, contractile vacuole; _st_, contractile stalk. _II_, the ciliated
-circle on the male individual has disappeared. The male has become
-firmly embedded in the female by means of a sucker-like retraction
-of its lower end. _III_, the fusion of the two individuals has been
-completed; the bristly residue of the male (_ct_) is about to be thrown
-off; the stalk (_st_) is contracted into a spiral. Magnified about 300
-times.]
-
-As these differences are only adaptations to the necessity that the
-animals or cells shall find each other and unite, so also are all the
-other differences of a sexual kind, the thousand-fold differences
-between the sperm-cell and the egg-cell, and the not less numerous
-differences between male and female animals, both in 'primary' and
-especially in the diverse 'secondary' sexual characters which we have
-previously discussed; all these are only means for bringing about the
-process of the union of two germ-cells to form a fertilized 'ovum'
-which is capable of development. The essential part of this so-called
-'sexual reproduction' does not, however, depend on these differences,
-neither on the sexual differences of the germ-cells nor on those of
-the whole organism; it lies solely in the actual union of the two
-germ-cells. Let us remember the idea we have already emphasized, that
-the _essential part_ of the so-called 'sexual reproduction' does not
-depend on these differences, and let us hold fast to the idea already
-indicated, that the chromosomes of the nucleus are the real bearers of
-the hereditary tendencies; then we see that the mingling, or, better,
-the union of the hereditary substances of two different individuals,
-whether single-celled or many-celled, is the result of the process
-which we have hitherto called fertilization or conjugation, but which
-we shall henceforward designate by the more general term 'Amphimixis'
-which means the mingling of substances contributed from two distinct
-sources.
-
-Having made ourselves acquainted with the phenomena of amphimixis in
-animals, plants, and unicellular organisms, we have to face the problem
-of the significance of this remarkable and complicated process. What is
-it that happens, and what meaning can we attach to it?
-
-The first thing to be done is to show that the old and long-prevailing
-conception of fertilization as _a life-awakening process_ must be
-entirely abandoned. That a new individual can arise even among highly
-organized animals, quite independently of fertilization, is proved by
-the parthenogenetic eggs of insects and crustaceans; fertilization
-is not the spark 'which falls into the powder-cask' and causes the
-explosion; it is only an indispensable condition of development. As we
-have seen, there are germ-cells which are not sexually differentiated,
-such as the spores of the lower plants, which are all capable of
-development without amphimixis; and parthenogenetic ova prove that
-even differentiated female germ-cells, that is, germ-cells originally
-adapted for amphimixis, may in certain circumstances develop without
-it; amphimixis is thus not the fundamental cause of development, but
-is only, for many germ-cells, one of the conditions which must be
-fulfilled before development can set in. It is a condition which, under
-certain circumstances, may be dispensed with.
-
-If, then, the multiplication of individuals by single-celled germs
-can take place independently of amphimixis, we may conclude that the
-establishment of amphimixis has nothing to do with the capacity for
-multiplication, that it is not a life-awakening process, but is a
-process of a unique kind, which means something quite different. The
-whole conception of the awakening of life in the germ is antiquated and
-out of harmony with the present state of our knowledge. _Life never
-begins anew_, as far as we can see, and apart from the possibility
-that, unknown to us, a spontaneous generation (_Urzeugung_) of the
-lowest forms of life is still taking place, life is continuous and
-consists of an infinite series of living forms between which there
-is no real interruption. Life, in fact, is like a continuous stream,
-the larger and smaller waves of which are particular species and
-individuals. Only a few decennia ago a morphologist, who was rightly
-held in high esteem, could champion the idea that the mature ovum of
-animals was lifeless material, which had to be quickened in order to
-develop, but now such a theory is untenable, since we have become
-aware of the phenomena of maturation in the ovum, and know that most
-important vital processes, the reducing divisions, take place at the
-time of maturation, quite independently of fertilization.
-
-Thus we do not even require to take into account the conjugation of
-unicellular organisms to make it clear that amphimixis is not the cause
-of the origin of new individuals, but a process, _sui generis_, which
-may indeed be associated with the beginning of embryonic development,
-but which may also occur independently of it, as we see in the case of
-unicellular organisms. If, on the one hand, we see development taking
-place in spores and parthenogenetic ova independently of amphimixis,
-and on the other hand amphimixis occurring without reproduction in
-unicellular organisms, we must regard the two phenomena, amphimixis
-and reproduction, as processes of a distinct kind, which may, however,
-occur in association with and interdependence upon each other.
-
-It was by chance that human observation brought the latter fact to
-light first, and therefore led us for so long to accept the idea that
-_fertilization_, that is, amphimixis, and _development_, that is,
-reproduction, are one and the same; and thus it happens that even now
-there are many naturalists who cannot rid themselves of the idea that
-amphimixis, if not a life-awakening, is at least a _life-renewing_
-process, a so-called 'process of rejuvenescence.'
-
-More than ten years ago[14] I disputed this view, and since then
-the facts which make it untenable have become more and more clear.
-Notwithstanding this I see that it is still adhered to, at least in a
-modified form, by many esteemed naturalists, and so it does not seem
-superfluous to discuss it in more detail.
-
-[14] _Die Bedeutung der sexuellen Fortpflanzung für die
-Selektionstheorie_, Jena, 1886.
-
-I have already noted that we see in conjugation an amphimixis without
-reproduction, and in spores and parthenogenetic ova reproduction
-without amphimixis, and I do not doubt that every unprejudiced critic
-will admit this; many among us, however, are not unprejudiced, but are
-under the spell of earlier ideas, so that they cannot forget that it
-was long believed that fertilization was an indispensable condition
-of development; they therefore regard the divisions which recommence
-sooner or later after conjugation, and which may be repeated hundreds
-of times, _as conditioned by the conjugation which preceded them_,
-and compare them to the series of cells which, in the Metazoa, lead
-from the fertilized ovum to the fully-formed animal. They regard both
-series of cell-generations as a developmental cycle, which leads from
-fertilization to fertilization again, from conjugation to conjugation,
-and which would be impossible without either fertilization or
-conjugation.
-
-This play with the idea of a 'cycle' reminds me vividly of similar
-fantastic play from the time of the much-despised 'Naturphilosophie' of
-a hundred years ago. As men sought to find the analogues of 'solar' and
-'planetary' systems in animal and plant, and believed they had stated
-something when they compared the motile animals to planets and the
-sedentary plants to the sun (!), so it is now imagined that a deeper
-insight has been gained by the recognition of cycles of development.
-By all means let us regard the development of a multicellular organism
-as cyclic; it returns again to its starting-point, but this no more
-explains the forces which produce the cycle, and thus the meaning of
-fertilization, than a comparison with the circling planets explains
-the causes of locomotion in animals. With quite as much reason the
-cycle of development might be made to start from the parthenogenetic
-ovum, and then the whole conclusion of the fanciful cycle idea in
-regard to the meaning of fertilization falls to the ground, for in
-this case the cycle begins without fertilization. Attempts are made to
-get over this difficulty by showing that in many cases parthenogenesis
-alternates regularly or irregularly with sexual reproduction, as in
-the water-fleas (Daphnids), the Aphides, and so on. The mysterious
-rejuvenating power of amphimixis is supposed to suffice for several
-generations, a purely gratuitous assumption, which is also in open
-contradiction to the facts. For there are species which now reproduce
-exclusively by parthenogenesis, among plants for instance, a number
-of fungi, among animals a few species of Crustaceans. Of the latter
-it can be demonstrated that ages ago they reproduced sexually, for
-they still possess the sac which serves for receiving spermatozoa, but
-this sac remains empty, for there are now no males, at least in any
-habitat of the species known to us. To this set belongs an inhabitant
-of stagnant water, _Limnadia hermanni_, a species of Crustacean which
-was found thirty years ago in hundreds, all of the female sex, near
-Strassburg, and also many of the little Ostracods (_Cypris_) which
-inhabit especially the muddy bottom of our pools and marshes. I bred
-one of these (_Cypris reptans_) in numerous aquaria for sixteen years,
-during which there were about eighty generations, and throughout this
-time no male ever appeared, nor did the sperm-sac of the female ever
-contain spermatozoa. The after-effects of the 'rejuvenating' power of
-an amphimixis supposed to have taken place earlier must in this case
-have been enduring indeed!
-
-For these reasons it seems to me useless to make comparisons between
-the developmental cycle of unicellular organisms and the ontogeny of
-multicellular organisms. Both processes have indeed many points of
-resemblance--long series of cells, then interruption of the divisions
-and the occurrence of amphimixis--so that we may quite well speak of
-cyclic development in the physiological sense, in as far as certain
-internal conditions periodically recur and compel the organism to
-conjugation, but we must not suppose that there is more in this than,
-for instance, in the 'cyclic development' of Man, which consists in
-the fact that he finds himself periodically impelled to take food. The
-feeling of hunger which forces him to do so is the signal which warns
-the organism that it is time to supply fresh combustible material to
-the metabolism. In the same way, after a long series of generations
-of Infusorians the necessity for conjugation arises; the whole colony
-suffers an 'epidemic of conjugation,' and the animals unite in pairs;
-in the meantime we know not why, and must content ourselves with
-formulating what is observable, that _the nuclear substances of two
-individuals are thereby mingled in each conjugate_.
-
-Obviously the impulse to conjugation is a signal in the same sense as
-the feeling of hunger is, and we know well from the higher animals what
-a mighty influence it exerts, an influence hardly less potent than that
-of hunger. In Schiller's words, 'Durch Hunger und durch Liebe, erhält
-sich dies Weltgetriebe.'
-
-We can see clearly enough why Nature should have given animals the
-feeling of hunger, but the reason for the need of conjugation is not so
-plain; we can only say in the meantime that it must be of some value in
-maintaining the forms of life, for only that which fulfils a purpose
-can be permanently established.
-
-I shall return later to the problem of the meaning of 'sexual
-reproduction,' and try to probe more deeply into the meaning of its
-establishment; in the meantime I must restrict myself to having shown
-its significance in the union of the hereditary substances of two
-individuals, and at the same time to controverting the theory of the
-'rejuvenating power' of amphimixis. I use this expression in its
-original sense, which indicates that every life is gradually wearing
-itself away and would become extinct were it not fanned to flame again
-by amphimixis--by an artifice of Nature, we may say. This conception
-rests on the fact that the cells of the multicellular body possess for
-the most part only a limited length of life, for they are used up by
-the processes of life, and they break up and die, some sooner, some
-later. As it is observed that all true somatic cells, among higher
-animals at least, are subject to this law of mortality, but that the
-germ-cells are not, and that, furthermore, the germ-cells only develop
-when they are fertilized, the cause of the potential immortality of
-the germ-cells is believed to lie in amphimixis, and a 'rejuvenating'
-power in fertilization, or, more generally, in amphimixis, is inferred.
-Mystical as this sounds, and little as it agrees with our otherwise
-mechanical conceptions of the economy of life, it was until very
-recently a widespread view, although perhaps it is now abandoned by
-many who formerly held it, and has been imperceptibly modified into
-a quite different conception, for which the word 'rejuvenescence' is
-retained, but with the altered meaning of a mere 'strengthening of the
-metabolism' or 'of the constitution.' By many authors, indeed, the two
-meanings of the word are not clearly kept apart. I shall return later
-to the modified meaning of the word 'rejuvenescence,' and shall keep
-in the meantime to the original meaning of the word, which implies a
-renewal of life which would otherwise die out.
-
-This meaning seemed to gain a firm hold, when, about fifteen years ago,
-the French investigator Maupas published his remarkable observations
-on the conjugation of Infusorians. These seemed to show that colonies
-of Infusorians which were artificially prevented from conjugating
-gradually died out; not of course at once, but after many, often
-several hundred, generations; ultimately a degeneration of all the
-animals in such colonies set in, and ended only with their utter
-extinction. Maupas himself interpreted this as _a senile degeneration_
-which took place because conjugation had been prevented, and he
-therefore regarded conjugation as a '_rajeunissement karyogamique_,'
-a rejuvenescence, and therefore a means of preventing the ageing and
-final dying off of the individuals--of obviating, in short, the natural
-death to which in his opinion they would otherwise be subject. This
-conception was greeted with general approval, and there are many people
-who still regard conjugation as a process by which the capacity for
-life is renewed--a view which I must still dispute as emphatically as I
-did some years ago.
-
-In the first place, the observations on which this theory is based
-admit of another interpretation, quite different from that which has
-been assumed to be the only possible one. Maupas prevented conjugation,
-not perhaps because he had isolated individuals and their progeny, but
-by exposing the whole colony of near relatives to unusual conditions
-when conjugation was just about to set in, namely, by supplying
-them with particularly abundant food. The need for conjugation then
-disappeared, as, conversely, it could be called forth at any time in
-a colony by hunger. But these are artificial conditions, and indeed
-the breeding of Infusorians for months in a small quantity of water on
-the object-glass certainly does not correspond to natural conditions.
-We must admire the skill of the investigator who was able to keep his
-colonies alive for months and years under such artificial conditions,
-but we may venture to doubt whether the fate of extinction which did
-ultimately overtake them was really due to the absence of conjugation,
-and not to the unnaturalness of the conditions.
-
-In any case a repetition and modification of Maupas' experiments is
-very desirable, and would be of lasting value[15].
-
-[15] Since the above was written Calkins has made a series of new
-experiments, the results of which differed in several respects from
-those yielded by Maupas' experiments. When his infusorian-cultures
-began to grow weaker, as happened frequently and at irregular
-intervals, he was always able to restore them to more vigorous life by
-a change of diet, and especially by substituting grated meat, liver,
-and the like for infusions of hay. Certain salts, too, had the same
-effect: the animals became perfectly vigorous again. Calkins believes
-that chemical agents, and especially salts, must be supplied to the
-protoplasm from time to time. He reared 620 generations of Paramœcium
-without conjugation. But the 620th was weakly and without energy. The
-addition of an extract of sheep's brains made them perfectly fresh
-and vigorous again. Further experiments in this direction are to be
-desired, but, according to those of Calkins, it is probable that
-Infusorians can continue to live for an unlimited time even without
-conjugation.
-
-Let us, however, assume for the moment not only that Maupas'
-observations were correct, which I do not doubt, but also that they
-were rightly interpreted. Would they in that case afford a proof that
-amphimixis means a rejuvenescence of the power of life? To my thinking,
-not in the remotest degree.
-
-It certainly seems as if this were true at the first glance; the
-colony which is prevented from conjugating goes on multiplying for a
-considerable time, often indeed for hundreds of generations, but this
-may be compared with sufferers from hunger, whose life does not cease
-at once if the feeling of hunger is not appeased.
-
-It was certainly made evident by these experiments that Infusorians
-which were prevented from conjugating were incapable of unlimited
-persistence. But even this in no way proves that amphimixis has a
-power of rejuvenating life, but simply that these animals are adapted
-for conjugation, and that they degenerate without it, just as the
-sperm-cell or the ovum dies if it does not attain to amphimixis.
-
-My opponents take it as axiomatic that the life-movement must come to
-a standstill of itself, and that it therefore requires help. Even so
-distinguished a specialist on the Protozoa as Bütschli argues that
-organisms are not _perpetua mobilia_, and when one remembers the
-physicist's theory of the impossibility of a _perpetuum mobile_ this
-looks at first sight like a formidable objection. But does the organism
-always remain the same as long as it lives, like a pendulum which
-friction causes to swing more and more slowly till ultimately it comes
-to a standstill? We know surely that the phenomena of life arise from
-a continual process of combustion, which is followed by a constant
-replacement of the used-up particles by new particles; we know that
-life depends on an unceasing metabolism, which brings about changes
-in the material basis of the organism every moment, so that it is
-constantly becoming new again.
-
-I shall attempt to show later on that the cells cannot be the ultimate
-elements of the organism, but that the life-units visible with
-the microscope must be made up of smaller invisible units. These,
-therefore, undergo 'metabolism,' which conditions their multiplication
-and their destruction, and this 'metabolism' is not to be seen only in
-the building up and breaking down of 'albuminoid substances,' as the
-physiologists say, but in the alternation between the multiplication
-and the dissolution of these smallest vital particles. Therefore, it
-seems to me that the movement of life, whether in a single-celled or
-in a many-celled organism, is not to be compared to one pendulum,
-but to an endless number of pendulums which succeed one another
-imperceptibly in the course of the metabolism, always producing anew
-the same result, which therefore may continue _ad infinitum_. Suppose,
-then, that we possessed our present conception of life as a process of
-combustion, and of metabolism as the agency which continually provides
-new combustible material in the shape of new vital particles, but that
-we knew nothing about multicellular organisms and their transitory
-existence, but were acquainted only with unicellular organisms and
-their unlimited multiplication by division. If we were then to make
-the observation that all multicellular organisms are mortal, subject
-to natural and inevitable death, it would at first appear to us quite
-unintelligible, since we should be aware that in these also the fire
-of life is continually being fed by the supply of new combustible
-material. Not the potential immortality of unicellular organisms would
-then appear to us remarkable and surprising, but the limitation of the
-life of multicellular organisms--the occurrence of natural death. Who
-knows whether, in that case, many of those investigators trained in
-regard to unicellular organisms alone would not say just the opposite
-of what Bütschli has said, that there could be no natural death in
-many-celled organisms, since single-celled organisms prove to us that
-life is an endless chain of transitory minute vital units?
-
-Furthermore, our physiologists are still far from being able to explain
-the natural death of many-celled organisms from below--I mean from a
-knowledge of its necessary causes; on the contrary, they argue from
-the known occurrence of natural death to the causes which underlie
-it; and thus they have arrived at the idea, undoubtedly correct, that
-the somatic cells of the body are gradually so altered by their own
-activity that they are ultimately unable to function any longer and
-must die off. Therefore, if we were unacquainted with death, we should
-not have been able to infer it from our physiological knowledge, and
-still less from our knowledge of the unicellulars.
-
-As our insight has in point of fact grown by starting from the
-mortal many-celled organisms, and has only later penetrated down to
-the unicellular organisms, so we can understand the genesis of the
-conclusion, deduced from the mortality of the many-celled organisms,
-that unicellular organisms also are unable to continue without limit
-the renewal of material and of vital particles, and that consequently
-they would be subject to natural death if nature had not found in
-conjugation a 'remedy' for 'the physiological difficulties which ensue
-automatically and necessarily from the constitution and from the
-continual functioning' even of unicellular organisms.
-
-But we ask in vain for a shadow of proof of this remarkable conception;
-it is an axiom deduced from our knowledge of natural death among
-multicellular organisms, and bolstered up by a mistaken application of
-the idea of 'perpetual motion.' Or may we regard it as a proof of this
-if it should be found that all unicellular organisms are adapted for
-conjugation?
-
-We shall see later on that amphimixis has certainly quite a different
-and, undoubtedly, a very important effect, namely, that it increases
-the capacity of the species for adaptation; and a life-renewing
-effect in Bütschli's sense could only be ascribed to it in addition
-if the assumption of the necessity of a natural death in unicellular
-organisms were not directly contrary to the clear facts of the case;
-but this is just what it is.
-
-We are acquainted with such contradictory facts, not perhaps among
-the unicellulars themselves, where it is difficult to procure direct
-proof, but in regard to the germ-cells of many-celled organisms which
-correspond to unicellular organisms. We know that under certain
-circumstances the ovum is capable of persisting by itself--in cases
-of parthenogenesis--how then can we conclude that amphimixis is in
-the case of Metazoan germ-cells the cause of their capacity for
-development? We can only conclude, it seems to me, that their power of
-developing is usually bound up with the occurrence of amphimixis. So we
-may conclude in regard to the unicellulars that their unlimited power
-of multiplication is bound up with the occurrence of amphimixis, but
-not that amphimixis is the cause of this power, or that it implies a
-rejuvenescence of life. If unicellular organisms could have been made
-immortal through amphimixis, then what I maintain would be proved--that
-they possess potential immortality; but if they did not possess it, no
-artifice in the world could give it to them; amphimixis could be at
-most only the condition with the fulfilment of which the realization of
-their immortality was bound up.
-
-One may ask, How then can amphimixis be a condition of their survival?
-why should Infusorians which have not conjugated at the proper time be
-doomed to extinction? And from the standpoint of our present knowledge
-I am as little able to give a precise answer as my opponents. But I
-can give one in relation to the amphimixis of multicellular organisms,
-for in regard to these we know that each of the germ-cells--male and
-female--uniting in fertilization, is of itself incapable of development
-and doomed to perish, the sperm-cell because it is too small in mass
-to be able to develop the whole organism, and the ovum because, in
-order to become capable of being fertilized, it must undergo certain
-changes which make it incapable of independent development. We have
-seen that after the two maturing divisions in the egg-cell have been
-accomplished the ovum no longer contains a mechanism of division,
-as the centrosphere breaks up after the second division; embryonic
-development can therefore only begin when a new centrosphere has
-been introduced into the ovum, and this is normally brought about
-by fertilization, that is, by the entrance of the sperm-cell, whose
-nucleus is accompanied by a centrosphere.
-
-Thus amphimixis is seen to be really a condition of development. But
-we now know that the ovum can emancipate itself from this condition,
-by only going through a part of the processes of maturation which are
-related to the subsequent amphimixis, and by thus retaining its own
-centrosome. Nothing is more instructive in this connexion than the
-cases we have already briefly discussed of facultative or occasional
-parthenogenesis. We have seen that in some insects, for instance in
-the silk-moths, there are sometimes, among thousands of unfertilized
-eggs, a few that develop little caterpillars. If we examine a large
-number of such unfertilized eggs we not infrequently find among them
-several which, although they have not gone through the whole course of
-development, have at least gone through the earlier stages, and others
-which may have advanced somewhat further and then come to a standstill;
-in short, we can see that several of these eggs were capable of
-parthenogenetic development, although in varying degrees.
-
-The cause of this parthenogenetic capacity has not as yet been
-definitely determined by observation, but we shall hardly go wrong
-if we seek it in the fact that the centrosphere of the ovum does
-not always perish immediately and completely during maturation, and
-may persist, rarely in its integrity, but sometimes in a weakened
-state. Future observations will probably reveal some differences in
-the size or aster-forming power of the centrospheres of such eggs;
-in any case it is of the greatest interest that stimuli of various
-kinds--mechanical or chemical--can strengthen the disappearing
-centrosphere of the ovum, although as yet we are far from being able to
-say how this comes about.
-
-The experiments already mentioned of Tichomiroff, Loeb, and Winkler
-give us at least an indication how we must picture to ourselves the
-origin of parthenogenesis, namely, through the fact that the breaking
-up of the apparatus for division, introduced for the sake of compelling
-amphimixis, is prevented. Minute changes in the chemistry of the ovum,
-similar to those caused artificially in the ova of the sea-urchin by
-the introduction of an infinitesimal quantity of chloride of magnesium
-(Loeb), in the ovum of the silk-moth by friction or by sulphuric acid
-(Tichomiroff), or in the sea-urchin ovum by an extract of the sperm of
-the same animal (H. Winkler), will effect this modification, and normal
-parthenogenesis is induced.
-
-For the ovum, therefore, amphimixis is certainly not a life-renewing or
-rejuvenating factor; it only appears as such because the process has
-in the course of nature been made compulsory by making the two uniting
-cells each incapable of developing by itself. As we have seen, this is
-true also of the sperm-cell, for although it contains a centrosphere,
-and would be capable of division as far as that is concerned, yet in
-almost all animals and plants it consists of such a minimal quantity
-of living matter that it is unable to build up a new multicellular
-organism by itself. Only in one alga (_Ectocarpus siliculosus_) has
-it been observed that not only the female germ-cell can develop
-parthenogenetically under certain circumstances, but that the male-cell
-may also do so. In this case, however, the difference in size between
-the two is not great, and it is noteworthy that the male plant, in
-correspondence with the smaller size of the zoosperm, tends to be a
-somewhat poorly developed organism.
-
-If we are forced to the conclusion in regard to multicellular organisms
-that amphimixis does not supply the power of development to the ovum,
-but that, on the contrary, the power of development is withdrawn
-from the ovum, so that amphimixis can, so to speak, be forced, must
-we not assume something similar for unicellular organisms also? May
-not amphimixis be made compulsory in their case also, in that the
-Infusorians in preparation for conjugation go through changes which
-make their unlimited persistence possible only on condition that
-they conjugate? In my opinion the division of labour in the nucleus,
-which is differentiated into a macronucleus and a micronucleus, and
-the transitory nature of the former, may be regarded as an adaptation
-in this direction. In any case, it is striking that an organ which
-otherwise persists without limit among unicellular organisms, the
-nucleus, is here subject to natural death after the manner of the body
-of multicellular organisms, that it breaks up and must be reformed
-from the micronucleus which in this case is alone endowed with
-potential immortality. I am inclined to regard this as an arrangement
-for compelling conjugation, since it is only after conjugation that
-the micronucleus forms a new macronucleus, although the latter
-is indispensable to life, as we see from experiments in dividing
-Infusorians artificially.
-
-Suppose we had to create the world of life, and it was said to us that
-amphimixis must--wherever possible--be secured periodically to all
-unicellular and multicellular organisms, what better could we do than
-arrange devices which should exclude individuals which, by chance or
-constitution, could not attain to amphimixis from the possibility of
-further life? But would amphimixis then be the cause of persistence or
-a principle of rejuvenescence?
-
-I do not see that there can be any ground for such an assumption
-other than the tenacious and probably usually unconscious adherence
-to the inherited and deep-rooted idea of the dynamic significance
-of 'fertilization,' no longer, perhaps in its original form, which
-regarded the sperm as the vital spark which awakened new life in the
-dead ovum, but in the modified form of the 'rejuvenating' power of
-amphimixis.
-
-Quite recently an attempt has been made to modify the idea of the
-'rejuvenating' effect of amphimixis so that it should mean only
-an advantage, not an actual condition of persistence. Hartog, in
-particular, admits so much, that the occurrence of purely asexual and
-purely parthenogenetic reproduction excludes the possibility of our
-regarding the process of amphimixis as a condition of the maintenance
-of life. But then we must also cease to regard the 'ageing' and dying
-off of Infusorians which have been prevented from conjugating as an
-outcome of the primary constitution of the living substance, and should
-entirely abandon the misleading expression 'rejuvenescence.'
-
-If we fix our attention on the numberless kinds of cells in higher
-organisms and on multicellular organisms as intact unities, we see that
-they all die off, that they are subject to a natural death, that is, a
-cessation of vital movement from internal causes, yet no one is likely
-to refer their transitoriness to the fact that they do not enter into
-amphimixis. We find it quite 'intelligible' that the cells of our body
-should be used up sooner or later as a result of their own function,
-though we are very far from being able to demonstrate the necessity for
-this, and so really to 'understand' it.
-
-It is only from the standpoint of utility that we can understand the
-occurrence of natural death; we see that the germ-cells _must_ be
-potentially immortal like the unicellular organisms, but that the
-cells which make up the tissues of the body _may_ be transient, and
-indeed _must_ be so in the interests of their differentiation--often
-great and in one direction--which determines the services they render
-to the body. They required to become so differentiated that they
-could not continue to live on without limit, and they did become so
-differentiated because only thus could an ever-increasing functional
-capacity of the whole organism be rendered possible; but they die not
-because 'rejuvenescence through amphimixis is denied them, but because
-their physical constitution is what it is.' And we must explain the
-death of the whole many-celled individual in a similar way. When we
-were trying in a previous study to establish the unlimited continuance,
-the potential immortality, of unicellular organisms, we noted that an
-eternal continuance of the life of the body of multicellular organisms
-could certainly not be a necessity, since the continuance of these
-forms of life is secured by their germ-cells. A continuance of the
-body cannot even be regarded as useful from any point of view. And
-what is not useful for a form of life _does not arise as a lasting
-adaptation_, which is of course not to say that an immortality of
-multicellular organisms, such as they are now, would even have been
-possible. If these organisms were to attain to such a high degree of
-functional capacity and of structural complexity as they now exhibit,
-they obviously could not also have been adapted at the same time to an
-eternal persistence of life.
-
-This is in perfect harmony with our whole conception of the impelling
-forces in the development of the organic world; the ever-increasing
-functional capacity of the structure arose from the advantage which
-this afforded in the struggle for existence, in comparison with which
-the apparent advantage of the endless life of the individual was of no
-account whatever.
-
-I will not here follow out this idea. I have merely touched on it in
-order to make clear that the death of individuals in all multicellular
-organisms gives us no ground for thinking of the unlimited life of
-the germ-cells as dependent on a special artifice of nature, such as
-amphimixis is often supposed to be. Let us always remember that there
-is parthenogenesis, and that there are unicellular germs (spores)
-which are never fertilized, and that the reproduction of many species
-of animals and plants occurs in this way without the intervention of
-amphimixis at all.
-
-       *       *       *       *       *
-
-Attempts have recently been made to prove that parthenogenesis is
-a kind of self-fertilization, and these have been based on the
-observations of Blochmann and Brauer, which showed that in the bee and
-in the salt-water Crustacean, _Artemia salina_, the reducing second
-maturation division of the ovum-nucleus is not suppressed, but is
-regularly accomplished, and that the two daughter-nuclei which result
-from this division unite with each other subsequently. I have already
-noted that these statements do not hold true, at least with regard to
-the bee. In this case the second maturing division takes place without
-any subsequent fusion of the two daughter-nuclei. According to the
-observations of Dr. Petrunkewitsch, which I have already mentioned, and
-for the exactness of which I can vouch, the second maturation-spindle
-is unusually long, so that the two daughter-nuclei are pushed very far
-apart (Fig. 79, _Rsp 2_), and only the inner of the two nuclei (_K 4_)
-becomes a segmentation nucleus, while the outer undergoes a remarkable
-fate; it unites with the inner nucleus which results from the division
-of the _first maturation cell_ (_K 2_), and from this union the
-primitive _genital cells of the animal appear to arise_--an observation
-the eventual theoretical importance of which can only be estimated
-later.
-
-Meantime all we can gain from it is a certain mistrust of the
-interpretation of the processes of maturation in _Artemia_ which have
-hitherto been given; at least we are tempted to suppose that the
-copulation of two nuclei which Brauer observed in _Artemia_ may not
-have led to the formation of the segmentation nucleus there either, but
-may have had some other significance.
-
-But, even if we leave this point entirely out of account, there
-remain all the cases of regular parthenogenesis in which this mode of
-reproduction occurs alone and not in alternation with the sexual mode.
-In these only one maturing division is undergone, and only one polar
-body is formed, and thus there can lie no possibility of supposing a
-self-fertilization of the ovum.
-
-[Illustration: FIG. 79. The two maturation divisions in the
-unfertilized (drone-forming) egg of the bee, after Petrunkewitsch.
-_Rsp 1_, first polar-body in division. _K 1_ and _K 2_, the two
-daughter-nuclei thereof. _Rsp 2_, second directive spindle. _K 3_ and
-_K 4_, the two daughter-nuclei thereof. In the subsequent stage _K
-2_ and _K 3_ unite to form the primordial sex-cell nucleus. Highly
-magnified.]
-
-It is possible that we may yet discover species among unicellular
-organisms which multiply without limit in the absence of any
-amphimixis. R. Hertwig has recently observed phenomena in Infusorians
-which he is inclined to refer to the suppression of an earlier habit of
-conjugation, and so to a kind of parthenogenesis. But even if it should
-be shown that amphimixis plays a part regularly and without exception
-in the life of _all_ unicellular organisms, the facts in regard to
-multicellular organisms are not affected; and, finally, the process of
-amphimixis is one which we have not the slightest ground for assuming
-to be either an awakener or a maintainer of life, and so I return
-to the most essential part of the whole problem, the meaning of the
-chromatin structures, the combination of which is the undoubted result
-of amphimixis. Do they really represent, as we assumed earlier, _the
-hereditary substance_, and what do we mean by this term?
-
-As far as I know the literature and the development of biological
-theories, the botanist Nägeli was the first to deduce, from the
-considerable difference in size between the egg-cell and the
-sperm-cell, the conclusion that the material basis on which the
-hereditary tendencies depend must be a _minimal_ quantity of
-substance. The difference is especially great in animals, even in
-those species whose eggs may be called small, for instance, those of
-sea-urchins or of mammals; even in these the mass of spermatozoon is
-scarcely a thousandth part, often scarcely a hundred-thousandth part
-of the mass of the ovum. And yet the inheritance from the father and
-from the mother is equally great. Now as we know that vital powers
-have always a material basis, a minute quantity, such as is contained,
-for instance, in the spermatozoon of Man, must have implicitly in
-it all the hereditary tendencies of the father; and the conclusion
-is inevitable that in the ovum there can only be an equally minimal
-quantity of substance which is the bearer of the hereditary powers, for
-if there were a larger quantity of hereditary substance in the ovum its
-power of transmission would also be greater[16].
-
-[16] The improbable assumption that the hereditary substance of the
-father may be in quality altogether different from that of the mother,
-and so may have the same power of transmission, and yet take up much
-less room, I leave out of the question altogether.
-
-[Illustration: FIG. 69. Ovum of Sea-urchin (_Toxopneustes lividus_),
-after E. B. Wilson, _zk_, cell-substance. _k_, nucleus (so-called
-germinal vesicle). _n_, nucleolus (so-called germinal spot). Below
-there is a spermatozoon of the same animal (_sp_), magnified in the
-same proportion, about 750 times.]
-
-[Illustration: FIG. 68. Diagram of a spermatozoon. After E. B. Wilson.
-_sp_, apex. _n_, nucleus. _c_, centrosphere. _m_, middle portion. _ax_,
-axial filament. _e_, terminal filament.]
-
-If we inquire as to the part of the spermatozoon which bears this
-hereditary substance, we may exclude both the tail-thread and the
-middle piece (Fig. 68), the former because it obviously fulfils
-quite a specialized physiological function and is histologically
-adapted to this function, the latter because, from observation on the
-spermatozoon which has made its way into the ovum, we know that it
-contains the centrosome, the dividing apparatus of the nucleus. Thus
-there only remains the 'head' of the spermatozoon, which includes the
-nucleus, as the possible vehicle of the heritable substance. Therefore
-we are led to seek for the hereditary substance in the nucleus. But
-the hereditary substance cannot be a perishable substance which may
-at need be dissolved, in the literal sense of the word, and be formed
-anew; therefore we cannot look for it in the nuclear membrane, and just
-as little in the 'nuclear sap' which fills the meshes of the nuclear
-network, since the material on which heredity depends must necessarily
-be solid. Nägeli has clearly shown that we must assume a stable,
-that is, a solid molecular architecture. There thus remains only the
-nuclear reticulum with its chromatin granules, and when we remember
-what we have learnt of the behaviour of this chromatin substance during
-division and amphimixis we can entertain no doubt that the sought-for
-bearer of the inheritance is contained in the substance of the
-chromosomes.
-
-The great care with which the chromosomes are halved by means of the
-complicated division apparatus led us earlier to regard them as a
-substance of complex and manifold qualities and of great physiological
-importance; their constant number in any one species, and the reduction
-of that number to half by means of the reducing divisions, justify us
-in concluding that they are permanent structures, physiological and
-morphological units, which undergo no more than an apparent irregular
-dispersion during the resting state of the nucleus. Finally, the fact
-that these supposed vehicles of inheritance occur in equal numbers in
-each of the conjugating germ-cells, and that this number is _always_,
-both in animals and in plants, half of the normal number occurring in
-somatic cells, is decisive. The logical necessity that the hereditary
-substance of both parents should be transmitted to the offspring in
-equal quantity could not be more precisely met than it is by the
-fact that half the normal number of chromosomes occurs in each of
-the sex-nuclei in the ovum. Personally, I have long been certain, on
-these grounds, that the chromosomes of the nucleus are the hereditary
-substance, and I expressed my conviction on this point almost
-simultaneously with Strasburger and O. Hertwig[17].
-
-[17] More precisely, my conclusions were published several months
-later than those of the investigators named (1885). I think, however,
-that no one who is familiar with my writings for the years immediately
-preceding, which are collected in _Aufsätzen über Vererbung und
-verwandte biologische Fragen_ (Jena, 1892), will dispute that the idea
-was reached by me independently. I attach importance to this because
-all my later work is based upon this idea.
-
-But there is also a physiological proof of the meaning of the nuclear
-substance; and this we owe, again, to the simultaneous and independent
-researches of two investigators, M. Nussbaum and A. Gruber, the
-latter working in the Zoological Institute here (in Freiburg), and
-at my request. They made experiments on regeneration in unicellular
-organisms, and found that Infusorians which were artificially divided
-into two, three, or four pieces were able to build up a whole animal
-out of each piece, provided that it contained a portion of the
-nucleus (macronucleus). The large blue trumpet-animalcule, _Stentor
-cœruleus_, is well suited for such experiments, not only on account of
-its size, but because it possesses a very long rosary-like nucleus,
-which can be easily cut two or three times. When a piece is cut off
-which does not contain a portion of the nucleus, it may indeed live
-for some days and swim about and contract, but it is incapable of
-reconstructing the lost parts, and thus of forming a whole animal, and
-it perishes. It is in the nucleus, therefore, that we have to look
-for the substance which stamps the material of the cell-body with a
-particular form and organization, namely, the form and organization
-of its ancestors. But that is exactly the conception of a hereditary
-substance or idioplasm (Nägeli). Some modern biologists deny that there
-is any hereditary substance _per se_, and believe that the whole of
-the germ-cell, cell-body and nucleus together effects transmission.
-But though it must be admitted that the nucleus without the cell-body
-cannot express inheritance any more than the cell-body without the
-nucleus, this is dependent on the fact that the nucleus cannot live
-without the cell-body; if it be removed from the cell and put, say,
-into water, it bursts and is dissolved. But the cell-body without the
-nucleus lives on, though of course only for a few hours or days, and
-its metabolism ceases only when it is brought to a standstill by the
-failure to replace by nutrition the used-up material. Thus the argument
-used by those who deny the existence of a hereditary substance would be
-paralleled if we denied that Man possesses a thinking substance, and
-maintained that he thinks with his whole body, and even that the brain
-cannot think by itself without the body.
-
-I am convinced that it is just as mistaken to maintain that every
-part of an organism must contain the hereditary tendencies in the
-same degree, or that in unicellular organisms the cell-body is as
-important in inheritance as the nucleus (Conklin). If one feels any
-doubt on this point, one has only to call to mind Nägeli's inference,
-from the minuteness of the spermatozoon, that the hereditary substance
-must be minimal in quantity. But even theoretically there is not the
-smallest ground for the assumption that the cell-body as well as the
-nucleus contains the hereditary qualities, since we find in general
-that functions are distributed among definite substances and parts of
-the whole organism, and it is just on this division of labour that
-the whole differentiation of the body depends. And why should this
-principle not have been employed just here where the most important
-of all functions is concerned? Why should all living substance be
-hereditary substance? Although Nägeli thought of his 'idioplasm'
-otherwise than we now think of hereditary substance, although he
-wrongly imagined it in the form of strands running a parallel course
-through the cell-substance and forming a connected reticulum throughout
-the whole body, he recognized at least so much quite correctly,
-that there are two great categories of living substance--hereditary
-substance or idioplasm, and 'nutritive substance' or trophoplasm, and
-that the former is much smaller in mass than the latter. We now add
-to this, that the idioplasm must be sought for in the cell-nucleus,
-and indeed in the chromatin granules of the nuclear network and of the
-chromosomes.
-
-But incontrovertible proof of the fact that the nuclear substance
-_alone_ is the hereditary substance was furnished when it was found
-possible to introduce into a non-nucleated piece of a mature ovum of
-one species the nucleus of another related species, and when it was
-seen that the larva that developed from the ovum so treated belonged
-to the _second_ species. Boveri made this experiment with the ovum and
-spermatozoon of two species of sea-urchin, and believed that he had
-succeeded in getting from non-nucleated pieces of the ovum of the first
-species, fertilized with the sperm of the second, larvæ of this second
-species; but, unfortunately, later control-experiments made by several
-investigators, especially by Seeliger, have shown that this result
-cannot be regarded as quite certain and indubitable.
-
-I must emphasize again that I am far from regarding the cell-protoplasm
-of the ovum as an indifferent substance. It is certainly not only
-important but indispensable for the development of the embryo, and it
-has assuredly its own specific character, as in every other kind of
-cell. It represents, so to speak, the matrix and nutritive environment
-in which alone the hereditary substance can unfold its wonderful
-powers; it has developed historically, like every other kind of cell,
-but it contains nothing more than the inherited qualities of this one
-kind of cell-protoplasm, not those of the other cells of the body.
-
-But although the essence of fertilization lies, as we have seen, in
-the union of the hereditary substance of two individuals, and not in
-a 'quickening' of the ovum, we may quite well speak of a quickening by
-fertilization in another sense, if we mean the impulse to embryonic
-development, for this is really supplied by the entrance of the
-sperm-nucleus with its centrosphere into the ovum. But even this
-impulse can, under certain circumstances, be given in another way,
-and certainly the awakening of it is not the _end_ of fertilization,
-but only the condition without which the end, the union of two kinds
-of nuclear substance, could not be attained. There is no indication
-whatever that this 'quickening' of the ovum would be necessary for any
-other reason except that _the ovum was previously made incapable of
-development_. There would be no 'fertilization' were not the mingling
-of hereditary substances of fundamental importance for the organic
-world.
-
-Moreover, an ovum, or a fragment of an ovum, may also develop of
-itself, having only _one_ of the sex-nuclei, and the union of the
-hereditary substance of two cells is therefore not indispensable for
-the mere production of a new individual.
-
-What has been observed in regard to fragments of ova is particularly
-interesting in this connexion. Ernst Ziegler first succeeded in halving
-a newly fertilized sea-urchin ovum, so that one half contained the
-female and the other the male pronucleus. The latter alone contained
-a centrosphere, and developed a blastula larva. Delage carried these
-experiments further, and cut an unfertilized but mature sea-urchin
-ovum into pieces, and then 'fertilized' the non-nucleated pieces with
-spermatozoa. These pieces developed and yielded young larvæ of the
-relevant species; so it is clearly seen that even a piece of mature
-ovum-protoplasm may undergo embryonic development, provided that
-a nucleus furnished with a dividing apparatus penetrates into it.
-Unfortunately it is technically impossible to cut such a non-nucleated
-and then fertilized fragment of ovum so that one half shall contain
-the male nucleus the other its centrosphere. Even without this
-_experimentum crucis_ we may say that the half with the male nucleus
-would not multiply by division, and that the other probably would,
-though it would not go through the regular course of segmentation
-processes, because the hereditary substance absolutely necessary for
-these was wanting.
-
-But these and similar experiments prove something more, namely, that
-the nuclei of the sperm-cell and egg-cell do not, as was formerly
-believed, stand in a primary and essential contrast to each other,
-which may be described as male and female, but that both are alike in
-their deeper essence, and may replace each other. They only differ from
-each other as far as the cells to which they belong differ, in this,
-namely, that they are mutually attractive; they find each other and
-unite, and then go on to develop, which each was previously unable to
-do by itself. Widely as the sperm-cell and egg-cell differ in size,
-constitution, and behaviour, in regard to essential character they are
-alike; they bear the relation--as I expressed it twenty years ago--of
-1:1; that is, _they both contain an equal quantity of essentially
-similar hereditary substance_, and the quality of this substance is
-only individually variable. We should, therefore, speak not of a 'male'
-and 'female,' but of a 'paternal' and a 'maternal' nucleus.
-
-All the more recent experiments on 'merogony,' that is, on the
-development of fragments of the ovum, confirm this view. Thus Boveri
-had already observed that even small pieces of sea-urchin ova which did
-not contain the nucleus of the ovum developed, after the spermatozoon
-had entered them, into small but otherwise normal larvæ of the species.
-More recently Hans Winkler proved the same thing for the ova of plants,
-by dividing the ovum of a marine alga (_Cystosira_) into two pieces,
-then fertilizing these with water containing sperms, with the result
-that he got from both pieces, the nucleated and the non-nucleated, an
-embryo of normal appearance. In the latter it could only have been a
-'paternal' nucleus which directed the development.
-
-To sum up. Our investigation into the meaning of amphimixis has led
-us to the conclusion that it consists in the union of two equal
-complements of hereditary substance, contributed by two different
-individuals, into one unified nucleus, and that the sole immediate
-result of this is _the combination of the hereditary tendencies of two
-individuals in one_. Among multicellular organisms this one individual
-of dual origin always implies the beginning of a new life, since
-amphimixis is indissolubly associated with reproduction, and even
-among unicellular organisms it can hardly be disputed that the two
-Infusorians which separate after conjugation are no longer the same
-as they were before. After amphimixis they must contain a different
-combination of hereditary substance from what they had before, and
-this must reproduce the parts of the animal in a somewhat modified
-form. This is theoretically beyond doubt, although it can scarcely be
-established by observation.
-
-We thus know now what 'fertilization' is. Through the labours of the
-last decade the veil has been torn from a mystery of nature which for
-thousands of years confronted humanity as unapproachable; a riddle has
-been solved for the solution of which a few centuries ago men did not
-even dare to hope. Not a few have taken part in these labours; some I
-have already named, but it is impossible that I should here mention
-all who have shared in the achievement by observation or reflection.
-Whoever has helped it on even a single step may say to himself that
-he has taken an active part in bringing about what must be called
-essential progress in human knowledge.
-
-But in the science of nature every new solution implies the cropping up
-of a new riddle, and we are immediately confronted with the problem,
-Why should nature, in the course of evolution, have interpolated this
-process of the mingling of different hereditary substances almost
-everywhere in the organic world? This, however, is a problem which we
-cannot attack until we have first made ourselves more fully acquainted
-with the phenomena of inheritance, and have attempted to reason back
-from these to the nature of the hereditary substance. We must, in
-short, think out a theory of heredity.
-
-
-
-
-LECTURE XVII
-
-THE GERM-PLASM THEORY
-
- Conception of the 'id' deduced from the process of
- fertilization--Hereditary substance, 'idioplasm' and
- 'germ-plasm'--'Idants'--Evolution or Epigenesis--Herbert Spencer's
- uniform germinal substance--Determinants--Illustrations: _Lycæna
- agestis_--The leaf-butterflies--Insect metamorphosis, limbs of
- segmented animals--Heterotopia--The ultimate living units or
- biophors--Number of determinants--Stridulating organ of the
- grasshopper.
-
-
-IN proceeding to expound the theory of heredity which has shaped itself
-in my mind in the course of my own scientific development, I should
-like to begin by pointing out that the hereditary substance of the
-germ-cell of an animal or of a plant contains not only the primary
-constituents (_Anlagen_) of a single individual of the species, but
-rather those of several, often even of many individuals. That this is
-so can be proved in several ways.
-
-I start from what I hold to be the proved proposition, that the
-chromatin substance of the nucleus is the hereditary substance. We
-have seen that this is present in the germ-cells of every species in
-the form of a definite number of chromosomes, and that in germ-cells
-destined for fertilization, that is, in sex-cells, this number is first
-reduced to half, the reduction being effected, as is now proved in
-regard to a whole series of animals, by the two last cell-divisions,
-the so-called maturation divisions.
-
-We know that the full number is only reached again through amphimixis,
-by which process the half number of chromosomes in the male and female
-germ-cells are united in a single cell, the 'fertilized ovum,' and in a
-single nucleus, the so-called segmentation nucleus. Thus the hereditary
-substance of the child is formed half from the paternal, half from the
-maternal hereditary substance, and we have seen that this remains so
-during the whole development of the child, since, at every succeeding
-cell-division each of the paternal and each of the maternal chromosomes
-doubles by dividing, and the resulting halves are distributed between
-the two daughter-nuclei.
-
-Now if the complete hereditary substance of a germ-cell before the
-reducing divisions contains potentially all the primary constituents
-of the body, which it does as a matter of course, then it follows that
-after the reduction each germ-cell must either contain only half the
-primary constituents of the parents or all the primary constituents
-must be contained in the half number of chromosomes. The latter seems
-to me the only possible assumption, as I shall immediately proceed to
-show, and this is as much as to say that the primary constituents of at
-least two complete individuals must be contained in the chromosomes of
-the segmentation nucleus.
-
-That this conclusion is correct is obvious from the fact that a whole,
-that is, a perfect individual with all its parts, develops from the
-ovum, and not a defective one. For suppose that each mature germ-cell
-contained only half the primary constituents of the body, it would be
-impossible that these halves should always exactly complete themselves
-to form a whole embryo when they are brought together in fertilization,
-after having been halved by mere chance during the preceding reducing
-division; it would be much more likely to happen that they did not
-complete themselves, and that their union would therefore result
-in an individual with certain parts wanting. If, for instance, in
-the sperm-cell only the anterior half of the body was potentially
-present, and this united with an ovum which likewise contained only
-the primary constituents of the anterior half, the embryo resulting
-from their union would lack the posterior half of the body, and so
-on. Of course so rough a division of the primary constituents is not
-to be thought of, but however fine we can imagine the halving of the
-mass of primary constituents to be, there would never be any guarantee
-that the two cells uniting in amphimixis would complete the mass of
-primary constituents again; indeed, the chance that the two exactly
-complementary halves of the mass would meet would rather become less
-the finer and more complex one imagines the halving by reducing
-divisions to be. A perfect embryo with all its parts would rarely
-arise, but now one group of parts, now another would be wanting, while
-another group might be developed double, or at least would be doubly
-present in the primary constituents.
-
-But in addition to this the facts of inheritance show us that the
-resemblance to mother and father may express itself simultaneously
-in all the parts, or at least in the same parts of the child, as may
-be seen with especial clearness among plant-hybrids, and thus the
-conclusion is inevitable that even in the half number of chromosomes
-all the primary constituents of the whole body are present.
-
-Let us go a generation further. If the species possess four
-chromosomes the child will have in its cells two maternal chromosomes
-(_A_) and two paternal chromosomes (_B_); what form will this
-proportion take in the germ-cells produced by the child? The maturation
-division can effect the reduction to two chromosomes in different ways;
-there may, for instance, be two paternal chromosomes (_B_) left in the
-one, and two maternal chromosomes (_A_) in the other daughter-cell, or
-one paternal (_B_) and one maternal (_A_) in the one, and a similar
-combination in the other cell. Let us follow the latter case further.
-A sperm-cell which contained the combination _A_ and _B_ might meet
-in amphimixis with an egg-cell of different origin also containing a
-similar combination of chromosomes, let us say a chromosome _C_ from
-the mother, and a chromosome _D_ from the father. We should then have
-in the segmentation nucleus of the fertilized ovum four different
-chromosomes, each of which contained the hereditary substance of one
-grandparent; we should have the four chromosomes, _A_, _B_, _C_, _D_,
-as the hereditary substance of the grandchild.
-
-_But since, as we have seen, the halved hereditary substance still
-contains the whole mass of primary constituents, each one of these
-chromosomes must contain the collective primary constituents of the
-whole body of the relevant grandparent_[18]. _The hereditary substance
-in the fertilized ovum thus consists of several complexes of primary
-constituents (chromosomes) each of which (an 'id') comprises within
-itself all the primary constituents of a complete individual._
-
-[18] When I say the 'collective' primary constituents of the whole body
-of the grandparent this is not expressing it quite precisely, for, as
-we shall see later, each individual must arise from the co-operation
-of different chromosomes of different origin, not merely from one of
-the chromosomes contained in its germ-plasm. In the example given
-above, the body of each grandparent cannot have arisen only from
-a single chromosome, which was transmitted to his grandchild, but
-from the co-operation of this chromosome with three others, which
-have distributed themselves along other genealogical paths. But this
-does not affect the above chain of reasoning, for here it is not a
-question of whether all the primary constituents of the grandparent
-are present in the child--that can never be the case--but whether the
-primary constituents transmitted by him represent the whole body of an
-individual.
-
-It can be made clear in yet another way that, as a consequence of
-sexual reproduction, the germ-plasm of each species must be composed
-of several 'ids,' _individually different_. Let us assume that there
-was as yet no amphimixis, and that we could look on at its introduction
-into the organic world; the hereditary substance of the beings which
-had previously lived and multiplied by division would consist of
-more or less numerous chromosomes similar to each other, so that,
-for instance, each individual would contain sixteen identical 'ids.'
-But if amphimixis were now to take place for the first time, in the
-same manner as it does to-day--that is, after the reduction of the
-number of the ids to half--in the first amphimixis eight paternal
-ids would unite with eight maternal ids to form the germ-plasm of the
-new individual, as is indicated in Fig. 87 by a circle of spheres,
-of which ten are white and ten black as a sign of their difference.
-We may think of the figure as representing the 'equatorial plate'
-of a nuclear spindle with its ids arranged in a circle. Now, if
-two organisms of this generation, with two kinds of ids, unite in
-amphimixis after previous reduction of the ids, we have figure _B_,
-in which the paternal ids (_pJ_) are seen to the left of the line and
-the maternal ids (_mJ_) to the right, while each semicircle is in its
-turn made up of two kinds of ids, those of the grandparents (_p_^2_J_
-and _m_^2_J_, _p_^2_J_^1 and _m_^2_J_^1). The figures _C_ and _D_ show
-the two following generations, in which the number of identical ids is
-each time reduced to half, because eight strange ids are again mingled
-with them; in _C_ only two ids are still identical, and in _D_ all the
-ids are individually different, because they have come from different
-ancestors of the same species. Of course this would only be the case
-if inbreeding were excluded, because through it the ids of the same
-forefathers from two or more sides would meet; but prolonged inbreeding
-is a rare exception in free nature.
-
-[Illustration: FIG. 87. Diagram to illustrate the operation of
-amphimixis on the composition of the germ-plasm out of diverse
-ancestral plasms or 'ids.' _A_-_D_, the ids of the germ-plasm of
-four successive generations: _A_, consisting of only two kinds of
-ids; _B_, of four; _C_, of eight; _D_, of sixteen kinds. _pJ_ and
-_mJ_, paternal and maternal ids. _p_^2_J_, grandpaternal; _p_^3_J_,
-great-grandpaternal; _p_^4_J_, great-great-grandpaternal ids. The marks
-in the ids themselves indicate their individually distinct characters.]
-
-I shall now call the hereditary substance of a cell its 'idioplasm,'
-after Nägeli's example, although he sought it in the cell-substance,
-not in the nucleus, and had a different theoretical conception of its
-mode of action. It was he, however, who conceived and established
-the idea of the idioplasm as the bearer of the primary constituents,
-an _Anlagensubstanz_, determining the whole structure of the
-organism in contrast to the general nutritive protoplasm. Every cell
-contains idioplasm, since every cell-nucleus contains chromatin,
-but I call the idioplasm of the germ-cells _germ-plasm_, or the
-primary-constituent-substance of the whole organism, and the complexes
-of primary constituents necessary to the production of a complete
-individual--whose presence we have just shown to be theoretically
-necessary--I call _ids_. In many cases these 'ids' might be synonymous
-with chromosomes, at least in all the cases in which the chromosomes
-are simple, that is, are not composed of several similarly formed
-structures. Thus in the salt-water Crustacean, _Artemia salina_,
-which possesses 168 minute granular chromosomes, each of these
-chromosomes must be regarded as an id, for each can in certain
-circumstances be thrown out from the ovum by the reducing division,
-or it can be brought into the most various combinations with other
-chromosomes by fertilization. Each of them must therefore consist of
-perfect germ-plasm in the sense that all the parts of an individual
-are virtually contained in it; _each is a biological unity, an id_.
-But when we see in many animals larger band-shaped or rod-shaped
-'chromosomes,' and when these are composed of a series of granules, as
-they are, for instance, in the often mentioned _Ascaris megalocephala_,
-each of these granules is to be regarded as an id. In point of fact,
-we find, instead of the two or four large rod-shaped chromosomes
-of _Ascaris megalocephala_, a larger number of smaller spherical
-chromosomes in other species of _Ascaris_.
-
-Compound chromosomes consisting of several ids, such as all rod or
-band-like elements of the nuclear substance probably are, I designate
-'idants.' That they are composed of several individual ids is not
-always clearly apparent because of the smallness of the object, and
-even in larger ones this may only be seen in certain stages. Thus
-we have in Fig. 88, _A_ and _B_, two 'mother-sperm-cells' of the
-salamander; _A_ at an earlier stage, in which the individual ids are
-not visible; _B_ at a later stage, in which the band has split, and
-the rosary-like structure has become at once apparent. It is not
-possible, then, to see at once whether each chromosome corresponds to
-one or to several ids. A more exact investigation of the processes
-of reducing division has shown that there are chromosomes of simple
-spherical form, that is, composed of several ids whose 'plurivalence'
-cannot be directly recognized, but can only be inferred from their
-further development; there are bivalent chromosomes of double value
-and quadrivalent chromosomes of fourfold value, which we have to think
-of as made up of two or four ids. It would lead us too far to go into
-this more precisely, nor does it fall within the scope and intention
-of these lectures to inquire into these intimate and still disputed
-details.
-
-The germ-plasm of every species of plant or animal is thus composed
-of a larger or smaller number of ids or primary constituents of an
-individual, and it is through the co-operation of these that the
-individual which develops from the ovum is determined.
-
-[Illustration: FIG. 88. Sperm-mother-cells (spermatocytes) of the
-salamander. _A_, cross-section of the cell in the aster-stage; the
-chromosomes (_chr_) or idants do not reveal that they are compounded
-out of many ids, which are, however, quite distinctly seen in
-_B_ (_Jd_), where the chromosomes or idants (_chr_) are already
-longitudinally split. _zk_, cell-substance. _csp_, centrosome. _c_,
-centrosome in division. After Hermann and Drüner.]
-
-We have further to inquire what conception we can form of the
-constitution of an id and of its mode of operation. I have already
-spoken of 'primary constituents' (_Anlagen_) of which the germ-plasm
-consists, but what right have we to think of the parts of an animal as
-already contained in the germ in any form whatever? Is it not equally
-possible that the germ consists of parts, none of which bear any
-definite relation in advance to the parts of the finished animal? Might
-not the germ-cell, along with its nucleus, undergo transformations and
-regular changes which would successively give rise to new conditions,
-namely, the different stages of development, until finally the complete
-animal was attained?
-
-We stand here before an old problem, before the two opposed
-interpretations--the theory of 'Evolution' and the theory of
-'Epigenesis,' which were first ranged against each other long ago, and
-which are a cause of strife even now, although in somewhat different
-guise.
-
-The theory of 'Evolution' is especially associated with the name of
-Bonnet, who elaborated it in detail in the eighteenth century. It
-maintains that the development of the ovum to the perfect animal is not
-really a new creation, but only an unfolding of invisible small parts,
-which were already present in the ovum. It assumes that the parts of
-the perfect organism are already preformed in the ovum, and on this
-account it is called the 'Preformation Theory.' Bonnet often speaks
-of the preformation of the perfect animal in the germ as a 'miniature
-model,' although his conception of 'evolution' was not really so crude
-as has been often alleged. He expressly emphasized that this miniature
-model was not exactly like the perfect animal, but consisted of
-'elementary parts' only, which he thought of as a net whose meshes were
-filled up during development and by means of nutrition with an infinite
-number of other parts. But after all, his conceptions, and those of his
-time generally, were very far removed from the biological thinking of
-our own day, as may perhaps be most readily understood when I mention
-that he regarded death and decay as an 'involution,' as a folding back,
-so to speak, by means of which all the parts gained though nutrition
-were removed again, so that the net of the miniature model shrank
-together to the invisible minuteness that it had in the ovum. So it
-remained, he fancied, till it was reawakened at the resurrection,
-using the term in the religious sense! He afterwards dropped this
-fancy, because the objection was made to it that human beings who had
-lost a leg or an arm in this life would necessarily be maimed at the
-resurrection!
-
-In Bonnet's time the facts of development were quite unknown, and
-not even the stages of the development of the chick from the egg had
-been observed. When this was afterwards done the prevalent theory
-of 'evolution' necessarily collapsed, for men saw with their own
-eyes that a miniature model of the chick did not gradually grow into
-visibility and ultimately into the young chick, but that first of all
-parts showed themselves in the egg which bore no resemblance at all
-to the chick, that these first rudiments were then altered, and that
-through continual new formations and transformations the chick finally
-appeared. Upon this K. von Wolff based his theory of 'Epigenesis,' or
-development through new formations and transformations. He maintained
-that the doctrine of 'Evolutio' was false; that there is no miniature
-model invisibly contained within the egg; but that from the simple
-egg-substance there arises, through the agency of the formative powers
-inherent in it, a long series of stages of development, of which each
-succeeding one is more complex than the one before, until ultimately
-the perfect animal is reached.
-
-This certainly marked considerable progress, for it meant the
-beginning of a science of embryology, that is, the science of the
-form-development of the animal or plant from the ovum. The result was
-not so important in its theoretical aspect, for though the knowledge
-had been gained that the young animal goes through a long series of
-different stages, it had not been discovered how nature works this
-wonder and causes an animal of complex structure to arise from the
-apparently simple substance of the ovum. A solution of the difficulty
-was found by attributing to the ovum a formative power, afterwards
-called by Blumenbach the _nisus formativus_, which possessed the
-capacity of developing a complex animal from the simple 'slime,' or, as
-we should say, the simple protoplasm.
-
-If we contrast the strictly theoretical part of the two theories, we
-find that Bonnet regarded the ovum as something only apparently simple,
-but in reality almost as complex as the animal which developed from it,
-and that he thought of the latter, not as being formed anew, but as
-being unfolded or evolved. That is to say, he thought that rudiments
-present from the outset in the ovum gradually revealed themselves and
-became visible. Wolff, on the other hand, regarded the ovum as being
-what it seemed, something quite simple, out of which only the _nisus
-formativus_ could, by a series of transformations and new formations,
-build up a new organism of the relevant species.
-
-Wolff's Epigenesis routed Bonnet's theory so completely from the
-field that, until quite recently, epigenesis was regarded as the only
-scientifically justifiable theory, and a return to the 'evolutionist'
-position would have been looked upon as a retrograde step, as a
-reversion to a period of fancy which had been happily passed. I myself
-have been repeatedly told, with regard to my own 'evolutionistic'
-theory, that the correctness of epigenesis was indisputably
-established, that is, was a fact, verifiable at any time by actual
-observation!
-
-But what are the facts? Surely only that there is a succession of
-numerous developmental stages, which we know very precisely in the
-case of a great many animals, and that the miniature model which
-Bonnet assumed to be in the egg does not exist. Both these facts
-are now no longer called in question. But that does not furnish us
-with a theory of development, for theory is not the observation of
-phenomena or of a series of phenomena, _it is the interpretation of
-them_. Epigenesis, as formulated first by Aristotle and again by
-Harvey, Wolff, and Blumenbach, certainly offered an interpretation of
-development, not, however, by referring only to what was observable,
-but by going far beyond it; on the one hand taking the _appearance_ of
-a homogeneous germ-substance for reality, and, on the other, assuming
-a special power, which caused a heterogeneous organism to arise from a
-homogeneous germ.
-
-We cannot now accept either of these assumptions, for we know that the
-germ-substance is not homogeneous, and indeed is not merely a substance
-but a living cell of complex structure; and we no longer believe
-in a special vital force, and therefore not in a special 'power of
-development,' which could only be a modification of the former. We are
-thus as little able to accept the old epigenesis as the old evolution,
-and we must establish a theory of Development and Heredity on a new
-basis.
-
-What this basis must be is in a general way beyond doubt. Since it
-is the endeavour of the whole of modern biology to interpret life
-more and more through the interactions of the physical and chemical
-forces bound up with matter, development, too, comes within this aim,
-for development is an expression of life. We seek to understand the
-mechanism of life, and, as a part of that, the mechanism of development
-and of heredity which is closely associated with it.
-
-If we wished to attack the problem of heredity at its roots we
-should first of all have to try to understand the process of life
-itself as a series of physico-chemical sequences. Perhaps this will
-be achieved up to a certain point in the future, but if we were to
-wait for this we should in the meantime have to abandon all attempts
-at a theoretical interpretation of the phenomena of development and
-heredity, and might indeed have to postpone them to the Greek Kalends.
-That would be as though, in the practice and theory of medicine, all
-investigation into and speculation regarding disease had to wait until
-the normal, healthy processes of life were thoroughly understood. In
-that case we should now know nothing of bacteria diseases and the
-hundred other acquisitions of pathological science: physiology too
-would have remained far behind its present level if it had lacked the
-fruitful influence of experience in cases of disease, and the ideas
-and theories, true and false, which have been based thereon. In the
-same way we require a theory of development and heredity if we are to
-penetrate deeper into these phenomena, and must have it in spite of the
-fact that we are still very far from having a complete causal knowledge
-of the processes of life. For the raw material of observation, which
-is to some extent fortuitous, will never bring us any further on;
-observation must be guided by an idea, and thus directed towards a
-particular goal.
-
-It is, however, quite possible to leave aside for the present all
-attempts at an explanation of life, and simply to take the elements of
-life for granted, and on this basis to build up a theory of heredity.
-We have already taken a step in that direction by establishing that
-the whole substance of the fertilized ovum does not take part in
-heredity in the same degree, but that only a small part, the chromatin
-of the nucleus, is to be looked upon as the bearer of the hereditary
-qualities, and by deducing, further, that this chromatin is made
-up of a varying number of small but still visible units, the ids,
-each of which virtually represents the whole organism, or, as I have
-already expressed it, each of which contains within itself, as primary
-constituents, all the parts of a perfect animal.
-
-It was these 'primary constituents' which led us to the digression in
-regard to Bonnet's theory of 'Evolutio' and Wolff's 'Epigenesis.'
-
-Let us now inquire what must be the constitution of such a chromatin
-globule, an id, so that, shut up within the nucleus of a living
-reproductive cell, it can direct the development of a new organism
-which resembles its parent. Two fundamental assumptions present
-themselves, and these can be related to every conception of a
-'germ-plasm,' even independently of the assumption of ids. Either we
-may think of the id as made up of similar or of different kinds of
-parts, none of which has any constant relation to the parts of the
-perfect animal, or we think of it as composed of a mass _of different
-kinds of parts, each of which bears a relation to a particular part
-of the perfect animal_, and so to some extent represents its 'primary
-constituents' (_Anlagen_), although there may be no resemblance
-between these 'primary constituents' and the finished parts. The
-assumption of a germ-plasm composed of similar parts, which has
-been made, for instance, by Herbert Spencer, may be called the
-modern form of epigenesis, while the other assumption is the modern
-form of the 'evolution' theory. As the former theory can no longer
-call to its aid a 'formative power' as a _Deus ex machina_, it can
-only explain development as induced by the influence of external
-conditions--temperature, air, water, gravity, position of parts--upon
-the chemical components of the germ-plasm, which are everywhere
-uniformly mingled; and it makes no difference whether this uniform
-germ-plasm is thought of as composed of many different kinds of parts,
-as long as those parts are mingled uniformly to make the germ-plasm
-and bear no relation to definite parts of the developing animal.
-Oscar Hertwig has recently outlined such a theory. Although I cannot
-expound it here I must say at least so much with regard to it, and to
-all other theories of development founded on a similar basis, that
-they could not be accepted even if they were able to offer a workable
-explanation of the development of the individual, and for this reason,
-that ontogeny is not an isolated phenomenon which can be interpreted
-without reference to the whole evolution of the living world, for it
-is most intimately associated with this, being indeed a piece of it,
-having, as we shall see, arisen from it, and, furthermore, preparing
-for its continued progress. _Ontogeny must be explained in harmony
-with phylogeny and on the same principles._ The assumption of a
-germ-plasm without primary constituents, or of a completely homogeneous
-germ-plasm, as Herbert Spencer maintained, is irreconcilable with this,
-for, as will be seen, it contradicts certain facts of inheritance and
-variation. Therefore all theories founded on this assumption must be
-rejected.
-
-There is another and, I believe, weighty consideration which forbids us
-to assume a germ-substance without primary constituents. I shall return
-to this later, but in the meantime I wish to build up more completely
-my own 'germ-plasm' theory.
-
-I assume that the germ-plasm consists of a large number of different
-living parts, each of which stands in a definite relation to particular
-cells or kinds of cells in the organism to be developed, that is, they
-are 'primary constituents' in the sense that their co-operation in the
-production of a particular part of the organism is indispensable, the
-part being _determined_ both as to its existence and its nature by the
-predestined particles of the germ-plasm. I therefore call these last
-_Determinants_ (_Bestimmungsstücke_), and the parts of the complete
-organism which they determine _Determinates_, or hereditary parts.
-
-It is easy to show on what basis this assumption rests; the phenomena
-of inheritance taken in conjunction with those of variation seem to
-me to compel us to it. We know that all the parts of an organism are
-variable, and that in one individual the same part may be larger, in
-another smaller. Not all variations are transmissible, but many of
-them, and some very minute ones, are. Thus, for instance, in many human
-families there occurs a small pit, hardly as large as the head of a
-pin, in the skin of the ear, whose transmission I have observed from
-the grandmother to the son and to several grandchildren. In such a case
-there must be a minute something in the germ-plasm, not present in
-that of other human beings, which causes the origin, in the course of
-development, of this little abnormality in the skin.
-
-There are human families in which individuals occur repeatedly,
-and through several generations, who have a white lock of hair, in
-a particular spot, on an otherwise dark-haired head. This cannot be
-referred to external influences, it must depend on a difference in
-the germ, on one, too, which does not affect the whole body, not even
-all the hairs of the body, but only those of a particular spot on the
-surface of the head. It is a matter of indifference whether the white
-colouring of the hair-tuft is produced by an abnormal constitution of
-the matrix of the hair, or by other histological elements of the skin,
-as of the blood-vessels or nerves. It can only depend ultimately on a
-divergently constituted part of the germ-plasm, which can only affect
-this one spot on the head, and alter it, if it is itself different from
-what is usual. On this account I call _it_ the _determinant_ of the
-relevant skin-spot and hair-group. In Man such minute local variations
-are usually lost after a number of generations, but in animals there
-are innumerable phenomena which prove to us that single minute
-deviations can become permanent. Thus there lives in Central Europe a
-brown 'blue butterfly,' _Lycæna agestis_, which has a little black spot
-in the middle of its wing. The same species also occurs in Scotland,
-but there, instead of the black spot, it has a milk-white one, and
-so-called 'eye-spots' on the under surface of the wing have also
-lost their black centres. The species has thus varied transmissibly,
-but only in regard to these particular spots on the wing. A slight
-variation must therefore have taken place in the germ-plasm which only
-affects these few parts of the body, or, to express it otherwise, the
-germ-plasms of the ancestral species and of the variety can only be
-distinguished by a difference which determines exclusively the scale
-colour of these spots. The two germ-plasms differ, I should say, only
-as regards the _determinants_ of these wing-scales.
-
-We know from the artificial selection to which Man has subjected and
-still subjects his domesticated animals and useful plants, that any
-spots and parts of the body which he chooses can be hereditarily
-altered, if the desired variations which present themselves are always
-selected for breeding, and that this does not necessarily cause
-variation in other parts of the body. When, for instance, in the case
-cited by Darwin, the comb of a Spanish cock which had previously hung
-downwards was made to stand upright because a prize had been offered
-for this character, or when a certain breed of hens was 'furnished with
-beards,' the results were permanent variations affecting only the parts
-on which the fancier's attention had been fixed. In the same way, when
-the tail feathers of the Japanese cock are lengthened to three feet
-the rest of the plumage does not alter, still less any other part of
-the body. Of course there are numerous 'correlated' variations, and
-in very many cases the breeder causes a second or third character, on
-which he had not fixed his attention, to vary in addition to the one
-he was aiming at. But such concomitant variations are not necessary
-or inevitable in all cases; and indeed we need not refer them all to
-a true correlation of the parts, but may suppose that they depend not
-infrequently on the faultiness of our power of observation, which is
-not sufficiently keen to control several parts of the body at one
-time, and to notice minimal variations in parts on which we have not
-specially fixed our attention.
-
-[Illustration: FIG. 13. _Kallima paralecta_, from India; showing the
-right under surface in the resting pose. _K_, head. _Lt_, palps. _B_,
-limbs. _V_, fore wing. _H_, hind wing. _St_, 'tail' of the latter,
-representing the stalk of the leaf. _gl_^1 and _gl_^2, transparent
-spots, _Aufl_, remains of 'eye-spots.' _Sch_, a 'mould'-spot.]
-
-So much, at least, is certain, that in all these cases of the
-artificial alteration of individual characters the germ-plasm is in
-some way changed, but always in such a way that it differs from that
-of the ancestral form through such variations alone, and the effect of
-these is that only the altered parts are influenced thereby, and not
-the whole organism. This again is but another way of saying that only
-the _determinants_ of these parts have altered.
-
-We can see from a thousand cases that exactly the same happens in a
-state of nature, that there, too, one part changes after another,
-until the highest possible degree of adaptation to the conditions has
-been attained. In the mimetic resemblance to leaves exhibited among
-butterflies this is most clearly seen, for here we are familiar with
-the model--the leaf--and we see how one species approximates to it in
-a general way only in the total colour, how others develop a brown
-stripe crossing the posterior wing obliquely, so that, to a certain
-extent, it resembles the midrib of a leaf, how in a third species this
-stripe is continued for some distance forward across the anterior wing,
-in a fourth it goes a little further, until, finally, in a fifth, it
-is continued on to the tip of the anterior wing. This may be seen,
-for instance, in the genus _Anæa_, which is rich in species. But even
-then a still further increase of the resemblance is possible, for, as
-is well known, there are not infrequently imitations of the lateral
-veins of the leaf as well, or dark spots which faithfully reproduce the
-mould-spot on a damp, decaying leaf, or colourless transparent spots
-which probably simulate dewdrops, and so on. All these are variations
-relating to individual and distinct groups of wing-scales, which have
-varied transmissibly and independently, that is, each of them has been
-produced by a variation in the germ-plasm, which brought about a change
-in this particular area of the body and in no other.
-
-Let us for a moment assume the impossible, and suppose that we could
-look on at the evolution of such a leaf-butterfly; the beginning of the
-leaf-imitation might have its cause in the fact that an ancestral form
-of _Kallima_, which had previously lived in the meadows, exhibited on
-the part of some of its descendants a migration to the woods, and thus
-divided into two groups, with a different manner of life--a meadow form
-and a wood form. The latter adapted itself to sitting among leaves, and
-the midrib of a leaf developed on its wings. In a germ-plasm without
-'primary constituents' this variation could only depend on a uniform
-variation of all the parts, for these parts are either alike among
-themselves, or at any rate have the same value for every part of the
-finished organism. But the germ-plasm of the new breed must somehow
-differ from that of the ancestral form, otherwise it could produce no
-new variety, but only the ancestral form over again. But how could
-an animal differing only in one minute part arise from a germ-plasm
-which has varied in all its parts, and how could such little steps of
-variation be repeated many times in the course of the phylogeny without
-the corresponding variations of the germ-plasm becoming so intense
-that not only the wing-markings but everything about the animal would
-be altered likewise? And yet these 'leaf-pictures' have not originated
-suddenly, but by many small steps, so that the germ-plasm must have
-varied _in toto_ a hundred times in succession if there are no primary
-constituents.
-
-In the Indian species, _Kallima paralecta_, there are no fewer than
-five well-marked varieties, the differences between which depend solely
-on the manner in which the leaf-picture on the wing is elaborated,
-_for the upper surface of the wing is alike in all_. Even a cursory
-observation of a collection of these butterflies shows that the lateral
-veins of the leaf-picture are quite different in number, distinctness,
-and length in the different individuals. On the right half of the wing
-there may be as many as six of them indicated (Fig. 13); and it can
-be observed that the three middle ones are the longest, most sharply
-defined, and darkest, while those lying near the tip and the base of
-the mimic leaf are shorter and often even shadowy. On the left side
-the second lateral vein in particular distinctly shows indentations
-indicative of the rings, inherited from the ancestral forms, which
-surrounded the still visible eye-spots (_Aufl_); the third lateral
-vein is quite indefinite and shadowy, but nevertheless it runs exactly
-parallel to the first two, and thus heightens the deceptive effect. We
-can thus distinguish older and more recent elements in the marking--a
-proof of the slow and successive origin of the picture.
-
-This is not reconcilable with the conception of a germ-plasm without
-primary constituents, however complex a mixture it may otherwise
-be. A substance which had to undergo thousands upon thousands of
-variations, arising from each other according to law and in the
-strictest succession, in order that it might become a definite
-organism, predetermined as to all its thousands of parts down to the
-most minute, cannot vary over and over again in its whole constitution
-without the consequences showing themselves in numerous, or indeed in
-_all_, the parts of the body. Such variations in the germ-plasm would
-be comparable to many successive deviations of a ship from her course,
-which, although the single ones would only cause a minimal deviation
-from the true course, would, when summed up in a voyage of some length,
-land the vessel at quite another coast than the one intended. If each
-individual adaptation of the species depended on a variation of the
-whole germ-plasm the wood _Kallima_ would soon retain no resemblance
-to its ancestral form, the meadow species; yet we are acquainted with
-species of _Kallima_ which do not show the special resemblance to a
-leaf, but, for instance, still exhibit the perfectly developed eye-spot
-of the ancestral form, and so forth. It follows, therefore, that the
-origin of the leaf-picture has not greatly influenced the general
-character of the species; and the fact that the upper surface of the
-wings has remained the same in all the varieties is in itself enough to
-prove this.
-
-Since, then, the resemblance to a leaf cannot have arisen without
-something in the germ-plasm varying, since the germ-plasm of a forest
-_Kallima_ and a meadow _Kallima_ must be different in something, and
-cannot be any more alike than the germ-plasm of a fantail-pigeon and a
-carrier, there _must be 'primary constituents' in the germ-plasm_, that
-is, vital units whose variation occasions the variation of definite
-parts of the organism, and of these alone.
-
-[Illustration: FIG. 17. Caterpillar of _Selenia tetralunaria_ on a twig
-of birch. _K_, head. _F_, feet. _m_, protuberances resembling dormant
-buds. Natural size.]
-
-It is on such considerations as these that my assumption, that _the
-germ-plasm is composed of determinants_, depends. There must be as many
-of these as there are regions in the fully-formed organism capable
-of independent and transmissible variation, including all the stages
-of development. Every part, for instance, of the butterfly's wing,
-which is capable of independent and transmissible variation, must,
-so I conclude, be represented in the germ-plasm by an element which
-is likewise variable, the determinant; but the same must be true of
-every independently and transmissibly variable spot of the caterpillar
-from which the butterfly developed. We know how markedly caterpillars
-are adapted in form and colour to their environment. Let us assume
-that the caterpillar of the butterfly which we chose as an example
-of wing-marking had the habit of feeding only by night and during
-the daytime of resting on the trunk of a tree, or, more precisely,
-in the crevices of the bark. It would then resemble the caterpillar
-of the moths of the genus _Catocala_ or the Geometers (Geometridæ),
-and possess the colour of the bark of the tree in question; the
-determinants of the skin would thus have varied to correspond with this
-mode of life on the part of the caterpillar, so that the skin would
-appear grey or brown. But there cannot be only _one_ determinant of
-the caterpillar skin in the germ-plasm, for the bark-like colour of,
-for instance, a Geometer caterpillar is not a uniform grey, but has
-darker spots at certain places and lighter whitish spots at others,
-such as are to be seen on the bark of the twig on which the caterpillar
-is wont to rest, or brown-red spots, like those on the cover-scales of
-the buds, or little warts and protuberances which exactly correspond
-to similar roughnesses on the twigs, to cracks in the bark, and so on.
-All these markings are constant, and are to be found in the same spot
-in every caterpillar of the species. A large number of regions of the
-caterpillar skin must therefore be independently determined by the
-germ-plasm; the germ-plasm must contain parts the variations of which
-bring about variations only of an independently variable region of the
-caterpillar skin. In other words, in the germ-plasm of the butterfly
-ovum there must not only be determinants for many regions of the
-butterfly's wing, but also for many regions of the caterpillar's skin.
-
-This line of argument, of course, applies to all the bodily parts and
-organs of the butterfly and of the caterpillar, as well as to all the
-stages of development of the species as far as these parts are able
-to vary in such a way that the variation reappears in the following
-generation, that is to say, as far as it is transmissibly variable.
-
-But all parts must be transmissibly variable which have exhibited
-independent variation in relation to their ancestors. When, for
-instance, the eggs of a butterfly (_Vanessa levana_) bear a deceptive
-resemblance to the flower-buds of the stinging-nettle on which
-the caterpillar lives, not only in form and colour, but in their
-pillar-like arrangement, we may conclude that these eggs have varied
-transmissibly from those of their ancestors, which had not acquired
-the habit of living on the stinging-nettle, in these three respects
-independently, that is, uninfluenced by any other variations the
-species may have undergone; and that, consequently, the germ-plasm
-must contain determinants for the egg-shell, egg-colouring, and so
-on. The manner of laying the eggs in the form of pillars depends on a
-modification of the egg-laying instinct, which must in its turn depend
-on the variations of certain nerve-centres, and we learn from this that
-there must be in the germ-plasm determinants for the individual centres
-of the nervous system.
-
-It may, perhaps, be suggested that matters could be explained in a
-simpler way--that it is enough to assume the presence in the egg of
-determinants for all the parts of the caterpillar, and that those of
-the butterfly are only formed within the caterpillar.
-
-This suggestion seems justifiable if we confine ourselves to
-superficial considerations. We read in every handbook of entomology
-that the wings only arise during the life of the caterpillar, and
-in a certain sense this is true, for the primary constituents or
-primordia of wings only develop into the fully formed wing during the
-larval period. But even if these primordia were only formed during
-the caterpillar-stage, what could they develop from? Only out of the
-material parts of the caterpillar, that is, from some of its living
-cells or cell-groups. The constitution of the wings would therefore
-be dependent on that of the cells of the caterpillar from which they
-arose, so that if these varied transmissibly through the variation
-of their determinants contained in the germ, the determinants of the
-butterfly which were just developing would vary with them; every
-transmissible variation of the caterpillar would necessarily cause a
-similar variation in the butterfly, and this does not happen. If any
-one hazarded the assumption that the determinants of the butterfly
-develop only in the caterpillar, but quite independently of its
-constitution, he would either be making an absurd statement, namely,
-that the characters of the butterfly were not transmissible at all,
-or he would be unconsciously admitting that the determinants of the
-butterfly were already contained in the parts of the caterpillar, and
-come direct from the germ-plasm.
-
-That the characters of the butterfly do vary independently of those
-of the caterpillar I demonstrated many years ago, when we were still
-very far away from the idea of the germ-plasm or of determinants. I
-demonstrated then that the constancy of the markings of a species can
-be quite different in the two chief stages; that the caterpillar may be
-very variable, while the butterfly or the moth may be very constant in
-all its markings, or conversely. I called attention to the dimorphic
-caterpillars which are green or brown, and yet become the same moth
-(for instance, _Deilephila elpenor_ and _Sphinx convolvuli_); I cited
-the case of the spurge hawk-moth (_Deilephila euphorbiæ_), whose dark
-but at the same time motley caterpillars occur in the Riviera at
-Nice as a local variety (_Nicæa_), and there wear quite a different
-dress--pale clay-yellow, with a double row of large conspicuous dark
-yellow eye-spots--while the moth does not differ from our variety
-in a single definite character, except in its larger size. At that
-time, too, I instituted experiments with the caterpillars of the
-smallest of our indigenous Vanessa species (_Vanessa levana_), of
-which the majority are black with black thorns, while a minority are
-yellowish-brown with yellow thorns; reared separately, both yielded the
-same butterfly, though in this case one would be inclined to suppose
-that there was some internal connexion between the colour of the
-caterpillar and that of the butterfly, since the butterfly also occurs
-in two colours. It was shown, however, that the colour of the butterfly
-had nothing to do with that of the caterpillar, for it is known to be
-dependent on the season, and is a seasonal dimorphism, 'while the two
-forms of caterpillar may occur side by side at all times of the year.'
-
-Subsequently I made a similar experiment with the dimorphic
-caterpillars of the 'fire'-butterfly (_Polyommatus phlæas_), and it
-yielded the same result. The pure green caterpillars became the same
-butterflies as those marked with broad red longitudinal stripes, and in
-this case we can definitely describe both colours as protective, for
-the green form is adapted to the green under surface of the leaf, the
-red-striped to the green red-edged stalk of the lesser sorrel (_Rumex
-acetosella_).
-
-There was really no necessity for special proofs that the caterpillar
-and butterfly vary transmissibly in complete independence of each
-other, for the facts of metamorphosis alone are enough to prove it. How
-would it have been possible otherwise that the jaws adapted for biting
-should, in the primitive insects, and in the locusts which are nearest
-to them, remain as a biting apparatus throughout life, while in the
-caterpillar they are modified during its pupal stage into the suctorial
-proboscis of the butterfly? The parts of insects, therefore, must be
-capable of transmissible variation in the stages of life independently
-of each other. Not only have the jaws of the leaf-eating caterpillars
-remained unaltered, while in the sexually mature animal they have been
-gradually modified into a very long and extremely complex suctorial
-apparatus, but when at a much later time this proboscis became
-superfluous in a species, because the butterfly or moth, from some
-cause or another, lost the habit of taking any nourishment at all, its
-degeneration exercised no effect on the jaws of the caterpillar, as we
-can observe in many hawk-moths, silk-moths and Geometridæ. How could
-such a degeneration become transmissible if the caterpillar's jaws,
-from which those of the adult are developed, remain the same? We are
-thus forced to assume that there is something in the latter which can
-vary from the germ, without the jaws themselves being altered thereby.
-This 'something' it is which I call 'determinants,' vital particles,
-which--however we may try to picture them--are indeed contained in the
-cells of the caterpillar's jaws, but are there inactive and do not
-influence the structure of these, while, on the other hand, it is their
-constitution which determines the form and structure of the suctorial
-proboscis of the butterfly down to the minutest details. It must be
-these alone which cause the suctorial proboscis to develop, and in some
-cases to degenerate again, without bringing about any change in the
-corresponding parts in the caterpillar.
-
-[Illustration: FIG. 89. Anterior region of the larva of a Midge
-(_Corethra plumicornis_). _K_, head. _Th_, thorax. _ui_, inferior
-imaginal disks. _oi_, superior imaginal disks. _ui_^1, _ui_^2, and
-_ui_^3, the primordia of the limbs. _oi_^2 and _oi_^3, the primordia of
-the wings and 'balancers.' _g_, brain. _bg_, chain of ventral ganglia
-with nerves which enter the imaginal disks. _trb_, tracheal vesicle.
-Enlarged about 15 times.]
-
-This example seems to me to be preferable to that of the wings of
-insects in this respect, that there is no organ in the caterpillar
-with a specific function corresponding to the wing of the butterfly.
-Yet the two cases are exactly alike, and it would be a mistake to say
-that the first primordium of the wing within the caterpillar is not
-a part of the caterpillar at all. At first, certainly, it is only a
-group of cells on the skin, occurring at a particular spot on the
-dorsal surface of the second and third segments of the caterpillar,
-and doubtless arising from a single cell of the embryo, the 'primitive
-wing-cell,' which, however, has not as yet been demonstrated. But
-it is nevertheless an integral part of the caterpillar, which could
-neither be wanting, nor be larger or smaller, and so on; which, in
-short, does mean something for the caterpillar, although perhaps not
-more than any other of the skin-cells. For the butterfly, however, this
-area on the skin means the rudiment of the wing; for from it alone
-can there arise by multiplication the aggregate of cells which grows
-out into a hollow protuberance, enlarges by degrees into a disk, the
-imaginal disk, and eventually develops into the form of wing peculiar
-to the species. This imaginal disk is connected very early with nerves
-and with tracheæ, as may be beautifully seen especially in dipterous
-larvæ (Fig. 89, _oi_), and these become later the nerves and tracheæ
-of the wing, while thousands of peculiar scale-like hairs develop
-on the upper surface; in short, the rudiment becomes a perfect wing
-with its specific venation, and with the marking and colouring which
-is often so complicated in Lepidoptera. Almost every little spot and
-stripe of the latter is handed down with the most tenacious power
-of transmission from generation to generation, and each can at the
-same time be transmissibly varied; the same is true of the venation,
-which is so important systematically just because it is so strictly
-hereditary, yet it too can vary transmissibly, as can also the hooked
-bristles, the odoriferous apparatus, and, in short, the whole complex
-structure of the wing, with all its specific adaptations to the mode of
-flight, to the manner of life, and to the colour of the environment.
-How is it possible that all this can develop from a skin-cell? Is it
-the influence of position that effects it, and could any other cell
-of the caterpillar's skin do the same if it were placed in the same
-position? Could any neighbour-cell of the primitive wing-cell replace
-it if it were destroyed? It is hardly probable, and I think I can even
-prove that this is not so. The experiment of killing such a cell in
-the living animal has not yet been made; if it should succeed, we may
-venture to say in advance that none of the neighbouring skin-cells
-will be able to do its work and take its place in developing a wing;
-the wing in question will simply remain undeveloped. In the summer of
-1897 I hatched a specimen of _Vanessa antiope_ from the pupa, which,
-though otherwise normal and well-developed, lacked the left posterior
-wing altogether; no trace of it could be recognized. In this case, from
-some cause which could no longer be discovered, the first formative
-cell of the wing in the hypodermis, or its descendants, must have been
-destroyed, and no substitution of another took place, as the defect
-showed.
-
-The young science of developmental mechanics attributes to the position
-of a cell in the midst of a group of cells a determining value as
-regards its further fate, and as far as the cells of the segmenting
-ovum are concerned this seems to be true in certain cases, but the
-assumption cannot be generally true except in a very subordinate sense.
-The formative cell of the wing does not become what it is because of
-its relative position in the organism. If this were so it could not
-happen that a wing should develop instead of a leg, as was observed
-in a _Zygæna_, nor could there be any of those deformities already
-referred to, to which the name 'Heterotopia' is applied, and which
-consist in the development of organs of definite normal structure, or
-at any rate of apparently normal structure in quite unusual places, e.
-g. an antenna on the coxa of a leg, or of a leg instead of an antenna
-(in _Sirex_), or instead of a wing. It is therefore not some influence
-from without that makes that particular skin-cell of the caterpillar
-the rudiment of the wing, but the _reason lies within itself_, in its
-own constitution. As the whole mass of determinants for the whole body
-and for all the stages of its development must be contained within
-the ovum and the sperm-cell, so the primitive cell of the butterfly's
-wing must contain all the determinants for the building up of this
-complicated part; and if the cell gets into a wrong position in the
-course of development because of some disturbance or other, a wing
-may develop from it in that position if the conditions are not too
-utterly divergent. These heterotopic phenomena afford a further proof
-of the existence of determinants, because they are quite unintelligible
-without the assumption of 'primary constituents' or _Anlagen_.
-
-The hypothesis of determinants in the germ-plasm is so fundamental to
-my theory of development that I should like to adduce another case
-in its support and justification. The limbs of the jointed-footed
-animals, or Arthropods, originally arose as a pair on each segment
-of the body, and they were at first alike or very similar both in
-their function and in their form. We find illustration of this in
-the millipedes, and still more in the species of the interesting
-genus _Peripatus_, which resembles them externally, as well as in the
-swimming and creeping bristle-footed marine worms (Chætopods) belonging
-to the Annelid phylum. We can quite well picture to ourselves that the
-whole series of these appendages was represented in the germ-plasm by
-a single determinant or group of determinants, which only required
-to be multiplied in development. Without disputing whether this has
-really been the case in the primitive Arthropods or not, it is certain
-that it can no longer be the case in the germ-plasm of the Arthropods
-of to-day. In these each pair of appendages must be represented by
-a particular determinant. We must infer this from the fact that
-the several pairs of these appendages have varied transmissibly,
-independently of each other, for some are jaws, others swimming legs,
-or merely bearers of the gills or of the eggs; others are walking legs,
-digging legs, or jumping legs. In Crustaceans a forceps-like claw
-is often borne by the first of the otherwise similarly constructed
-appendages, or also by the second or the third, or there may be no
-forceps, and so on; in short, we see that each individual pair has
-adapted itself independently to the mode of life of its species. This
-could only have been possible if each was represented in the germ-plasm
-by an element, whose variations caused _a variation only in that one
-pair of legs, and in no other_.
-
-It may perhaps be objected that the differences in the appendages may
-quite well have had their origin simply during the development of the
-animal, while the primary constituents were the same for all, so that a
-single determinant in the germ-plasm would suffice. But this could only
-be the case if the differences depended not on internal but on external
-causes, that is, if the same primary constituents gave rise to a set
-of appendages which became different because they were subject in the
-course of their development to different modifying influences. But
-this is not the case, at least not to the extent that this supposition
-would necessitate. Can it be supposed that, for instance, the jumping
-legs of the water-flea (_Gammarus_) are a necessary consequence of the
-somewhat divergent form of the segments from which they grow? A direct
-proof to the contrary may be found in 'Heterotopia,' for in the place
-where a posterior limb, modified for holding the eggs, normally occurs
-in the crab an ordinary walking leg may exceptionally develop (Fig. 90,
-Bethe), or an appendage resembling an antenna may take the place of an
-extirpated eye (Herbst). But if there were really only one determinant
-in the germ-plasm for all the appendages these would of necessity be
-all alike, apart from the larger or smaller differences which might be
-stamped upon them by growing from segments different in size and in
-nutrition. Such differences, however, are far from being sufficient to
-explain the great deviations seen among the appendages of most kinds
-of Crustaceans, and still less to explain their adaptation to quite
-different functions.
-
-[Illustration: FIG. 90. The Common Shore-Crab (_Carcinus mænas_), seen
-from below, with the abdomen forced back. In place of the swimmeret,
-which ought to be borne by the fifth abdominal swimmeret, a walking leg
-has grown on the left side, and one which properly should belong to the
-right side (6). 1-5, thoracic limbs, _ps_1-4, swimmerets of the right
-side. _s_6, _s_7, posterior segments of the abdomen. After Bethe.]
-
-It need not be imagined that my argument can be controverted by
-saying that _one_ appendage-determinant in the germ may split
-itself in the course of development into a series of different
-appendage-determinants. The question would then arise, How is it
-able to do so? And the answer can be no other than that the single
-first determinant had within it several different kinds of elements,
-which subsequently separated to determine in different ways the
-various appendages. But that is just another way of saying that this
-single determinant actually includes within itself several different
-determinants. For a determinant means nothing more than an element
-of the germ-substance by whose presence in the germ the specific
-development of a particular part of the body is conditioned. If we
-could remove the determinants of a particular appendage from the
-germ-plasm this appendage would not develop; if we could cause it to
-vary the appendage also would turn out differently.
-
-In this general sense the determinants of the germ-plasm are not
-hypothetical, but actual; just as surely as if we had seen them with
-our eyes, and followed their development. Hypothesis begins when we
-attempt to make creatures of flesh and blood out of these mere symbols,
-and to say how they are constituted. But even here there are some
-things which may be maintained with certainty; for instance, that they
-are _not_ miniature models, in Bonnet's sense, of the parts which they
-determine; and, further, that they are not lifeless material, mere
-substances, but living parts, vital units. If this were not so they
-would not remain as they are throughout the course of development,
-but would be displaced and destroyed by the metabolism, instead of
-dominating it as living matter alone can do--doubtless undergoing
-oxidation, but at the same time assimilating material from without,
-and thereby growing. There cannot be lifeless determinants; they must
-be living units capable of nutrition, growth, and multiplication by
-division.
-
-And now we have arrived at the point at which a discussion of
-the organization of the living substance in general can best be
-interpolated.
-
-The Viennese physiologist, Ernst Brücke, forty years ago promulgated
-the theory that living matter could not be a mere mixture of chemical
-molecules of any kind whatever; it must be 'organized,' that is, it
-must be composed of small, invisible, vital units. If, as we must
-certainly assume, the mechanical theory of life is correct, if there
-is no vital force in the sense of the 'Natur-Philosophie,' Brücke's
-pronouncement is undoubtedly true; for a fortuitous mixture of
-molecules could no more produce the phenomena of life than a _single_
-molecule could, because, as far as our experience goes, molecules
-do not live; they neither assimilate, nor grow, nor multiply. Life
-can therefore arise only through a particular combination of diverse
-molecules, and all living substance must consist of such definite
-groups of molecules. Shortly after Brücke, Herbert Spencer likewise
-assumed the reality of such vital 'units,' and the same assumption
-has been made in more recent times by Wiesner, De Vries, and myself.
-In the meantime we can say nothing more definite about the composition
-of these bearers of life, or 'biophors,' as I call them, than that
-albumen-molecules, water, salts, and some other substances play the
-chief part in their composition. This has been found out by analysis
-of dead protoplasm; but in what form these substances are contained in
-the biophors, and how they affect each other in order to produce the
-phenomena of life by going through a ceaseless cycle of disruptions and
-reconstructions, is still entirely hidden from us.
-
-We have, however, nothing to do with that here; we content ourselves
-with recognizing in the biophors the characteristics of life, and
-picturing to ourselves that all living substance, cell-substance, and
-nuclear substance, muscle-, nerve-, and gland-substance, in all their
-diverse forms, consist of biophors, though, of course, of the most
-varied composition. There must be innumerable kinds of biophors in all
-the diverse parts of the millions of forms of life which now live upon
-the earth; but all must be constructed on a certain fundamental plan,
-which conditions their marvellous capacity for life; all possess the
-fundamental characters of life--dissimilation, assimilation, growth,
-and multiplication by division. We must also ascribe to them in some
-degree the power of movement and sensibility.
-
-As to their size, we can only say that they are far below the limits
-of visibility, and that even the minutest granules which we can barely
-perceive by means of our most powerful microscopes cannot be small
-individual biophors, but must be aggregates of these. On the other
-hand, the biophors must be larger than any chemical molecule, because
-they themselves consist of a group of molecules, among which are some
-of complex composition, and therefore of relatively considerable size.
-
-It may now be asked whether the determinants, whose existence we have
-already inferred, are not identical with these 'biophors' or smallest
-living particles; but that is not the case, at least not generally. We
-called determinants those parts of the germ-substance which determine a
-'hereditary character' of the body; that is, whose presence in the germ
-determines that a particular part of the body, whether it consists of
-a group of cells, a single cell, or a part of a cell, shall develop in
-a specific manner, and whose variations cause the variations of these
-particular parts alone.
-
-Again, it may be asked how large and how numerous such 'hereditary
-parts' may be, whether they correspond to every distinct part of a
-cell, or to every cell of the body, or only to the larger cell groups.
-Obviously the areas which are individually determined from the germ
-must differ in size, according as we have to do with an organism which
-is small or large, simple or more complex. Unicellular organisms,
-such as Infusorians, probably possess special determinants for a
-number of cell-organs and cell-parts, although we cannot directly
-observe the independent and transmissible variation of these organs;
-lowly multicellular animals, such as the calcareous sponges, will
-require a relatively small number of determinants, but in the higher
-multicellular organisms, as, for instance, in most Arthropods, the
-number must be very high, reaching many thousands if not hundreds of
-thousands, for in them almost everything in the body is specialized,
-and must have varied through independent variation from the germ. Thus
-in many Crustaceans the smelling-hairs occur singly on special joints
-of the antennæ, and the number of joints furnished with a smelling-hair
-is different in different species; the size, too, of the smelling-hairs
-themselves varies greatly, being, for instance, much smaller in our
-common Asellus than in the blind form from the depths of our lakes, in
-which the absence of sight is compensated for by an increased acuteness
-of the sense of smell. Thus the smelling-hairs may vary transmissibly
-in themselves, while any joint of the antennæ may also produce one
-independently through variation. In this case accordingly we must
-assume that there are special determinants for the smelling-hairs, and
-for the joints of the antennæ. But we cannot always and everywhere
-refer identical or approximately similar organs, when there are many
-of them, to a corresponding number of determinants. Certainly the
-hairs of mammals or the scales of butterflies' wings do not all vary
-individually and independently, but those of a certain region vary
-together, and are therefore probably represented in the germ-plasm by
-a single determinant. These regions often appear to be very small,
-as is best seen by the fine lines, spots, and bands which compose
-the marking of a butterfly's wing, and still more in the odoriferous
-scales occurring in some butterflies, as, for instance, in the blue
-butterflies (_Lycæna_). These little lute-shaped scales do not occur
-in all species, and they occur in very unequal numbers even in those
-which possess them; there are certain species which exhibit only about
-a dozen, and these are all on one little spot of the wing. Since these
-odoriferous scales must have arisen as modifications of the ordinary
-hair-like scales, as one of my pupils, Dr. Köhler, has demonstrated by
-comparative studies, these ordinary hair-like scales must have varied
-transmissibly at certain spots, that is, their determinants have varied
-while those of the surrounding scales have not.
-
-The case is the same in respect to the sound-producing apparatus of
-many insects. Many grasshoppers produce sounds by fiddling with the
-thigh of the hind leg on the wing, others by rubbing one anterior wing
-upon the other, and, indeed, always with one particular vein in one
-upon a particular vein in the other. One of these serves as the bow,
-the other as the string, of the violin, and the bow is furnished with
-teeth (Fig. 91), ranged beside each other in a long row, which have
-the same function as the colophonium of the violin, that is, to grasp
-and release the strings alternately, and thus to produce resounding
-vibrations. My pupils, Dr. Petrunkewitsch and Dr. Georg von Guaita,
-have recently proved that these teeth have arisen as modifications of
-the hairs which are scattered everywhere over the wing and leg. But
-only in this one place, on the so-called 'stridulating-vein,' have
-they been modified to form stridulating teeth (_schr_). Thus this
-vein must be capable of transmissible variation by itself alone, that
-is, there must be parts contained in the germ-plasm, the variation of
-which causes a variation solely of this individual vein and its hairs,
-possibly even a variation only on certain hairs on this vein.
-
-[Illustration: FIG. 91. Hind leg of a Locustid (_Stenobothrus
-protorma_), after Graber. _fe_, femur. _ti_, tibia. _ta_, tarsal
-joints. _schr_, the stridulating ridge.]
-
-On the other hand, there are also large regions, whole cell-masses
-of the body, which in all probability vary only _en bloc_, as, for
-instance, the milliards of blood-cells in Man, the hundreds of
-thousands or millions of cells in the liver and other glandular organs,
-the thousands of fibres of a muscle, or of the sinews or fascia, the
-cells of a cartilage or a bone, and so on. In all these cases a single
-determinant, or at least a few in the germ-plasm, may be enough. But in
-numerous cases it is impossible to say how large the region is which
-is controlled by a single determinant, and it is, of course, of no
-importance to the theory. In unicellular organisms the determinants
-will control parts of cells, in multicellular organisms often whole
-cells and groups of cells.
-
-Perhaps an inference as to the nature of the determinants may be drawn
-from this with some probability, in as far as mere parts of cells may
-be supposed to have simpler determinants than whole cells and groups of
-cells. The determinants in the chromosomes of unicellular organisms
-may therefore often consist of single biophors, so that in this case
-the conception of biophors would coincide with that of determinants.
-In multicellular organisms, on the other hand, I should be inclined
-on the whole to picture the determinant as a group of biophors, which
-are bound together by internal forces to form a higher vital unity.
-This determinant must live as a whole, that is, assimilate, grow, and
-multiply by division, like every vital unit, and its biophors must be
-individually variable, so that the separate parts of a cell controlled
-by them may also be capable of transmissible variation. That they are
-so, every highly differentiated cell of a higher animal teaches us;
-even the smelling-hairs of a crab exhibit a stalk, a terminal knob, and
-an internal filament, and many muscle-, nerve-, and gland-cells are
-much more complex in structure.
-
-
-
-
-LECTURE XVIII
-
-THE GERM-PLASM THEORY (_continued_)
-
- Structure of the germ-plasm--Vital affinities--Division--O.
- Hertwig's chief objections to this theory--Male and female eggs
- in the Phylloxera show differential division--Dispersal of the
- germ-plasm in the course of Ontogeny--Active and passive state
- of the determinants--Predetermination of cells--There are no
- determinants of characters--Liberation of the determinants--Accessory
- idioplasm--Herbst's lithium larvæ--Plant galls--Cells with several
- facultative determinants--Connective tissue in vertebrates--Mesoderm
- cells of Echinoderms--Sexual dimorphism--Female and male
- ids--Polymorphism (_Papilio merope_)--Ants.
-
-
-I HAVE endeavoured to prove that the germ-substance proper must be
-looked for in the chromatin of the nucleus of the germ-cell, and more
-precisely still in those ids or chromosomes which we conceive of as
-containing the primary constituents (_Anlagen_) of a complete organism.
-Such ids in larger or smaller numbers make up the whole germ-plasm of
-a germ-cell, and each id in its turn consists of primary constituents
-or determinants, i.e. of vital units, each of which determines the
-origin and development of a particular part of the organism. We have
-now to make an attempt to picture to ourselves how these determinants
-predetermine those cells or cell-groups to which they correspond. In
-doing so we have to fall back upon mere hypotheses, and in stating any
-such hypothesis I wish expressly to emphasize that I am only following
-up one of the possibilities which our imaginative faculty suggests.
-Nevertheless, to endeavour to form such a conception is certainly not
-without use, for it is only by elaborating a theory to the utmost
-that we are able to use it in application to concrete cases, thus
-stimulating the search for corroboratory or contradictory facts, and
-leading gradually to a recognition of the gaps or mistakes in the
-theory.
-
-The first condition that must be fulfilled in order that a determinant
-may be able to control a cell or cell-group is that it should
-succeed in getting into it. It must be guided through the numerous
-cell-divisions of ontogeny so that it shall ultimately come to lie
-in the cells which it is to control. This presupposes that each
-determinant has from the very beginning its definite position in
-relation to the rest, and that the germ-plasm, therefore, is not a mere
-loose aggregate of determinants, but that it possesses a structure, an
-architecture, in which the individual determinants have each their
-definite place. The position of the determinants in relation to one
-another cannot be due to chance, but depends partly on their historical
-development from earlier ancestral determinants, partly on internal
-forces, such as we have already assumed for keeping the determinants
-together. We may best designate these hypothetical forces 'affinities,'
-and in order to distinguish them from mere chemical affinities we
-may call them 'vital.' There must be forces interacting among the
-different determinants which bind them together into a living whole,
-the id, which can assimilate, grow, and multiply by division, in the
-same manner as we were forced to assume for the smaller units, the
-biophors and single determinants. In the ids, however, we can observe
-the working of these forces quite directly, since each chromosome
-splits into two halves of equal size at every nuclear division, and not
-through the agency of external forces, e.g. the attraction which we may
-assume to be exerted by the fibrils of the nuclear spindle, but through
-purely internal forces, often long before the nuclear spindle has been
-formed at all.
-
-But if the determinants must separate from each other in the course
-of development so as to penetrate singly into the cells they are
-to control, the id must not only have the power of dividing into
-daughter-ids of identical composition, it must also possess the power
-of dividing under certain influences into dissimilar halves, so that
-the two daughter-ids contain different complexes of determinants. The
-first mode of division of the id, and with it of the nucleus and of the
-cell, I call _erbgleich_, or integral, the second _erbungleich_, or
-differential. The first form of multiplication is the usual one, which
-we observe everywhere when unicellular organisms divide themselves into
-two equal daughter-units, or when the cells of multicellular bodies
-produce their like by division into two. The second is not directly
-observable, because a dissimilarity of the daughter-cells, as long as
-it lies only in the idioplasm, cannot be actually seen; it can only
-be inferred from the different rôle which the two daughter-cells play
-in the building up of the individual. When, for instance, one of two
-sister-cells of the embryo gives rise to the cells of the alimentary
-canal and the other to those of the skin and the nervous system,
-I infer that the mother-cell divided its nuclear substance in a
-differential way between the two daughter-cells, so that one contained
-the determinants of the endoderm, the other those of the ectoderm; or
-when a red and a black spot lie side by side and under exactly the same
-conditions on the wing of a butterfly, I conclude that the ancestral
-cells of these two spots have divided differentially, so that one
-received the 'red,' the other the 'black' determinants. Our eyes can
-perceive no difference between the nuclear substance of the two cells,
-but the same is true of the chromosomes of the paternal and maternal
-nuclei in the fertilized ovum, although we know in this case that
-they contain different tendencies. In any case we are not justified
-in concluding from the apparent similarity of the chromosome-halves
-in nuclear division that there cannot be differential division. The
-theoretical possibility that there is such differential division cannot
-be disputed; indeed, I am inclined to say that it is more easily
-imagined than the division of the ids into absolutely similar halves.
-Both are only conceivable on the assumption that there are forces which
-control the mutual position of the determinants in the ids, that is, on
-the assumption of 'affinities.' I shall not follow this further, but
-that there are forces operative within the ids which are still entirely
-unknown to us is proved at every nuclear division by the _spontaneous_
-splitting of the chromosomes.
-
-It has been objected to my theory that such a complex whole as the
-id could not in any case multiply by division, since there is no
-apparatus present which can, in the division into two daughter-units,
-re-establish the architecture disturbed by the growth. But this
-objection is only valid if we refuse to admit the combining forces, the
-'vital affinities' within the ids, and the same is true for the smaller
-vital units. An ordinary chemical molecule cannot increase by division;
-if it be forcibly divided it falls into different molecules altogether;
-it is only the living molecule, that is, the biophor, which possesses
-this marvellous property of growth and division into two halves similar
-to itself and to the ancestral molecule, and we may argue from this
-that in the division of the ids forces of attraction and repulsion must
-likewise be operative[19].
-
-[19] In my book _The Germ-Plasm_ I have already assumed the existence
-of 'forces of attraction' in the determinants and biophors, as in the
-cells. I did not, indeed, enter into details, but I argued on the same
-basis as now (_Germ-Plasm_, p. 64, English edition). My critics have
-overlooked this.
-
-I see no reason why we should not assume the existence of such
-forces, when we make the assumption that the hundreds of atoms which,
-according to our modern conceptions, compose the molecule of albumen
-and determine its nature, are kept by affinities in this definite and
-exceedingly complex arrangement. Or must we suppose that between the
-atom-complex of the molecule and the next higher atom-complex of the
-biophor, determinant, and id there is an absolute line of demarcation,
-so that we must assume quite different forces in the latter from those
-we conceive of as operative in the former? The biophor is ultimately
-only a group of molecules, the determinants a group of biophors, the
-id a group of determinants, and all the three inferred stages of vital
-organization only become real units through the forces operating within
-them and combining them into a whole. What compels the chromatin
-granules of the resting nucleus to approach each other at the time of
-cell-division, to unite into a long, band-like thread, and what is it
-that subsequently causes this thread to break up again into a definite
-number of pieces? Obviously only internal forces of which we know
-nothing further than that they are operative.
-
-We shall see later that this assumption of vital affinities must be
-made not only in regard to the cells, but also in regard to entire
-organisms whose parts are united by an internal bond, and whose
-co-ordination is regulated by forces of which we have as yet no secure
-knowledge. In the meantime we may designate these forces by the name of
-'vital affinities.'
-
-It must be admitted, however, that some objections of a fundamental
-nature have been urged against the assumption of a differential nuclear
-division of the hereditary substance. O. Hertwig holds that the
-assumption of differential division is essentially untenable, because
-it is contradictory to 'one of the first principles of reproduction,'
-for 'a physiologically fundamental character of every living being is
-the power of maintaining its species.'
-
-This certainly seems so, but a closer examination shows that this
-'principle,' although correct enough when taken in a very general
-sense, does not really cover the facts, and is therefore incapable
-of supporting the inferences drawn from it. If the proposition
-expressed the whole truth there could have been no evolution from
-the primitive organisms to higher ones, every living being must have
-simply reproduced exact copies of itself. Whether the transformations
-of species have been sudden or gradual, whether they have been brought
-about by large steps or by very small ones, they could only have come
-about by breaking through this so-called 'principle' of like begetting
-like. In fact, we may with more justice maintain the exact converse
-of the principle, and say that 'no living being is able to produce an
-exact copy of itself,' and this is true not only of sexual, but of
-asexual reproduction.
-
-In ontogeny we see exactly the same thing. There are no two
-daughter-cells of a mother-cell which are exactly alike, and the
-differences between them, if they increase in the same direction, may
-lead in later descendants to entire differences of structure. Indeed
-the whole process of development depends on such an augmentation
-of the differences between two daughter-cells--on differences which
-proceed from within and are definitely pre-established. Here, again,
-the facts do not justify us in making a dogma of the proposition that
-it is a 'fundamental power' of every living being to maintain its
-species by producing replicas of itself. If we look at two directly
-successive cell-generations, we can hardly, it is true, in most cases,
-perceive any difference between them, just as in the generations
-of species; but if we compare the end of a long cell-lineage with
-the beginning, then the difference is marked, and we recognize that
-the difference is due to a gradual summing up of minute, invisible
-deviations. In my opinion these steps of difference cannot possibly
-depend merely on direct external influences; they proceed rather from
-the hereditary substance the cell receives from the ovum, which,
-therefore, in order to attain to such many-sided and far-reaching
-differentiation, must have undergone a frequently repeated splitting
-up of its qualities. That this splitting is not merely a variation
-to which the whole of the hereditary substance of the daughter-cells
-is uniformly subject, according to the influences dependent on their
-position in relation to other cells of the embryo, will be made clear
-from the case of the Ctenophora referred to in the next lecture. A
-scarcely less striking example is that of those animals in which the
-ova contain the primary constituents for only one sex, in which, in
-other words, there are 'male' ova and 'female' ova. This is the case,
-for instance, among Rotifers, and in plant-lice such as the vine-pest,
-_Phylloxera_. Here the eggs from which males develop are smaller than
-those which produce females. The primary constituents for both male
-and female are not, as in most animals, contained in the same ovum,
-to be liberated on one side or the other by influences unknown to
-us, but in each ovum there is only one of the two sets of primary
-constituents present, and in this case, therefore, the development of
-hermaphrodites, which not infrequently occur in other animals, would
-be impossible. But all these ova have been produced by one primitive
-reproductive cell, and consequently, at one of the divisions implied
-in the multiplication of this first cell, a separation of the male
-from the female primary constituents must have taken place, that is, a
-differential division of hereditary substance, for which no external
-and no intercellular influences can possibly account.
-
-If there is, then, a differential division of the ids and with them
-of the whole idioplasm, the germ-plasm of the fertilized ovum must
-be broken up in the course of ontogeny into ever smaller groups of
-determinants. I conceive of this as happening in the following manner.
-
-In many animals the fertilized ovum divides at the first segmentation
-into two cells, one of which gives rise predominantly to the outer,
-the other to the inner germinal layer, as in molluscs, for instance.
-Let us now assume that this is the case altogether, so that one of the
-first two blastomeres gives rise to the whole of the ectoderm, the
-other to the whole of the endoderm; we should here have a differential
-division, for the developmental import (the 'prospective' of Driesch)
-of the primitive ectoderm-cell is quite different from that of the
-primitive endoderm-cell, the former giving origin to the skin and the
-nervous system, with the sense organs, while the second gives rise
-to the alimentary canal, with the liver, &c. Through this step in
-segmentation, I conclude, the determinants of all the ectoderm-cells
-become separated from those of the endoderm-cells; the determinant
-architecture of the ids must be so constructed in such species that
-it can be segregated at the first egg-cleavage into ectodermal and
-endodermal groups of determinants. Such differential divisions will
-always occur in embryogenesis when it is necessary to divide a cell
-into two daughter-cells having dissimilar developmental import,
-and consequently they will continue to occur until the determinant
-architecture of the ids is completely analysed or segregated out into
-its different kinds of determinants, so that each cell ultimately
-contains only one kind of determinant, the one by which its own
-particular character is determined. This character of course consists
-not merely in its morphological structure and chemical content, but
-also in its collective physiological capacity, including its power of
-division and duration of life[20].
-
-[20] Emery has lately called attention to another direct proof of the
-existence of differential cell- and nucleus-division. According to
-observations made by Giardina, in the water-beetle (_Dytiscus_), one
-primitive ovum-cell gives rise, through four successive divisions,
-to fifteen nutritive cells and one well-defined ovum-cell. But only
-half of the nuclear substance takes part in these divisions, the rest
-remains inactive in a condensed, cloudy condition. 'The meaning of
-the whole process is obviously that the germ-plasm mass as a whole is
-handed over to the ovum-cell, while the nutritive cells receive only
-the nuclear constituents which belong to them' (_Biol. Centralbl._, May
-15, 1903).
-
-But embryogenesis does not proceed by differential divisions alone,
-for integral divisions are often interpolated between them, always,
-for instance, when in a bilateral animal an embryonic cell has to
-produce by division into two a corresponding organ for the right and
-left sides of the body; for instance, in the division of the primitive
-genital cell into the rudiments of the right and left reproductive
-organs, or in the division of the primitive mesoderm-cell into the
-right and left initial mesoderm-cell, but also later on in the course
-of embryogenesis, when, for instance, the right or the left primitive
-reproductive cell multiplies into a large number of primitive
-germ-cells, or in the multiplication of the blood-cells, or of the
-epithelial cells of a particular region; in short, whenever mother and
-daughter-cells have the same developmental import, that is, when they
-are to become nothing more than they already are. In all such cases a
-similar group of determinants, or a similar single determinant, must in
-the nuclear division penetrate into each of the two daughter-cells.
-
-It is in this way, it seems to me, that the determinants gain entrance
-into the cells they are to control, by a regulated splitting up of the
-ids into ever smaller groups of determinants, by a gradual analysis
-or segregation of the germ-plasm into the idioplasms of the different
-ontogenetic stages. When I first developed this idea I assumed that the
-splitting process would in all cases set in at the same time, namely,
-at the first division of the ovum. But since then, in the controversies
-excited by the theory, many facts have been brought to light which
-prove that the ova of the different animal groups behave differently,
-and that the splitting up of the aggregate of primary constituents may
-sometimes begin later--but I shall return to this later on.
-
-If we accept the segregation hypothesis, which is similar in purport
-to that advanced by Roux as the' mosaic theory,' it must strike us
-as remarkable that the chromatin mass of the nucleus does not become
-notably smaller in the course of ontogeny, and even ultimately sink to
-invisibility. Determinants lie far below the limits of visibility, and
-if there were really only a single determinant to control each cell
-there would be no chromatin visible in such a case. This objection has
-in point of fact been urged against me, although I expressly emphasized
-in advance the assumption that the determinants are continually
-multiplying throughout the whole ontogeny, so that in proportion as the
-number of the _kinds_ of determinants lying within a cell diminishes
-the number of resting determinants of each kind increases. When,
-finally, only one kind of determinant is present there is a whole army
-of determinants of that kind.
-
-It follows from this conception of the gradual segregation of the
-components of the id in the course of development that we must
-attribute to the determinants two different states, at least in regard
-to their effect upon the cell in which they lie: an active state,
-in which they control the cell, and a passive state, in which they
-exert no influence upon the cell, although they multiply. From the
-egg onwards, therefore, a mass of determinants is handed on by the
-cell-divisions of embryogenesis, which will only later become active.
-
-My conception of the manner in which the determinants become active is
-similar to that suggested by De Vries in regard to his 'Pangens,' very
-minute vital particles which play a determining part in his 'pangen
-theory,' similar to that filled by the determinants in my germ-plasm
-theory. It seems to me that the determinants must ultimately break
-up into the smallest vital elements of which they are composed, the
-biophors, and that these migrate through the nuclear membrane into the
-cell-substance. But there a struggle for food and space must take place
-between the protoplasmic elements already present and the newcomers,
-and this gives rise to a more or less marked modification of the
-cell-structure.
-
-It might be supposed that the structure of these biophors corresponded
-in advance to certain constituent parts of the cell, that there were,
-for instance, muscle biophors, which make the muscle what it is, or
-that the plant-cells acquired their chlorophyll-making organs through
-chlorophyll biophors. De Vries gave expression to this view in his
-'pangen theory,' and I confess that at the time there seemed to me
-much to be said for it, but I am now doubtful whether its general
-applicability can be admitted. In the first place, it does not seem
-to me theoretically necessary to assume that the particles which
-migrate into the cell-bodies should themselves be chlorophyll or muscle
-particles; they may quite well be only the architects of these, that
-is to say, particles which by their co-operation with the elements
-already present in the cell-body give rise to chlorophyll or muscle
-substance. As we are as yet unacquainted with the forces which dominate
-these smallest vital particles, as well as the processes which lead
-to the histological differentiation of the cells, it is useless in
-the meantime to make any further hypotheses in regard to them. But in
-any case the biophors which transform the general character of the
-embryonic cells into the specific character of a particular tissue-cell
-must themselves possess a specific structure different from that of
-other biophors, for they must keep up the continuity of the structures
-handed on from ancestors, chlorophyll and muscle-substance and the
-like, since we cannot assume that these structures, so peculiar and so
-complex in their chemical and physical constitution, are formed afresh,
-so to speak, by spontaneous generation in each new being, as De Vries
-has very rightly emphasized. A specific biophor, for instance, of
-muscle substance will produce this substance as soon as it has found
-its way into the appropriate cell-body, even though it may not be a
-contractile element itself.
-
-To this must be added that the structure of the body and the
-distinctive features of an organism do not depend merely on the
-histological differentiation of the cells, but quite as much on
-their number and arrangement, and on the size and on the frequency
-of repetition of certain parts. These distinctive characters are just
-as constant and as strictly transmissible, and may be as heritably
-variable as those which depend on specific cell-differentiation, and
-they must therefore likewise be determinable by definite elements of
-the germ-plasm. Obviously enough, however, these elements are not
-of the same nature as the known specific histological elementary
-particles; they can be neither nerve-, muscle-, nor gland-biophors.
-They must rather be vital units of such a kind that they communicate
-to the cells and lineage of cells, into whose bodies they migrate from
-within the nucleus, a definite vital power, that is, an organization
-which regulates the size, form, number of divisions, and so on, of
-these cells--in short their whole prospective significance. Always,
-however, they act in co-operation with the cell-body into which they
-have penetrated.
-
-Throughout we must hold ourselves aloof from the idea that 'characters'
-are transmissible. It is customary, indeed, to speak as if this
-were so, and it is also necessary, because we can only recognize
-the 'characters' of a body, and not the essential 'nature' on which
-these characters depend; but the determinants are not seed-grains of
-individual characters, but co-determinants of the nature of the parts
-which they influence. There are not special determinants of the size
-of a cell, others of its specific histological differentiation, and
-still others of its duration of life, power of multiplication, and so
-on; there are only determinants of the whole physiological nature of a
-cell, on which all these and many other 'characters' depend. For this
-reason alone I should object to the assumption that the determinants of
-the germ are ready-made histological substances. That is as unlikely
-as that their groups in the germ-plasm are 'miniature models' of the
-finished parts of the body.
-
-I conceive of the process of cell-differentiation as follows: at every
-cell-stage in the ontogeny determinants attain to maturity, and break
-up so that their biophors can migrate into the cell-bodies, so that the
-quality of each cell is thus kept continually under control, and may
-be more or less modified, or may remain the same. By the 'maturity'
-of a determinant I mean its condition when by continual division it
-has increased in number to such a point that its disintegration into
-biophors and their migration into the cell-substance can take place.
-
-One more point I must touch upon here, the question of the 'liberation'
-or 'stimulation' of the determinants. The activity of an organ never
-depends on itself alone; the contraction of a muscle is induced by
-a nerve stimulus or by an electric current; the activity of the
-nerve-cells of the brain requires the continual stimulus of the
-blood-stream, and cannot continue to exist without it; the specific
-sensory-nerves and sense-cells of the eye, ear, olfactory organ, and
-so on, are all prompted to activity by adequate stimuli. The same is
-true in regard to the determinants, they must be 'liberated' if they
-are to distribute themselves and migrate into the cell-body; and we
-have to ask how that happens, whether it is possibly due only to their
-own internal condition, which again would, of course, depend on the
-nutritive conditions of the cell in which they lie, or whether it is
-perhaps due to some specific stimulus which is necessary in addition to
-the fact of 'maturity,' just as a muscle is always ready to contract,
-yet only does so when it is affected by a specific stimulus.
-
-From the very first, therefore, I have considered whether it would
-not be better to elaborate the determinant theory in such a way that
-it would not be necessary to assume a disintegration of the id in
-the course of ontogeny, but simply to conceive of every expression
-of activity on the part of a determinant as dependent on a specific
-stimulus, which in many cases can only be supplied by a definite cell,
-that is, by internal influences, and in other cases may be due to
-external influences.
-
-Darwin assumed the first of these alternatives in his theory of
-Pangenesis, which we have still to outline. In it he attributes to
-his 'gemmules' the power of giving rise to particular cells, which,
-however, they can only accomplish when they reach the cells which are
-the genetic antecedents of those which the gemmules are to control.
-Translated into the language of our theory this view would read
-as follows: the whole complex of determinants is contained within
-every cell, as it is contained in the germ-cell, but at every stage
-of ontogeny, that is, in each of the developing cells, only the
-determinant which is to control the immediately successive cells is
-'liberated,' and that through the stimulus which the specific nature
-of the cell supplies to the determinant. In that case there would
-necessarily be in every species of animal as many specific stimuli for
-determinants as there are determinants. This appeared to me improbable,
-and I rejected the hypothesis because of the enormous number of
-specific stimuli which it demands, but also on other grounds which will
-be touched upon in the course of these lectures.
-
-Although the assumption of an autonomic dissolution of the determinant
-complexes of the id in the course of ontogeny seems to me imperative,
-I do not by any means reject the interposition of liberating stimuli,
-indeed I regard their co-operation as indispensable. Later on we shall
-discuss cases in which it is definitely demonstrable that there may be
-two alternative sets of homologous determinants present in a cell, but
-that on any occasion only one of these becomes active, a fact which we
-can only explain on the assumption that only one of these is affected
-by the specific liberating stimulus. The phenomena of regeneration, of
-polymorphism, of germ-cell formation, &c., compel us to the assumption
-that numerous cells, even after the completion of the building up of
-the body, contain two or more kinds of determinants, as in a sense
-inactive 'accessory idioplasm,' each of which could control the cell
-alone, though in reality it only does control it when it is affected by
-the appropriate liberating stimulus. I stated this view some years ago
-when I attempted to define more precisely the rôle played by 'external
-influences as developmental stimuli[21]'. It is not, then, that I
-underrate the importance of external influences on the organism, but
-I believe that a still larger part of the determination of what shall
-happen at a particular point depends on the primary constituents, and
-that these are not alike at all parts of the body.
-
-[21] _Äussere Einflüsse als Entwicklungsreize_ [External Influences as
-Stimuli to Development], Jena, 1894.
-
-All living processes, therefore, both those of growing and of
-differentiation, depend always upon the interaction of external and
-internal factors, of the environment and the living substance, and
-the resultants of the interaction, namely, the structure of the body
-and its parts must necessarily turn out differently, not only when
-the germ-substance is different, but when the essential conditions
-of development are changed. But that the constitution of the germ is
-by far the most potent factor, and that the nature of the results of
-development depends on it in a much greater degree than on the external
-conditions, has long been known. The conditions, such as warmth, may
-vary within certain limits, and yet the frog's egg becomes a frog;
-though it does not follow that the result of development may not be
-modified through certain changes in the conditions. The interesting
-experiments made by Herbst with the eggs of sea-urchins have shown
-that, in artificially altered sea-water in which sodium-salts are to a
-slight extent replaced by lithium-salts, these eggs develop into larvæ
-which only remotely suggest the normal structure, and diverge widely
-from it both in external shape and in the form of the skeleton.
-
-Such larvæ are not able to survive, but soon perish; they are,
-however, of great interest from the point of view of our theory, for
-they show that determinants do not bring forth the same structure
-under all circumstances, but that, as I have already said, they are
-vital units of specific composition, which play a part in the course
-of development, and give rise under normal external influences to
-normal parts, while under unusual influences, if these are not such
-as to prohibit development altogether, they may give rise to an
-abnormally formed part. It must not be forgotten that most composite
-parts--indeed, strictly speaking, all the parts--of an animal are not
-controlled by a single determinant, but by the many which successively
-determine the character of the cells and define the path of development
-of the part in question. There are no determinants of 'characters,'
-but only of parts; the germ-plasm no more contains the determinants
-of a 'crooked nose' than it does those of a butterfly's tailed wing,
-but it contains a number of determinants which so control the whole
-cell-group in all its successive stages, leading on to the development
-of the nose, that ultimately the crooked nose must result, just as the
-butterfly's wing with all its veins, membranes, tracheæ, glandular
-cells, scales, pigment deposits, and pointed tail arises through the
-successive interposition of numerous determinants in the course of
-cell-multiplication.
-
-But in both processes we must presuppose _normal conditions of
-development_. In regard to the butterfly we know that abnormal
-conditions, such as cold during the pupal period, can cause
-considerable variation in the colour and marking of the wing, and in
-regard to the nose it can scarcely be doubted that, for instance,
-persistent pressure on the nasal region would result in a considerable
-deviation from the hereditary form.
-
-The case of the lithium-larvæ is similar. Here the chemical conditions
-of the first segmentation-cells are modified by the presence of the
-lithium-salts, and the determinants which make their way out of the
-nucleus in the first and in subsequent cell-generations find an
-unusual soil for their activity, which diverges further and further
-from the normal with each successive cell-generation. Thus the whole
-animal is abnormally formed. The process may perhaps be compared to a
-plant which is negatively geotropic and positively heliotropic, that
-is, the stem of which tends to grow straight upwards, while all its
-green parts grow towards the light. If a plant of this kind have light
-shed on it from one side only, the stem with its leaves will grow
-obliquely towards that side. If the plant be then turned round so that
-it receives light from the other side, the stem in its further growth
-will curve in a direction opposite to that which it took before, and so
-by continually changing the position of the plant in relation to the
-light one could--theoretically at least--produce a plant with a zigzag
-stem. But this would not furnish any evidence against the presence of
-determinants; there are no 'upright determinants' any more than there
-are 'zigzag determinants' or 'crooked nose determinants,' but there are
-determinants controlling the nature of the cells which give rise, under
-normal conditions of development, to the straight stem, under abnormal
-conditions to the zigzag stem, or to a flat nose instead of a crooked
-one, and so on.
-
-This consideration should make it clear that plant-galls are not in
-the remotest degree a stone of stumbling for the determinant theory,
-as some have supposed. Of course there can be no 'gall-determinants,'
-for galls are not transmissible adaptations of the plants on which they
-occur; they arise solely through the larvæ of the gall-insect which
-has laid its eggs within the tissues of the plant. But the specific
-nature of the different kinds of plant-cells, predetermined by their
-determinants, is such that, through the abnormal influences exercised
-upon them by the larvæ, they are compelled to a special reaction which
-results in the formation of galls. It is marvellous enough that these
-abnormal stimuli should be so precisely graded and adjusted that such a
-specifically definite structure should result, and in this case there
-is obviously a very different state of matters from that obtaining in
-most other processes of development, in which the chief determining
-factor is rather implied in the nature of the idioplasm, that is, of
-the determinants, than in the nature of the external influences. Here,
-however, the specific structure of the gall depends mainly on the
-quality, variety, and successive effects of the external influences or
-stimuli. In discussing the influences of surroundings I shall return
-once more to the galls.
-
-My determinants have generally been regarded as if they were like
-grains of seed, from which either nothing may arise, under unfavourable
-conditions, or just the particular kind of plant from which the seed
-itself originated.
-
-This simile is, however, to be taken _cum grano salis_. The whole ovum
-is certainly comparable to a grain of seed, but single determinants
-or groups of determinants will always be able to adapt themselves
-to different influences, and to remain active even under slightly
-abnormal conditions, though in that case the resulting structures may
-be somewhat divergent. This relative plasticity is indispensable even
-in relation to the ceaseless mutual adaptations of the growing parts
-of the organism. Not only do the cells which live beside each other
-at the same time influence each other mutually, but the influence
-extends to the whole cell-lineage. No cell or group of cells develops
-independently of all the others in the body, but each has its ancestral
-series of cells on whose determinants it is so far dependent, since
-these have taken part in determining its own nature, in, so to speak,
-supplying the soil in which ultimately its own determinants will
-be sown from the nucleus, and whose influence modifies these last
-according to its quality. We might therefore say that every part is
-determined by all the determinants of its cell-ancestors.
-
-If there be urged against the doctrine of determinants the undoubted
-fact of the dependence of individual development on external
-conditions, or the capacity that organisms have of functional
-adaptation, or especially the power that some parts of the organism
-have of taking a different form in response to different stimuli, I can
-only say that I see no reason why certain cells and masses of cells
-should not be adapted from the first for responding differently to
-different stimuli.
-
-Therefore I see no contradiction of the determinant theory when, for
-instance, among the higher vertebrates, the cells of the connective
-tissue exhibit a great diversity of form, becoming a loose 'filling'
-connective tissue in one place, a tense fascia, ligament, or tendon
-tissue in another, according as they are subjected to slight pressure
-on all sides or to stronger pressure on one side. I see no difficulty
-in the fact that this connective tissue forms in one case bone-tissue
-with the most accurate adaptation of its microscopic structure to the
-conditions of stress and pressure which affect the relevant spot,
-or in another case cartilaginous tissue, when the cells are exposed
-to varying pressure (as on the surface of joints), or even that it
-gives rise to blood-vessels when the pressure of the circulating
-blood and the tension of the surrounding tissues supply the necessary
-stimulus. It is easy to see how important, indeed how necessary, the
-many-sidedness of these cells is for the organism, even leaving out
-of account such violent interference as the breaking of a bone, the
-irregular healing of broken ends of bones, new joint formation, and
-so on, and thinking only of the normal phenomena of growth. While the
-bone grows it is continually breaking up in the inside and forming anew
-on the surface, and this occurs through the power of the connective
-tissue-cells to form different tissues under different influences or
-stimuli.
-
-We must therefore assume that there are side by side in the connective
-cells of higher vertebrates determinants of bone, of cartilage, of
-connective tissue in the narrower sense, and of blood-vessels, and
-that one or other of these is liberated to activity according to the
-stimulus affecting it. Phenomena occur also in the development of lower
-animals which lead us to the same assumption.
-
-Among these is the remarkable behaviour of the primary mesoderm-cells
-in the young embryo (gastrula) of the Echinoderms (Fig. 92). At the
-point where the primitive gut or archenteron invaginates into the
-interior of the hitherto single-layered blastula (Fig. 92, _A_), some
-cells are separated off (_M_), and move independently, constantly
-multiplying the while, into the clear gelatinous fluid (_G_) which
-fills the cavity of the larva, and there they fix themselves, some
-on the outer ectodermic layer, others to the various regions and
-outgrowths of the archenteron (_Ms_). According as these cells have
-established themselves at one or another point, they become connective
-tissue, muscle, or skeleton cells of the dermis, or contribute to
-the muscular layer of the food-canal and water-vascular system, or,
-finally, become skeleton-forming cells of the calcareous ring which
-surrounds the gullet of the sea-cucumber. In all this there is nothing
-to indicate a determination of the cells in one direction; on the
-contrary it seems as if the fate of the individual cells depended on
-the chance conditions which may lead them to one place or to another.
-
-[Illustration: FIG. 92. Echinoderm-larvæ. _A_, blastula-stage; the
-primary mesoderm-cells (_M_) are being formed at the subsequent
-invagination-area of the endoderm (_Ent_). _Ekt_, the ectoderm. _B_,
-gastrula-stage; the archenteron (_UD_) has been invaginated (_Ent_),
-and between it and the ectoderm (_Ekt_) the mesoderm-cells (_Ms_)
-migrate into the gelatinous fluid which fills this cavity. There they
-attach themselves partly to the ectoderm, and partly to the endoderm.
-After Selenka.]
-
-There are thus three possibilities of development, three kinds of
-reaction, implied in these cells, which are all outwardly alike, and
-we can only understand their rôle in the building up of this very
-symmetrical animal if we assume that of these three only one is in
-each case liberated, by the specific stimulus exerted by the immediate
-surroundings of the cell, so that it may become, according to the
-chance position it takes up after its migration, either a skin-cell, a
-muscle-cell, or a skeleton-forming cell.
-
-This case may be compared in some respects with the permanent
-colour-adaptation of those caterpillars, in regard to which Poulton
-demonstrated that they become almost black if they are reared on
-blackish-brown bark, light brown on light bark, and green if they are
-kept among leaves, and in all cases permanently so. In this case also
-the implicated pigment-cells of the skin may develop in three ways,
-according to whether this or that quality of the light releases this or
-that determinant.
-
-But in many cases we do not know the quality of the liberating
-stimulus, and must content ourselves with imagining it. This is so in
-the case of dimorphism of the sexes. It is clear that in the males
-of a species the germ-cells develop quite otherwise than they do in
-the females, that different determining elements attain to activity
-in each sex, and since the primary constituents of both sexes must be
-contained in most animals in the ovum and in the spermatozoon, we must
-assume that in both there are at once 'ovogenic' and 'spermogenic'
-determinants, of which, however, only _one_ kind becomes active in a
-given individual. There are, however, both among plants and animals
-hermaphrodite individuals, in which both kinds of sexual products are
-developed simultaneously or successively.
-
-It is not only the primary sexual characters, however, that compel
-us to the assumption of double determinants in the germ-plasm, the
-secondary sexual characters do so too. We know very well in relation
-to ourselves that 'the beautiful soprano voice of the mother may be
-transmitted through the son to the grand-daughter, and that the black
-beard of the father may pass through the daughter to the grandson.'
-Thus both kinds of sexual characters _must be present in every sexually
-differentiated being_, some visible, others latent. In animals the
-determinants are sometimes handed on from germ-plasm to germ-plasm
-through several generations in a latent state, and only make their
-appearance again in a subsequent generation. This is the case in the
-water-fleas (Daphnids) and the plant-lice (Aphides), in which several
-exclusively female generations succeed one another, and only in the
-last of them do males occur again side by side with the females.
-
-The germ-plasm of the ovum which is ripe for development must thus
-contain not only the determinants of the specific ova and sperms of the
-species, but also those of all the male and female sexual characters,
-which we discussed at length in the section on sexual selection. I
-then showed that these secondary sexual characters differ greatly
-in range and in strength, that among lower animals they are almost
-entirely absent, and that among higher forms, such Crustaceans,
-Insects, and Birds, they attain to very different grades of development
-even among the same species. Thus the birds of Paradise are in most
-species brilliantly coloured and adorned with decorative feathers
-only in the male sex, while the females are simply blackish-grey, but
-there is a single species in which the males are almost as soberly
-coloured as the females. Conversely, too, we find that in parrots both
-sexes are usually coloured alike, but a few species exhibit a totally
-different colouring in the two sexes. In the same way the secondary sex
-differences may affect only a few parts of the animal or many, while
-in a few species the sexes are so divergent in structure that almost
-everything about them may be called different. Examples of this are
-the dwarf males of most Rotifers, and the males, more minute still in
-proportion to the females, of the marine worm _Bonellia viridis_ (p.
-227).
-
-We have now to inquire what theoretical explanation of these facts we
-can arrive at in accordance with the germ-plasm theory. That double
-determinants, male and female, for the differently formed parts of the
-two sexes must be assumed to exist in the germ-plasm has been already
-said, and we have to suppose that the same stimulus--usually unknown
-to us--which incites the determinants of the primary sexual characters
-to activity also liberates those of the secondary characters. But we
-may safely go a step further and conclude that there are male and
-female _ids_, that is, that the male and female determinants belong
-to different ids. I infer this from the fact that in some groups,
-such as the Rotifers and certain plant-lice, the ova are sexually
-differentiated even at the time of their origin. Males and females
-of these animals arise from different kinds of eggs, which are even
-externally recognizable. Both develop parthenogenetically, so that
-fertilization has nothing to do with it; from the first, therefore,
-they must contain ids which consist of determinants of one sex alone.
-
-If this conclusion be correct, then the sexual equipment of the
-determinants of the sexual characters must have taken place in the
-course of phylogeny in such a way that each id was affected in one
-direction only, and we should thus have to assume male and female ids,
-even before the separation of the sexes as males and females, and the
-same conclusion must be extended to the primary sexual characters. Only
-in this way can we understand the fact that differences between the
-sexes, at first small, have increased in the course of phylogeny to
-such complete divergence of structure as is now exhibited in the forms
-we have named, _Bonellia_, the Rotifers, and some parasitic worms.
-
-But there is not only sexual dimorphism, there is also dimorphism
-of larvæ, e.g. green and brown caterpillars in certain species of
-hawk-moth (_Sphinx_), and there are sometimes not only two but three
-or more forms of a species; and in all these cases determinants of the
-differential parts must be represented twice, thrice, or several times
-in each germ-plasm, in each fertilized ovum, at least in all cases in
-which the different forms live together on the same area. In discussing
-mimicry we spoke of species of butterfly which were everywhere alike
-or nearly so in the male sex, while the females were not only quite
-different from the males, but differed greatly in many respects among
-themselves. Three different forms of females of _Papilio merope_ occur
-in the same region of Cape Colony, each of these resembling a protected
-model. All three forms have been obtained from the eggs of one female.
-In this case the female ids of the germ-plasm must be represented by
-three different sets, one of which, when it is in the majority in the
-fertilized ovum, gives rise to the _Danais_-form, the second to the
-_Niavius_-form, and the third to the _Echeria_-form of the species.
-Phylogenetically considered, it is probable that each of these three
-kinds of ids originated by itself, on a more limited area on which the
-protected model lived in abundance; but with a wider distribution the
-different female ids mingled together, were united through the males
-into a single germ-plasm, and now occasionally exhibit all three forms
-on the same area. I doubt whether there is any other theory that can
-offer an interpretation of these facts, and I regard them, therefore,
-as affording further evidence of the real existence of ids.
-
-The polymorphism of social insects must be thought of as similarly
-based in the germ-plasm.
-
-In bees there are in addition to the males and females the so-called
-workers, and this can only depend on the existence of special kinds
-of ids. Those of the workers were originally truly female, but as
-many of their determinants underwent variations advantageous for
-the maintenance of the species, they were modified into special
-'worker-ids.' I postpone for the present any inquiry into the causes by
-which these ids come to dominate the ontogeny; obviously it cannot be
-by the mere fact of being in a majority over the rest of the ids, as I
-indicated in the case of the butterflies with polymorphic females.
-
-In many ants the division of labour goes further still; there are two
-kinds of workers in the colony, the ordinary workers and the so-called
-'soldiers,' and in this case the worker-id must have developed in two
-different directions in the course of phylogeny, and have separated
-into two kinds of ids, so that the germ-plasm of these species must
-contain four kinds of ids.
-
-I might cite many more cases in regard to which the assumption of two
-or more kinds of determinants seems imperative, but I believe that what
-has been said is enough to enable any one to think out other cases for
-himself.
-
-
-
-
-LECTURE XIX
-
-THE GERM-PLASM THEORY (_continued_)
-
- Co-operation of the determinants to form an organ: insect
- appendages--Venation of the insect-wing--Deformities in
- Man--Apex of the fly's leg--Proofs of the existence of
- determinants--Claws and adhesive lobes--Difference between a
- theory of development and a theory of heredity--Metamorphosis of
- the food-canal in insects--Delage's theory--Reinke's theory of the
- organism-machine--Fechner's views--Apparent contradiction by the facts
- of developmental mechanics--Formation of the germ-cells--Displacement
- of the germinal areas in the hydro-medusoid polyps, a proof of the
- existence of germ-tracks.
-
-
-IT would be futile to attempt to guess at the arrangement of the
-determinants in the germ-plasm, but so much at least we may say, that
-the determinants do not lie beside each other in the same disposition
-as their determinates exhibit in the fully-formed organism. This may be
-inferred from the complex formative processes of embryogenesis in which
-many groups of cells, which in their origin were far apart, combine
-together to form an organ. Thus the arrangement of the determinants
-in the germ-plasm does not correspond to the subsequent arrangement
-of the whole animal, nor are primary constituents of the _complete_
-organs contained within the germ-plasm. The organ is undoubtedly
-_predetermined_ in the germ-plasm, but it is not _preformed_ as such.
-
-Here, again, the history of development gives us a certain basis of
-fact from which to work. Let us consider, for instance, the origin
-of the appendages in those insects which in the larval state possess
-neither legs nor wings, but exhibit a gradual emergence of these
-structures from concealment underneath the integumentary skeleton.
-In these cases, as I have already shown in regard to the wings, the
-development of the limbs arises from definite groups of cells in the
-skin. These must therefore be regarded as the formative, and therefore
-as the most important and indispensable, parts of the rudiments, and
-may be designated the imaginal disks, as I many years ago proposed[22]
-(Fig. 89, _ui_ and _oi_).
-
-[22] _Die Entwicklung der Dipteren_, Leipzig, 1864.
-
-But these disks of cells do not contain the _whole_ leg, but only the
-skin-layer of it, the 'hypodermis,' which, however, in this case
-undoubtedly determines the form. But the internal parts of the leg,
-especially the nerves, tracheæ, and probably also the muscles, are
-formed from other cell-groups and grow into the imaginal disk from
-outside. Something similar probably takes place in the case of all
-organs which are made up of many parts; they are, so to speak, shot
-together from several points of origin, from various primordia; and
-determinants are brought into co-operation whose relative value in
-determining the form and function of the organ may be very diverse.
-
-[Illustration: FIG. 89. Anterior region of the larva of a Midge
-(_Corethra plumicornis_). _K_, head. _Th_, thorax. _ui_, inferior
-imaginal disks. _oi_, superior imaginal disks. _ui_^1, _ui_^2, and
-_ui_^3, the primordia of the limbs. _oi_^2 and _oi_^3, the primordia of
-the wings and 'balancers.' _g_, brain. _bg_, chain of ventral ganglia
-with nerves which enter the imaginal disks. _trb_, tracheal vesicle.
-Enlarged about 15 times.]
-
-For it is undoubtedly a very different matter whether a cell bears
-within it the elements which compel it in the course of growth
-to develop an organ, for instance a leg, of quite definite size,
-sculpture, number of joints, and so on, or whether it only bears the
-somewhat vague power of determining that connective tissue or fatty
-tissue is to be produced. In the first case it controls the whole
-formation of the part, in the second it only fills up gaps or lays
-down fat or other substances within itself if these be presented
-to it. Between these two extremes of determining power there are
-many intermediate stages. Cells which contain the determinants of
-blood-vessels, tracheæ, or nerves need not be so definitely determined
-that they always give rise to precisely the same blood-vessels, the
-same branching of the tracheæ, or the same bifurcation of nerves; they
-may probably possess no more than the general tendency to the formation
-of such parts, and the special form taken by the nerves, tracheæ, or
-blood-vessels may be essentially determined by their environment. Thus
-in the morbid tumours of Man, nerves, and especially blood-vessels,
-may develop in a quite characteristic manner, which was certainly not
-determined in advance, but has been called forth by the stimulus, the
-pressure, and other influences of the cellular basis of the tumour.
-In short, the cells were only determined to this extent, that they
-contained the tendency to give rise to blood-vessels under particular
-influences.
-
-It would be a mistake, however, to think of the primary constituents
-of all cell-groups as so indefinite. Let us call to mind, for instance,
-the venation of the insect wing. It is well known that this is not
-only quite different in beetles, bugs, and Diptera from that in the
-Hymenoptera, and different again in the butterflies, but that it is
-quite characteristic in every individual family of butterflies, and
-indeed in every genus. We cannot conceive of the absolute certainty of
-development of these very characteristic and constant branchings as
-having its roots elsewhere than in the determinants of the germ-plasm,
-which, lying within certain series of cells, ultimately cause
-particular cell-series of the wing-rudiment to become the wing-veins.
-If this were not so, how would it be possible to understand the fact
-that every minute deviation in the course of these veins is repeated
-in exactly the same way in all the individuals of a genus, while in
-all the individuals of an allied genus the venation turns out slightly
-different with equal constancy.
-
-But it is quite certain that all determinations are in some degree
-susceptible to modifying influences, that they are in very different
-degrees capable of variation.
-
-Many deformities of particular parts in Man and the higher animals
-may be referred to imperfect or inhibited nutrition of the part
-in question during embryonic development; the determinants alone
-cannot make the part, they must have a supply of formative material,
-and according as this material is afforded more abundantly or more
-scantily the part will turn out larger or smaller. In the same way the
-pressure conditions of the surrounding parts must in many cases have a
-furthering or inhibiting influence, or may even determine the shape.
-But it is quite possible, indeed even probable, that other specific
-influences are exerted by the cells or cell-aggregates surrounding an
-organ which is in process of being formed, just as the stake on which a
-twining plant is growing may prompt it to coil. If the stake be absent,
-the predetermined twining of the plant cannot attain to more than very
-imperfect expression, if indeed it finds any. The spirally coiled
-sheath of muscle-cells which occurs so often around blood-vessels
-in worms, Echinoderms, and Vertebrates is probably due to similar
-processes, that is, on the one hand, to a specific mode of reaction
-characteristic of these cells, and predetermined from the germ; on the
-other hand, to the external influence of the cell-surroundings without
-which the determination of the muscle-cell is not liberated, that is,
-is not excited to activity.
-
-[Illustration: FIG. 93. The development of a limb in the pupa of a Fly
-(_Sarcophaga carnaria_). _A_, apex of the limb from a pupa four days
-old; the jointing is hinted at; _hy_, hypodermis; _ps_, pupal sheath;
-_ph_, phagocytes; _tr_, tracheal branch. _B_, the same on the fifth
-day; the lumen of the limb is quite filled with phagocytes (_ph_); the
-last tarsal joint (_t_^5) is beginning to show a bifid apex. _C_, the
-same on the seventh day; the claws (_Kr_) and the adhesive lobes (_hl_)
-are formed.]
-
-But even if every determinant requires a stimulus to liberate it,
-whether this stimulus consists in currents of particular nutritive
-fluids, in contact with other cells, or, conversely, on the removal
-of some pressure previously exerted on the cell by its surroundings,
-the material cause of a structure is to be sought for not in these
-conditions of its appearance, but in the primary constituents which
-have been handed on to the relevant cell or cell-group from the germ,
-in other words, through its determinants. How, for instance, could
-the blunt rounded knob of the rough and clumsily jointed sac of cells
-which represents the insect's leg at the beginning of the pupal period
-(Fig. 93, _A_) be incited to thicken, to constrict at the root (_B_),
-and to form a joint-surface, to broaden out at the end, and produce
-two sharply cut points (_C_), which become incurved and form claws
-(_kr_), while beneath these a broad flat lobe (_hl_) grows forward, and
-with its regularly disposed cells gradually forms the characteristic
-adhesive organ of the fly--how could all this happen if there were not
-contained within these cells special formative forces which determine
-them not only in their form and the rest of their constitution, but
-above all in their power of multiplication? No special external
-stimulus affects the still unfinished knob of the fly's leg unless it
-be the removal of pressure; but this operates regularly, and cannot
-be the cause of the growth, at definite places, of claws and adhesive
-lobes with all their characteristically placed hairs.
-
-We require to assume that each of the cells composing the primary
-rudiment of the limb possessed a determining power which made it grow
-and multiply under the given conditions of nutrition and pressure in a
-prescribed manner and at a prescribed rate; and we must make the same
-assumption in regard to all the daughter and grand-daughter-cells,
-and so on. The strictest regulation of the power of multiplication of
-each of the implicated cells is a necessary condition of the constant
-production of the same two claws and adhesive lobes, the same form of
-tarsal joint, the same regular covering of hair, and so on. This exact
-determination of the cells can only take place through material vital
-particles, and it is these which I call determinants.
-
-I have already said so much about the assumed 'determinants' of the
-germ-plasm that it might perhaps be supposed that we have now exhausted
-the topic; but the assumption of such 'primary constituents' is so
-fundamental, not only for my own germ-plasm theory of to-day and
-to-morrow, but also--unless I am much mistaken--for all future theories
-of development and inheritance. In point of fact, the conception of
-determinants has as yet penetrated so little into the consciousness of
-biologists, that I cannot remain content with what I have already said,
-but must endeavour to test and to corroborate my thesis by additional
-illustrations.
-
-As far as I am aware, only a few zoologists have expressly and
-unconditionally agreed with the assumption of determinants; on the
-other hand, several biologists have rejected it as fanciful and
-untenable, while others have set it aside as a useless playing with
-ideas. The last, I am inclined to believe, have not taken the trouble
-to think out what the idea is. It has even been objected that there
-can be no determinants because we can see nothing of them, and that
-they must therefore be pure figments of the imagination, invented to
-explain facts which could be explained much more easily and simply in
-some other way. From the very first I have stated emphatically that
-they have not been, and never will be seen, because they lie far below
-the limit of visibility, and thus can at best only become visible
-when they are collected in large aggregates like chromatin granules.
-Nor have I any objections to make if any one chooses to describe all
-the details of their activity as mere hypotheses, such, for instance,
-as their distribution during development, their 'maturation,' their
-migration from the nucleus, and the manner in which they control the
-cell. All this is really an imaginative picture which may be correct
-to a certain degree, but may also be erroneous; no formal proof of it
-can be obtained at present; and I am content if it be simply admitted
-to be possible. On the other hand, the existence of determinants seems
-to me to be, in the sense indicated, indubitable and demonstrable.
-
-Let us return for a moment to the claws and adhesive lobes which
-are developed on the foot of the fly. It may perhaps be thought
-that it is possible to do without the assumption of determinants
-for these parts, by assuming that although 'external' influences in
-the ordinary sense could not possibly have determined that certain
-cells of the apex of the leg should form claws and others adhesive
-lobes, the result might be due to the differences of intercellular
-pressure within the apical knob; these may have been stronger in one
-direction, weaker in another, thus prompting the cells to grow here
-into claws and there into adhesive lobes. If we had merely to explain
-from the constitution of the germ-plasm the ontogeny or development
-of these parts in an individual fly there might perhaps be no radical
-objection to this view, though it would hardly be possible to explain
-the assumed differences in pressure otherwise than as due to a
-different intensity of growth in the cells in the various regions of
-the limb-apex, which again would have to be referred to differences in
-the germ-plasm. But when we reflect that these parts vary hereditarily
-and independently of other parts, and owe their present form to their
-power of doing so, and that they are differently formed in every
-genus and species, we see at once that they must be represented in
-the germ-plasm by particular vital particles, which are the roots of
-their transmissible variability, that is, which must have previously
-undergone a corresponding variation if the relevant parts themselves
-are to vary. Without previous variation of the determinants of the germ
-no transmissible independent deviation on the part of the claws or
-adhesive lobes of the animal is conceivable.
-
-All the opponents of my theory have overlooked this fact; both Oscar
-Hertwig and Kassowitz have forgotten that a theory of development
-is not a theory of heredity; they only aim at the former, and
-they therefore dispute the logical necessity for an assumption of
-determinants.
-
-But as this is the very foundation of the theory, let me further submit
-the following considerations in its favour.
-
-In insects which undergo metamorphosis, not only the external but the
-internal parts of the caterpillar or larva go through a more or less
-complete transformation. In the flies (Muscidæ), for instance, the
-whole intestinal tract of the larva is reconstructed in the pupa; in
-fact it breaks up into a loose, flocculent, dead, but still coherent
-mass of tissue. Within this there arises a new intestine, as I have
-shown in an early work (1864); and Kowalewsky and Van Rees have since
-made us aware of the interesting details of this reconstruction,
-showing that the new intestine arises from definite cells of the
-old one, which are present in the larval gut at certain fairly wide
-distances, and which do not share in the general destruction, but
-remain alive, grow, and multiply, and form islands of cells in the dead
-mass. These living islands, continually extending, ultimately come
-into contact and again form a closed intestinal canal which differs
-entirely from that of the larva in its form, in its various areas,
-and in its differentiation. In this case those formative cells of the
-imago-intestine must have contained the elements which determined
-their descendants in number, power of multiplication, arrangement,
-and histological differentiation. In other words, each of these
-cells must contain the determinants of a particular limited section
-of the intestine of the imago. The other cells of the intestinal
-epithelium could not do this, even though they were under exactly the
-same conditions, were included in the same intimate cell-aggregate,
-and had the same nutritional opportunities. They break up when the
-formative cells begin to be active, for till then the latter had
-remained inactive, and had not multiplied, although they lay regularly
-distributed among the other cells. Whence, then, could the entire
-difference in the behaviour of these two sets of cells arise, if it
-does not depend on the _nature of the cells themselves_, and how could
-this difference of nature have developed during the racial history of
-insect-metamorphosis if determinants did not reach the cell from the
-germ-plasm--determinants which conditioned that some cells should be
-hereditarily modified into the cells of the imago-intestine and others
-into the larval intestine? Quite similar processes have been recently
-demonstrated in regard to the reconstruction of the larval intestine
-in other insect-groups. Deegener has done this, for instance, for the
-water-beetle (_Hydrophilus piceus_); and it is certain that all these
-reconstructions start from particular cells, which lie indifferently
-between the active cells during the larval period, and contain the
-primary constituents for the formation of a section of the intestine,
-but which only become active when their hitherto living neighbours die
-and break up.
-
-The whole of the reconstruction of the external form of the fly takes
-place in a similar manner. Not only the limb, the head, the stigmata,
-but the skin itself is formed anew from imaginal disks. In each of
-the abdominal segments three pairs of little cell-islands are formed
-during larval life, and these only enter on the stage of formative
-activity after pupation, when they multiply rapidly and grow together
-to form a segment, whose size, form, and external nature is determined
-by them. But it is well known that the abdominal segments of the fly
-differ from those of the larva very markedly and in every respect,
-so that each cell-island must contain determinants which are quite
-different from those in the skin-cells of the corresponding larval
-segments. These last break up at the beginning of pupahood, while the
-former begin to grow vigorously, and to spread themselves out. The most
-remarkable fact about the whole business, and it seems to me also the
-most instructive, is that these imaginal disks frequently appear for
-the first time during larval life, as I found in the case of a midge,
-_Coretha plumicornis_, in regard to the disks of the thorax, and as
-Bruno Wahl[23] has recently demonstrated in the case of the abdominal
-cell-islands. Since in the young larva the position of the subsequent
-imaginal disks is occupied by cells which apparently in no way differ
-from the rest of the skin-cells, and are also exposed to precisely the
-same external and internal influences, the origination of the imaginal
-cells from these can only depend on differential cell-division; the
-primordial cell of each imaginal disk must have separated at the
-beginning of disk-formation into a larval and an imaginal skin-cell.
-
-[23] Bruno Wahl, _Ueber die Entwickelung der hypodermalen
-Imaginalscheiben im Thorax und Abdomen der Larve von 'Eristalis' L.,
-Zeitschr. f. wiss. Zool._, Bd. lxx. 1901.
-
-In insects in which the larva and the imago differ widely, the perfect
-insect, as regards all its principal parts, is already represented
-in the larva, namely, in particular cells which lie among those of
-the corresponding larval parts, and do not visibly differ from these,
-although they are equipped with quite different determinants, and
-consequently enter on their formative activity much later, and give
-rise to quite different structures. As the determinants of the whole
-animal with all its parts are contained in the ovum, so those of
-the parts of its imaginal phase are contained in these cells of the
-imaginal disks.
-
-In addition to all this, we have incontrovertible evidence in favour
-of the theory of determinants in the independent phyletic variations
-of the individual stages of development, on which depends the whole
-phenomenon of 'metamorphosis' which we have just been considering. How
-could the larval stage have become so different from the imago-stage,
-if the one were not alterable by variation arising in the germ
-without the other being affected? If this absolute independence of
-the transmissible variability of the individual stages were not an
-indispensable assumption in the explanation of metamorphosis and other
-phenomena of development, I should regard an attempt at a theory of
-development without determinants as justifiable. But I am forced to
-see in this fact alone an invalidation of all epigenetic theories of
-development, that is, of all theories which assume a germ-substance
-without primary constituents, which can produce the complicated
-body solely by varying step by step under the influence of external
-influences, both extra- and intra-somatic. It is possible to conceive
-of an ovum in which the living substance is of such a kind that it
-must vary in a definite manner under the influence of warmth, air,
-pressure, and so on, that it must divide into similar, and subsequently
-also into dissimilar parts, which then interact upon each other in
-diverse ways and give rise to further variations, which in their turn
-result in differentiations and variations, till ultimately we have the
-whole complicated organic machine complete and 'finished' in every
-detail. Certainly no mortal could make any pronouncement as to the
-constitution of such a substance, but even if we assume it, for the
-nonce, as possible, how can we account for the transmissible variation
-of the individual parts and developmental stages, on which the whole
-phylogenetic evolution depends?
-
-As the development of the butterfly exhibits the three main stages of
-caterpillar, pupa, and perfect insect, each of which is independently
-and hereditarily variable, and therefore implies a something in the
-germ, whose variation brings about a change in the one stage only, so
-the ontogeny of every higher animal is made up of numerous stages,
-which are all capable of independent and transmissible variation. How
-else should we human beings, in our embryonic phase, still possess the
-gill-arches of our fish-like ancestors, although much modified and
-without the gills? Truly, he who would seek to deny that the stages of
-individual development are capable of independent and transmissible
-variation must know very little about embryology. But if the facts
-are as stated, how can they be reconciled with the conception of a
-germinal substance developing in epigenetic fashion? Every variation
-in this substance would affect not only the whole _succession of
-stages, but the whole organism with all its parts_. In this way too,
-then, we are driven to the conclusion that there must be something in
-the germ whose variation causes variation only in a particular part
-of a particular stage. This something we define in our conception of
-the 'primary constituents' (_Anlagen_)--the determinants. These are
-not to be thought of either as 'miniature models,' or even as the
-'seeds' of the parts; they alone cannot produce the part which they
-determine, but they effect changes in the cell in which they become
-active, causing it to vary in such a manner that the formation of the
-relevant part results. While I conceive of development as a continuous
-process, I supplement this with the idea that from within, namely, from
-the nuclear substance, new, directive, 'determining' influences are
-continually being exerted on the developing cells.
-
-I can hardly think of a better proof of the necessity of this
-assumption than that furnished by Delage, one of the most acute
-biologists of France, who, in his comprehensive book on _Heredity_, has
-striven to replace the theory of determinants by something simpler.
-Delage rejects all 'primary constituents' (_Anlagen_) in the germ, all
-'particules représentatives,' as much too complicated an assumption,
-and thinks it possible to work with the conception of a germ-plasm
-which is about as simple as the cell-substance of a Rhizopod, that
-is to say, a protoplasm of definite chemico-physical constitution
-and composition. Leaving out of account the consideration that the
-protoplasm of an amœba is scarcely of such extreme simplicity, but is
-certainly made up of numerous differentiated and definitely arranged
-biophors, how could such an extremely simple ('éminemment simple')
-constitution of the ovum as is here assumed give rise to such a
-complicated organism, the individual parts of which are capable of
-independent and transmissible variation? According to Delage it does so
-because the ovum, though not containing 'all the factors requisite for
-its ultimate resultant,' does contain 'un certain nombre des facteurs
-nécessaires à la détermination de chaque partie et de chaque caractère
-de l'organisme futur'! Determinants after all, it may be said, but
-that is far from the truth! It is not primary constituents that the
-germ contains, according to Delage, it is chemical substances, for
-instance muscle substances, probably 'les substances caractéristiques
-des principales catégories de cellules, c'est-à-dire, celles qui, dans
-ces cellules, sont la condition principale de leur fonctionnement.' All
-these must be contained in the ovum. How they are to reach their proper
-place in the organism, how the 'characteristic chemical substance' of a
-mole is to land just behind the right or left ear of the fully formed
-man, is not stated. But apart from this, there is a much deeper error
-in this assumption of specific chemical substances in the ovum as an
-explanation of the phenomena of local hereditary variation, and I have
-already touched upon it: chemical substances are not vital units, which
-feed and reproduce, which assimilate and which bear a charm against
-the assimilating power of the surrounding protoplasm. They would
-necessarily be modified and displaced in the course of ontogeny, and
-would therefore--no matter where they had been placed at first--be
-incapable of performing all that Delage ascribes to them. Either the
-germ contains 'living' primary constituents, or it is, as Delage
-maintains, determined chemico-physically; but in the latter case there
-is no scope for hereditary local variation. Delage must either renounce
-the attempt to explain this, or he must transform his 'substances
-chimiques' into real and actually living determinants.
-
-Thus from all sides we are forced to the conclusion that the
-germ-substance on the whole owes its marvellous power of development
-not only to its chemico-physical constitution, whether that be
-eminently simple or marvellously complex, but to the fact that it
-consists of many and different kinds of 'primary constituents'
-(_Anlagen_), that is, of groups of vital units equipped with the forces
-of life, and capable of interposing actively and in a specific manner,
-but also capable of remaining latent in a passive state, until they
-are affected by a liberating stimulus, and on this account able to
-interpose successively in development. The germ-cell cannot be merely
-a simple organism, it must be a fabric made up of many different
-organisms or units, a microcosm.
-
-Yet another train of thought leads us to the same idea, and this has
-its roots in the extraordinary complexity of the machine which we call
-the organism.
-
-The botanist Reinke has recently called attention once again to
-the fact that machines cannot be directly made up of primary
-physico-chemical forces or energies, but that, as Lotze said, forces of
-a superior order are indispensable, which so dispose the fundamental
-chemico-physical forces that they must act in the way aimed at by
-the purpose of the machine. To produce a watch it is not enough to
-bring together brass, steel, gold, and stones; to produce a piano it
-is not enough to lay wood, iron, leather, ivory, steel, &c., side by
-side, but these stuffs must be brought together in a definite form and
-combination. In the same way, the mere juxtaposition of carbon and
-water does not result in a carbohydrate like sugar or illuminating gas;
-the component elements only yield what is desired when they are placed
-in a particular and absolutely definite relation to each other, in
-which they so act upon and with one another that sugar or illuminating
-gas results, and the same is true of the component elements of a watch
-or of a piano. In the watch and in the piano this relation is arranged
-by human intelligence, by the workmen who form the different materials
-and put them together in the proper manner. In this case, then, human
-intelligence is, as Reinke says, the 'superior force' which compels the
-energies to work together in a particular way.
-
-But organisms also are machines which perform a particular and
-purposeful kind of work, and they are only capable of doing so because
-the energies which perform the work are forced into definite paths by
-superior forces; these superior forces are thus 'the steersmen of the
-energies.' There is undoubtedly a kernel of truth in this view, and I
-shall return to it. Reinke, however, uses it in a way which I cannot
-follow; that is, he infers from it a 'cosmic intelligence' which puts
-these superior forces into the organisms, and thus controls these
-machines to purposeful work, as the watchmaker puts 'superior forces'
-into the watch by means of wheels, cylinders, and levers. In one case
-it is human intelligence which controls the 'superior forces,' in
-the other 'cosmic' intelligence. I cannot regard this reasoning from
-analogy as convincing, because, in the first place, these 'superior
-forces' are not 'forces' at all. They are constellations of energy,
-co-ordinations of matter and the energies immanent therein under
-complex and precisely defined conditions, and it is a matter of
-indifference whether chance or human intelligence has brought them
-together. If we take Reinke's own example of carbohydrates it is
-certain that our coal-gas is due to the intelligence of man, which
-brings together the carbon and the water in such a way that coal-gas
-must arise. The 'superior forces' must here be looked for in the
-arrangements of the coke-stove, and, in the second place, in the
-intelligence of man. But when decaying plants in the marsh form another
-carbon-compound, marsh-gas, where do the directing 'superior forces'
-come in? Surely only in the fortuitous concomitance of the necessary
-materials and the necessary conditions. Or may 'cosmic' intelligence
-have established this laboratory in the marsh? If not, what can compel
-us to refer the formation of dextrin or starch in the cells of the
-green leaves of plants to 'superior forces' which are placed in them by
-'cosmic' intelligence? I am far from believing that the great and deep
-problem here touched upon can be put aside in any off-hand manner, but
-I feel sure that it will never be solved by word-play about energies
-and 'superior forces.'
-
-Let us return to the kernel of truth in Reinke's thesis; it lies in
-this, that, while the working of a machine does really depend on
-the forces or energies which are bound up with the stuffs of which
-it consists, it also depends on a particular combination of these
-stuffs and forces, on a particular 'constellation' of them, as Fechner
-expressed it. In the watch these 'constellations' are the springs,
-the wheels, &c., and their position in relation to each other; but in
-the organism they are the organs, down to the cells and cell-parts;
-for the cell too is a machine, indeed a very complex one, as its
-functions prove. There are thousands of kinds of 'constellations' of
-elementary substances and forces which condition the activity of the
-living machine, and only when all these constellations are present in
-the proper manner and in the proper relations to each other can the
-functions of the organism be properly discharged.
-
-But the living machine differs essentially from other machines in
-the fact that it constructs itself; it arises by development from a
-cell, by going through numerous 'stages of development.' But none
-of these stages is a dead thing, each is itself a living organism
-whose chief function is to give rise to the next stage. Thus each
-stage of the development may be compared to a machine whose function
-consists in producing a similar but more complex machine. Each stage
-is thus composed, just like the complete organism, of a number of such
-'constellations' of elementary substances and elementary forces, whose
-number in the beginning is relatively small, but increases rapidly with
-each new stage.
-
-But whence come these 'constellations' or, to keep to our metaphor,
-the levers, wheels, and cranks of each successive stage in the making
-of the organic machine? The epigenetic theory of a germ-plasm without
-primary constituents answers by pointing to internal and external
-influences which cause the germ-plasm, originally homogeneous, to
-differentiate gradually more and more, bringing it into the most
-diverse 'constellations.' But how can such influences introduce new
-springs, levers, and wheels of a quite specific kind, as must be the
-case if apparently similar germinal substances are to give rise to two
-such different animals as a domestic duck and a teal? The cause must
-lie in the invisible differences in the protoplasm, opponents will
-answer, and we with them. But our studies up to this point have shown
-us that the differences cannot be merely elementary differences, cannot
-be merely of a physico-chemical nature depending on the composition of
-the raw material and the implicated energies; they must depend on the
-definite co-ordination of substances and energies, in other words, on
-the occurrence of 'constellations' of these. Thus the germ-plasm must
-be composed of definite and very diverse combinations of living units,
-which are themselves bound up in a higher 'constellation,' so that they
-act as a living machine at the first stage of development, and liberate
-into activity the already existing constellations of the second
-stage. The second stage in the series of living machines which arise
-successively from each other liberates the sleeping 'constellations'
-for the third, and so on.
-
-These 'constellations' of matter and energy are the biophors, the
-determinants, and the 'groups of determinants' which we may think
-of as disposed in a manifold overlapping series. That they do not
-enter into activity all at once, but successively take their part in
-development, seems to me a necessary consequence of their successive
-origin in the phylogeny; and the ontogeny, as we shall see later,
-arises through a modified condensation of the phylogeny. Now since
-every new determinant that arises in the course of phylogeny can only
-develop by division and subsequent variation from the determinants
-which were previously active at the same place in the organism, it is
-quite intelligible that later on, when the phylogeny has been condensed
-in the ontogeny, they should not enter upon their active stage at
-the same time as their phyletic predecessors, but after them. The
-theory of Oscar Hertwig, who starts from a germ-plasm without primary
-constituents, that all parts of the germ-plasm become active at the
-same time, seems to me quite untenable. How could the wheels, levers,
-and springs of the complete vital machine, which arose so very slowly
-in the course of phylogeny, arise to-day in the ontogeny in such rapid
-succession unless they were already present in the germ-plasm and only
-required to be incited to activity, that is, liberated by the stage
-preceding them? Even Fechner supported this view when he supposed that
-the interaction and mutual influences of the parts in the organism,
-that is, of the 'constellations,' gave rise of themselves to the
-succeeding stage, that is to say, to the new constellations peculiar
-to the succeeding stage. To this Reinke reasonably objected that it
-was like expecting the window frames of a house in process of building
-to produce the panes of glass. The panes in the organism only develop
-in the window frames if their determinants have been present in the
-germ-plasm from the beginning, and are liberated by the development of
-the frames, just as the activity of the glazier is liberated by the
-sight of the completed frames. Neither new panes nor new determinants
-could be produced rapidly; the former must be manufactured in the
-glass factory, the latter in the developmental workshop of the form
-of life in question, which workshop we call its phylogeny. But just
-as it is unnecessary to erect a new glass factory for each new house
-that is built, so the development of each individual does not require
-the establishment each time of those numberless life-factories--the
-constellations--whose business it is to produce anew the wheels,
-levers, springs, and cylinders of the developmental machinery at each
-stage, for they are all provided for in the germ-plasm, and it is only
-on this account that they are capable of hereditary variation.
-
-I have already directed attention to some embryological facts which
-seem to be contradictory, if not to the germ-plasm theory itself, at
-least to the assumption it makes that the germ-plasm is analysed out
-during the ontogeny; and something more must be said on this head.
-I refer to the numerous facts brought to light through the science
-of developmental mechanics founded by Wilhelm Roux, and particularly
-to the investigations as to the prospective significance of the
-segmentation-cells of the animal ovum.
-
-Among these investigations we find experiments in compressing
-certain eggs (sea-urchin's) in the early stages of segmentation.
-The blastomeres are prevented by artificial pressure from grouping
-themselves in the normal manner; they are compelled to spread out side
-by side in the _same plane_. If the pressure is removed, they change
-their grouping, and yield a normal embryo. I will not here discuss
-whether these results can only be interpreted as showing that each
-segmentation-cell has the same prospective significance, and that it
-is only its relative position which decides what part of the embryo is
-to be formed from it; this could not be done without going into great
-detail; I therefore assume it to be true, and confine my survey to the
-second group of experiments, those on isolated segmentation-cells.
-
-It has been shown that in the eggs of the most diverse animals,
-for instance in the sea-urchin once more, each of the two first
-blastomeres, if separated from one another, can develop into a complete
-larva. Indeed, in the eggs of sea-urchin and some other animals each
-of the first four, or any of the first eight, blastomeres, and indeed
-any segmentation-cell during the earlier stages, possesses the power
-of developing to a certain point, namely, as far as the so-called
-'blastula-larva.' This seems to contradict a theory which assumes that
-the primary constituents become separated in the successive stages of
-ontogeny. But in the first place the blastomeres of all animals do
-not behave in this way, and, moreover, the facts can be quite well
-explained without entirely renouncing the assumption of the segregation
-of the determinant-complexes. It is only necessary to assume that the
-segmentation-cells, which develop in the isolated condition as if they
-were intact eggs, still contain the complete germ-plasm, and that the
-differential segregation into groups of determinants with dissimilar
-hereditary tendencies takes place later. This would certainly load
-the theory with further complications, and I shall not enter into the
-question here, since the facts which we should have to consider are as
-yet by no means undisputed.
-
-But in any case the facts of developmental mechanics referred to, which
-we owe to numerous excellent observers of the last decade,--I need only
-name W. Roux, O. Hertwig, Chun, Driesch, Barfurth, Morgan, Conklin,
-Wilson, Crampton, and Fischel--not only leave the essential part of the
-germ-plasm theory untouched, but rather strengthen than endanger its
-more subordinate points, such as the assumption of a segregation of the
-components of the germ-plasm in the course of ontogenesis.
-
-As to the fundamental ideas expressed in the theory, I have already
-shown that these remain unaltered, even if we do not assume a
-disintegration or segregation of the germ-plasm, but think of all the
-developing cells as equipped with the complete germ-plasm. In that case
-the determinants would be liberated to activity solely by specific
-stimuli. But in regard to the assumption of disintegration, it must be
-noted that the facts cited relative to the sea-urchin's ova do not by
-any means hold true of the eggs of all animals.
-
-In various animal types each of the first two segmentation-cells,
-when separated from its neighbour, produces only a half-embryo, and
-any one of the first four cells a quarter-embryo. This 'fractional
-embryo' is, however, in some cases able later to develop into a whole
-embryo (to 'postgenerate' itself, as W. Roux says). The isolated
-blastomere shows, to begin with, an activity of only a half of the
-primary constituents of the animal, as was first established by W. Roux
-and maintained conclusively, in spite of many attacks, until it was
-established beyond doubt by the detailed corroboratory investigations
-of Endres. The secondary completion of the embryo, which, however, is
-still disputed, must be regarded as a regeneration, and, to explain it,
-a co-operation of the complete but not yet wholly active germ-plasm in
-both segmentation-cells must therefore be assumed.
-
-It would carry us too far if I were to deal in detail even with the
-most important of the numerous facts that the last decade has brought
-to light; I shall restrict myself to the most essential.
-
-That isolated segmentation-cells have the capacity of developing into
-embryos which are complete but correspondingly smaller in size has been
-demonstrated in animals of various groups, though it does not seem to
-go to the same length in all. In the Medusæ we find that not only one
-of the first two, but one of the first four, eight, and even sixteen
-segmentation-cells may develop a whole larva when isolated (Zoja). In
-the sea-urchin at least any one of the first eight blastomeres may do
-so. And Driesch's experiments in cutting up the young larvæ at the
-blastula-stage (a single-layered ball of cells) leads us to assume that
-each of these cells still possesses the complete germ-plasm. Beyond
-that stage, however, the primary constituents obviously divide into
-those of the ectoderm and those of the endoderm, for the subsequent
-two-layered stage in the sea-urchin's development, the gastrula, does
-not complete itself if it be artificially divided into fragments which
-consist only of cells from the outer, or only of cells from the inner
-layer. In corroboration of this experiment made by Barfurth, Samassa
-was able to demonstrate in regard to the egg of the frog that, even
-after the third division of the ovum, the segmentation-cells are so
-different from each other in respect of their primary constituents
-that they were not able to replace each other mutually. When this
-investigator killed the ectoderm-cells alone by means of an induction
-current, or the endoderm-cells alone, the dead half could not be
-replaced by the half which remained alive, and the whole ovum perished.
-
-If these facts may be adduced in favour of a separation of the primary
-constituents at an earlier or later stage, we find even stronger proofs
-among the Ctenophores, Gastropods, Bivalves, and Annelids. In the
-last-named group Wilson has shown it to be probable that development
-is really a 'mosaic work,' as Roux and I had assumed. The older
-observations made by Chun at an earlier date on the Ctenophora, and
-the more recent experiments of Fischel on the same animals, prove the
-same thing for this group. In this case complete larvæ are easily
-distinguished from mere 'partial developments' by the number of the
-characteristic 'ciliated meridional rows' or ribs, which extend from
-one pole of the larva to another. In the complete larva there are
-eight of these, but in larvæ from one of the first two blastomeres
-(isolated) there are only four, and in those which have arisen from one
-of the first four blastomeres there are only two. If an ovum at the
-eight-cell stage can be successfully divided into separate blastomeres,
-each of these will form an 'eighth larva,' always with only one
-ciliated rib. Even in the succeeding sixteen-cell stage it could still
-be demonstrated that the substance responsible for the formation of
-the ribs only lies in particular places and always suffices only for
-eight ribs. The sixteen-cell stage consists of eight large cells and
-eight small ones, the 'macromeres' and the 'micromeres'; if an ovum
-at this stage be cut so that one piece contains five macromeres and
-five micromeres, a partial larva will develop which possesses only
-five ribs, while the larva from the other portion will have only
-three. But the localizing of the rib-determinants can be followed
-still further, for in larvæ in which individual micromeres have
-been displaced from their normal position there is a correlated
-displacement of the corresponding ribs, and a dislocation of their
-ciliated comb-plates. The determinants of the ribs must therefore
-lie in the micromeres, and we must conclude that at the antecedent
-division they were only imparted to one daughter-nucleus, while the
-other, that of the macromere, did not receive this kind of determinant.
-Here then we have an example of dissimilar or differential division.
-Those who oppose this theory of qualitative division will hardly be
-likely to admit this, but will rather seek to maintain that 'external
-influences,' such as relative position, determine which cells are to
-give rise to the ciliated ribs and which are not. But the fact that
-artificial displacement of the micromeres leads to a disarrangement of
-the ciliated comb-plates, of which the ribs are made up, invalidates
-this suggestion, and at the same time overthrows the interpretation
-that it may be the cells which lie on particular meridians that are
-determined by this position to the production of ciliated plates.
-Obviously, the converse of this is true; those cells which contain
-the rib-determinants come to lie in the regular course of development
-in these eight meridians, and the cells lying between them, though of
-the same descent (from micromeres), contain no such determinants and
-therefore form no ribs. But if those cells which are equipped with
-rib-determinants be artificially displaced, then they give rise to
-swimming-plates elsewhere than on the aforesaid meridians.
-
-The experiments made by Crampton on a marine Gastropod, _Ilyanassa_,
-likewise go to prove that a disintegration or segregation of the
-primary constituents does occur in the course of development. In
-this case, when the first two or first four segmentation-cells were
-artificially separated from each other, they developed exactly as
-if they still belonged to the complete ovum, that is, each isolated
-segmentation-cell yielded, respectively, a half or a quarter-embryo.
-And these 'partial embryos' were not able in this case to give rise
-subsequently to the missing parts or to form complete embryos.
-
-There are thus two contrasted groups of animals, in one of which a
-segregation of the mass of primary constituents apparently takes place
-at the very beginning, while in the other it does not take place in the
-first stages of development, but apparently occurs later on. We may
-distinguish these two groups, with Heider, as those having 'regulation
-ova' and those having 'mosaic ova.' But I do not see that this affords
-any reason why we should give up our conception of the successive
-segregation of the germ-plasm into its determinants, even although, as
-I said before, I may modify it so far as to say that the segregation
-does not necessarily take place in all groups and species of animals at
-the same time, but occurs earlier in some and later in others.
-
-Now that I have shown how the germ-plasm theory may be brought into
-harmony with the phenomena of ontogeny, I wish to go on to show what
-the theory can accomplish in clarifying our understanding of the
-phenomena of reproduction and heredity. I shall at the same time give a
-brief exposition of some of the most important of these phenomena.
-
-First, a few words in regard to the development of the reproductive
-cells. We may leave aside in the meantime the question whether they
-are sexually differentiated or not; we are only concerned just now
-with the main problem: How is it possible for the organism to produce
-germ-cells, that is, cells which contain the complete germ-plasm with
-all its determinants, when the building up of the body in ontogeny,
-according to our theory, involves a disintegration or segregation of
-the determinant-architecture into smaller and smaller groups? It is
-impossible that specific determinants should arise _de novo_, just
-as an animal cannot arise otherwise than from its germ, nor a cell
-otherwise than from a cell, nor a nucleus otherwise than from an
-already existing nucleus. If vital units ever originate _de novo_
-at all, it is only conceivable in the case of the very simplest
-biophors, as we shall see later when we come to speak of 'Spontaneous
-Generation.' Specific biophors and the determinants composed of them
-have behind them a phylogeny, a history, which conditions that they
-shall arise only from their like.
-
-Thus we see that germ-cells can only arise where all the determinants
-of the relevant species arranged as ids are already present. If we
-could assume that the ovum, just beginning to develop, divides at
-its first cleavage into two cells, one of which gives rise to the
-whole body (soma) and the other only to the germ-cells lying in this
-body, the matter would be theoretically simple. We should say, the
-germ-plasm of the ovum first doubles itself by growth, as the nuclear
-substance does at every nuclear division, and then divides into two
-similar halves, one of which, lying in the primordial somatic cell,
-becomes at once active and breaks up into smaller and smaller groups
-of determinants corresponding to the building up of the body, while
-the germ-plasm in the other remains in a more or less 'bound' or 'set'
-condition, and is only active to the extent of gradually stamping as
-germ-cells the cells which arise from the primordial germ-cell.
-
-As yet, however, only one group of animals is known to behave
-demonstrably in this manner, the Diptera among insects; in all others
-the cell from which the germ-cells exclusively arise, the 'primordial
-germ-cell,' makes its appearance later in development, usually
-during embryogenesis and often very early in it, after the first few
-divisions of the ovum, but sometimes not till long after the end of
-embryogenesis, and not even in the individual which arises from the
-ovum, but in descendants which arise from it by budding. This last
-case occurs especially in the colonial hydroid polyps, which multiply
-by budding. Here the primordial germ-cell is separated from the ovum
-by a long series of cell-generations, and the sole possibility of
-explaining the presence of germ-plasm in this primordial cell is to
-be found in the assumption that in the divisions of the ovum the
-whole of the germ-plasm originally contained in it was not broken up
-into determinant groups, but that a part, perhaps the greater part,
-was handed on in a latent state from cell to cell, till sooner or
-later it reached a cell which it stamped as the primordial germ-cell.
-Theoretically it makes no difference whether these 'germ-tracks,' that
-is, the cell-generations which lead from the ovum to the primordial
-germ-cell, are short or very long, whether they consist of three or
-six or sixteen cells, or of hundreds and thousands of cells. That
-all the cells of the germ-track do not take on the character of
-germ-cells must, in accordance with our conception of the 'maturing'
-of determinants, be referred to the internal conditions of the cells
-and of the germ-plasm, perhaps in part also to an associated quantum
-of somatic idioplasm which is only overpowered in the course of the
-cell-divisions.
-
-This splitting up of the substance of the ovum into a somatic half,
-which directs the development of the individual, and a propagative
-half, which reaches the germ-cells and there remains inactive, and
-later gives rise to the succeeding generation, constitutes _the theory
-of the continuity of the germ-plasm_, which I first stated in a work
-which appeared in the year 1885. Its fundamental idea had already been
-expressed much earlier by Francis Galton (1872), without however being
-fully appreciated at the time or having any influence on the course
-of science, and the same is true with the later theoretical views of
-Jäger, Rauber, and Nussbaum, all of whom reached the same idea quite
-independently of each other, and sought to elaborate it more or less
-fully.
-
-The hypothesis does not depend for support merely on a recognition of
-its theoretical necessity; on the contrary, there is a whole series of
-facts which may be adduced as strongly in its favour.
-
-Thus, even the familiar fact that the excision of the reproductive
-organs in all animals produces sterility proves that no other cells
-of the body are able to give rise to germ-cells; germ-plasm cannot be
-produced _de novo_. An unmistakable corroboration of this, it seems
-to me, is to be found in the conditions of germ-cell formation in
-the medusoids and hydroid polyps, for here it is apparent that the
-birthplace of the germs, that is, the place at which the germ-cells
-of the animal are formed, has been shifted backwards in the course
-of phylogenetic evolution, that is, has been moved nearer to the
-starting-point of development. This shifting has exactly followed
-the 'germ-tracks.' as we shall see, although in some cases it would
-have been more advantageous if the birthplace of the germ-cells could
-have lain outside of these. Obviously, then, it is only the existing
-cell-generations of the germ-track which were able to give rise to
-germ-cells, or, in other words, they alone contained the indispensable
-germ-plasm. With the help of Figs. 94 and 95 I hope to be able to make
-this matter clear.
-
-[Illustration: FIG. 94. Diagram to illustrate the phylogenetic shifting
-back of the origins of the germ-cells in medusoids and hydroids. A
-composite picture. _A_, branch of a polyp colony. _P_, polyp-head with
-mouth (_m_) and tentacles. _St_, stalk of the polyp. _M_, medusoid-bud
-with the bell (_Gl_). _T_, marginal tentacle. _m_, mouth. _Mst_,
-manubrium. _GphK_, a gonophore-bud. _GH_, gastric cavity. _ekt_,
-ectoderm. _ent_, endoderm. _st_, supporting lamella. The germ-cells
-(_kz_) arise in the medusoid in the ectoderm of the manubrium--first
-phyletic stage--where they also attain maturity. In the gonophore-bud
-(_GphK_) they arise in the ectoderm (_kz´_), or further down in the
-stalk of the polyp at _kz´´_--third phyletic stage, or in the ectoderm
-of the branch from which the polyp has arisen, at _kz´´´_--fourth
-phyletic stage of the shunting of the originative area of the
-germ-cells. In the two last cases the germ-cells migrate until they
-reach their primitive place of origination in the medusoid, or in the
-corresponding layer of the medusoid gonophore, as may be more clearly
-seen in Fig. 95. Drawn from my sketch by Dr. Petrunkewitsch.]
-
-In the hydroid polyps and their medusoids the germ-cells always
-arise in the ectoderm; in species which produce sexual medusoids by
-budding, the germ-cells arise in the ectoderm of the manubrium of these
-medusoids (Fig. 94, _M_, _kz_). But in many species these sexual stages
-have degenerated in the course of phylogeny into so-called gonophores,
-that is, to medusoids which still exhibit more or less complete bells,
-but neither mouth (_m_) nor marginal tentacles (_T_), and which no
-longer break away from the colony to swim freely about, to feed
-independently, and to produce and ripen germ-cells. The degeneration
-of the 'gonophores' often goes even further; in many the medusoid bell
-is represented only by a thin layer of cells, and in some even this
-token of descent from medusoid ancestry is absent, and they are mere
-single-layered closed brood-sacs (Fig. 95, _Gph_).
-
-The adherence of the sexual animal to the hydroid colony has,
-however, made a more rapid ripening of the germ-cells possible, and
-nature has taken advantage of this possibility in all the cases
-known to me, for the germ-cells no longer arise in the manubrium
-of the mature degenerate medusoid, that is, of the gonophore, but
-_earlier_, before the bud which becomes a gonophore possesses a
-manubrium. The birthplace of the germ-cells is thus shifted back from
-the manubrium of the medusoid to the young gonophore-bud (Fig. 94,
-_M_, _kz_). The same thing occurs in species in which the medusoids
-are liberated, but live only for a short time, for instance, in the
-genus _Podocoryne_. Although perfect medusoids are formed, these have
-their germ-cells fully developed at the time of their liberation from
-the hydroid colony. But in species in which the medusoid-buds have
-really degenerated and are no longer liberated, the birthplace of the
-germ-cells is shifted _even further back_, and in the first place into
-the stalk (_St_, _kz´´_) of the polyp from the gonophore-buds. This
-is the case in the genus _Hydractinia_. In the further course of the
-process the birthplace of the germ-cells has shifted as far back as to
-the branch from which the polyp has grown out (Fig. 94, _A_, _kz´´´_);
-and finally, in the cases in which the medusoid has degenerated to
-a mere brood-sac (Fig. 95, _Gph_), even to the generation of polyps
-immediately before, that is, into the polyp-stem from which the branch
-arises that bears the polyps producing the gonophore-bud (Fig. 95,
-_kz´´´_). Then we find the birthplace of the germ-cells _still_ further
-back (Fig. 95, _kz´´´´_), for the egg and sperm-cells arise in the stem
-of the principal polyps (the main stem of the colony). The advantage of
-this arrangement is easily seen, for the principal polyp is present
-earlier than those of the secondary branches, and these again earlier
-than the polyp which bears the sexual buds, and this, finally, earlier
-than the sexual bud which it bears. Thus this shunting backwards of the
-birthplace of the germ-cells means an earlier origin of the primordium
-(_Anlage_) of the germ-cells, and consequently an earlier maturing of
-these.
-
-[Illustration: FIG. 95. Diagram to illustrate the migration of the
-germ-cells in hydro-medusæ from their remotely shunted place of origin
-to their primitive place of origin in the gonophore, in which they
-attain to maturity. The state of affairs in Eudendrium is taken as the
-basis of the diagram. _HP_, one of the principal polyps. _mu_, mouth.
-_ma_, gut-cavity. _t_, tentacle. _Sta_, its stem. _A_, a branch of the
-polyp colony. _SP_, lateral polyp. _Gph_, a medusoid-bud completely
-degenerated into a mere gonophore. _Ei_, ovum. _GH_, gastric cavity.
-_st_, supporting lamella. The originative area of the germ-cells
-lies in the stem of the principal polyp at _kz´´´´_, whence the
-germ-cells first migrate into the endoderm of the branch (_A_) at
-_kz´´´_, creeping within which they reach _kz´´_ in the lateral polyp
-(blastostyle), finally reaching the gonophore (_kz_) and passing again
-into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.]
-
-But none of all these germ-cells come to maturity in the birthplace to
-which they have been shifted, for they migrate independently from it to
-the place at which they primitively arose, namely, into the manubrium
-of the medusoid, which is still present even when great degeneration
-has occurred, or even--in the most extreme cases of degeneration--into
-the ectoderm of the brood-sac. This is the case in the genus
-_Eudendrium_, of which Fig. 95 gives a diagrammatic representation.
-
-The most interesting feature of this migration of the germ-cells is
-that the cells invariably arise in the ectoderm (_kz´´´´_), then pierce
-through the supporting lamella (_st_) into the endoderm (_kz´´´_), and
-then creep along it to their maturing-place. Once there they break
-through again to the outer layer of cells, the ectoderm (_kz_), and
-come to maturity (_Ei_). That they make their way through the endoderm
-is probably to be explained by the fact that they are there in direct
-proximity to the food-stream which flows through the colony (_GH_ =
-gastric cavity), and they are thus more richly nourished there than
-in the ectoderm. But although this is the case, they never arise in
-the endoderm; in no single case is the birthplace of the germ-cells to
-be found in the endoderm, but always in the ectoderm, no matter how
-far back it may have been shunted. Even when the germ-cells migrate
-through the endoderm, their first recognizable appearance is invariably
-in the ectoderm, as, for instance, in _Podocoryne_ and _Hydractinia_.
-The course of affairs is thus exactly what it would necessarily be if
-our supposition were correct, that only definite cell-generations--in
-this case the ectoderm-cells--contain the complete germ-plasm. If the
-endoderm-cells also contained germ-plasm it would be hard to understand
-why the germ-cells never arise from them, since their situation offers
-much better conditions for their further development than that of
-the ectoderm-cells. It would also be hard to understand why such a
-circuitous route was chosen as that exhibited by the migration of
-the young germ-cells into the endoderm. Something must be lacking in
-the endoderm that is necessary to make a cell into a germ-cell: that
-something is the germ-plasm.
-
-If we accept the theory of the continuity of the germ-plasm as in the
-main correct, it appears that higher animals and plants are constructed
-of two kinds of elements, the somatic cells and the germ-cells; both
-owe their being to the germ-plasm of the ovum, but the former do not
-contain it complete but only in individual determinants[24], and
-therefore can never give rise again to the rank of germ-cells; the
-others contain the latent germ-plasm intact, and can therefore produce
-not only cells like themselves for a certain time by division, but
-have also the power, when they are mature and the necessary conditions
-have been fulfilled, of bringing forth a new individual of the same
-species. The former have only a limited length of life, they die--they
-must necessarily die--when the life of the individual to which they
-belong is at an end; the latter are potentially immortal, like the
-unicellular organisms, that is, they can in favourable circumstances
-give rise to the germ-cells of a new individual, and so on for all
-time, as far as we can see. The germ-plasm of a species is thus never
-formed _de novo_, but it grows and increases ceaselessly; it is handed
-on from one generation to another like a long root creeping through
-the earth, from which at regular distances shoots grow up and become
-plants, the individuals of the successive generations. If these
-conditions be considered from the point of view of reproduction, the
-germ-cells appear the most important part of the individual, for they
-alone maintain the species, and the body sinks down almost to the
-level of a mere cradle for the germ-cells, a place in which they are
-formed, and under favourable conditions are nourished, multiply, and
-attain to maturity. But the matter can also be looked at in an opposite
-light, and then the endless root of the germ-plasm, with its germ-cells
-ever forming new individuals, may be regarded as the means by which
-alone nature was able to create multicellular organisms, individuals
-of higher and higher differentiation and capacity, able to adapt
-themselves to all possible conditions, and to make the fullest use of
-all the possibilities of life.
-
-[24] Boveri has recently made an observation upon the thread-worm of
-the horse, which points to the correctness of the conception of the
-germ-plasm. The two first segmentation-cells both receive the four
-chromosomes of the species, but, in one of the two, a portion of the
-chromatin breaks off and degenerates, or dissolves, at least as far
-as can be seen. The other cell retains the whole mass of chromatin,
-and from this there arise later the primitive genital-cells. In the
-germ-track, therefore--so we must interpret it--the whole of the
-germ-plasm is retained, while a part of it is withdrawn from the soma.
-I have only partly described the process, and I do not wish to enter in
-detail on an interpretation of it, since it seems to me obscure and to
-require further observations before an interpretation can be attempted
-with any confidence.
-
-
-
-
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-
-<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Evolution Theory, Vol. 1 of 2, by August Weismann</div>
-
-<div style='display:block; margin:1em 0'>
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online
-at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
-are not located in the United States, you will have to check the laws of the
-country where you are located before using this eBook.
-</div>
-
-<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Evolution Theory, Vol. 1 of 2</div>
-
-<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: August Weismann</div>
-
-<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Translator: J. Arthur Thomson and Margaret R. Thomson</div>
-
-<div style='display:block; margin:1em 0'>Release Date: January 06, 2021 [eBook #64227]</div>
-
-<div style='display:block; margin:1em 0'>Language: English</div>
-
-<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div>
-
-<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Constanze Hofmann, Alan, Marilynda Fraser-Cunliffe and the Online Distributed Proofreading Team at https://www.pgdp.net (This book was produced from images made available by the HathiTrust Digital Library and The Internet Archive.)</div>
-
-<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION THEORY, VOL. 1 OF 2 ***</div>
-
-
-
-<h1>
-<span class="more">THE</span><br />
-EVOLUTION THEORY</h1>
-
-<p class="c"><b>VOLUME I</b>
-</p>
-
-<div class="figcenter">
-<img src="images/cover.jpg" alt=""/>
-</div>
-
-
-<p class="c p6">
-<span class="xlarge">THE</span><br />
-<span class="xxxlarge gesperrt">EVOLUTION THEORY</span></p>
-
-<p class="c p2">
-BY</p>
-
-<p class="c xlarge">
-<span class="smcap">Dr.</span> AUGUST WEISMANN</p>
-
-<p class="c more">
-PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU</p>
-
-<p class="c p4">
-TRANSLATED WITH THE AUTHOR'S CO-OPERATION</p>
-
-<p class="c half">
-BY</p>
-
-<p class="c large">
-J. ARTHUR THOMSON</p>
-
-<p class="c half">
-REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN</p>
-
-<p class="c half p2">
-AND</p>
-
-<p class="c large">
-MARGARET R. THOMSON</p>
-
-<p class="c p4">
-ILLUSTRATED</p>
-
-<p class="c p2">
-IN TWO VOLUMES</p>
-
-<p class="c">
-VOL. I</p>
-
-<p class="c xlarge p4">
-LONDON<br />
-EDWARD ARNOLD</p>
-
-<p class="c">
-41 &amp; 43 MADDOX STREET, BOND STREET, W.</p>
-
-<p class="c">
-1904</p>
-
-<p class="c">
-<i>All rights reserved</i>
-</p>
-
-
-<hr class="full" />
-
-<div class="chapter">
-<p class="ph2">AUTHOR'S PREFACE</p>
-</div>
-
-
-<p><span class="smcap">When</span> a life of pleasant labour is drawing towards a close,
-the wish naturally asserts itself to gather together the main
-results, and to combine them in a well-defined and harmonious
-picture which may be left as a legacy to succeeding generations.</p>
-
-<p>This wish has been my main motive in the publication
-of these lectures, which I delivered in the University of
-Freiburg in Breisgau. But there has been an additional
-motive in the fact that the theory of heredity published
-by me a decade ago has given rise not only to many investigations
-prompted by it, but also to a whole literature
-of 'refutations,' and, what is much better, has brought to
-light a mass of new facts which, at first sight at least, seem
-to contradict my main theory. As I remain as convinced
-that the essential part of my theory is well grounded as
-I was when I first sketched it, I naturally wish to show how
-the new facts may be brought into harmony with it.</p>
-
-<p>It is by no means only with the theory of heredity by
-itself that I am concerned, for that has served, so to speak,
-as a means to a higher end, as a groundwork on which to
-base an interpretation of the transformations of life through
-the course of the ages. For the phenomena of heredity, like
-all the functions of individual life, stand in the closest
-association with the whole evolution of life upon our earth;
-indeed, they form its roots, the nutritive basis from which all
-its innumerable branches and twigs are, in the long run,
-derived. Thus the phenomena of the individual life, and
-especially those of reproduction and inheritance, must be
-considered in connexion with the Theory of Descent, that
-the latter may be illumined by them, and so brought nearer
-our understanding.</p>
-
-<p>I make this attempt to sum up and present as a harmonious<span class="pagenum"><a id="Page_vi"></a>[Pg vi]</span>
-whole the theories which for forty years I have been gradually
-building up on the basis of the legacy of the great workers of
-the past, and on the results of my own investigations and
-those of many fellow workers, not because I regard the picture
-as complete or incapable of improvement, but because I believe
-its essential features to be correct, and because an eye-trouble
-which has hindered my work for many years makes it
-uncertain whether I shall have much more time and strength
-granted to me for its further elaboration. We are standing
-in the midst of a flood-tide of investigation, which is ceaselessly
-heaping up new facts bearing upon the problem of evolution.
-Every theory formulated at this time must be prepared
-shortly to find itself face to face with a mass of new facts
-which may necessitate its more or less complete reconstruction.
-How much or how little of it may remain, in face of the facts
-of the future, it is impossible to predict. But this will be so
-for a long time, and it seems to me we must not on that
-account refrain from following out our convictions to the
-best of our ability and presenting them sharply and definitely,
-for it is only well-defined arguments which can be satisfactorily
-criticized, and can be improved if they are imperfect,
-or rejected if they are erroneous. In both these processes
-progress lies.</p>
-
-<p>This book consists of 'Lectures' which were given publicly
-at the university here. In my introductory lecture in 1867
-I championed the Theory of Descent, which was then the
-subject of lively controversy, but it was not till seven years
-later that I gave, by way of experiment, a short summer
-course with a view to aiding in the dissemination of Darwin's
-views. Then very gradually my own studies and researches
-and those of others led me to add to the Darwinian edifice,
-and to attempt a further elaboration of it, and accordingly
-these 'Lectures,' which were delivered almost regularly every
-year from 1880 onwards, were gradually modified in accordance
-with the state of my knowledge at the time, so that they
-have been, I may say, a mirror of the course of my own
-intellectual evolution.</p>
-
-<p><span class="pagenum"><a id="Page_vii"></a>[Pg vii]</span></p>
-
-<p>In the last two decades of the nineteenth century much
-that is new has been introduced into biological science;
-Nägeli's idea of 'idioplasm'&mdash;the substance which determines
-form; Roux's <i>Struggle of the Parts</i>, the recognition of a special
-hereditary substance, 'the germ-plasm,' its analysis into chromosomes,
-and its continuity from generation to generation;
-the potential immortality of unicellular organisms and of the
-germ-cells in contrast to the natural death of higher forms and
-'bodies'; a deeper interpretation of mitotic nuclear division,
-the discovery of the centrosphere&mdash;the marvellous dividing
-apparatus of the cell&mdash;which at once allowed us to penetrate
-a whole stratum deeper into the unfathomable mine of
-microscopic vital structure; then the clearing up of our ideas
-in regard to fertilization, and the analysis of this into the two
-processes combined in it, reproduction and the mingling of
-the germ-plasms (Amphimixis); in connexion with this, the
-phenomena of maturation, first in the female and then in the
-male cell, and their significance as a reduction of the hereditary
-units:&mdash;all this and much more we have gained during this
-period. Finally, there is the refutation of the Lamarckian
-principle, and the consequent elaboration of the principle
-of selection by applying it to the hitherto closed region of
-the ultimate vital elements of the germ-plasm.</p>
-
-<p>The actual form of these lectures has developed as they
-were transcribed. But although the form is thus to some
-extent new, I have followed in the main the same train
-of thought as in the lectures of recent years. The lecture-form
-has been adhered to in the book, not merely because
-of the greater vividness of presentation which it implies,
-but for many other reasons, of which the greater freedom
-in the choice of material and the limiting of quotation to
-a minimum are not the least. That all polemics of a personal
-kind have thus been excluded will not injure the book, but
-it is by no means lacking in discussions of opinion, and will,
-therefore, I trust, contribute something towards the clearing
-up of disputed points.</p>
-
-<p>I have endeavoured to introduce as much of the researches<span class="pagenum"><a id="Page_viii"></a>[Pg viii]</span>
-and writings of others as possible without making the book
-heavy; but my aim has been to write a book to be read, not
-merely one to be referred to.</p>
-
-<p>If it be asked, finally, for whom the book is intended,
-I can hardly answer otherwise than 'For him whom it
-interests.' The lectures were delivered to an audience consisting
-for the most part of students of medicine and natural
-science, but including some from other faculties, and sometimes
-even some of my colleagues in other departments.
-In writing the book I have presupposed as little special
-knowledge as possible, and I venture to hope that any one
-who <i>reads</i> the book and does not merely skim it, will be
-able without difficulty to enter into the abstruse questions
-treated of in the later lectures.</p>
-
-<p>It would be a great satisfaction to me if this book were
-to be the means of introducing my theoretical views more
-freely among investigators, and to this end I have elaborated
-special sections more fully than in the lectures. Notwithstanding
-much controversy, I still regard its fundamental
-features as correct, especially the assumption of 'controlling'
-vital units, the determinants, and their aggregation into
-'ids'; but the determinant theory also implies germinal
-selection, and without it the whole idea of the guiding of
-the course of transformation of the forms of life, through
-selection which rejects the unfit and favours the more fit,
-is, to my mind, a mere torso, or a tree without roots.</p>
-
-<p>I only know of two prominent workers of our day who
-have given thorough-going adherence to my views: Emery
-in Bologna and J. Arthur Thomson in Aberdeen. But
-I still hope to be able to convince many others when the
-consistency and the far-reachingness of these ideas are better
-understood. In many details I may have made mistakes
-which the investigations of the future will correct, but as
-far as the basis of my theory is concerned I am confident:
-<i>the principle of selection does rule over all the categories of
-vital units</i>. It does not, indeed, create primary variations,
-but it determines the paths of evolution which these are to<span class="pagenum"><a id="Page_ix"></a>[Pg ix]</span>
-follow, and thus controls all differentiation, all ascent of
-organization, and ultimately the whole course of organic
-evolution on the earth, for everything about living beings
-depends upon adaptation, though not on adaptation in the
-sense in which Darwin used the word.</p>
-
-<p>The great prominence thus given to the idea of selection
-has been condemned as one-sided and exaggerated, but the
-physicist is quite as open to the same reproach when he
-thinks of gravity as operative not on our earth alone, but
-as dominating the whole cosmos, whether visible to us or
-not. If there is gravity at all it must prevail everywhere,
-that is, wherever material masses exist; and in the same way
-the co-operation of certain conditions with certain primary
-vital forces must call forth the same process of selection
-wherever living beings exist; thus not only are the vital
-units which we can perceive, such as individuals and cells,
-subject to selection, but those units the existence of which
-we can only deduce theoretically, because they are too minute
-for our microscopes, are subject to it likewise.</p>
-
-<p>This extension of the principle of selection to all grades
-of vital units is the characteristic feature of my theories;
-it is to this idea that these lectures lead, and it is this&mdash;in
-my own opinion&mdash;which gives this book its importance.
-This idea will endure even if everything else in the book
-should prove transient.</p>
-
-<p>Many may wonder, perhaps, why in the earlier lectures
-much that has long been known should be presented afresh,
-but I regard it as indispensable that the student who wishes
-to make up his own mind in regard to the selection-idea
-should not only be clear as to what it means theoretically,
-but should also form for himself a conception of its sphere
-of influence. Many prejudiced utterances in regard to
-'Natural Selection' would never have been published if
-those responsible for them had known more of the facts;
-if they had had any idea of the inexhaustible wealth of
-phenomena which can only be interpreted in the light of this
-principle, in as far, that is, as we are able to give explanations<span class="pagenum"><a id="Page_x"></a>[Pg x]</span>
-of life at all. For this reason I have gone into the subject
-of colour-adaptations, and especially into that of mimicry,
-in great detail; I wished to give the reader a firm foundation
-of fact from which he could select what suited him when he
-wished to test by the light of facts the more difficult problems
-discussed in the book.</p>
-
-<p>In conclusion, I wish to thank all those who have given
-me assistance in one way or other in this work: my former
-assistant and friend Professor V. Häcker in Stuttgart, my
-pupils and fellow workers Dr. Gunther and Dr. Petrunkewitsch,
-and the publisher, who has met my wishes in the most
-amiable manner.</p>
-
-<p class="l">
-<span class="smcap">Freiburg-I-Br.</span>,<br />
-<span class="l"><i>February 20, 1902</i>.</span>
-</p>
-
-<hr class="full" />
-
-<p><span class="pagenum"><a id="Page_xi"></a>[Pg xi]</span></p>
-
-<div class="chapter">
-<p class="ph2">PREFATORY NOTE TO ENGLISH EDITION</p>
-</div>
-
-
-<p><span class="smcap">Professor Weismann's</span> <i>Evolution Theory</i>, here translated
-from the second German edition (1904), is a work of compelling
-interest, the fruit of a lifetime of observation and
-reflection, a veteran's judicial summing up of his results, and
-certainly one of the most important contributions to Evolution
-literature since Darwin's day.</p>
-
-<p>As the author's preface indicates, the salient features of
-his crowning work are (1) the illumination of the Evolution
-process with a wealth of fresh illustrations; (2) the vindication
-of the 'Germ-plasm' concept as a valuable working
-hypothesis; (3) the final abandonment of any assumption of
-transmissible acquired characters; (4) a further analysis of
-the nature and origin of variations; and (5), above all, an
-extension of the Selection principle of Darwin and Wallace,
-which finds its logical outcome in the suggestive theory of
-Germinal Selection.</p>
-
-<p>The translation will be welcomed, we believe, not only
-by biological experts who have followed the development of
-'Weismannism' during the last twenty years, and will here
-find its full expression for the time being, but also by those
-who, while acquainted with individual essays, have not
-hitherto realized the author's complete system. Apart from
-the theoretical conceptions which unify the book and mark
-it as an original contribution of great value, there is a lucid
-exposition of recent biological advances which will appeal to
-those who care more for facts than theories. To critics of
-evolutionism, who are still happily with us, the book ought
-to be indispensable; it will afford them much material for
-argumentation, and should save them many tilts against<span class="pagenum"><a id="Page_xii"></a>[Pg xii]</span>
-windmills. But, above all, the book will be valued by workers
-in many departments of Biology, who are trying to help
-in the evolution of Evolution Theory, for it is characteristic
-of the author, as the history of recent research shows, to
-be suggestive and stimulating, claiming no finality for his
-conclusions, but urging us to test them in a mood of 'thätige
-Skepsis.'</p>
-
-<p>The translation of this book&mdash;the burden of which has
-been borne by my wife&mdash;has been a pleasure, but it has also
-been a serious responsibility. We have had fine examples
-set us by previous translators of some of Weismann's works,
-Meldola, Poulton, Shipley, Parker, and others; and if we
-have fallen short of their achievements, it has not been for
-lack of endeavour to follow the original with fidelity, nor for
-lack of encouragement on the part of the author, who revised
-every page and suggested many emendations.</p>
-
-<p class="r">
-J. ARTHUR THOMSON.</p>
-
-<p class="l">
-<span class="smcap">University of Aberdeen</span>,<br />
-<span class="l"><i>October, 1904</i>.</span>
-</p>
-
-<hr class="full" />
-
-
-<p><span class="pagenum"><a id="Page_xiii"></a>[Pg xiii]</span></p>
-
-<div class="chapter">
-<p class="ph2">CONTENTS</p>
-</div>
-
-
-<table>
-
-<tr><td class="tdr"><span class="half">LECTURE</span></td>
-    <td class="tdl"></td>
-    <td class="tdr"><span class="half">PAGE</span></td></tr>
-
-<tr><td class="tdr">I.</td>
-    <td class="tdl"><span class="smcap">Introductory</span></td>
-    <td class="tdr"><a href="#LECTURE_I">1</a></td></tr>
-
-<tr><td class="tdr">II.</td>
-    <td class="tdl"><span class="smcap">The Darwinian Theory</span></td>
-    <td class="tdr"><a href="#LECTURE_II">25</a></td></tr>
-
-<tr><td class="tdr">III.</td>
-    <td class="tdl"><span class="smcap">The Darwinian Theory</span> (<i>continued</i>)</td>
-    <td class="tdr"><a href="#LECTURE_III">42</a></td></tr>
-
-<tr><td class="tdrt">IV.</td>
-    <td class="tdl"> <span class="smcap">The Coloration of Animals and its relation to the<br />
-                     Processes of Selection</span></td>
-    <td class="tdrb"><a href="#LECTURE_IV">57</a></td></tr>
-
-<tr><td class="tdr">V.</td>
-    <td class="tdl"><span class="smcap">True Mimicry</span></td>
-    <td class="tdr"><a href="#LECTURE_V">91</a></td></tr>
-
-<tr><td class="tdr">VI.</td>
-    <td class="tdl"><span class="smcap">Protective Adaptations in Plants</span></td>
-    <td class="tdr"><a href="#LECTURE_VI">119</a></td></tr>
-
-<tr><td class="tdr">VII.</td>
-    <td class="tdl"><span class="smcap">Carnivorous Plants</span></td>
-    <td class="tdr"><a href="#LECTURE_VII">132</a></td></tr>
-
-<tr><td class="tdr">VIII.</td>
-    <td class="tdl"><span class="smcap">The Instincts of Animals</span></td>
-    <td class="tdr"><a href="#LECTURE_VIII">141</a></td></tr>
-
-<tr><td class="tdr">IX.</td>
-    <td class="tdl"><span class="smcap">Organic Partnerships or Symbiosis</span></td>
-    <td class="tdr"><a href="#LECTURE_IX">161</a></td></tr>
-
-<tr><td class="tdr">X.</td>
-    <td class="tdl"><span class="smcap">The Origin of Flowers</span></td>
-    <td class="tdr"><a href="#LECTURE_X">179</a></td></tr>
-
-<tr><td class="tdr">XI.</td>
-    <td class="tdl"><span class="smcap">Sexual Selection</span></td>
-    <td class="tdr"><a href="#LECTURE_XI">210</a></td></tr>
-
-<tr><td class="tdr">XII.</td>
-    <td class="tdl"><span class="smcap">Intra-Selection or Selection among Tissues</span></td>
-    <td class="tdr"><a href="#LECTURE_XII">240</a></td></tr>
-
-<tr><td class="tdr">XIII.</td>
-    <td class="tdl"><span class="smcap">Reproduction in Unicellular Organisms</span></td>
-    <td class="tdr"><a href="#LECTURE_XIII">253</a></td></tr>
-
-<tr><td class="tdr">XIV.</td>
-    <td class="tdl"><span class="smcap">Reproduction by Germ-cells</span></td>
-    <td class="tdr"><a href="#LECTURE_XIV">266</a></td></tr>
-
-<tr><td class="tdr">XV.</td>
-    <td class="tdl"><span class="smcap">The Process of Fertilization</span></td>
-    <td class="tdr"><a href="#LECTURE_XV">286</a></td></tr>
-
-<tr><td class="tdrt">XVI.</td>
-    <td class="tdl"><span class="smcap">Fertilization in Plants and Unicellular Organisms<br />
-                    and its immediate significance</span></td>
-    <td class="tdrb"><a href="#LECTURE_XVI">312</a></td></tr>
-
-<tr><td class="tdr">XVII.</td>
-    <td class="tdl"><span class="smcap">The Germ-plasm Theory</span></td>
-    <td class="tdr"><a href="#LECTURE_XVII">345</a></td></tr>
-
-<tr><td class="tdr">XVIII.</td>
-    <td class="tdl"><span class="smcap">The Germ-plasm Theory</span> (<i>continued</i>)</td>
-    <td class="tdr"><a href="#LECTURE_XVIII">373</a></td></tr>
-
-<tr><td class="tdr">XIX.</td>
-    <td class="tdl"><span class="smcap">The Germ-plasm Theory</span> (<i>continued</i>)</td>
-    <td class="tdr"><a href="#LECTURE_XIX">392</a></td></tr>
-</table>
-
-<hr class="full" />
-
-
-<p><span class="pagenum"><a id="Page_xiv"></a>[Pg xiv]</span></p>
-
-<div class="chapter">
-<p class="ph2">LIST OF ILLUSTRATIONS</p>
-</div>
-
-<table>
-
-<tr><td class="tdr"><span class="half">FIGURE</span></td>
-    <td class="tdl"></td>
-    <td class="tdr"><span class="half">PAGE</span></td></tr>
-
-<tr><td class="tdr">1.</td>
-    <td class="tdl">Group of various races of domestic pigeons</td>
-    <td class="tdr"><a href="#f1">35</a></td></tr>
-
-<tr><td class="tdr">2.</td>
-    <td class="tdl">Longitudinally striped caterpillar of a Satyrid</td>
-    <td class="tdr"><a href="#f2">67</a></td></tr>
-
-<tr><td class="tdr">3.</td>
-    <td class="tdl">Full-grown caterpillar of the Eyed Hawk-moth (<i>Smerinthus ocellatus</i>)</td>
-    <td class="tdr"><a href="#f3">67</a></td></tr>
-
-<tr><td class="tdrt">4.</td>
-    <td class="tdl">Full-grown caterpillar of the Elephant Hawk-moth (<i>Chærocampa
-                    elpenor</i>)</td>
-    <td class="tdrb"><a href="#f4">68</a></td></tr>
-
-<tr><td class="tdr">5.</td>
-    <td class="tdl">The Eyed Hawk-moth in its 'terrifying attitude'</td>
-    <td class="tdr"><a href="#f5">69</a></td></tr>
-
-<tr><td class="tdr">6.</td>
-    <td class="tdl">Under surface of the wings of <i>Caligo</i></td>
-    <td class="tdr"><a href="#f6">70</a></td></tr>
-
-<tr><td class="tdr">7.</td>
-    <td class="tdl">Caterpillar of a North American <i>Darapsa</i></td>
-    <td class="tdr"><a href="#f7">71</a></td></tr>
-
-<tr><td class="tdr">8.</td>
-    <td class="tdl">Caterpillar of the Buckthorn Hawk-moth (<i>Deilephila hippophaës</i>)</td>
-    <td class="tdr"><a href="#f8">73</a></td></tr>
-
-<tr><td class="tdr">9.</td>
-    <td class="tdl"><i>Hebomoja glaucippe</i>, from India; under surface</td>
-    <td class="tdr"><a href="#f9">76</a></td></tr>
-
-<tr><td class="tdr">10.</td>
-    <td class="tdl"><i>Xylina vetusta</i>, in flight and at rest</td>
-    <td class="tdr"><a href="#f10">77</a></td></tr>
-
-<tr><td class="tdr">11.</td>
-    <td class="tdl"><i>Tropidoderus childreni</i>, in flying pose</td>
-    <td class="tdr"><a href="#f11">79</a></td></tr>
-
-<tr><td class="tdr">12.</td>
-    <td class="tdl"><i>Notodonta camelina</i>, in flight and at rest</td>
-    <td class="tdr"><a href="#f12">80</a></td></tr>
-
-<tr><td class="tdr">13.</td>
-    <td class="tdl"><i>Kallima paralecta</i>, from India, right under side of the butterfly at
-                    rest</td>
-    <td class="tdr"><a href="#f13">83</a>, <a href="#f13a">357</a></td></tr>
-
-<tr><td class="tdr">14.</td>
-    <td class="tdl"><i>Cœnophlebia archidona</i>, from Bolivia, in its resting attitude</td>
-    <td class="tdr"><a href="#f14">85</a></td></tr>
-
-<tr><td class="tdr">15.</td>
-    <td class="tdl"><i>Cærois chorinæus</i>, from the lower Amazon, in its resting attitude</td>
-    <td class="tdr"><a href="#f15">86</a></td></tr>
-
-<tr><td class="tdr">16.</td>
-    <td class="tdl"><i>Phyllodes ornata</i>, from Assam</td>
-    <td class="tdr"><a href="#f16">87</a></td></tr>
-
-<tr><td class="tdr">17.</td>
-    <td class="tdl">Caterpillar of <i>Selenia tetralunaria</i>, seated on a birch twig</td>
-    <td class="tdr"><a href="#f17">90</a>, <a href="#f17a">360</a></td></tr>
-
-<tr><td class="tdrt">18.</td>
-    <td class="tdl">Upper surfaces of <i>Acræa egina</i>, <i>Papilio ridleyanus</i>, and <i>Pseudacræa
-                    boisduvalii</i></td>
-    <td class="tdrb"><a href="#f18">102</a></td></tr>
-
-<tr><td class="tdr">19.</td>
-    <td class="tdl">Barbed bristles of <i>Opuntia rafinesquii</i></td>
-    <td class="tdr"><a href="#f23">123</a></td></tr>
-
-<tr><td class="tdr">20.</td>
-    <td class="tdl">Vertical section through a piece of a leaf of the Stinging-nettle
-                   (<i>Urtica dioica</i>)</td>
-    <td class="tdr"><a href="#f24">123</a></td></tr>
-
-<tr><td class="tdr">21.</td>
-    <td class="tdl">A piece of a twig of Barberry (<i>Berberis vulgaris</i>)</td>
-    <td class="tdr"><a href="#f25">124</a></td></tr>
-
-<tr><td class="tdr">22.</td>
-    <td class="tdl">Tragacanth (<i>Astragalus tragacantha</i>)</td>
-    <td class="tdr"><a href="#f26">125</a></td></tr>
-
-<tr><td class="tdr">23.</td>
-    <td class="tdl">Bladderwort (<i>Utricularia grafiana</i>)</td>
-    <td class="tdr"><a href="#f27">133</a></td></tr>
-
-<tr><td class="tdr">24.</td>
-    <td class="tdl">Pitcher of <i>Nepenthes villosa</i></td>
-    <td class="tdr"><a href="#f28">134</a></td></tr>
-
-<tr><td class="tdr">25.</td>
-    <td class="tdl">Butterwort (<i>Pinguicula vulgaris</i>)</td>
-    <td class="tdr"><a href="#f29">136</a></td></tr>
-
-<tr><td class="tdr">26.</td>
-    <td class="tdl">The Sundew (<i>Drosera rotundifolia</i>)</td>
-    <td class="tdr"><a href="#f30">137</a></td></tr>
-
-<tr><td class="tdr">27.</td>
-    <td class="tdl">A leaf of the Sundew</td>
-    <td class="tdr"><a href="#f31">137</a></td></tr>
-
-<tr><td class="tdr">28.</td>
-    <td class="tdl">Leaf of Venus Fly-trap</td>
-    <td class="tdr"><a href="#f32">138</a></td></tr>
-
-<tr><td class="tdr">29.</td>
-    <td class="tdl"><i>Aldrovandia vesiculosa</i></td>
-    <td class="tdr"><a href="#f33">138</a></td></tr>
-
-<tr><td class="tdr">30.</td>
-    <td class="tdl"><i>Aldrovandia</i>, its trap apparatus</td>
-    <td class="tdr"><a href="#f34">139</a></td></tr>
-
-<tr><td class="tdr">31.</td>
-    <td class="tdl">Sea-cucumber (<i>Cucumaria</i>)</td>
-    <td class="tdr"><a href="#f35">148</a></td></tr>
-
-<tr><td class="tdr">32.</td>
-    <td class="tdl">Metamorphosis of <i>Sitaris humeralis</i>, an oil-beetle</td>
-    <td class="tdr"><a href="#f36">150</a></td></tr>
-
-<tr><td class="tdr">33.</td>
-    <td class="tdl">Cocoon of the Emperor Moth (<i>Saturnia carpini</i>)</td>
-    <td class="tdr"><a href="#f37">158</a></td></tr>
-
-<tr><td class="tdr">34.</td>
-    <td class="tdl">Hermit-crab</td>
-    <td class="tdr"><a href="#f38">163</a></td></tr>
-
-<tr><td class="tdr">35.</td>
-    <td class="tdl"><i>Hydra viridis</i>, the Green Freshwater Polyp</td>
-    <td class="tdr"><a href="#f39">169</a></td></tr>
-
-<tr><td class="tdr">36.</td>
-    <td class="tdl"><i>Amœba viridis</i></td>
-    <td class="tdr"><a href="#f40">170</a></td></tr>
-
-<tr><td class="tdr">37.</td>
-    <td class="tdl">Twig of an Imbauba-tree, showing hair cushions</td>
-    <td class="tdr"><a href="#f41">172</a></td></tr>
-
-
-<tr><td class="tdr">38.<span class="pagenum"><a id="Page_xv"></a>[Pg xv]</span></td>
-    <td class="tdl">A fragment of a Lichen</td>
-    <td class="tdr"><a href="#f42">173</a></td></tr>
-
-<tr><td class="tdr">39.</td>
-    <td class="tdl">A fragment of a Silver Poplar root</td>
-    <td class="tdr"><a href="#f43">176</a></td></tr>
-
-<tr><td class="tdr">40.</td>
-    <td class="tdl"><i>Potentilla verna</i></td>
-    <td class="tdr"><a href="#f44">181</a></td></tr>
-
-<tr><td class="tdr">41.</td>
-    <td class="tdl">Flower of Meadow Sage</td>
-    <td class="tdr"><a href="#f45">183</a></td></tr>
-
-<tr><td class="tdr">42.</td>
-    <td class="tdl">Alpine Lousewort (<i>Pedicularis asplenifolia</i>)</td>
-    <td class="tdr"><a href="#f46">184</a></td></tr>
-
-<tr><td class="tdr">43.</td>
-    <td class="tdl">Flower of Birthwort (<i>Aristolochia clematitis</i>)</td>
-    <td class="tdr"><a href="#f47">185</a></td></tr>
-
-<tr><td class="tdr">44.</td>
-    <td class="tdl">Alpine Butterwort (<i>Pinguicula alpina</i>)</td>
-    <td class="tdr"><a href="#f48">185</a></td></tr>
-
-<tr><td class="tdr">45.</td>
-    <td class="tdl"><i>Daphne mezereum</i> and <i>Daphne striata</i></td>
-    <td class="tdr"><a href="#f49">187</a></td></tr>
-
-<tr><td class="tdr">46.</td>
-    <td class="tdl">Common Orchis (<i>Orchis mascula</i>)</td>
-    <td class="tdr"><a href="#f50">188</a></td></tr>
-
-<tr><td class="tdr">47.</td>
-    <td class="tdl">Head of a Butterfly</td>
-    <td class="tdr"><a href="#f51">190</a></td></tr>
-
-<tr><td class="tdr">48.</td>
-    <td class="tdl">Mouth-parts of the Cockroach</td>
-    <td class="tdr"><a href="#f52">191</a></td></tr>
-
-<tr><td class="tdr">49.</td>
-    <td class="tdl">Head of the Bee</td>
-    <td class="tdr"><a href="#f53">192</a></td></tr>
-
-<tr><td class="tdr">50.</td>
-    <td class="tdl">Flowers of the Willow</td>
-    <td class="tdr"><a href="#f54">194</a></td></tr>
-
-<tr><td class="tdr">51.</td>
-    <td class="tdl">The Yucca-moth (<i>Pronuba yuccasella</i>)</td>
-    <td class="tdr"><a href="#f55">201</a></td></tr>
-
-<tr><td class="tdr">52.</td>
-    <td class="tdl">The fertilization of the Yucca</td>
-    <td class="tdr"><a href="#f56">202</a></td></tr>
-
-<tr><td class="tdr">53.</td>
-    <td class="tdl">Scent-scales of diurnal Butterflies</td>
-    <td class="tdr"><a href="#f57">217</a></td></tr>
-
-<tr><td class="tdr">54.</td>
-    <td class="tdl">A portion of the upper surface of the wing of a male 'blue' (<i>Lycæna
-                    menalcas</i>)</td>
-    <td class="tdr"><a href="#f58">218</a></td></tr>
-
-<tr><td class="tdr">55.</td>
-    <td class="tdl"><i>Zeuxidia wallacei</i>, male</td>
-    <td class="tdr"><a href="#f59">218</a></td></tr>
-
-<tr><td class="tdr">56.</td>
-    <td class="tdl"><i>Leptodora hyalina</i></td>
-    <td class="tdr"><a href="#f60">224</a></td></tr>
-
-<tr><td class="tdr">57.</td>
-    <td class="tdl"><i>Moina paradoxa</i>, male</td>
-    <td class="tdr"><a href="#f61">225</a></td></tr>
-
-<tr><td class="tdr">58.</td>
-    <td class="tdl"><i>Moina paradoxa</i>, female</td>
-    <td class="tdr"><a href="#f62">226</a></td></tr>
-
-<tr><td class="tdr">59.</td>
-    <td class="tdl">An Amœba: the process of division</td>
-    <td class="tdr"><a href="#f63">253</a></td></tr>
-
-<tr><td class="tdr">60.</td>
-    <td class="tdl"><i>Stentor rœselii</i>, trumpet-animalcule</td>
-    <td class="tdr"><a href="#f64">254</a></td></tr>
-
-<tr><td class="tdr">61.</td>
-    <td class="tdl"><i>Holophrya multifiliis</i></td>
-    <td class="tdr"><a href="#f65">256</a></td></tr>
-
-<tr><td class="tdr">62.</td>
-    <td class="tdl"><i>Pandorina morum</i></td>
-    <td class="tdr"><a href="#f66">257</a></td></tr>
-
-<tr><td class="tdr">63.</td>
-    <td class="tdl"><i>Volvox aureus</i></td>
-    <td class="tdr"><a href="#f67">270</a></td></tr>
-
-<tr><td class="tdr">64.</td>
-    <td class="tdl"><i>Fucus platycarpus</i>, brown sea-wrack</td>
-    <td class="tdr"><a href="#f68">272</a></td></tr>
-
-<tr><td class="tdr">65.</td>
-    <td class="tdl">Copulation in a Daphnid (Lyncæid)</td>
-    <td class="tdr"><a href="#f69">276</a></td></tr>
-
-<tr><td class="tdr">66.</td>
-    <td class="tdl">Spermatozoa of various Daphnids</td>
-    <td class="tdr"><a href="#f70">277</a></td></tr>
-
-<tr><td class="tdr">67.</td>
-    <td class="tdl">Spermatozoa of various animals</td>
-    <td class="tdr"><a href="#f71">278</a></td></tr>
-
-<tr><td class="tdr">68.</td>
-    <td class="tdl">Diagram of a spermatozoon</td>
-    <td class="tdr"><a href="#f72">279</a>,&nbsp;<a href="#f72a">338</a></td></tr>
-
-<tr><td class="tdr">69.</td>
-    <td class="tdl">Ovum of the Sea-urchin</td>
-    <td class="tdr"><a href="#f73">281</a>,&nbsp;<a href="#f73a">338</a></td></tr>
-
-<tr><td class="tdr">70.</td>
-    <td class="tdl"><i>Daphnella</i></td>
-    <td class="tdr"><a href="#f74">283</a></td></tr>
-
-<tr><td class="tdr">71.</td>
-    <td class="tdl"><i>Bythotrephes longimanus</i></td>
-    <td class="tdr"><a href="#f75">283</a></td></tr>
-
-<tr><td class="tdr">72.</td>
-    <td class="tdl"><i>Sida crystallina</i>, a Daphnid</td>
-    <td class="tdr"><a href="#f76">284</a></td></tr>
-
-<tr><td class="tdr">73.</td>
-    <td class="tdl">Diagrammatic longitudinal section of a hen's egg before incubation</td>
-    <td class="tdr"><a href="#f77">285</a></td></tr>
-
-<tr><td class="tdr">74.</td>
-    <td class="tdl">Diagram of nuclear division</td>
-    <td class="tdr"><a href="#f78">288</a></td></tr>
-
-<tr><td class="tdr">75.</td>
-    <td class="tdl">Process of fertilization in <i>Ascaris megalocephala</i></td>
-    <td class="tdr"><a href="#f79">296</a></td></tr>
-
-<tr><td class="tdr">76.</td>
-    <td class="tdl">Diagram of the maturation divisions of the ovum</td>
-    <td class="tdr"><a href="#f80">299</a></td></tr>
-
-<tr><td class="tdr">77.</td>
-    <td class="tdl">Diagram of the maturation divisions of the sperm-cell</td>
-    <td class="tdr"><a href="#f81">301</a></td></tr>
-
-<tr><td class="tdr">78.</td>
-    <td class="tdl">Diagram of the maturation of a parthenogenetic ovum</td>
-    <td class="tdr"><a href="#f82">305</a></td></tr>
-
-<tr><td class="tdr">79.</td>
-    <td class="tdl">The two maturation divisions of the 'drone eggs' of the Bee</td>
-    <td class="tdr"><a href="#f83">307</a>,&nbsp;<a href="#f83a">337</a></td></tr>
-
-<tr><td class="tdr">80.</td>
-    <td class="tdl">Fertilization of the ovum of a Gasteropod</td>
-    <td class="tdr"><a href="#f84">310</a></td></tr>
-
-<tr><td class="tdr">81.</td>
-    <td class="tdl">Formation of polar bodies in a Lichen</td>
-    <td class="tdr"><a href="#f85">313</a></td></tr>
-
-<tr><td class="tdr">82.</td>
-    <td class="tdl">Fertilization in the Lily</td>
-    <td class="tdr"><a href="#f86">314</a></td></tr>
-
-<tr><td class="tdr">83.</td>
-    <td class="tdl">Conjugation of Noctiluca</td>
-    <td class="tdr"><a href="#f87">317</a></td></tr>
-
-<tr><td class="tdr">84.</td>
-    <td class="tdl">Conjugation and polar body formation in the Sun-animalcule</td>
-    <td class="tdr"><a href="#f88">319</a></td></tr>
-
-<tr><td class="tdr">85.<span class="pagenum"><a id="Page_xvi"></a>[Pg xvi]</span></td>
-    <td class="tdl">Diagram of the conjugation of an Infusorian</td>
-    <td class="tdr"><a href="#f89">321</a></td></tr>
-
-<tr><td class="tdr">86.</td>
-    <td class="tdl">Conjugation of an Infusorian</td>
-    <td class="tdr"><a href="#f90">323</a></td></tr>
-
-<tr><td class="tdr">87.</td>
-    <td class="tdl">Diagram to illustrate the operation of amphimixis</td>
-    <td class="tdr"><a href="#f91">348</a></td></tr>
-
-<tr><td class="tdr">88.</td>
-    <td class="tdl">Sperm-mother-cells (spermatocytes) of the Salamander</td>
-    <td class="tdr"><a href="#f92">350</a></td></tr>
-
-<tr><td class="tdr">89.</td>
-    <td class="tdl">Anterior region of the larva of a Midge</td>
-    <td class="tdr"><a href="#f93">364</a>,&nbsp;<a href="#f93a">393</a></td></tr>
-
-<tr><td class="tdr">90.</td>
-    <td class="tdl">The Common Shore-Crab, seen from below</td>
-    <td class="tdr"><a href="#f94">367</a></td></tr>
-
-<tr><td class="tdr">91.</td>
-    <td class="tdl">Hind leg of a Locustid</td>
-    <td class="tdr"><a href="#f95">371</a></td></tr>
-
-<tr><td class="tdr">92.</td>
-    <td class="tdl">Echinoderm-larvæ</td>
-    <td class="tdr"><a href="#f96">387</a></td></tr>
-
-<tr><td class="tdr">93.</td>
-    <td class="tdl">Development of a limb in the pupa of a Fly</td>
-    <td class="tdr"><a href="#f97">395</a></td></tr>
-
-<tr><td class="tdrt">94.</td>
-    <td class="tdl">Diagram to illustrate the phylogenetic shifting back of the origins
-                    of the germ-cells in medusoids and hydroids</td>
-    <td class="tdrb"><a href="#f98">412</a></td></tr>
-
-<tr><td class="tdr">95.</td>
-    <td class="tdl">Diagram to illustrate the migration of the germ-cells in Hydromedusæ</td>
-    <td class="tdr"><a href="#f99">414</a></td></tr>
-
-</table>
-
-<hr class="full" />
-
-<p class="ph2">COLOURED PLATES</p>
-
-<p class="c">
-SOME MIMETIC BUTTERFLIES AND THEIR IMMUNE MODELS</p>
-
-
-<table class="autotable" summary="">
-<tr>
-<td class="tdl"><span class="smcap">Plate I</span></td>
-<td class="tdr"></td>
-<td class="tdr"><i>to face page</i></td>
-<td class="tdr"><a href="#f19">112</a></td>
-</tr>
-<tr>
-<td class="tdl"><span class="smcap">Plate II</span></td>
-<td class="tdr"></td>
-<td class="tdc">" "</td>
-<td class="tdr"><a href="#f21">114</a></td>
-</tr>
-<tr>
-<td class="tdl"><span class="smcap">Plate III</span></td>
-<td class="tdr"></td>
-<td class="tdc">" "</td>
-<td class="tdr"><a href="#f22">116</a></td>
-</tr>
-
-
-</table>
-
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_1"></a>[Pg 1]</span></p>
-
-<h2 class="nobreak" id="LECTURE_I">LECTURE I</h2>
-</div>
-
-<p class="c">INTRODUCTORY</p>
-
-
-<p><span class="smcap">Every</span> one knows in a general way what is meant by the doctrine
-of descent&mdash;that it is the theory which maintains that the forms of
-life, animals and plants, which we see on our earth to-day, have not
-been the same from all time, but have been developed, by a process of
-transformation, from others of an earlier age, and are in fact descended
-from ancestors specifically different. According to this doctrine of
-descent, the whole diversity of animals and plants owes its origin to
-a transformation process, in the course of which the earliest inhabitants
-of our earth, extremely simple forms of life, were in part
-evolved in the course of time into forms of continually increasing
-complexity of structure and efficiency of function, somewhat in the
-same way as we can see every day, when any higher animal is
-developed from a single cell, the egg-cell, not suddenly or directly,
-but connected with its origin by a long series of ever more complex
-transformation stages, each of which is the preparation for, and leads on
-to the succeeding one. The theory of descent is thus a theory of
-development or evolution. It does not merely, as earlier science did,
-take for granted and describe existing forms of life, but regards them
-as having become what they are through a process of evolution, and
-it seeks to investigate the stages of this process, and to discover the
-impelling forces that lie behind it. Briefly, the theory of descent is
-an attempt at a scientific interpretation of the origin and diversity of
-the animate world.</p>
-
-<p>In these lectures, therefore, we have not merely to show on what
-grounds we make this postulate of an evolution process, and to
-marshall the facts which necessitate it; we must also try to penetrate
-as far as possible towards the causes which bring about such transformations.
-For this reason we are forced to go beyond the limits
-of the theory of descent in the narrow sense, and to deal with
-the general processes of life itself, especially with reproduction and
-the closely associated problem of heredity. The transformation of
-species can only be interpreted in one of two ways; either it depends
-on a peculiar internal force, which is usually only latent in the
-organism, but from time to time becomes active, and then, to a<span class="pagenum"><a id="Page_2"></a>[Pg 2]</span>
-certain extent, moulds it into new forms; or it depends on the
-continually operating forces which make up life, and on the way in
-which these are influenced by changing external conditions. Which
-of these alternatives is correct we can only undertake to determine
-when we know the phenomena of life, and as far as possible their
-causes, so that it is indispensable to make ourselves acquainted with
-these as far as we can.</p>
-
-<p>When we look at one of the lowest forms of life, such as an
-Amœba or a single-celled Alga, and reflect that, according to the theory
-of evolution, the whole realm of creation as we see it now, with Man
-at its head, has evolved from similar or perhaps even smaller and
-simpler organisms, it seems at first sight a monstrous assumption, and
-one which quite contradicts our simplest and most certain observations.
-For what is more certain than that the animals and plants
-around us remain the same, as long as we can observe them, not
-through the lifetime of an individual only, but through centuries, and
-in the case of many species, for several thousand years?</p>
-
-<p>This being so, it is intelligible enough that the doctrine of
-evolution, on its first emergence at the end of the eighteenth century,
-was received with violent opposition, not on the part of the laity only,
-but by the majority of scientific minds, and instead of being followed
-up, was at first opposed, then neglected, and finally totally forgotten,
-to spring up anew in our own day. But even then a host of
-antagonists ranged themselves against the doctrine, and, not content
-with loftily ignoring it, made it the subject of the most violent and
-varied attacks.</p>
-
-<p>This was the state of affairs when, in 1858, Darwin's book on
-<i>The Origin of Species</i> appeared, and hoisted the flag of evolution
-afresh. The struggle that ensued may now be regarded as at an end,
-at least as far as we are concerned&mdash;that is, in the domain of science.
-The doctrine of descent has gained the day, and we can confidently
-say that the Evolution theory has become a permanent possession of
-science that can never again be taken away. It forms the foundation
-of all our theories of the organic world, and all further progress must
-start from this basis.</p>
-
-<p>In the course of these lectures, we shall find at every step fresh
-evidence of the truth of this assertion, which may at first seem all too
-bold. It is not by any means to be supposed that the whole question
-in regard to the transformation of organisms and the succession of
-new forms of life has been answered in full, or that we have now
-been fortunate enough to solve the riddle of life itself. No! whether
-we ever reach that goal or not, we are a long way from it as yet, and<span class="pagenum"><a id="Page_3"></a>[Pg 3]</span>
-even the much easier problem, how and by what forces the evolution
-of the living world has proceeded from a given beginning, is far from
-being finally settled; antagonistic views are still in conflict, and there
-is no arbitrator whose authoritative word can decide which is right.
-The <i>How?</i> of evolution is still doubtful, but not the <i>fact</i>, and this
-is the secure foundation on which we stand to-day: The world of
-life, as we know it, has been evolved, and did not originate all at
-once.</p>
-
-<p>Were I to try to give, in advance, even an approximate idea of the
-confidence with which we can take our stand on this foundation,
-I should be almost embarrassed by the wealth of facts on which
-I might draw. It is hardly possible nowadays to open a book on the
-minute or general structural relations, or on the development of any
-animal whatever, without finding in it evidences in favour of the
-Evolution theory, that is to say, facts which can only be understood
-on the assumption of the evolution of the organic world. This, too,
-without taking into account at all the continually increasing number
-of facts Palæontology is bringing to light, placing before our eyes the
-forms which the Evolution theory postulates as the ancestors of the
-organic world of to-day: birds with teeth in their bills, reptile-like
-forms clothed with feathers, and numerous other long-extinct forms
-of life, which, covered up by the mud of earlier waters, and preserved
-as 'fossils' in the later sedimentary rocks, tell us plainly how the
-earlier world of animals and plants was constituted. Later, we shall
-see that the geographical distribution of plant and animal species of
-the present day can only be understood in the light of the Evolution
-theory. But meantime, before we go into details, what may justify
-my assumption is the fact that the Evolution theory enables us to
-predict.</p>
-
-<p>Let us take only a few examples. The skeleton of the wrist in
-all vertebrate animals above Fishes consists of two rows of small
-bones, on the outer of which are placed the five bones of the palm,
-corresponding to the five fingers. The outer row is curved, and there
-is thus a space between the two rows, which, in Amphibians and
-Reptiles, is filled by a special small bone. This 'os centrale' is
-absent in many Mammals, notably, for instance, in Man, and the
-space between the two rows is filled up by an enlargement of one
-of the other bones. Now if Mammals be descended from the lower
-vertebrates, as the theory of descent assumes, we should expect to
-find the 'os centrale' even in Man in young stages, and, after many
-unsuccessful attempts, Rosenberg has at last been able to demonstrate
-it at a very early stage of embryonic development.</p>
-
-<p><span class="pagenum"><a id="Page_4"></a>[Pg 4]</span></p>
-
-<p>This prediction, with another to be explained later, is based upon
-the experience that the development of an individual animal follows,
-in a general way, the same course as the racial evolution of the
-species, so that structures of the ancestors of a species, even if they
-are not found in the fully developed animal, may occur in one of its
-earlier embryonic stages. Further on, we shall come to know this
-fact more intimately as a 'biogenetic law,' and it alone would be
-almost enough to justify the theory of evolution. Thus, for instance,
-the lowest vertebrates, the Fishes, breathe by means of gills, and these
-breathing organs are supported by four or more gill-arches, between
-which spaces, the gill-slits, remain open for the passage of water.
-Although Reptiles, Birds, and Mammals breathe by lungs, and at no
-time of their life by gills, yet, in their earliest youth, that is, during
-their early development in the egg, they possess these gill-arches and
-gill-slits, which subsequently disappear, or are transformed into other
-structures.</p>
-
-<p>On the strength of this 'biogenetic law' it could also be predicted
-that Man, in whom, as is well known, there are twelve pairs of ribs,
-would, in his earliest youth, possess a thirteenth pair, for the lower
-Mammals have more numerous ribs, and even our nearest relatives,
-the anthropoid Apes, the gorilla and chimpanzee, have a thirteenth
-rib, though a very small one, and the siamang has even a fourteenth.
-This prediction also has been verified by the examination of young
-human embryos, in which a small thirteenth rib is present, though it
-rapidly disappears.</p>
-
-<p>During the seventies I was engaged in investigating the development
-of the curious marking which adorns the long body of many
-of our caterpillars. I studied in particular the caterpillars of our
-Sphingidæ or hawk-moths, and found, by a comparison of the various
-stages of development from the emergence of the caterpillar from the
-egg on to its full growth, that there is a definite succession of
-different kinds of markings following each other, in a whole range
-of species, in a similar manner. From the standpoint of the
-Evolution theory, I concluded that the markings of the youngest
-caterpillars, simple longitudinal stripes, must have been those of the
-most remote ancestors of our present species, while those of the later
-stages, oblique stripes, were those of ancestors of a later date.</p>
-
-<p>If this were the case, then all the species of caterpillar which
-now exhibit oblique stripes in their full-grown stage must have had
-longitudinal stripes in their youngest stages, and because of this
-succession of markings in the individual development, I was able to
-predict that the then unknown young form of the caterpillar of our<span class="pagenum"><a id="Page_5"></a>[Pg 5]</span>
-privet hawk-moth (<i>Sphinx ligustri</i>) must have a white line along each
-side of the back. Ten years later, the English zoologist, Poulton,
-succeeded in rearing the eggs of <i>Sphinx ligustri</i>, and it was then
-demonstrated that the young caterpillar actually possessed the postulated
-white lines.</p>
-
-<p>Such predictions undoubtedly give the hypothesis on which
-they are based, the Evolution theory, a high degree of certainty, and
-are almost comparable to the prediction of the discovery of the
-planet Neptune by Leverrier. As is well known, this, the most
-distant of all the planets, whose period of revolution round the sun is
-almost 165 of our years, would probably never have been recognized
-as a planet, had not Adams, an astronomer at the Greenwich Observatory,
-and afterwards Leverrier, deduced its presence from slight
-disturbances in the path of Jupiter's moons, and indicated the spot
-where an unknown planet must be looked for. Immediately all
-telescopes were directed towards the spot indicated, and Galle, at the
-Berlin Observatory, found the sought-for planet.</p>
-
-<p>We might with justice regard as lacking in discernment those
-who, in the face of such experiences, still doubt that the earth
-revolves round the sun, and we might fairly say the same of any one
-who, in the face of the known facts, would dispute the truth of the
-Evolution theory. It is the only basis on which an understanding of
-these facts is possible, just as the Kant-Laplace theory of the solar
-system is the only basis on which an adequate interpretation of the
-facts of the heavens can be arrived at.</p>
-
-<p>To this comparison of the two theories it has been objected that
-the Evolution theory has far less validity than the other, first, because
-it can never be mathematically demonstrated, and secondly, because
-at the best it can only interpret the transformations of the animate
-world, and not its origin. Both objections are just: the phenomena
-of life are in their nature much too intricate for mathematics to deal
-with, except with extreme diffidence; and the question of the origin
-of life is a problem which will probably have to wait long for
-solution. So, if it gives pleasure to any one to regard the one
-theory as having more validity than the other, no one can object; but
-there is no particular advantage to be gained by doing so. In any
-case, the Evolution theory shares the disadvantage of not being able
-to explain everything in its own province with the Kant-Laplace
-cosmogony, for that, too, must presuppose the first beginning, the
-rotating nebula.</p>
-
-<p>Although I regard the doctrine of descent as proved, and
-hold it to be one of the greatest acquisitions of human knowledge,<span class="pagenum"><a id="Page_6"></a>[Pg 6]</span>
-I must repeat that I do not mean to say that everything is clear
-in regard to the evolution of the living world. On the contrary,
-I believe that we still stand merely on the threshold of investigation,
-and that our insight into the mighty process of evolution, which has
-brought about the endless diversity of life upon our earth, is still
-very incomplete in relation to what may yet be found out, and that,
-instead of being vainglorious, our attitude should be one of modesty.
-We may well rejoice over the great step forward which the dominant
-recognition of the Evolution theory implies, but we must confess that
-the beginnings of life are as little clear to us as those of the solar
-system. But we can do this at least: we can refer the innumerable
-and wonderful inter-relations of the organic cosmos to their causes&mdash;common
-descent and adaptation&mdash;and we can try to discover the ways
-and means which have co-operated to bring the organic world to
-the state in which we know it.</p>
-
-<p>When I say that the theory of descent is the most progressive
-step that has yet been taken in the development of human knowledge,
-I am bound to give my reasons for this opinion. It is justified,
-it seems to me, even by this fact alone, that the Evolution idea is not
-merely a new light on the special region of biological science, zoology
-and botany, but is of quite general importance. The conception of
-an evolution of the world of life upon the earth reaches far beyond
-the bounds of any single science, and influences our whole realm of
-thought. It means nothing less than the elimination of the miraculous
-from our knowledge of nature, and the placing of the phenomena
-of life on the same plane as the other natural processes, that is, as
-having been brought about by the same forces, and being subject
-to the same laws. In the domain of the inorganic, no one now
-doubts that out of nothing nothing can come: energy and matter
-are from everlasting to everlasting, they can neither be increased or
-decreased, they can only be transformed&mdash;heat into mechanical
-energy, into light, into electricity, and so on. For us moderns, the
-lightning is no longer hurled by the Thunderer Zeus on the head
-of the wicked, but, careless alike of merit or guilt, it strikes where
-the electric tension finds the easiest and shortest line of discharge.
-Thus to our mode of thought it now seems clear that no event in the
-world of the living depends upon caprice, that at no time have organisms
-been called forth out of nothing by the mighty word of a Creator,
-but they have been produced at all times by the co-operation of the
-existing forces of nature, and every species must have arisen just
-where, and when, and in the form in which it actually did arise, as
-the necessary outcome of the existing conditions of energy and<span class="pagenum"><a id="Page_7"></a>[Pg 7]</span>
-matter, and of their interactions upon each other. It is this
-correlation of animate nature with natural forces and natural laws
-which gives to the doctrine of evolution its most general importance.
-For it thus supplies the keystone in the arch of our interpretation of
-nature and gives it unity; for the first time it makes it possible to
-form a conception of a world-mechanism, in which each stage is the
-result of the one before it, and the cause of the succeeding one.</p>
-
-<p>How deeply all our earlier opinions are affected by this doctrine
-will become clear if we fix our attention on a single point, the
-derivation of the human understanding from that of animal ancestors.
-What of the reason of Man, of his morals, of his freedom of will? may
-be asked, as it has been, and still is often asked. What has been
-regarded as absolutely distinct from the nature of animals is said to
-differ from their mental activities only in degree, and to have evolved
-from them. The mind of a Kant, of a Laplace, of a Darwin&mdash;or to
-ascend into the plane of the highest and finest emotional life, the
-genius of a Raphael or a Mozart&mdash;to have any real connexion, however
-far back, with the lowly psychical life of an animal! That is
-contrary to all our traditionary, we might say our inborn, ideas, and it
-is not to be wondered at that the laity, and especially the more cultured
-among them, should have opposed such a doctrine whose dominating
-power was unintelligible to them, because they were ignorant of
-the facts on which it rests. That a man should feel his dignity
-lowered by the idea of descent from animals is almost comical to the
-naturalist, for he knows that every one of us, in his first beginning,
-occupied a much lowlier position than that of our mammalian
-ancestors&mdash;was, in fact, as regards visible structure, on a level with
-the Amœba, that microscopically minute unicellular animal, which
-can hardly be said to possess organs, and whose psychical activities
-are limited to recognizing and engulfing its food. Very gradually at
-first, and step by step, there develop from this single cell, the ovum,
-more and more numerous cells; this mass of cells segregates into
-different groups, which differentiate further and further, until at last
-they form the perfect man. This occurs in the development of every
-human being, and we are merely unaccustomed to the thought that it
-means nothing else than an incredibly rapid ascent of the organism
-from a very low level of life to the highest.</p>
-
-<p>Still less is it to be wondered at that the Evolution doctrine
-met with violent opposition on the part of the representatives of
-religion, for it stood in open contradiction to that remarkable and
-venerable cosmogony, the Mosaic story of Creation, which people had
-been accustomed to regard, not as what it is&mdash;a conception of nature<span class="pagenum"><a id="Page_8"></a>[Pg 8]</span>
-at an early stage of human culture&mdash;but as an inalienable part of our
-own religion. But investigation shows us that the doctrine of
-evolution is true, and it is only a weak religion which is incapable
-of adapting itself to the truth, retaining what is essential, and letting
-go what is unessential and subject to change with the development of
-the human mind. Even the heliocentric hypothesis was in its day
-declared false by the Church, and Galilei was forced to retract; but
-the earth continued to revolve round the sun, and nowadays any one
-who doubted it would be considered mentally weak or warped. So
-in all likelihood the time is not far distant when the champions of
-religion will abandon their profitless struggle against the new truth,
-and will see that the recognition of a law-governed evolution of the
-organic world is no more prejudicial to true religion than is the
-revolution of the earth round the sun.</p>
-
-<hr class="tb" />
-
-<p>Having given this very general orientation of the Evolution
-problem, which is to engage our attention in detail, I shall approach
-the problem itself by the historical method, for I do not wish to bring
-the views of present-day science quite suddenly and directly into
-prominence. I would rather seek first to illustrate how earlier
-generations have tried to solve the question of the origin of the
-living world. We shall see that few attempts at solution were made
-until quite recently, that is, until the end of the eighteenth and the
-beginning of the nineteenth century. Only then there appeared a
-few gifted naturalists with evolutionist ideas, but these ideas did not
-penetrate far; and it was not till after the middle of the nineteenth
-century that they found a new champion, who was to make them
-common property and a permanent possession of science. It was
-the teaching of Charles Darwin that brought about this thorough
-awakening, and laid the foundations of our present interpretations,
-and his work will therefore engross our attention for a number of
-lectures. Only after we have made ourselves acquainted with his
-teaching shall we try to test its foundations, and to see how far this
-splendid structure stands on a secure basis of fact, and how deeply its
-power of interpretation penetrates towards the roots of phenomena.
-We shall examine the forces by which organisms are dominated, and
-the phenomena produced, and thereby test Darwin's principles of
-interpretation, in part rejecting them, in part accepting them, though
-in a much extended form, and thus try to give the whole theoretic
-structure a more secure foundation. I hope to be able to show
-that we have made some real progress since Darwin's day, that<span class="pagenum"><a id="Page_9"></a>[Pg 9]</span>
-deductions have been drawn from his theory which even he did not
-dream of, which have thrown fresh light on a vast range of phenomena,
-and, finally, that through the more extended use of his own
-principles, the Evolution theory has gained a completeness, and
-an intrinsic harmony which it previously lacked.</p>
-
-<p>This at least is my own opinion, but I cannot ignore the fact
-that it is by no means shared by all living naturalists. The obvious
-gaps and insufficiencies of the Darwinian theory have in the last few
-decennia prompted all sorts of attempts at improving it. Some of
-these were lost sight of almost as soon as they were suggested, but
-others have held their own, and can still claim numerous supporters.
-It would only tend to bewilder if I gave an account of those of the
-former class, but those which still hold their own must be noticed
-in these lectures, though it is by no means my intention to expound
-the confused mass of opinions which has gathered round the doctrine
-of evolution, but rather to give a presentation of the theory as it
-has gradually grown up in my own mind in the course of the last
-four decades. Even this will not be the last of which science will
-take knowledge, but it will, I hope, at least be one which can be
-further built upon.</p>
-
-<p>Let us, then, begin at once with that earliest forerunner of the
-modern theory of descent, the gifted Greek philosopher Empedocles,
-who, equally important as a leader of the state of Agrigentum, and as
-a thinker in purely theoretical regions of thought, advanced very
-notable views regarding the origin of organisms. We must, however,
-be prepared to hear something that is hardly a theory in the modern
-scientific acceptation of that term; and we must not be repelled by
-the unbridled poetical fancy of the speculative philosopher; we have
-to recognize that there is a sound kernel contained in his amusing
-pictures&mdash;a thought which we meet with later, in much more concrete
-form, in the Darwinian theory, and which, if I mistake not, we shall
-keep firm hold of in all time to come.</p>
-
-<p>According to Empedocles the world was formed by the four
-elements of the ancients, Earth, Water, Fire, and Air, moved and
-guided by two fundamental forces, Hate and Love, or, as we should
-now say, Repulsion and Attraction. Through the chance play of
-these two forces with the elements, there arose first the plants, then
-the animals, in such a manner that at first only parts and organs
-of animals were formed: single eyes without faces, arms without
-bodies, and so on. Then, in wild play, Nature attempted to put
-together these separate parts, and so created all manner of combinations,
-for the most part inept monsters unfit for life, but in a few<span class="pagenum"><a id="Page_10"></a>[Pg 10]</span>
-cases, where the parts fitted, there resulted a creature capable not
-only of life, but, if the juxtaposition was perfect, even of reproduction.</p>
-
-<p>This phantastic picture of creation seems to us mad enough, but
-there slumbers in it, all unsuspected though it may have been by the
-author, the true idea of selection, the idea that much that is unfit
-certainly arises, but that only the fit endures. The mechanical
-coming-to-be of the fit is the sound kernel in this wondersome
-doctrine.</p>
-
-<p>The natural science of the ancients, in regard to life and its
-forms, reached its climax in Aristotle (died 322 <span class="allsmcap">B. C.</span>). A true polyhistorian,
-his writings comprehended all the knowledge of his time,
-but he also added much to it from his own observation. In his
-writings we find many good observations on the structure and habits
-of a number of organisms, and he also had the merit of being the
-first to attempt a systematic grouping of animals. With true insight,
-he grouped all the vertebrates together as Enaimata or animals with
-blood, and classed all the rest together as Anaimata or bloodless
-animals. That he denied to the latter group the possession of blood
-is not to be wondered at, when we take into account the extremely
-imperfect means of investigation available in his time, nor is it
-surprising that he should have ranked this motley company, in
-antithesis to the blood-possessing animals, as a unified and equivalent
-group. Two thousand years later, Lamarck did exactly the same
-thing, when he divided the animals into backboned and backboneless,
-and we reckon this nowadays as a merit only in so far that he was
-the first, after Aristotle, to re-express the solidarity of the classes of
-animals which we now call vertebrates.</p>
-
-<p>Aristotle was, however, not a systematic zoologist in our sense
-of the term, as indeed was hardly possible, considering the very small
-number of animal forms that were known in his time. In our day
-we have before us descriptions of nearly 300,000 named species
-wherefrom to construct our classification, while Aristotle knew hardly
-more than 200. Of the whole world of microscopic animals he could,
-of course, have no idea, any more than of the remains of prehistoric
-animals, of which we now know about 40,000 named and adequately
-described species. One would have thought that it would have
-occurred to a quick-witted people like the Greeks to pause and ponder
-when they found mussel-shells and marine snail-shells on the hills far
-above the sea; but they explained these by the great flood in the
-time of Deucalion and Pyrrha, and they did not observe that the
-fossil molluscs were of different species from the similar animals
-living in the sea in their own day.</p>
-
-<p><span class="pagenum"><a id="Page_11"></a>[Pg 11]</span></p>
-
-<p>Thus there was nothing to suggest to Aristotle and others of
-his time the idea that a transformation of species had been going
-on through the ages, and even the centuries after him evoked no such
-idea, nor did there arise new speculations, after the manner of
-Empedocles, in regard to the origin of the organic world. On the
-whole, the knowledge of the living world retrograded rather than
-advanced until the beginning of the Roman Empire. What Aristotle
-had known was forgotten, and Pliny's work on animals is a catalogue
-embellished with numerous fables, arranged according to a purely
-external principle of division. Pliny divided animals into those
-belonging to earth, water, and air, which is not very much more
-scientific than if he had arranged them according to the letters of the
-alphabet.</p>
-
-<p>During the time of the Roman Empire, as is well known, the
-knowledge of natural history sank lower and lower; there was no
-more investigation of nature, and even the physicians lost all scientific
-basis, and practised only in accordance with their traditional esoteric
-secrets. As the whole culture of the West gradually disappeared,
-the knowledge of nature possessed by earlier centuries was also
-completely lost, and in the first half of the Middle Ages Europeans
-revealed a depth of ignorance of the natural objects lying about them,
-which it is difficult for us now to form any conception of.</p>
-
-<p>Christianity was in part responsible for this, because it regarded
-natural science as a product of heathendom, and therefore felt bound
-to look coldly on it, if not even to oppose it. Later, however, even
-the Christian Church felt itself forced to give the people some mental
-nourishment in the form of natural history, and under its influence,
-perhaps actually composed by teachers of the Church, there appeared
-a little book, the so-called <i>Physiologus</i>, which was meant to instruct
-the people in regard to the animal world. This remarkable work,
-which has been preserved, must have had a very wide distribution
-in the earlier Middle Ages, for it was translated into no fewer than
-twelve languages, Greek, Armenian, Syriac, Arabic, Ethiopic, and so
-on. The contents are very remarkable, and come from the most
-diverse sources, that is, from the most different writers of antiquity,
-from Herodotus, from the Bible, and so forth, but never from original
-observation. The compilation does not really give descriptions of
-animals or of their habits, but, of each of the forty-one animals which
-the <i>Physiologus</i> recognizes, something remarkable is briefly related
-in true lapidary style, sometimes a mere curiosity without further
-import, or sometimes a symbolical interpretation. Thus the book
-says of the panther: 'he is gaily coloured; after satiating himself he<span class="pagenum"><a id="Page_12"></a>[Pg 12]</span>
-sleeps three days, and awakes roaring, giving forth such an agreeable
-odour that all animals come to him.' Of the pelican the well-known
-legend is related, that it tears open its own breast to feed its young
-with its blood, thus standing as a symbol of mother-love. Fabulous
-creatures, too, appear in these pages. Of the Phœnix, that bird
-whose plumage glitters with gold and precious stones, which was
-known even to Herodotus, and which has survived through Eastern
-fairy-tales on to the time of our own romanticists (Tieck), we read: 'it
-lives a thousand years, because it has not eaten of the tree of knowledge';
-then 'it sets fire to itself and arises anew from its own ashes,'
-a symbol of nature's infinite power of renewing its youth.</p>
-
-<p>But while among the peoples of Europe all the science of the
-ancients was lost, except a few barely recognizable fragments, the old
-lore was preserved, both as regards organic nature and other orders
-of facts, among the Arabs, through whom so many treasures of
-antiquity have eventually been handed down to us, coming in the
-track of the Arabian conquests across North Africa and Spain to the
-nations of Europe.</p>
-
-<p>It was in this way, too, that the writings of Aristotle again
-found recognition, after having been translated into Latin at Palermo
-at the order of that enthusiast for Science and Art, the Hohenstaufen
-Emperor, Frederick the Second. Our Emperor presented
-one copy of Aristotle's writings to the University of Bologna, and
-thus the wisdom of the ancient Greeks again became the common
-property of European culture. From the thirteenth century to the
-eighteenth, the study of natural science was limited to repeating and
-extending the work of Aristotle. Nothing new, depending upon
-personal observation, was added, and it does not even seem to have
-occurred to any one to subject the statements of the Stagirite to any
-test, even when they concerned the most familiar objects. No one
-noticed the error which ascribed to the fly eight legs instead of six;
-there was in fact as yet no investigation, and all knowledge of
-natural history was purely scholastic, and gave absolute credence
-to the authority of the ancients.</p>
-
-<p>A revulsion, however, occurred in the century of the Reformation,
-with the breaking down of the blind belief in authority which
-had till then prevailed in all provinces of human knowledge and
-thought. After a long and severe struggle, dry scholasticism was
-finally overcome, and natural science, with the rest, turned from a
-mere reliance on books to original thinking and personal observation.
-Thenceforward interpretations of natural processes were sought
-for no longer in the writings of the ancients, but in Nature herself.<span class="pagenum"><a id="Page_13"></a>[Pg 13]</span>
-Of the magnitude of this emancipation, and of the severity of the
-struggle against deep-rooted authority, one could form a faint idea
-from experience even in my own youth. Our young minds were so
-deeply imbued with the involuntary feeling that the ancients were
-superior to us moderns in each and every respect, that not only the
-hardly re-attainable plastic art of the Greeks and the immortal songs
-of Homer, but all the mental products of antiquity seemed to us
-models which could never be equalled; the tragedies of Sophocles
-were for us the greatest tragedies that the world had ever seen, the
-odes of Horace the most beautiful poems of all time!</p>
-
-<p>In the domain of natural science the new era began with the
-overthrow of the Ptolemaic cosmogony, which, for more than a
-thousand years, had served as a basis for astronomy. When the
-German canon, Nicolas Copernicus (born at Thorn, 1473, died 1543),
-reversed the old theory, and showed that the sun did not revolve
-round the earth, but the earth round the sun, the ice was broken and
-the way paved for further progress. Galilei uttered his famous
-'e pur si muove,' Kepler established his three laws of the movements
-of the planets, and Newton, a century later, interpreted their courses
-in terms of the law of gravitation.</p>
-
-<p>But we have not here to do with a history of physics or
-astronomy, and I only wish to recall these well-known facts, in
-order that we may see how increased knowledge in this domain was
-always accompanied by advances in that of biology.</p>
-
-<p>Here, however, we cannot yet chronicle any such thoroughgoing
-revolution of general conceptions; the basis of detailed empirical
-knowledge was not nearly broad enough for that, and it was in the
-acquiring of such a foundation that the next three centuries, from
-the sixteenth to the end of the eighteenth, were eagerly occupied.</p>
-
-<p>The first step necessary was to collate the items of individual
-knowledge in regard to the various forms of life, and to bring the
-whole in unified form into general notice. This need was met for
-the first time by Conrad Gessner's <i>Thierbuch</i>, a handsome folio
-volume, printed at Zurich in 1551, and embellished with numerous
-woodcuts, some of them very good. This was followed, in 1600, by
-a great work in many volumes, written in Latin, by a professor of
-Bologna, Aldrovandi. Not native animals alone but foreign ones also
-were described in these works, for, after the discovery of America
-and the opening up of communication with the East Indies, many
-new animal and plant forms came to the knowledge of European
-nations by way of the sea. Thus Francesco Hernandez (died 1600),
-physician in ordinary to Philip II, described no fewer than forty new<span class="pagenum"><a id="Page_14"></a>[Pg 14]</span>
-Mammals, more than two hundred Birds, and many other American
-animals.</p>
-
-<p>Again, in a quite different way, the naturalist's field of vision
-was widened, namely, by the invention of the simple microscope, with
-which Leeuwenhoek first discovered the new world of Infusorians, and
-Swammerdam made his notable observations on the structure and
-development of the very varied minute animal inhabitants of fresh
-water. In the same century, the seventeenth, anatomists like Tulpius,
-Malpighi, and many others extended the knowledge of the internal
-structure of the higher animals and of Man, and a foundation was laid
-for a deeper insight into the nature of vital functions by the discovery
-of the circulation of the blood in Man and the higher animals. In
-the following century, the eighteenth, this path of active research was
-eagerly followed, and we need only mention such names as Réaumur,
-Rösel von Rosenhof, De Geer, Bonnet, J. Chr. Schäfer, and Ledermüller,
-to be immediately reminded of the wealth of facts about the structure,
-life, and especially the development of our indigenous animals, which
-we owe to the labours of these men.</p>
-
-<hr class="tb" />
-
-<p>All these advances, great and many-sided as they were, did not
-at once lead to a renewal of the attempt of Empedocles to explain the
-origin of the organic world. This was as yet not even recognized as
-a problem requiring investigation, for men were content to take the
-world of life simply as a fact. The idea of getting beyond the naïve,
-poetic standpoint of the Mosaic story of Creation was as yet remote
-from the minds of naturalists, partly because they were wholly
-fascinated by the observation of masses of details, but chiefly because,
-first by the English physician, John Ray (died 1678), then by the
-great Swede, Carl Linné, the conception of organic 'species' had been
-formulated and sharply defined. It is true enough that before the
-works of these two men 'species' had been spoken of, but without
-being connected with any definite idea; the word was used rather in
-the same vague sense as the word 'genus,' to designate one of the
-smaller groups of organic forms, but without implying any clear
-idea of its scope or of its limitations. Now, however, for the first
-time, the term 'species' came to be used strictly to mean the
-smallest homogeneous group of individual forms of life upon the earth.
-John Ray held that the surest indication of a 'species' was that its
-members had been produced from the same seed; that is, 'forms
-which are of different species maintain this specific nature constantly,
-and one species does not arise from the seed of another.' Here we
-have the germ of the doctrine of the absolute nature and the<span class="pagenum"><a id="Page_15"></a>[Pg 15]</span>
-immutability of species which Linné briefly characterized in these
-words: 'Species tot sunt, quot formæ ab initio creatæ sunt,'
-'there are just so many species as there were forms created in the
-beginning.' It is here clearly implied, that species as we know them
-have been as they are from all time, that, therefore, they exist in
-nature as such and unchangeably, and have not been merely read into
-nature by man.</p>
-
-<p>This view, though we cannot now regard it as correct, was
-undoubtedly reasonable, and thoroughly in accordance with the spirit
-of the time; it was congruent with the knowledge, and above all with
-the scientific endeavours of the age. In the eighteenth century there
-was danger that all outlook on nature as a whole would be lost&mdash;smothered
-under the enormous mass of isolated facts, and especially
-under the inundation of diverse animal and plant forms which were
-continually being recognized. It must therefore have been regarded
-as a real deliverance, when Linné reduced this chaos of forms to
-a clearly ordered system, and relegated each form to its proper place
-and value in relation to the whole. How, indeed, could the great
-systematist have performed his task at all, if he had not been able to
-work with definite and sharply circumscribed groups of forms, if he
-had not been able to regard at least the lowest elements of his system,
-the species, as fixed and definite types? On the other hand, Linné was
-much too shrewd an observer not to entertain, in the course of his
-long life, and under the influence of the continually accumulating
-material, doubts as to the correctness of his assumption of the fixity
-and absoluteness of his species. He discovered from his own
-experience, what is fully borne out by ours, that it is easy enough to
-define a species when there are only a few specimens of a form to deal
-with, but that the difficulty increases in proportion to the number
-and to the diversity of habitat of those that are to be brought under
-one category. In the last edition of the <i>Systema Naturæ</i> we find very
-noteworthy passages in which Linné wonders whether, after all, a
-species may not change, and in the course of time diverge into
-varieties, and so forth. Of these doubts no notice was taken at the
-time; the accepted doctrine of the fixity of species was held to and
-even raised to the rank of a scientific dogma. Georges Cuvier, the
-great disciple of the Stuttgart 'Karlschule,' accentuated the doctrine
-still further by his establishment of animal-types, the largest groups
-of forms in the animal kingdom within which a definite and fundamentally
-distinct plan of architecture prevails. His four types,
-Vertebrates, Molluscs, Articulate and Radiate animals, furnished a
-further corroboration of the absolute nature of species, since they<span class="pagenum"><a id="Page_16"></a>[Pg 16]</span>
-seemed to show that even the highest and most comprehensive groups
-are sharply defined off from one another.</p>
-
-<p>Let me add that this doctrine of the absolute nature of species
-was not fully elaborated till our own day, when the Swiss (afterwards
-American) naturalist, Louis Agassiz, went so far as to maintain that not
-only the highest and the lowest categories, but all those coming between
-them, were categories established and sharply separated by Nature
-herself. But in spite of much ingenuity and his wide and comprehensive
-outlook he exerted himself in vain to find satisfactory and
-really characteristic definitions of what was to be considered a class,
-an order, a family, or a genus. He did not succeed in finding a
-rational definition of these systematic concepts, and his endeavour
-may be regarded as the last important attempt to prop up an
-interpretation of nature already doomed to fall. But in referring to
-Louis Agassiz I have anticipated the historical course of scientific
-development, and must therefore go back to the last quarter of the
-eighteenth century.</p>
-
-<p>The first unmistakable pioneer of the theory of descent, which
-now emerged for the first time as a scientific doctrine, was our great
-poet Goethe. He has indeed been often named as the founder of the
-theory, but that seems to me saying too much. It is true, however,
-that the inquiring mind of the poet certainly recognized in the
-structure of 'related' animals the marvellous general resemblances
-amid all the differences in detail, and he probed for the reason of
-these form-relations. Through the science of 'comparative anatomy,'
-as it was taught at the close of the century by Kielmeyer, Cuvier's
-teacher, and later by Cuvier himself, Blumenbach, and others,
-numerous facts had become known, which paved the way for such
-questions. It had, for instance, been recognized that the arm of man,
-the wing of the bird, the paddle of the seal, and even the foreleg of
-the horse, contain essentially the same chain of bones, and Goethe had
-already expressed these relations in his well-known verse,</p>
-
-<div class="poem-container">
-<div class="poem">
-<div class="stanza">
-<div class="i0">'Alle Gestalten sind ähnlich, doch keine gleichet der andern,</div>
-<div class="i0">Und so deutet der Chor auf ein geheimes Gesetz.'</div>
-</div>
-</div>
-</div>
-
-<p>As to what this law was he did not at that time pronounce an opinion,
-though he may even then have thought of the transformation of species.
-At first he contented himself with seeking for an ideal archetype or
-'Urtypus' which was supposed to lie at the foundation of a larger or
-smaller group. He discovered the archetypal plant or 'Urpflanze,' when
-he rightly recognized that the parts of the flower are nothing more
-than modified leaves. He spoke plainly of the 'metamorphosis of
-plants,' meaning by that the transformation of his 'archetype' into the<span class="pagenum"><a id="Page_17"></a>[Pg 17]</span>
-endless diversity of actual plant forms. But at first he certainly
-thought of this transformation only in the ideal sense, and not as
-a factual evolutionary process.</p>
-
-<p>The first who definitely maintained the latter view was, remarkably
-enough, the grandfather of the man who, in our own day, made
-the theory of descent finally triumphant, the English physician
-Erasmus Darwin, born 1731. This quiet thinker published, in 1794,
-a book entitled <i>Zoonomia</i>, and in it he takes the important step of
-substituting for Goethe's 'secret law' a real relationship of species. He
-proclaims the gradual establishment and ennobling of the animal
-world, and bases his view mainly on the numerous obvious adaptations
-of the structure of an organ to its use. I have not been able
-to find any passage in the book in which he has expressly indicated
-that, because many of the conditions of life could not have existed
-from the beginning, these adaptations are therefore, as such, an
-argument for the gradual transformation of species. But he assumed
-that such exact adaptations to the functions of an organ could only
-arise through the exercise of that function, and in this he saw a proof
-of transformation. Goethe had expressed the same idea when he
-said, 'Thus the eagle has conformed itself through the air to the air,
-the mole through the earth to the earth, and the seal through the
-water to the water,' and this shows that he too at one time thought
-of an actual transformation. But neither he nor Erasmus Darwin
-were at all clear as to <i>how</i> the use of an organ could bring about its
-variation and transformation. The latter only says that, for instance,
-the snout of the pig has become hard through its constant grubbing in
-the ground; the trunk of the elephant has acquired its great mobility
-through the perpetual use of it for all sorts of purposes; the tongue
-of the herbivore owes its hard, grater-like condition to the rubbing to
-and fro of the hard grass in the mouth, and so on. How acute and
-thoughtful an observer Erasmus Darwin was, is shown by the fact
-that he had correctly appreciated the biological significance of many
-of the colour-adaptations of animals to their surroundings, though it
-was reserved for his grandson to make this fully clear at a much
-later date. Thus he regarded the varied colouring of the python,
-of the leopard, and of the wild cat as the best adapted for concealing
-them from their prey amid the play of light and shadow in a leafy
-thicket. The black spot in front of the eye of the swan he considered
-an arrangement to prevent the bird from being dazzled,
-as would happen if that spot were as snow-white as the rest of the
-plumage.</p>
-
-<p>At the end of the book he sums up his views in the following<span class="pagenum"><a id="Page_18"></a>[Pg 18]</span>
-sentences: 'The world has been evolved, not created; it has arisen
-little by little from a small beginning, and has increased through the
-activity of the elemental forces embodied in itself, and so has rather
-grown than suddenly come into being at an almighty word.' 'What
-a sublime idea of the infinite might of the great Architect! the Cause
-of all causes, the Father of all fathers, the Ens entium! For if we
-could compare the Infinite it would surely require a greater Infinite
-to cause the causes of effects than to produce the effects themselves.'</p>
-
-<p>In these words he sets forth his position in regard to religion,
-and does so in precisely the same terms as we may use to-day when
-we say: 'All that happens in the world depends on the forces that
-prevail in it, and results according to law; but where these forces
-and their substratum, Matter, come from, we know not, and here we
-have room for faith.'</p>
-
-<p>I have not been able to discover whether the <i>Zoonomia</i>, with its
-revolutionary ideas, attracted much attention at the time when it
-appeared, but it would seem not. In any case, it was afterwards so
-absolutely forgotten, that in an otherwise very complete <i>History of
-Zoology</i>, published in 1872 by Victor Carus, it was not even
-mentioned. About a year after the appearance of <i>Zoonomia</i>, Isidore
-Geoffrey St.-Hilaire in Paris expounded the view that what are called
-species are really only 'degenerations,' deteriorations from one and
-the same type, which shows that he too had begun to have doubts as
-to the fixity of species. Yet it was not till the third decade of the
-nineteenth century that he clearly and definitely took up the position
-of the doctrine of transformation, and to this we shall have to return
-later on.</p>
-
-<p>But as early as the first decade of the century this position was
-taken up by two noteworthy naturalists, a German and a Frenchman,
-Treviranus and Lamarck.</p>
-
-<p>Gottfried Reinhold Treviranus, born at Bremen in 1776, an
-excellent observer and an ingenious investigator, published, in 1802,
-a book entitled <i>Biologie, oder Philosophie der lebenden Natur</i>
-[<i>Biology, or Philosophy of Animate Nature</i>], in which he expresses and
-elaborates the idea of the Evolution theory with perfect clearness.
-We read there, for instance: 'In every living being there exists
-a capacity for endless diversity of form; each possesses the power of
-adapting its organization to the variations of the external world, and
-it is this power, called into activity by cosmic changes, which has
-enabled the simple zoophytes of the primitive world to climb to
-higher and higher stages of organization, and has brought endless<span class="pagenum"><a id="Page_19"></a>[Pg 19]</span>
-variety into nature.' But where the motive power lies, which brings
-about these transformations from the lowliest to ever higher forms
-of life, was a question which Treviranus apparently did not venture
-to discuss. To do this, and thus to take the first step towards
-a causal explanation of the assumed transformations, was left for
-his successor.</p>
-
-<p>Jean Baptiste de Lamarck, born in 1744 in a village of Picardy,
-was first a soldier, then a botanist, and finally a zoologist. He won
-his scientific spurs first by his <i>Flora of France</i>, and zoology holds
-him in honour as the founder of the category of 'vertebrates.' Not
-that he occupied himself in particular detail with these, but he
-recognized the close alliance of the classes of animals in question&mdash;an
-alliance which was subsequently expressed by Cuvier by the
-systematic term 'type' or 'embranchement.'</p>
-
-<p>In his <i>Philosophie zoologique</i>, published in 1809, Lamarck set
-forth a theory of evolution whose truth he attempted to vindicate
-by showing&mdash;as Treviranus had done before him&mdash;that the conception
-of species, on the immutability of which the whole hypothesis of
-creation had been based, was an artificial one, read into nature by us;
-that sharply circumscribed groups do not exist in nature at all; and
-that it is often very difficult, and not infrequently quite impossible,
-to define one species precisely from allied forms, because it is connected
-with these on all sides by transition stages. Groups of forms
-which thus melted into one another indicated that the doctrine of
-the fixity of species could not be correct, any more than that of their
-absolute nature. Species, he maintained, are not immutable, and are
-not so old as nature; they are fixed only for a certain time. The
-shortness of our life prevents our directly recognizing this. 'If we
-lived a much shorter time, say about a second, the hour-hand of the
-clock would appear to us to stand still, and even the combined
-observations of thirty generations would afford no decisive evidence
-as to the hand's movement, and yet it had been moving.'</p>
-
-<p>The causes on which, according to Lamarck, the transformation
-of species, their modification into new species, depends, lie in the
-changes in the conditions of life which must have occurred ceaselessly
-from the earliest period of the earth's history till our own day, now
-here, now there, due in part to changes in climate and in food-supply,
-in part to changes in the earth's crust by the rising or sinking of
-land-masses, and so forth. These external changes have sometimes
-been the <i>direct</i> cause of changes in bodily structure, as in the
-case of heat or cold; but they have sometimes and much more
-effectively operated <i>indirectly</i>. Thus changed conditions may have<span class="pagenum"><a id="Page_20"></a>[Pg 20]</span>
-prompted an animal of a given species to use certain parts of its body
-in a new way, more vigorously, or less actively, or even not at all,
-and the more vigorous use, or, conversely, the disuse, has brought
-about variations in the organ in question.</p>
-
-<p>Thus the whales lost their teeth when they abandoned their fish
-diet, and acquired the habit of feeding on minute and delicate
-molluscs, which they swallowed whole without seizure or mastication.
-Thus, too, the eyes of the mole degenerated through its life in the
-dark, and a still greater degeneration of the eyes has taken place in
-animals, like the proteus-salamander, which always inhabit lightless
-caves. In mussels both head and eyes degenerated because the
-animals could no longer use them after they became enclosed in
-opaque mantles and shells. In the same way snakes lost their legs
-<i>pari passu</i> with the acquisition of the habit of moving along by
-wriggling their long bodies, and of creeping through narrow fissures
-and holes. On the other hand, Lamarck interpreted the evolution of
-the web-feet of swimming birds by supposing that some land-bird or
-other had formed the habit of going into the water to seek for food,
-and consequently of spreading out its toes as widely as possible so as
-to strike the water more vigorously. In this way the fold of skin
-between the toes was stretched, and as the extension of the toes was
-very frequent and was continued through many generations, the web
-expanded and grew larger, and thus formed the web-foot.</p>
-
-<p>In the same way the long legs of the wading birds have been,
-according to Lamarck, gradually evolved by the continual stretching
-of the limbs by wading in deeper and deeper water, and similarly for
-the long necks and bills of the waders, the herons and the storks.
-Finally we may mention the case of the giraffe, whose enormously
-long neck and tall forelegs are interpreted as due to the fact that the
-animal feeds on the foliage of trees, and was always stretching as far
-as possible, in order to reach the higher leaves.</p>
-
-<p>We shall see later in what a different way Charles Darwin
-explained this case of the giraffe. Lamarck's idea is at once clear; it
-is true that exercising an organ strengthens it, that disuse makes
-it weaker. Through much gymnastic exercise the muscles of the arm
-become thicker and more capable, and memory too may be improved,
-that is to say, even a definite part of the brain may be considerably
-strengthened by use. Indeed, we may now go so far as to admit that
-every organ is strengthened by use and weakened by disuse, and so
-far the foundations of Lamarck's interpretations are sound. But he
-presupposes something that cannot be admitted so readily, namely,
-that such 'functional' improvement or diminution in the strength of<span class="pagenum"><a id="Page_21"></a>[Pg 21]</span>
-an organ can be transmitted by inheritance to the succeeding
-generation. We shall have to discuss this question in detail at a later
-stage, and I shall only say now that opinions as to whether this is
-possible or not are very much divided. I myself doubt this possibility,
-and therefore cannot admit the validity of the Lamarckian
-evolutionary principle in so far as it implies the directly transforming
-effect of the functioning of an organ. But even if we recognize the
-Lamarckian factor as a <i>vera causa</i>, it is easy to show that there are
-a great many characters which it is not in a position to interpret.
-Many insects which live upon green leaves are green, and not a few
-of them possess exactly the shade of green which marks the plant on
-which they feed; they are thus protected in a certain measure from
-injuries. But how could this green colour of the skin have been
-brought about by the activity of the skin, since the colour of the
-surroundings does not usually stimulate the skin to activity at all? Or
-how should a grasshopper, which is in the habit of sitting on dry
-branches of herbs, have thereby been incited to an activity which
-imparts to it the colour and shape of a dry twig? Just as little, or
-perhaps still less, can the protective green colour of a bird's or insect's
-eggs be explained through the direct influence of their usually green
-surroundings, even if we disregard the fact that the eggs are green
-when they are laid&mdash;that is, before the environment can have had any
-influence on them.</p>
-
-<p>The Lamarckian principle of modification through use does not,
-in any case, nearly suffice as an interpretation of the transformations
-of the organic world. It must be allowed that Lamarck's theory
-of transformation was well founded at the time when it was
-advanced; it not only attacked the doctrine of the immutability of
-species, but sought for the first time to indicate the forces and
-influences which must be operative in the transformations of species;
-it was therefore well worth careful testing. Nevertheless it did not
-divert science from its chosen path; very little notice was taken of it,
-and in the great Cuvier's chronicle of scientific publications for 1809,
-not a syllable is devoted to Lamarck's book, so strong was the
-power of prejudice.</p>
-
-<p>But, although the new doctrine was thus ignored, it did not
-altogether fall to the ground; it glimmered for a while in Germany,
-where it found its champions in the 'Naturphilosophie' of the time,
-and especially in Lorenz Oken, a peasant's son, born at Ortenau, near
-Offenburg, in 1783.</p>
-
-<p>Oken professed views similar to those of Erasmus Darwin,
-Treviranus, and Lamarck, though they were not clothed in such<span class="pagenum"><a id="Page_22"></a>[Pg 22]</span>
-purely scientific garb, being, in fact, bound up with the general
-philosophical speculations which came increasingly into favour at
-that time, chiefly through the writings of Schelling. In the same
-year, 1809, in which Lamarck published his <i>Philosophie zoologique</i>,
-Oken's <i>Lehrbuch der Naturphilosophie</i> appeared.</p>
-
-<p>This book is by no means simply a theory of descent; its scope
-is much wider, including the phenomena of the whole cosmos; on
-the other hand, it goes too little into details and is too indefinite to
-deserve its title. Its way of playing with ideas, its conjectures and
-inferences from a fanciful basis, make it difficult for us now to think
-ourselves into its mode of speculation, but I should like to give some
-indication of it, for it was just these speculative encroachments
-of the 'categories' of the so-called 'Naturphilosophie' which played
-a fatal part in causing the temporary disappearance of the Evolution-theory
-from science, so that, later on, it had to be established anew.</p>
-
-<p>Oken defines natural science as 'the science of the everlasting
-transmutations of God (the Spirit) in the world': Every thing,
-considered in the light of the genetic process of the whole, includes,
-besides the idea of being, that of not-being, in that it is involved in
-a higher form. 'In these antitheses the category of polarity is included.
-The simpler elementary bodies unite into higher forms, which are
-thus merely repetitions at a potential higher than that of their causes.
-Thus the different genera of bodies form parallel and corresponding
-series, the reasonable arrangement of which results as an intrinsic
-necessity from their genetic connexion. In individuals these lowlier
-series make their appearance again during development. The contrasts
-in the solar system between planets and sun are repeated in
-plants and animals, and, as light is the principle of movement, animals
-have the power of independent movement in advance of the plants
-which belong to the earth.'</p>
-
-<p>Obviously enough, this is no longer the study of nature; it is
-nature-construction from a basis of guesses and analogies rather than of
-knowledge and facts. Light is the principle of motion, and as animals
-move, they correspond to the sun, and plants to the planets! Here
-there is not even a hint of a deepening of knowledge, and all these
-deductions now seem to us quite worthless.</p>
-
-<p>On the other hand, it must be allowed that good ideas are by no
-means absent from this 'philosophy,' nor can we deny to this restlessly
-industrious man a great mind always bent on discovering what
-was general and essential. Much of what we now <i>know</i> he even
-then guessed at and taught, as, for instance, that the basis of all forms
-of life in this infinitely diverse world of organisms was one and the<span class="pagenum"><a id="Page_23"></a>[Pg 23]</span>
-same substance&mdash;'primitive slime,' 'Urschleim' as he called it, or,
-as we should now say, 'protoplasm.' We can therefore, <i>mutatis
-mutandis</i>, agree with Oken when he says,'Everything organic has
-come from slime, and is nothing but diversely organized slime.'
-Many naturalists of the present day would go further, and agree with
-Oken when he suggests that 'this primitive slime has arisen in the
-sea, in the course of the planet's (the earth's) evolution out of inorganic
-material.'</p>
-
-<p>Thus Oken postulated, as the specific vehicle of life, a primitive
-substance, in essence at least homogeneous. But he went further,
-and maintained that his 'Urschleim' assumed <i>the form of vesicles</i>,
-of which the various organisms were composed. 'The organic world
-has as its basis an infinitude of such vesicles.' Who is not at
-once reminded of the now dominant <i>Cell-theory</i>? And, in fact, thirty
-years later, when the cell was discovered, Oken did claim priority for
-himself. In so doing, he obviously confused the formulating of
-a problem with the solving of it; he had, quite rightly, divined
-that organisms must be built up of very minute concentrations of the
-primitive substance, but he had never seen a cell, or proved the
-necessity for its existence, or even attempted to prove it. His vesicle-theory
-was a pure divination, a prevision of genius, but one which
-could not directly deepen knowledge; it did not prompt, or even
-hasten, the discovery of the cell. Here, as throughout in his natural
-philosophy, Oken built, not from beneath upwards, by first establishing
-facts and then drawing conclusions from them, but, inversely, he
-invented ideas and principles, and out of them reconstructed the
-world. In this he differs essentially from his predecessors Erasmus
-Darwin, Treviranus, and Lamarck, who all reasoned inductively, that
-is, from observed data.</p>
-
-<p>Thus the whole evolutionary movement was lost in indefiniteness;
-because men wanted to find a reason for everything, they
-missed even what might then have been explained. Moreover, the
-theory of evolution still lacked a sufficiently broad basis of facts;
-the 'Naturphilosophie,' by its want of moderation, robbed it of all
-credit; and it is not to be wondered at that men soon ceased to occupy
-themselves with the problem of the evolution of the living world.
-A few indeed held fast to the doctrine of evolution during the first
-third of the century, but then it disappeared completely from the realm
-of science.</p>
-
-<p>Its last flicker of life was seen in France, in 1830, at the time
-of the July revolution, when the legitimate sovereignty of Charles X
-was overthrown. It is interesting to note the lively interest that<span class="pagenum"><a id="Page_24"></a>[Pg 24]</span>
-Goethe, the first forerunner of the theory, and then aged eighty-one,
-had in the intellectual combat that took place in the French Academy
-between Cuvier and Isidore Geoffroy St.-Hilaire. A friend of Goethe's,
-Soret, relates that on August 2, 1830, he went into the poet's room,
-and was greeted with the words: 'Well, what do you think of this
-great event? The volcano is in eruption, and all is in flames. There
-can no longer be discussion with closed doors.' Soret replied: 'It is
-a terrible business! But what else was to be expected with things
-as they are, and with such a ministry, than that it should end in
-the expulsion of the reigning family?' To which Goethe answered:
-'We don't seem to understand each other, my dear friend. I am not
-talking of these people at all; I am thinking of quite different affairs.
-I refer to the open rupture in the Academy between Cuvier and
-Geoffroy St.-Hilaire; it is of the utmost importance to science.'</p>
-
-<p>In this conflict of opinions, Cuvier opposed Geoffroy's conception
-of the unity of the plan of structure in all animals, confronting him
-with the four Cuvierian types, in each of which the plan of structure
-was altogether different, and strongly insisting on the doctrine of the
-fixity of species, which he maintained to be the necessary postulate of
-a scientific natural history.</p>
-
-<p>The victory fell to Cuvier, and it cannot be denied that there
-was much justification for his opinions at the time, for the knowledge
-of facts at that stage was not nearly comprehensive enough to give
-security to the Evolution theory, and moreover the quiet progress of
-science might have been hindered rather than furthered by premature
-generalization and theorizing. It had now been seen how far the
-interpretation of general biological problems could be carried with the
-available material; the 'Naturphilosophie' had not merely exploited
-it as far as possible, but had burdened it much beyond its carrying
-power, and the world was weary of insecure speculations. The
-'Naturphilosophie' was for the time quite worked out, and a long
-period set in, during which all energies were devoted to detailed
-research.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_25"></a>[Pg 25]</span></p>
-
-<h2 class="nobreak" id="LECTURE_II">LECTURE II</h2>
-</div>
-
-<p class="c">THE DARWINIAN THEORY</p>
-
-<div class="blockquot">
-
-<p>Period of detailed research&mdash;Appearance of Darwin's <i>Origin of Species</i>&mdash;Darwin's
-life&mdash;Voyage round the world&mdash;His teaching&mdash;Domesticated animals, dog, horse&mdash;Pigeons&mdash;Artificial
-selection&mdash;Unconscious selection&mdash;Correlated variations.</p></div>
-
-
-<p><span class="smcap">The</span> period of wholly unphilosophical, purely detailed research
-may be reckoned as from about 1830 to 1860, though, of course,
-many of the labours of the earlier part of the century must be
-counted among the investigations which were carried out without
-any reference to general questions, and even after 1860 numerous
-such works appeared. Nor could it be otherwise, for the basis of
-all science must be found in facts, and the thorough working up
-of the fact-material will always remain the first and most indispensable
-condition of our scientific progress. During the period
-referred to, however, it had become the sole end to be striven for;
-and all energies were concentrated exclusively on the accumulation
-of facts.</p>
-
-<p>The previous century had added much to the knowledge of the
-inner structure of animals, the so-called 'comparative anatomy,' and
-in the nineteenth century this line of investigation was pursued even
-more extensively and energetically, so that the knowledge increased
-enormously. Up till this time it was chiefly the structure of the
-backboned animals and of a few 'backboneless' animals, so called,
-that had been studied, but now all the lower groups of the animal
-kingdom were also investigated, and became known better and in
-more detail as the methods of research improved.</p>
-
-<p>Not content, however, with a knowledge of the adult animal,
-naturalists began to investigate its development. In the year
-1814 the first great work on development appeared, on the development
-of the chick, by Pander and Von Baer. It was there shown
-for the first time, how the chick begins as a little disk-shaped
-membrane on the surface of the yolk of the egg, at first simply as
-a pale streak, the 'primitive streak,' then as a groove, the 'primitive
-groove,' by the side of which arise two folds, the 'medullary folds,'
-and further how a system of blood-vessels is developed around this
-primitive rudiment on the upper surface of the yolk, how a heart<span class="pagenum"><a id="Page_26"></a>[Pg 26]</span>
-arises before the rest of the body is complete, and how the blood begins
-to circulate; in short, there was disclosed all the marvel of development
-to which we are now so much accustomed, that we can hardly
-understand the sensation it made at that time.</p>
-
-<p>Later on, attention was turned to the development of Fishes and
-Amphibians (Agassiz and Vogt, later Remak), then to that of the
-Worms (Bagge), of Insects (Kölliker), and gradually the development
-of all the groups of the animal-kingdom&mdash;from Sponges to Man&mdash;was
-so thoroughly investigated that it almost seems to-day as if there
-could not be much that is new to discover in this department. This
-impression may indeed be true as far as the less complex processes
-and the more obvious questions are concerned, but it is impossible
-to predict what new problems may confront us, whose solution will
-depend on a still more detailed study of development.</p>
-
-<p>As embryology is a science of the nineteenth century, so also
-is histology, the science of tissues. Its pioneer was Bichat, but its
-real foundations were not laid till Schwann and Schleiden formulated
-the conception of the 'cell,' and proved that all animals and plants
-were composed of cells. What Oken had only guessed at they now
-proved, that there are very minute form-elements of life which build
-up all the parts of animals and plants or produce them by processes
-of secretion. New light was thus shed on embryonic development,
-and this gradually led to the recognition of the fact that the egg, too,
-is a cell, and that development depends on a cell-division process in
-this egg-cell. This led further to the conception of many-celled and
-single-celled organisms, and so on to many items of knowledge to
-speak of which here would carry us too far.</p>
-
-<p>For it is not my intention to attempt a complete review of the
-development of biology in the nineteenth century, or even in the
-period which we have mentioned as devoted to detailed research;
-it is rather my desire to convey a general impression of the enormous
-extent and many-sidedness of the progress that was made in this
-time. Let us therefore briefly recall the entirely new facts which
-were brought to light in this period with regard to the reproduction
-of animals. Asexual reproduction by budding and division was
-already known, but parthenogenesis is a discovery of this period,
-and so also is alternation of generations, so far-reaching in its
-bearing on general problems. It was first observed (1819) by
-Chamisso in Salpa, then by Steenstrup in Medusæ and trematodes,
-and was later made fully clear in its most diverse forms and relations
-by the researches of Leuckart, Vogt, Kölliker, Gegenbaur, Agassiz,
-and other illustrious investigators. Reproduction by heterogony, too,<span class="pagenum"><a id="Page_27"></a>[Pg 27]</span>
-which occurs in many crustaceans, and in aphides and certain worms,
-was recognized at that time, and in the sixties Carl Ernst von Baer
-added to the list precocious reproduction, or pædogenesis, which is
-illustrated in certain insects which reproduce in the larval state.</p>
-
-<p>This may suffice to convey some idea of the great mass of new,
-and in some cases startling facts previously unguessed at, which were
-then brought to light in the department of animal biology alone. To
-this must be added the vast increase in the number of known species
-and varieties, their distribution on the earth, and all this, <i>mutatis
-mutandis</i>, for plants also. Nor can we omit to mention the rapidly
-growing number of fossil species of animals and plants.</p>
-
-<p>Thus there gradually accumulated a new mass of material;
-investigation became more and more specialized, and the danger
-became imminent that workers in the various departments would
-be unable to understand each other, so completely were they independent
-of one another in their specialist researches. There was lack
-of any unifying bond, for workers had lost sight of the general
-problem in which all branches of the science meet, and through which
-alone they can be united into a general science of biology. The time
-had come for again combining and correlating the details, lest they
-should grow into an unconnected chaos, through which it would be
-impossible to find one's way, because no one could overlook it and
-grasp it as a whole. In a word, it was high time to return to general
-questions.</p>
-
-<hr class="tb" />
-
-<p>Though I have called the period from 1830 to 1860 that of
-purely detailed research, I do not mean to ignore the fact that,
-during that time, there were a few feeble attempts to return to
-the great questions which had been raised at the beginning of the
-century. But the point is, that all such attempts remained unnoticed.
-Thus there appeared, in 1844, a book entitled <i>Vestiges of the Natural
-History of Creation</i>, the anonymous author of which revealed himself
-much later as Robert Chambers, an Edinburgh publisher. In this
-book the evolution of species was ascribed to two powers, a power
-of transformation and a power of adaptation. Two Frenchmen,
-Naudin and Lecoq, also published a work in which the theory of
-evolution was set forth, and from 1852 to 1854 the well-known
-German anthropologist Schaafhausen was writing on similar lines.
-But all these calls sounded unheard, so deeply were naturalists
-plunged in detailed investigations, and it required a much mightier
-voice to command the ear of the scientific world.</p>
-
-<p>It is impossible to estimate the effect of Darwin's book on <i>The<span class="pagenum"><a id="Page_28"></a>[Pg 28]</span>
-Origin of Species</i>, published in English in 1858, in German in 1859
-unless we fully realize how completely the biologists of that time had
-turned away from general problems. I can only say that we, who
-were then the younger men, studying in the fifties, had no idea that
-a theory of evolution had ever been put forward, for no one spoke
-of it to us, and it was never mentioned in a lecture. It seemed as
-if all the teachers in our universities had drunk of the waters of
-Lethe, and had utterly forgotten that such a theory had ever been
-discussed, or as if they were ashamed of these philosophical flights on
-the part of natural science, and wished to guard their students from
-similar deviations. The over-speculation of the 'Naturphilosophie'
-had left in their minds a deep antipathy to all far-reaching deductions,
-and, in their legitimate striving after purely inductive
-investigation, they forgot that the mere gathering of facts is not
-enough, that the drawing of conclusions is an essential part of the
-induction, and that a mass of bare facts, however enormous, does not
-constitute a science.</p>
-
-<p>One of my most stimulating teachers at that time, the gifted
-anatomist, Jacob Henle, had written as a motto under his picture,
-'There is a virtue of renunciation, not in the province of morality
-alone, but in that of intellect as well,' a sentence which expressly
-indicated the desirability of refraining from all attempts to probe
-into the more general problems of life. Thus the young students
-of that time were nourished only on the results of detailed research,
-in part indeed interesting enough, but in part dry and, because
-uncorrelated, unintelligible in the higher sense, and only here and
-there awakening a deeper interest, when, as in physiology and in
-embryology, they formed a connected system in themselves. Without
-being fully clear as to what was lacking, we certainly missed the
-deeper correlation of the many separate disciplines.</p>
-
-<p>It is therefore not to be wondered that Darwin's book fell like
-a bolt from the blue; it was eagerly devoured, and while it excited
-in the minds of the younger students delight and enthusiasm, it
-aroused among the older naturalists anything from cool aversion
-to violent opposition. The world was as though thunderstruck, as
-we can readily see from the preface with which the excellent zoologist
-of Heidelberg, Bronn, introduced his translation of Darwin's book,
-where he asks this question among others, 'How will it be with you,
-dear reader, after you have read this book?' and so forth.</p>
-
-<p>But before I enter on a detailed examination of the contents of
-this epoch-making book, I should like to say a few words about the
-man himself, who thus revolutionized our thinking.</p>
-
-<p><span class="pagenum"><a id="Page_29"></a>[Pg 29]</span></p>
-
-<p>Charles Darwin was born in 1809, the year of the publication
-of Lamarck's <i>Philosophie zoologique</i>, and of Oken's <i>Lehrbuch der
-Naturphilosophie</i>. There was thus a whole generation between
-the first emergence of the Evolution theory and its later revival.
-Darwin's father was a physician, and his education was not a regular
-one. In his youth he seems to have devoted much time and enthusiasm
-to hunting, and only very slowly to have taken up regular studies
-towards a definite end. In accordance with his father's wishes, he
-studied medicine for a time, but soon abandoned it to devote himself
-to botany and zoology. Before he had had time to distinguish
-himself in any special way in these subjects, he was offered, in his
-twenty-first year, the post of naturalist on an English war-ship
-which was to make a voyage round the world, and that at a
-leisurely rate.</p>
-
-<p>This was decisive not only for Darwin's immediate studies, but
-for the work of his life, for, as he tells us himself, it was during this
-voyage on the <i>Beagle</i> that the idea of the Evolution theory first came
-to him. While the vessel made a stay at the Galapagos Islands, west
-of South America, he noticed that quite a number of little land-birds
-occurred there which closely resembled those of the neighbouring
-mainland, but yet were different from them. Almost every little
-island had its own species, and so he concluded that all these might
-be descended from representatives of a few species which had long
-before drifted over from the mainland to these volcanic islands,
-become established there, and in the course of time taken on the
-character of new species. The problem of the transformation of
-species opened up before him, and he made up his mind to follow
-up the idea after his return, in the hope that by a patient collecting
-of facts, he would by and by arrive at some security with regard
-to this great question.</p>
-
-<p>I need not linger over any detailed account of his travels;
-one can readily understand how a voyage round the world, lasting
-for five years, would offer to the inquiring mind of a Darwin rich
-opportunities for the most varied observations. That he did not
-fail to make use of these is evidenced not only by his book on <i>The
-Origin of Species</i>, but by several more special works, published
-shortly after his return&mdash;his natural history of those remarkable
-sessile crustaceans, the barnacles or Cirripedia, and his studies on
-the origin of coral reefs. The first-named book still holds its own
-as a classic monograph on this animal group, with its wealth of forms;
-and the theory of the origin of coral reefs which Darwin elaborated
-has still many adherents, in spite of various rival interpretations.</p>
-
-<p><span class="pagenum"><a id="Page_30"></a>[Pg 30]</span></p>
-
-<p>But Darwin would hardly have achieved what he did if he had
-been compelled to secure for himself a professional position in order
-to obtain bread and butter. Such great problems demand not only
-the whole of a man's mental energy, they monopolize his time.
-Studies of detail may well be taken up in leisure hours, but big
-problems absorb all the thoughts and must always be present to the
-mind, lest the connexion between the many individual inquiries,
-which make up the whole task, be lost sight of. Darwin had the
-good fortune to be a free investigator, and to be able to retire, on
-his return from his travels, to a small property at Down in Kent,
-there to live for his family and his work. Here he followed up the
-idea of evolution which he had already formulated, and it has always
-seemed to me the most remarkable thing about him, that he was able
-to keep in mind and work up the hundreds of isolated inquiries that
-were eventually to be brought together to form the main fabric of
-his theory. When one studies his many later writings, one cannot
-but be surprised afresh by the number of different sets of facts he
-collected at the same time, partly from others, partly from personal
-observation, and continually also from his own experiments. He
-made experiments on plants and on animals, and the number of
-people with whom he carried on a scientific correspondence is simply
-astounding. In this way he brought together, in the course of twenty
-years, an extraordinarily rich material of facts, from the fullness of
-which he was able later to write his book on <i>The Origin of Species</i>.
-Never before had a theory of evolution been so thoroughly prepared
-for, and it is undoubtedly to this that it owed a great part of its
-success; not to this alone, however, but still more, if not mainly,
-to the fact that it presented a principle of interpretation that had
-never before been thought of, but whose importance was apparent
-as soon as attention was called to it&mdash;the principle of selection.</p>
-
-<p>Charles Darwin championed, in the main, the same fundamental
-ideas as had been promulgated by his grandfather, Erasmus Darwin,
-by Treviranus, and by Lamarck: species only seem to us immutable;
-in reality they can vary, and become transformed into other species,
-and the living world of our day has arisen through such transformations,
-through a sublime process of evolution which began with the
-lowest forms of life, but by degrees, in the course of unthinkably long
-ages, progressed to organisms more and more complex in structure,
-more and more effective in function.</p>
-
-<p>It is interesting to note at what point Darwin first put in his
-lever to attempt the solution of the problem of evolution. He started
-from quite a different point from the investigators of the early part<span class="pagenum"><a id="Page_31"></a>[Pg 31]</span>
-of the century, for he began with forms of life which had previously
-been markedly neglected by science, the varieties of our domesticated
-animals and cultivated plants.</p>
-
-<p>Previously these had been in a sense mere step-children of
-biology, inconvenient existences which would not fit properly into
-the system, which were therefore as far as possible ignored or dismissed
-as outside the scope of 'the natural,' because it was difficult
-to know what else to do with them. I can quite well remember that,
-even as a boy, I was struck by the fact that one could find nothing in
-the systematic books about the many well-established garden forms
-of plants, or about our domestic animals, which seemed to be regarded
-as in a sense artificial products, and as such not worthy of scientific
-consideration. But it was in these that Darwin particularly interested
-himself, making them virtually the basis of his theory, for he
-led up from them to the very principle of transformation, which was
-his most important addition to the earlier presentations of the
-Evolution theory.</p>
-
-<p>He started from the existence of varieties which may be
-observed in so many wild species. His line of thought was somewhat
-as follows: If species have really arisen through a gradual process
-of transformation, then varieties must be regarded as possible first
-steps towards new species; if, therefore, we can only succeed in finding
-out the causes which underlie the formation of any varieties whatever,
-we shall have discovered the causes of the transformation of
-species. Now we find by far the greatest number of varieties, and
-the most marked ones, among our domesticated animals and plants,
-and unless we are to assume that each of these is descended from
-a special wild species, the reason why there has been such a wealth of
-variety-formation among them must lie in the conditions which
-influence the relevant species in the course of domestication; and
-it remains for us to analyse these conditions till we come upon the
-track of the operative factors. With this conviction, Darwin devoted
-himself to the study of domesticated animals and plants.</p>
-
-<p>The first essential was to prove that every variety had not
-a separate wild species as ancestor, but that the whole wealth of our
-domesticated breeds originated, in each case, from one, or at least from
-a few wild species. Of course I cannot here recapitulate the multitudinous
-facts which were marshalled by Darwin, especially in his
-later works, notably his <i>Animals and Plants under Domestication</i>,
-but this is not necessary to an understanding of his conclusions,
-and I shall therefore restrict myself to a few examples.</p>
-
-<p>Let us take first the domestic dog, <i>Canis familiaris</i>, Linné. We<span class="pagenum"><a id="Page_32"></a>[Pg 32]</span>
-have at the present day no fewer than seven main breeds, each of
-which has its sub-breeds, often numerous. Thus there are forty-eight
-sub-breeds which are used as guardians of our houses, 'house-dogs'
-in the restricted sense, thirty sub-breeds of dogs with silk-like hair
-(King Charles dogs, Newfoundland dogs, &amp;c.), twelve of terriers,
-and thirty-five of sporting dogs, among them such different forms
-as the deerhound and the pointer. We have further nineteen sub-breeds
-of bulldogs, thirty-five of greyhounds, and six of naked or hairless
-dogs. Not only the main breeds, but even the sub-breeds often
-differ as markedly from one another as wild species do, and the
-question must first be decided whether each of the very distinct
-breeds has not a special wild species as ancestor.</p>
-
-<p>Obviously, however, this cannot be maintained, for so many
-species of wild dog have never existed on the earth at any time. We
-know, too, that 4,000 or 5,000 years ago a large number of breeds
-of dogs were in existence in India and Egypt. There were Pariah
-dogs, coursers, greyhounds, mastiffs, house-dogs, lapdogs and terriers.
-It is not possible that the products of all lands could, at that time,
-have been gathered into one, and it is inconceivable that so many
-wild species could have existed in the one country of India.</p>
-
-<p>On the other hand, however, it cannot be maintained that all
-our present breeds have descended from <i>a single</i> wild species; it is
-much more probable that several wild species were domesticated in
-different countries.</p>
-
-<p>It has often been supposed that the manifold diversity of our
-present breeds has been brought about by crossing the various tamed
-species. That cannot be the case, however, because crossing gives
-rise only to hybrid mongrel forms, not to distinct breeds with quite
-new characters. It is true that all breeds of dogs can be very readily
-crossed with each other, but the result is not new breeds, but those
-numberless and transient intermediate forms which the dog-breeder
-despises as worthless for his purpose. It must therefore have been
-through the influence of domestication, combined with crossing, that
-a few wild species gave rise to the various breeds of dogs.</p>
-
-<p>The pedigree of the horse is rather more clear than that of
-the dog. Even in this case, indeed, one cannot definitely name
-the ancestral wild form, but it is very probable that it was of a grey-brown
-colour, and similar to the wild horses of our own day. Darwin
-supposes that it must also have had the black stripe on the back
-which is exhibited by the domestic ass, and by several wild species of
-ass, basing his opinion on the fact that the spinal stripe often occurs
-in foals, especially in those of a grey-brown colour.</p>
-
-<p><span class="pagenum"><a id="Page_33"></a>[Pg 33]</span></p>
-
-<p>But though there can be no doubt that this is to be interpreted
-as a reversion to a character of a remote ancestor, it by no means
-follows that the <i>direct</i> ancestral form must have had this stripe.
-I am more inclined to believe that the ancestor which bore this mark
-was considerably more remote, and lived before the differentiation
-of the horse from the ass. Darwin himself noted the remarkable fact
-that in rare cases, especially in foals, not only may the stripe on the
-back be present, but there may be more or less distinct zebra-striping
-on the legs and withers: this, however, must be interpreted as a
-reversion to the character of a very much more remote ancestor, to
-a common ancestor of all our present-day horses and asses, which
-must have been striped over its whole body, like the zebra living in
-Africa now.</p>
-
-<p>It cannot be proved of any of the wild horses of to-day that
-they are not descended from domesticated ancestors; indeed, we can
-say with certainty that the thousands of wild horses which roam the
-plains of North and South America are descended from domestic
-horses, for there was no horse in America at the time it was discovered
-by the Europeans. In all probability our horse originated in
-Middle Asia, was there first domesticated, and has thence been
-gradually introduced into other countries. In Egypt it appears
-first on the monuments in the seventeenth century <span class="allsmcap">B.C.</span>, and it seems
-to have been introduced by the conquering Hyksos. On the ancient
-Assyrian monuments the chase after wild horses is depicted, and
-they were not caught, but killed with arrow and lance, like the lion
-and the gazelle.</p>
-
-<p>But even if two wild species of horse had been tamed in different
-parts of the great continent of Asia, these two domesticated animals
-would have varied much and in the most diverse manner, as we may
-infer from our different breeds of horses at the present day. There
-are a great many of these, and many of them differ very considerably
-from each other. If we think of the lightly built Arab horse, and
-place beside it the little pony, or the enormous Percheron, the
-powerful cart-horse from the old French province of La Perche,
-which easily draws a load of fifty kilograms, we are face to face with
-differences as great as those between natural species. And we may
-realize how many breeds of horses there are now upon the earth if we
-remember that nearly every oceanic island has its special breed of
-ponies. Not only in the cold Shetland Islands, England, Sardinia
-and Corsica, but in almost every one of the larger islands of the
-extensive Indian Archipelago there is one, and Borneo and Sumatra
-have several.</p>
-
-<p><span class="pagenum"><a id="Page_34"></a>[Pg 34]</span></p>
-
-<p>But the most conclusive proof of descent from a single wild
-species is afforded by the pigeons, and as the production of new
-breeds among them has been, and will continue to be, carried on with
-particular enthusiasm and deliberateness, I propose to deal with them
-somewhat more in detail.</p>
-
-<p>Darwin's work proves beyond a doubt that all our present-day
-breeds of pigeons are descended from a single wild species, the rock-dove,
-<i>Columba livia</i>. In appearance, this form, which still lives in
-a wild state, differs little from our half-wild blue-grey field-pigeon.
-It has the same metallic shimmer on the feathers of the neck, the
-same two black cross-bars on the wings as well as the band over the
-tail, and it has also the same slate-blue general colour. Now, all
-breeds of pigeons are without restriction fertile <i>inter se</i>, so that any
-breed can be crossed with any other, and it often happens that, in
-the products of such crossing, characters appear which the parents,
-that is, the two or more crossed breeds, did not possess, but which
-are among the characters of the rock-dove. Thus Darwin obtained,
-by crossing a pure white fantail with a black barb, hybrids which
-were partly blackish brown, partly mixed with white, but when he
-crossed these hybrids with others from two breeds which were
-likewise not blue, and had no bars, he obtained a slate-blue rock-pigeon,
-with bars on the wings and tail. We shall inquire later on
-how far it is correct to regard such cases as reversions to remote
-ancestors, but if we take it for granted in the meantime, we have
-here a proof of the descent of our breeds from a single wild species.
-This is corroborated, too, by everything that we know about the
-distribution of the rock-pigeon and the place and time of its
-domestication. It still lives on the cliff-guarded shores of England,
-Brittany, Portugal, and Spain, and both in India and in Egypt there
-were tame pigeons at a very early period. Pigeons appear on the
-menu of a Pharaoh of the fourth dynasty (3000 <span class="allsmcap">B.C.</span>), and of India
-we know at least that in 1600 <span class="allsmcap">A.D.</span> there were 20,000 pigeons
-belonging to the court of one of the princes.</p>
-
-<p>The beauty of this bird, and the ease with which it can be tamed,
-obviously called man's attention to it at a very early date, and it has
-been one of man's domestic companions for several thousands of years.
-Now we can distinguish at least twenty main races (<a href="#f1">Fig. 1</a>), which
-differ from each other as markedly as, if not more markedly than, the
-most nearly allied of the 288 wild species of pigeons which inhabit
-the earth. We have carriers and tumblers, runts and barbs, pouters,
-turbits and Jacobins, trumpeters and laughers, fantails, swallows,
-Indian pigeons, &amp;c.</p>
-
-<p><span class="pagenum"><a id="Page_35"></a>[Pg 35]</span></p>
-
-<div class="figcenter" id="f1">
-<a id="fig1" href="images/fig1big.jpg">
-<img src="images/fig1.jpg" alt=""/></a>
-
-<p class="caption"><span class="smcap">Fig. 1.</span> Group of various races of domestic pigeons (after Prütz). 1. Pouter. 2. Indian barb. 3. Bucharest trumpeter with a whorl
-of feathers (<i>Nelke</i>) on its forehead. 4. Nürnberger swallow. 5. Nürnberger bagadotte. 6. English carrier. 7. Fantail. 8. Eastern
-turbit. 9. Schmalkaldener Jacobin. 10. Chinese owl. 11. German turbit.</p>
-</div>
-
-
-
-<p><span class="pagenum"><a id="Page_36"></a>[Pg 36]</span></p>
-
-<p>Each of these races falls into sub-races; thus there is a German,
-an English, and a Dutch pouter-pigeon. The books on pigeons
-mention over 150 kinds which are quite distinct from one another,
-and breed true, that is, always produce young similar to themselves.</p>
-
-<p>Without entering upon a detailed description of any of these,
-I should like to call attention to the way in which certain characters
-have varied among them. Colour is a subordinate race-character,
-in so far that colour alone does not constitute a race, yet the colouring
-within a particular sub-race is usually very sharply defined, and in
-every breed there are sub-races of different colours. Thus there
-are white, black, and blue fantails, there are white turbits with
-red-brown wings, but also red ones with white heads, and white
-tumblers with black heads, &amp;c. Very unusual colours and colour-markings
-sometimes occur. Thus one sub-race of tumblers exhibits
-a peculiar clayey-yellow colour splashed with black markings, otherwise
-rare among pigeons, and almost suggestive of a prairie-hen; there is
-also a copper-red spot-pigeon, a cherry-red 'Gimpel'-pigeon, lark-coloured
-pigeons, &amp;c. Then we find all possible juxtapositions of
-colours, limited to quite definite regions of the body; thus we have
-white tumblers with a red head, red tail, and red wing-tips, or white
-tumblers with a black head, red turbits with white head, Indian
-pigeons quite black except for white wing-tips, and so on. The
-distribution of colour is often very complicated, but nevertheless, all
-the individuals of the breed show it in exactly the same manner. Thus
-there are the so-called blondinettes in which almost the whole body
-is copper-red, but the wings white, save that each quill bears at the
-rounded end of its vane a black and red fringe. I should never come
-to an end, if I were to try to give anything like a complete idea of the
-diversity of colouring among the various breeds of pigeons.</p>
-
-<p>Even such an important and, among wild species, unusually
-constant organ as the bill has varied among pigeons to an astonishing
-degree. Carrier-pigeons (<a href="#f1">Fig. 1</a>, No. 6) have an enormously long and
-strong bill, which is moreover covered with a thick red growth of the
-cere, while in the turbits and owls (<a href="#f1">Fig. 1</a>, Nos. 8 and 10) the bill is
-shorter than any we find among wild birds. In many breeds even the
-<i>form</i> of the bill deviates far from the normal, as in the bagadottes
-(No. 5) with crooked bill.</p>
-
-<p>Like the bill, the legs vary in regard to their length. The
-pouters (No. 1) stand on their long legs as on stilts, while the legs of
-the 'Nürnberger swallow' are strikingly small. Remarkable, too, and
-very different from the wild species, is the thick growth of feathers
-on the feet and toes of the pouters and trumpeters (<a href="#f1">Fig. 1</a>, No. 1),<span class="pagenum"><a id="Page_37"></a>[Pg 37]</span>
-as well as of some other breeds, which suggests the arrangement of
-feathers on a wing.</p>
-
-<p>Furthermore, the number and size of wing and tail-feathers in
-the different breeds often deviate considerably from the normal.
-The fantail (No. 7) in its most perfect form possesses forty tail-feathers,
-instead of the twelve usual in the wild rock-pigeon, and they
-are carried upright like a fan, while the head and neck of the bird are
-bent sharply backwards. In the hen-like pigeons the tail-feathers
-are few and short, so that they show an upright tail like that of a hen.
-I have already referred to the extraordinary carunculated skin-growth
-on the bill of many breeds; such folds also often surround the eye,
-and, as in the Indian barb (No. 3), are developed into well-formed thick
-circular ridges, while in the English carrier (No. 6) they lie about the
-bill as a formless mass of flesh.</p>
-
-<p>Even the skull has undergone many variations, as can be
-observed even in the living bird in many of the breeds with short
-forehead. Differences are to be found, too, in the number and breadth
-of the ribs, the length of the breast-bone, the number and size of the
-tail-vertebræ in different breeds. Of the internal organs, the crop in
-many breeds, but particularly in the pouters (No. 1), has attained an
-enormous size, and with this size is usually associated the habit of
-blowing it out with air, and assuming the characteristically upright
-position.</p>
-
-<p>That variations have taken place, too, in the most delicate
-structure of the brain, is shown by certain new instincts, such as the
-trumpeting of the trumpeters, the cooing of others, and the silence
-of yet other breeds, as well as by the curious habit of the tumblers of
-ascending quickly and vertically to a considerable height, and then
-turning over once, or even several times, in the course of their descent.
-In contrast to this, other breeds like the fantails have altogether
-given up the habit of flying high, and usually remain close to the
-dove-cot.</p>
-
-<p>Lastly, let me mention that the unusual development of individual
-feathers, or of groups of feathers, has become a race-character, upon
-which depend such remarkable structures as the feather-mantle
-turned over the head in the Jacobins (No. 9), the cap or plume on
-the head of various breeds, the white beard in the bearded tumbler,
-the collars which lie like a shirt-collar on the breast, or run down the
-sides of the neck (Nos. 8 and 10), and the circle of feathers which
-marks the root of the bill in the Bucharest trumpeter (No. 3).</p>
-
-<p>After what has been said, it is hardly necessary to add that the
-size of the whole body differs in different races. But the differences<span class="pagenum"><a id="Page_38"></a>[Pg 38]</span>
-are very considerable, for, according to Darwin, one of the largest
-runt-pigeons weighed exactly five times as much as one of the
-smallest tumblers with short forehead, and in the illustration (<a href="#f1">Fig. 1</a>)
-the pouter looks a giant beside the little barb to its left.</p>
-
-<p>Thus we see that nearly every part of the body of the pigeon has
-varied under domestication in the most diverse ways, and to a high
-degree; and the same is true of several other domesticated animals,
-poultry, horses, sheep, cattle, pigs, and so on, though the matter is not
-altogether so clear in their case, since descent from a single wild
-species cannot be proved, and is in many cases improbable. But in
-the case of pigeons this common descent is certain, and we have now
-to inquire in what manner all these variations from the parent form
-have been brought about.</p>
-
-<p>The answering of this question is rendered easier by the fact
-that new breeds arise even now, and that, to some extent at least,
-they can be caused to arise, consciously and intentionally. In
-England, as well as in Germany and France, there are associations for
-the breeding of birds, and in England especially pigeon and poultry
-clubs are numerous and highly developed. These by no means confine
-themselves to simply preserving the purity of existing breeds, they
-are continually striving to improve them, by increasing and accentuating
-their characters, or even by introducing quite new qualities,
-and in many cases they succeed even in this last. Prizes are offered
-for particular new variations, and thus a spirit of rivalry is fostered
-among the breeders, and each strives to produce the desired character
-as quickly as possible. Darwin says: 'The English judges decided
-that the comb of the Spanish cock, which had previously hung
-limply down, should stand erect, and in five years this end was
-achieved; they ordained that hens should have beards, and six years
-later fifty-seven of the groups of hens exhibited at the Crystal Palace
-in London were bearded.' The transformation does not always come
-about so quickly, however; thus, for instance, it required thirteen
-years before a certain breed of tumblers was furnished with a white
-head. But the breeders cause every visible part of the body to vary
-as seems good to them, and within the last fifty years they have
-really brought about very considerable changes in many breeds.
-Their method of procedure is carefully to select for breeding those
-birds which already possess a faint beginning of the desired character.
-Domesticated animals have on the whole a higher degree of variability
-than wild species, and the breeder takes advantage of this. Suppose
-it is a question of adding a crown of feathers to a smooth-headed
-breed, a bird is chosen which has the feathers on the back of the<span class="pagenum"><a id="Page_39"></a>[Pg 39]</span>
-head a little longer than usual, and mated for breeding. Among its
-descendants there will probably be some which also exhibit these
-slightly prominent feathers, and possibly there may be one or other
-of them which has these feathers considerably lengthened. This one
-is then used for breeding, and by continually proceeding thus, and
-selecting for breeding, from generation to generation, only the
-individuals which approach most nearly to the desired end, the
-wished-for character is at last secured.</p>
-
-<p>Thus it is not by crossing of different breeds, but by a patient
-accumulating of insignificant little variations through many generations,
-that the desired transformations are brought about. That is
-the magic wand by means of which the expert breeder produces his
-different breeds, we might almost say, as the sculptor moulds and
-remoulds his clay model according to his fancy. Quite according to
-his fancy the breeder has brought about all the fantastic forms we
-are familiar with among pigeons, mere variations which are of no
-use either to the bird itself or to man, which simply gratify man's
-whim without in many cases even satisfying his sense of beauty.
-For many of the existing breeds of pigeons, hens, and other domesticated
-animals, are anything but beautiful, the body being often
-unharmonious in structure and sometimes actually monstrous.</p>
-
-<p>Among pigeons, as well as among other domesticated animals,
-some changes have been brought about, which are not only of no use
-to their possessors, but would be actually disadvantageous if they
-were living under natural conditions. Some of the very short-billed
-breeds of pigeons have the bill so short and soft that the young can
-no longer use it to scratch and break the egg-shell, and would perish
-miserably if human aid were not at hand. The Yorkshire pig has
-become such a colossus of fat on weak, short legs, that if it were
-dependent on its own resources, it could not secure its food, much less
-escape from a beast of prey; and among horses the heavy cart-horse
-and the racer are alike unfit to cope with the dangers of a wild life,
-or the vicissitudes of weather.</p>
-
-<p>Breeding has done much to bring about variations useful to man.
-Thus we have breeds of cattle which excel in flesh, or in milk, or as
-draught animals, and sheep which excel in flesh or in wool, and to
-what a height the perfecting of a useful quality can be brought is
-shown, in regard to fineness of wool, by that finest breed of sheep,
-the merino, which instead of the 5,500 hairs borne by the old German
-sheep on a square inch, possesses 48,000.</p>
-
-<p>Not infrequently it is a particular stage of a species that has
-been bred by man, and the other stages have remained more or less<span class="pagenum"><a id="Page_40"></a>[Pg 40]</span>
-unaltered. Thus it is with one of the few domesticated insects, the
-silk-moth. Only the cocoon is of use to man, and according to the
-cocoon different breeds are distinguished, differing in fineness, colour,
-&amp;c.; but no breeds can be distinguished in reference to the larvæ, or
-the perfect insects. Among gooseberries there are about a hundred
-varieties distinguished according to the form, colour, size, thickness
-of skin, hairiness, &amp;c., of the fruits, but the little, inconspicuous,
-green blossoms, of which the breeders take no account, are alike in
-them all. In the pansies (<i>Viola tricolor</i>), on the other hand, it is
-only by the flowers that the varieties are distinguished, while the
-seeds have remained alike in all.</p>
-
-<p>It may be asked how it could have occurred to any one, when
-pigeons, for instance, first began to be domesticated, to wish to
-produce fantails or pouters, since he could have no mental picture
-of them in advance. Darwin replies to this objection, that it was not
-always conscious and methodical artificial selection, such as is now
-practised, that brought about the origin of breeds, but that they have
-very often resulted, and at first perhaps always, from unconscious
-selection. When savages tamed a dog, they used the 'best' of their
-dogs for breeding, that is, they chose those which had in the highest
-degree the qualities they valued, watchfulness, for instance, or if the
-dog were intended for the chase, keen scent and swiftness. In this
-way the body of the animal would be changed in a definite direction,
-especially if rivalry helped, and if it was the ambition of each to
-possess a dog as good as, or better than those of his tribal companions.
-That perfectly definite changes in bodily form can thus be brought
-about unconsciously is well illustrated by the case of a racehorse.
-This has arisen within the last two hundred years simply because the
-fleetest of the products of crossing between the Arab and the English
-horse were always chosen for breeding. It could not have been
-predicted that horses with thin neck, small head, long rump, and
-slender legs would necessarily be the swiftest runners; but this is
-the form which has resulted from the selection,&mdash;a very ugly, but
-very swift horse. This unconscious selection must undoubtedly have
-played a large part in the early stages of the evolution of the breeds
-of our domestic animals.</p>
-
-<p>But even in the fully conscious and methodical selective breeding
-of particular characters, the breeder rarely alters only the one his
-attention is fixed on; generally quite a number of other characters
-alter apart from his intention as an inevitable accompaniment of the
-desired variation on which attention was riveted. There are breeds
-of rabbits whose ears hang limply down instead of standing erect,<span class="pagenum"><a id="Page_41"></a>[Pg 41]</span>
-and in these so-called lop-eared rabbits the ear-muscles are partly
-degenerated, and as a consequence of this lack of muscular strain the
-skull has assumed another form. Thus the variation of one part may
-influence the development of a second and a third organ, and may
-even not stop there, for very often the influence has penetrated much
-deeper and affected quite remote parts of the body.</p>
-
-<p>If any one were to succeed in adding a heavy pair of horns to
-a breed of hornless sheep, there would run parallel with the course of
-this variation, which was directly aimed at, a long series of secondary
-changes which would affect at least the whole of the anterior half
-of the body; the skull would become thicker and stronger to support
-the weight of the heavy horns; the neck-tendon (<i>ligamentum nuchæ</i>)
-would have to become thicker to hold up the heavy head, and so also
-with the muscles of the neck; the spinous processes of the cervical
-and dorsal vertebrae would become longer and stronger, and the forelegs,
-too, would need to adapt themselves to the heavier burden.
-Every organism thus resembles, as it were, a mosaic, out of which no
-individual group of pieces can be taken and replaced by another
-without in some measure disturbing the correlation and harmony of
-the whole: in order to restore this, the pieces all round about the
-changed part must be moved or replaced by others.</p>
-
-<p>According to Darwin, it is to this correlation of parts that we must
-refer the variation of other parts besides the one intentionally altered in
-the course of breeding. It must be admitted that the mutual dependence
-of the parts plays a very important rôle in the economy and development
-of the animal body, as we shall see later, and these connexions
-still remain very mysterious to us. Especially is this the case with
-the connexion between the reproductive organs and the so-called
-secondary sexual characters. Removal of the reproductive organs
-or gonads induces, in Man, for instance, if it be effected in youth,
-the persistence of the childish voice and the non-development of the
-beard; in the stag the antlers do not appear, and in the cock the
-comb does not develop perfectly, &amp;c., but we are not yet able to
-understand clearly why this should be so.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_42"></a>[Pg 42]</span></p>
-
-<h2 class="nobreak" id="LECTURE_III">LECTURE III</h2>
-</div>
-
-<p class="c">THE DARWINIAN THEORY (<i>continued</i>)</p>
-
-<div class="blockquot">
-
-<p>Natural selection&mdash;Variation&mdash;Struggle for existence&mdash;Geometric ratio of rate of
-increase&mdash;Normal number and ratio of elimination in a species&mdash;Accidental causes
-of extinction&mdash;Dependence of the strength of a species on enemies&mdash;Struggle for
-existence between individuals of the same species&mdash;Natural selection affects all organs
-and stages&mdash;Summary.</p></div>
-
-
-<p><span class="smcap">In</span> artificial selection, through which, with or without conscious
-intention, our domesticated animals and cultivated plants have arisen,
-there must obviously be three kinds of co-operative factors: first,
-the <i>variability</i> of the species; second, the capacity of the organism
-for <i>transmitting</i> its particular characters to its progeny; and third,
-the <i>breeder</i> who selects particular qualities for breeding. No one of
-the factors can be dispensed with; the breeder could effect nothing,
-were there not presented to him the variations of parts in the
-particular direction in which he wishes them to vary; an indefinite
-variation, that is, a variation not guided by selection, would never
-lead to the formation of new breeds; the species would probably
-become in time a motley mixture of all sorts of variations, but
-a breed with definite characters, transmissible in their purity to its
-descendants, could never be formed. Finally, every process of selective
-breeding would be futile, if the variations which appeared could
-not be transmitted.</p>
-
-<p>Darwin assumes that processes of transformation quite similar
-to those which take place under the guidance of Man occur also in
-nature, and that it is mainly these which have brought about and
-guided the transformations of species which have taken place in
-the course of the earth's history. This process he calls <i>natural
-selection</i>.</p>
-
-<p>It will readily be admitted that two out of the three factors
-necessary to a process of selective breeding are present also in the
-natural conditions of the life of species. Variability in some degree
-or other is absent from no species of animal or plant, though it may
-be greater in one than in another, and it cannot be doubted that the
-inborn differences which distinguish one individual from another are
-capable of transmission. It is only to untrained observers that all the
-individuals of a species appear alike; for instance, all garden whites,
-or all the individuals of the small tortoiseshell butterfly (<i>Vanessa<span class="pagenum"><a id="Page_43"></a>[Pg 43]</span>
-urticæ</i>), or all the chaffinches. If the individuals are carefully compared
-it will be recognized that, even in these relatively constant
-species, no individual exactly resembles another; that even among
-butterflies twenty black scales may go to form a particular spot on
-the wings in one individual and thirty or twenty-five in others; that
-the length of the body, the legs, the antennæ, the proboscis exhibit
-minute differences; and it is probable that the same combination of
-quite similar parts never occurs twice. In many animals this cannot,
-of course, be proved, because our power of diagnosis is not fine
-enough to be able to estimate the differences directly, and because
-a comparison of measurements of all the parts in detail is not practicable.
-So we may here confine ourselves to the differences in the
-human race, which we can recognize with ease and certainty. Even
-as regards the face alone, all men differ from one another, and,
-numerous and complete as likenesses may be, it is impossible to find
-two human beings in which even the characters of the face are
-exactly similar. Even so-called 'identical twins' can always be
-distinguished if they are directly compared either in person or in
-a photograph, and if the rest of the body be also taken into consideration
-we find numerous small, sometimes even measurable
-differences.</p>
-
-<p>The same is true of animals, and it is only our lack of practice
-that is at fault if we frequently fail to detect their individual
-differences. The Bohemian shepherds are said to know personally,
-and be able to distinguish from all the rest, every sheep in their herds
-of many thousands. Thus the factors of variability and transmissibility
-must be granted, and it remains only to ask: Who plays the
-part of selecting breeder in wild nature? The answer to this question
-forms the kernel to the whole Darwinian theory, which ascribes this
-rôle to the conditions of life, to definite relations of individuals to the
-external influences which they meet with during the course of their
-lives, and which together make up their 'struggle for existence.'</p>
-
-<p>To make this idea clear I must to some extent diverge.</p>
-
-<p>It is a generally observed fact that, in every species of animals
-or of plants, more germs and more individuals are produced than
-grow to maturity, or become capable of reproduction. Numerous
-young individuals perish at an early stage, often because of unfavourable
-circumstances&mdash;cold, drought, damp, or through hunger, or at the
-hands of their enemies. When we ask which of the progeny perish
-early, and which survive to carry on the species, we are at first sight
-inclined to suppose that this is entirely a matter of chance; but this
-is just what Darwin disputed. It is not chance alone, it is, above all,<span class="pagenum"><a id="Page_44"></a>[Pg 44]</span>
-the differences between individuals, which enable them to withstand
-adverse circumstances better or worse, and thus decide, according to
-his view, which shall perish and which shall survive. If this be so,
-then we have a veritable process of selection, and one which secures
-that the 'best,' that is, the most capable of resistance, survive to breed,
-being thus, so to speak, 'selected.'</p>
-
-<p>It may be asked, however, why so many individuals must perish
-in youth, and whether it could not have been arranged that all, or at
-least most, should survive till they had reproduced. But this is an
-impossibility, unrealizable for this among other reasons, that organisms
-multiply in geometrical progression, and that their progeny
-would very soon exceed the limits of computability. This does not
-occur, for there is a limit set which they can in no case overstep,&mdash;which,
-indeed, as we shall see, they never reach&mdash;I mean the limits
-of space and food-supply. Every species, by the natural requirements
-of its life, is restricted to a particular habitat, to land or to water, but
-most are still more strictly limited to a definite area of the earth's
-surface, which alone affords the climate suited to them, or where alone
-the still more specialized conditions of their existence can be realized.
-Thus, for instance, the occurrence of a particular species of plant
-determines that of the animal which is dependent on it for its food-supply.
-If they could multiply unchecked, that is, without the loss
-of many of their progeny, every species would fill up its area of
-occurrence and exhaust the whole of its food-supply, and thus bring
-about its own extermination. This seems to be prevented in some
-way, for as a matter of fact it does not happen.</p>
-
-<p>It may, perhaps, be imagined that this might be prevented by
-a regulation of the productivity of the species, and that those which
-have not a large area of distribution, or can only count on a relatively
-limited food-supply, have also a low rate of multiplication, but this is
-not the case; even the lowest rate of multiplication would very soon
-suffice to make any species fill up its whole available space and completely
-exhaust its food-supply. Darwin takes as an example the
-elephant, which only begins to breed at thirty years of age, and
-continues to do so till about ninety, but so slowly that in these sixty
-years only three pairs of young are produced. Nevertheless, in 500
-years an elephant pair would be represented by fifteen millions of
-descendants, if all the young survived till they were capable of reproduction.
-A species of bird with a duration of life of five years,
-during which it breeds four times, producing and rearing four young
-each time, would in the course of fifteen years have 2,000 millions of
-descendants.</p>
-
-<p><span class="pagenum"><a id="Page_45"></a>[Pg 45]</span></p>
-
-<p>Thus, although the fertility of each species is, as a matter of fact,
-precisely regulated, a low rate of multiplication is not in itself
-sufficient to prevent the excessive increase of any species, nor is the
-quantity of the relevant food-supply. Whether this be very large or
-very small, we see that in reality it is never entirely used up, that, as
-a matter of fact, a much greater quantity is always left over than has
-been consumed. If increase depended only on food-supply, there
-would, for instance, be food enough in their tropical home for many
-thousand times more elephants than actually occur; and among
-ourselves the cockchafers might appear in much greater numbers
-than they do even in the worst cockchafer year, for all the leaves of
-all the trees are never eaten up; a great many leaves and a great
-many trees are left untouched even in the years when the voracious
-insects are the most numerous. Nor do the rose-aphides, notwithstanding
-their enormously rapid multiplication, ever destroy all the
-young shoots of a rose-bush, or all the rose-bushes of a garden, or
-of the whole area in which roses grow.</p>
-
-<p>At the same time it must be noted, that the number of individuals
-in a species undoubtedly does bear some relation to the amount of the
-food-supply available; for instance, it is very low among the large
-carnivores, the lion, the eagle, and the like. In our Alps the eagles have
-become rarer with the decrease of game, and where one eagle pair
-make their eyrie they rule alone over a hunting territory of more than
-sixty miles, a preserve on which no others of the same species are
-allowed to intrude. If there were several pairs of eagles in such a
-preserve, they would soon have so decimated the food-supply that
-they would starve. On the other hand, numerous herbivores, e.g.
-chamois and marmots, live within the bounds of the pair of eagles'
-hunting grounds, since the food they require is present in enormously
-greater quantity.</p>
-
-<p>While it is true that the number of individuals of a given species
-which live in a particular area is not exactly the same year in
-year out, being subject to small, and sometimes, as in the case of the
-aphides and cockchafers, to very great fluctuations, nevertheless we
-may assume that the <i>average number</i> remains the same, that in the
-course of a century, or, let us say, of a thousand years, the number of
-mature individuals inhabiting the particular area remains the same.
-This, of course, only holds true on the supposition that there has been
-no great change in the external conditions of life during this period.
-But before Man began to interfere with nature, these external
-conditions would remain uniform for much longer periods than we
-have assumed. Let us call the average number of individuals<span class="pagenum"><a id="Page_46"></a>[Pg 46]</span>
-occurring on such a uniform area, <i>the normal number</i> of the species;
-this number will be determined in the first instance by the number
-of offspring that are annually brought forth, and secondly by the
-number that annually perish before reaching maturity. As the
-fertility of a species is a definite quantity, so also will its elimination
-be definite, or, as we may say, when the normal number under
-uniform conditions of life remains constant, the ratio of elimination
-will also remain constant. Each species is therefore subject to a
-perfectly definite ratio of elimination which remains on the average
-constant, and this is the reason why a species does not multiply
-beyond its normal number notwithstanding the great excess of the
-food-supply, and notwithstanding the fertility which, in all species, is
-sufficient to lead to boundless multiplication.</p>
-
-<p>It is not difficult to calculate the ratio of elimination for a
-particular species, if one knows its rate of multiplication; for if the
-normal number remains constant, it follows that only two of all
-the offspring which a pair brings forth in the course of its life can
-attain to reproductive maturity, and that all the rest must perish.</p>
-
-<p>Suppose, for instance, a pair of storks produced four young ones
-annually for twenty years, of these eighty young ones which are born
-within this period, on an average seventy-eight must perish, and only
-two can become mature animals. If more than two attained maturity
-the total number of storks would increase, and this is against the
-presupposition of constancy in the normal number. It is important,
-in reference to the fact on which we are now focusing our attention,
-that we should consider some other illustrations from the same point
-of view. The female trout yearly produces about 600 eggs; let us
-assume that it remains capable of reproduction for only ten years,
-then the elimination-number of the species will be 6,000 less two,
-that is, 5,998, for of the 6,000 eggs only two can become mature
-animals. But in the majority of fishes the ratio of extermination is
-enormously greater than this. Thus a female herring brings forth
-40,000 eggs annually, the duration of life is estimated at ten years,
-and this means an elimination number of 400,000 less two, that is,
-399,998. The carp produces 200,000 eggs a year, and the sturgeon
-two millions, and both species live long, and remain capable of
-reproduction for at least fifty years. But of all the 100 million eggs
-which are produced by the sturgeon, only two reach their full
-development and reproduce; all others perish prematurely.</p>
-
-<p>But even with these examples we have not reached the highest
-elimination number, for many of the lower animals&mdash;not to speak
-of many plants&mdash;produce an even greater number of offspring.<span class="pagenum"><a id="Page_47"></a>[Pg 47]</span>
-Leuwenhoek calculated the fertility of a thread-worm at sixty
-million eggs, and a tape-worm produces hardly less than 100 millions.</p>
-
-<p>There exists, therefore, a constant relation between fertility and
-the ratio of elimination; the higher the latter is, the greater must the
-former be, if the species is to survive at all. The example of the
-tape-worm makes this very obvious, for here we can readily understand
-why the fertility must be so enormous, as we are aware of the
-long chain of chances on which the successful development of this
-animal depends. The common tape-worm of Man, <i>Tænia solium</i>,
-does not lay its eggs, they remain enclosed within one of the
-liberated joints or 'proglottides.' Only if this liberated joint or one
-of the embryos within it happens to be fortuitously eaten by a pig or
-other mammal can there be successful development, and even then
-under difficulties and possible failures, and not right away into adult
-animals, but first into microscopically minute larvæ which may bore
-their way into the walls of the intestine, or, if they are fortunate
-enough, may get into the blood-stream and be carried by it to
-a remote part of the body. There they develop into 'measles,' the
-so-called bladder-worms, within which the head of the tape-worm
-arises. But in order that this may become a complete and reproductive
-adult worm the pig must die, and the next step necessary is that
-a piece of the flesh of the infected first host must happen to be
-swallowed raw by a man or other mammal! Only then does the
-fortunate bladder-worm&mdash;swallowed with the flesh&mdash;attain the goal
-of its life, that is, a suitable place to mature in, the food-canal of
-a human being. It is obvious that countless eggs must be lost for
-one that succeeds in getting through the whole course of a development
-depending so greatly on chance. Hence the necessity for such
-enormous productivity of eggs.</p>
-
-<p>In many cases the causes of elimination, which keep a species
-within due bounds, are very difficult to determine. Enemies, that is
-to say, other species which use the species in question as food, play an
-important rôle; often, however, the cause lies in the unfavourableness
-of external conditions, in chance, which is favourable only to one of
-a thousand. The oak would only require to produce one seed in the
-500 years of its life, if it were certain that that one would grow into
-an oak-tree; but most of the little acorns are eaten up by pigs, squirrels,
-insects, &amp;c., before they have had time to sprout, thousands fall on
-ground already thickly covered with growth where they cannot take
-root, and even if they do succeed in finding an unoccupied space in
-which to germinate, the young plants are still surrounded by a thousand
-dangers&mdash;the possibility of being devoured by many animals large and<span class="pagenum"><a id="Page_48"></a>[Pg 48]</span>
-small, of being suffocated by the surrounding vegetation, and so on.
-We can thus understand, to some extent, though only approximately,
-why it is that the oak must year by year produce thousands of seeds
-in order that the species may maintain its normal number, and not be
-exterminated; for it is obvious that a constant, even though slow
-diminution of the normal number, a regular deficit, so to speak, can
-end in nothing else than the gradual extinction of the species.</p>
-
-<p>But even this prodigality of seeds is not the greatest reach of
-fertility that we meet with in nature; it is, perhaps, amongst the
-simpler flowerless plants that we find the climax. It has been
-calculated that a single frond of the beautiful fern so common in our
-woods, <i>Aspidium filix mas</i>, produces about fourteen million spores.
-They serve to distribute the species, and are carried as motes by the
-wind, but comparatively few of the millions ever get the length of
-germinating at all, much less of attaining to full development into
-adult plants. Thus we see that the apparent prodigality of nature is
-a real necessity, an indispensable condition of the maintenance of the
-species; the fertility of each species is related to the actualities of
-elimination to which it is exposed. This is clearly seen when
-a species is placed under new and more favourable conditions of life,
-in which it has an abundant food-supply and few enemies. This
-was the case, for instance, with the horses introduced from Europe
-into South America, where they reverted to a feral state, and are now
-represented by herds of many thousands roaming the great grassy
-plains. If the small singing-birds of a region diminish in number,
-there is a great increase of caterpillars and other injurious insects
-which form part of their food-supply. The colossal destruction
-which the much-dreaded nun-moth from time to time brings about
-in our woods probably depends in part on the diminution of
-one or another of the many animals inimical to insects; but the
-occurrence of several years of weather-conditions favourable to the
-larvæ must also be taken into account. How enormously, indeed
-almost inconceivably, the number of larvæ may increase under
-favourable conditions is shown by such devastations as that in
-Prussia in 1856, when many square miles of forest were absolutely
-eaten up. The caterpillars were so numerous that even from some
-distance the falling excrement could be heard rustling like rain, and
-ten hundredweights of the eggs were collected, with an average of
-20,000 eggs to the half-ounce!</p>
-
-<p>But it would be a great mistake to conclude, from this enormous
-and sudden increase in the number of individuals, that the normal
-number of individuals is determined by the number of enemies alone.<span class="pagenum"><a id="Page_49"></a>[Pg 49]</span>
-The average number of individuals in a species depends on many
-other conditions, especially on the extent of the available area, and
-on the amount of the food-supply in relation to the size of body
-in the species. I cannot dwell on this now, but I wish to point out
-that, for the continuance of a species, it is indifferent whether it is
-'frequent' or 'rare,' if we presuppose that its normal number remains
-on an average constant for centuries, that is, that its fertility suffices
-to make good the continual losses through enemies and other causes
-of elimination. One would be inclined to conclude from such cases
-of sudden and enormous increase in the number of individuals as
-these caterpillar-blights, that enemies and other causes of destruction
-played the major part in the regulation of the normal number of the
-species. But this is only apparently the case. Enemies necessitate
-a certain fertility in the species on which they prey, so that the
-elimination in each generation may be made good; but the number of
-pairs capable of reproduction is not thereby decisively determined.
-We must not forget that the number of enemies is also, on the other
-hand, dependent on the number of victims, and that the normal
-number of enemies must rise and fall with that of the species preyed
-upon.</p>
-
-<p>For this reason, such an enormous increase as that of the caterpillars
-cannot last long; it carries its corrective in itself. The
-appearance of the caterpillars in such enormous numbers in itself
-increases the host of their enemies; singing-birds, ichneumon-flies,
-beetle-grubs, and predaceous beetles find abundant and available
-food, and therefore reproduce and multiply so rapidly, that, with the
-help of the caterpillar's plant-enemies, especially the insect-destroying
-fungi, they soon reduce the caterpillars to their normal number, or
-even below it. But then the reverse process begins; the enemies of
-the caterpillars diminish because their food has become scarce, and
-their normal number is lowered, while that of the caterpillars
-gradually rises again.</p>
-
-<p>When the number of foxes in a hunting district increases, the
-number of the hares that they prey upon diminishes, and, on the other
-hand, the decimating of the foxes by Man brings about an increase in
-the number of hares in the district. Under natural conditions, that
-is, without the intervention of Man, there would be a constant
-balancing of the numbers of hares and foxes, for every noteworthy
-increase of the hares would be followed by a similar increase of foxes,
-and this, in its turn, would diminish the number of hares, so that
-they would no longer suffice for the support of so many foxes, and
-these would decrease in number again, until the number of hares had<span class="pagenum"><a id="Page_50"></a>[Pg 50]</span>
-again increased because of the lessened persecution and elimination.
-In nature the case is not quite so simple, because the fox does not
-live on hares alone, and the hare is not preyed upon only by the fox;
-but the illustration may serve to elucidate the point that a moving
-equilibrium is maintained between the species of a district, between
-persecutors and persecuted, in such a way that the number of
-individuals in the two species is always varying a little up and
-down, and that each influences the other so that a regulative process
-results. Throughout periods of considerable length the average
-remains the same; that is to say, a <i>normal number</i> is established.
-This normal strength of population is the mean above and below
-which the number of individuals is constantly varying. It is, of
-course, seldom that the mutual influences and regulations are so
-simple as in the example given; usually several or even many
-species interact upon each other, and not beasts of prey and their
-victims alone, but the most diverse species of animals and plants,
-which do not stand in any obvious relation to one another at
-all. Moreover, the physical, and especially the climatic conditions, also
-cause the normal number of the species to rise and fall.</p>
-
-<p>The inter-relations between species living together on the same
-area are so intricate that I should like to give two other illustrations.
-Let us first take Darwin's famous instance of the fertility of clover,
-which depends on the number of cats. It is of course only an
-imaginary one, but the facts it is based upon are quite correct. The
-number of cats living in a village to a certain extent determines the
-number of field-mice in the neighbourhood. These again destroy the
-nests of the humble-bees, which live in holes in the ground, and thus
-the number of humble-bees depends on that of the field-mice and cats.
-But the clover must be pollinated by insects if it is to produce fertile
-seed, and only the humble-bee has a proboscis long enough to effect
-the pollination. Therefore the quantity of clover-seed annually
-produced depends on the number of humble-bees, and ultimately upon
-the number of cats. And, as a matter of fact, humble-bees were introduced
-into New Zealand from England, because without them the
-clover would produce no fertile seeds.</p>
-
-<p>On the grassy plains of Paraguay there are no wild cattle and
-horses, because of the presence of a fly which has a predilection for
-laying its eggs in the navel of the newly-born calves and foals, with
-the result that the calves or foals are killed by the emerging maggots.
-We may reasonably assume that the numerical strength of this fly-species
-depends on the distribution of insect-eating birds, whose
-numbers in turn are determined by certain beasts of prey. These<span class="pagenum"><a id="Page_51"></a>[Pg 51]</span>
-again vary in number in relation to the extent of the forest-land, and
-this is determined by the number of ruminants which browse on the
-young growth of the woods (Darwin).</p>
-
-<p>That forests can actually be totally destroyed by ruminants is
-proved by the case of the island of St. Helena among others. On its
-discovery the island was covered with thick wood, but in the course
-of 200 years it was transformed into a bare rock by goats and pigs,
-which devoured the young growth so completely that trees which
-were felled or which died were not replaced.</p>
-
-<p>This point is vividly illustrated by Darwin's observation of a
-wide heath on which stood only a few groups of old pine-trees. The
-mere fencing in of a portion of the heath sufficed to call forth a thick
-growth of young seedling pines within the enclosure, and an examination
-of the open part of the heath revealed that the grazing cattle had
-eaten up all the young pine-trees which sprang from seed, and that
-again and again. In one small space thirty-two little trees stood concealed
-in the grass, and several of these showed as many as twenty-six
-yearly rings.</p>
-
-<p>How definitely the number of individuals in different species
-living on the same area mutually limit and thereby regulate each
-other, Darwin sought to illustrate also by the case of the primitive
-forest, where the numerous species of plants occur, not mixed together
-irregularly, but in a definite proportion. We can find examples of the
-same kind wherever the plant-growth of a district has been left to
-itself. If we walk along the banks of our little river, the Dreisam, we
-see a wild confusion of the most diverse trees, shrubs and herbaceous
-plants. But, even though it cannot be demonstrated, we may be
-certain that these are represented in definite numerical proportions,
-dependent on the natural qualities and requirements of each species,
-on the number of their seeds and the facilities for their distribution,
-on the favourable or unfavourable season at which they ripen, and on
-their varying capacity for taking root in the worst ground, and
-springing quickly up, &amp;c. They limit each other mutually, so that
-the whole flora of the river-bank will be made up of one per cent. of
-this species, one per cent. of that, and, it may be, five per cent. of a
-third, and the same combination will repeat itself in the same
-proportions on the banks of other rivers of our country in as far as
-the external conditions are the same. The same must be true of the
-fauna of such a plant-thicket; the animal species also limit one another
-mutually, and thereby regulate the number of individuals, which
-becomes relatively stable over any area on which the conditions remain
-the same. That is to say, a 'normal number' is attained and persists.</p>
-
-<p><span class="pagenum"><a id="Page_52"></a>[Pg 52]</span></p>
-
-<p>Thus we see that the capacity for boundless multiplication
-inherent in every species is limited by the co-existence of other
-species; there is, metaphorically speaking, a continuous struggle
-going on between species, plant and animal alike; each seeks as far
-as possible to multiply, and each is hemmed in by the others and
-as far as possible prevented from doing so. The 'struggle' is by no
-means only the <i>direct</i> limitation of the number of individuals, which
-consists in the use of one species by another as food, as in beasts of
-prey and their victims, or locusts and plants; it is much more the
-indirect limitation&mdash;figuratively speaking, the struggle for space, for
-light, for moisture among plants, for food among animals. But all this,
-important as it is, does not yet exhaust the content of that 'struggle
-for existence' to which Darwin and Wallace ascribe the rôle of the
-breeder in the process of natural selection. The struggle, that is, the
-mutual limiting of species, may indeed restrict a species in its distribution,
-and may reduce its normal number possibly to nil. In
-other words, it may bring about extinction, but it cannot make a
-species other than it is. This can only be done by a struggle within
-the limits of the species itself, and this struggle is due to the fact that
-of the numerous offspring, on an average those survive&mdash;that is,
-attain to reproduction&mdash;which are the most fit, whose constitution
-makes it most possible for them to overcome the difficulties and
-dangers of life, and so to reach maturity. We see, in fact, that a
-large percentage of each generation in all species always perishes
-before attaining maturity. If, then, the decision as to which is to
-perish and which is to reach maturity is <i>not a matter of chance alone</i>,
-but is in part due to the constitution of the growing individual; if
-the 'fittest' do <i>on the average</i> survive, and the 'least fit' are on the
-average eliminated, we have here a process of selection entirely comparable
-to that of artificial selection, and one whose result must be
-the 'improvement' of the species, whether that depends on one set of
-characters or on another. The victorious qualities, which earlier were
-peculiar to certain individuals, must gradually become the common
-property of the species, if in each generation the individuals which
-attained to reproduction all possessed them, and thus could transmit
-them to their progeny. But those of the descendants which did not
-inherit them would again be at a disadvantage in the struggle for
-existence, or rather for reaching maturity, if in each generation a
-higher percentage of individuals which possess these characters reach
-maturity than of those which do not possess them. This percentage
-must increase in each generation, because, in each, natural selection
-again chooses out the fittest, and it must finally rise to 100 per<span class="pagenum"><a id="Page_53"></a>[Pg 53]</span>
-cent., that is to say, none but individuals of this fittest type will be
-left surviving.</p>
-
-<p>This does not yet exhaust the process, however, for we can infer
-from the results of artificial breed-forming that the selected characters
-may intensify from generation to generation, and that they will
-continue to do so as long as it gives them any advantage in the struggle
-for existence, for so long will it lead to the more frequent survival
-of its possessors. The increase will only stop when it has reached
-the highest degree of usefulness, and in this way new characters
-may be formed, just as, in artificial selection, the short upward-turning
-feathers of the Jacobin pigeon have been intensified into
-the peruke, a feather canopy covering the head.</p>
-
-<p>A few examples of natural selection will make the process
-clearer. Our hare is well secured from discovery by his fur of
-mixed brown, yellow, white, and black, when he cowers in his form
-among the dry leaves of the underwood. It is easy to pass close
-to him without seeing him. But if the ground and the bushes are
-covered with snow, he contrasts conspicuously with them. Suppose,
-now, that our climate became colder, and that the winter brought
-lasting snow, the hares which had the largest mixture of white
-in their fur would have an advantage in their 'struggle for
-existence' over their darker fellows; they would be less easily
-discovered by their enemies&mdash;the fox, the badger, the horned owl,
-and the wild cat. Of the numerous hares which would annually
-become the prey of these enemies, there would be, on an average, more
-dark than light individuals. The percentage of light-coloured hares
-would, therefore, increase from generation to generation, and the
-longer the winter the keener would be the selection between dark
-and light hares, until finally none but light ones would remain. At
-the same time, the colour of the hares would become increasingly
-light, first, because it would happen more and more frequently that
-two light hares would pair, and secondly, because, after a time, the
-struggle for existence would no longer be between light and dark
-hares, but between light hares and still lighter ones. Thus ultimately
-a race of white hares would arise, as has actually happened
-in the Arctic regions and on the Alps.</p>
-
-<p>Or let us think of a herbaceous plant, in appearance something
-like a belladonna, rich in leaves and very juicy, but not poisonous.
-It would doubtless be a favourite food with the animals of the forest,
-and it would not, therefore, attain to more than a sparse occurrence,
-since few of the individuals would be able to form seeds. But now
-let us assume that a stuff of very unpleasant taste develops in<span class="pagenum"><a id="Page_54"></a>[Pg 54]</span>
-the stem and leaves of some of the individuals, as may easily happen
-through very slight changes in the chemical metabolism of the plant,
-what, then, could result but that such individuals would be less
-readily eaten than the others? A process of selection must, therefore,
-ensue, and the unpleasant-tasting specimens of the plant would be
-much more frequently spared, and consequently would bear seed
-much oftener than the palatable ones. Thus the number of unpalatable
-plants would increase from year to year. If the stuff in
-question were not only unpalatable but poisonous, or gradually
-became so, a plant would in time be evolved which would be
-absolutely safe from being devoured by animals, just as the deadly
-nightshade (<i>Atropa belladonna</i>) actually is.</p>
-
-<p>Or let us suppose that a stretch of water is inhabited by a species
-of carp, which have hitherto had no large enemy, and so have become
-lazy and slow, and that there migrates from the sea into this stretch
-of water a large species of pike. At first numerous carp will fall
-victims to the pike, and the pike will rapidly increase in number.
-But if all the carp were not equally lazy and dull-witted, if some of
-them were quicker and more intelligent, these would, on an average,
-become more rarely the victims of the pike, and numerous individuals
-with these better qualities would survive in each generation, till
-ultimately there were no others, and the useful characters would
-gradually become intensified, and so a more active and wary race of
-carp would arise.</p>
-
-<p>Let us suppose, however, that the increased activity and
-wariness would not alone suffice to preserve the colony from extinction;
-it might require also an increased fertility to prevent the
-normal number from being permanently lowered; but even this could
-eventually be brought about by natural selection, if the nature of
-the species and the general conditions of its life permitted. For there
-are variations of fertility in every species, and if the chance of seeing
-some of its eggs become mature animals were greater for the more
-fertile female than for the less fertile, <i>ceteris paribus</i>, a process
-of selection must take place, which would result in an increase of
-fertility as far as that was possible.</p>
-
-<p>Obviously, such processes of natural selection can affect all parts
-and characters&mdash;size and form of the body, as well as isolated
-parts, the external skin and its colour, every internal organ&mdash;and
-not bodily characters alone, but psychical ones as well, such as
-intelligence and instincts. According to this principle, it is only
-characters which are biologically indifferent that cannot be altered
-through natural selection.</p>
-
-<p><span class="pagenum"><a id="Page_55"></a>[Pg 55]</span></p>
-
-<p>Natural selection can also bring about changes at every age,
-for the elimination of individuals begins from the egg, and any kind
-of egg which is in some way better able to escape elimination
-will transmit its useful characters to its descendants, because the
-resulting young animals will thus more frequently reach full development
-than the young from other eggs. In the same way, at every
-succeeding stage of development, every character favourable to the
-preservation of the individual will be maintained and intensified.</p>
-
-<p>We see from all this that natural selection is vastly more
-powerful than artificial selection by Man. In the latter, only one
-character at a time can be caused to change, while natural selection
-may influence a whole group of characters at the same time, as
-well as all the stages of development. Through the weeding out
-of the individuals which are annually exterminated, it is always
-on an average the 'fittest' which survive, that is to say, those which
-have the greatest number of bodily parts and rudiments of parts
-in the fittest possible condition of development at every stage. The
-longer this process of selection continues, the smaller will be the
-deviations of the individual from this standard, and the more minute
-will be the differences of fitness determining which is to be eliminated
-and which is to survive to reproduce its characteristics. In the
-immeasurable periods of time which are at the disposal of natural
-selection, and in the inestimable numbers of individuals on which
-it may operate, lie the essential causes of superiority of natural
-selection over the artificial selection of Man.</p>
-
-<p>To sum up briefly: Natural selection depends essentially on
-the cumulative augmentation of the most minute useful variations
-in the direction of their utility; only the useful is developed and
-increased, and great effects are brought about slowly through the
-summing up of many very minute steps. Natural selection is a self-regulation
-of the species which secures its preservation; its result
-is the ceaseless adaptation of the species to its life-conditions. As soon
-as these vary, natural selection changes its mode of action, for what
-was previously the best is now no longer so; parts that before had to
-be large must now perhaps be small, or vice versa; muscle-groups
-which were weak must now become strong, and so on. The conditions
-of life are, so to speak, the mould into which natural selection
-is continually pouring the species anew.</p>
-
-<p>But the philosophical significance of natural selection lies in
-the fact, that it shows us how to explain the origin of useful, well-adapted
-structures purely by mechanical forces and without having
-to fall back on a <i>directive</i> force. We are thus for the first time in<span class="pagenum"><a id="Page_56"></a>[Pg 56]</span>
-a position to understand, in some degree, the marvellous adaptation
-of the organism to an end, without having to call to our aid any supernaturally
-intrusive force on the part of the Creator. We understand
-now how, in a purely mechanical way, through the forces always
-at work in nature, all forms of life must conform to, and adapt
-themselves precisely to the conditions of their life, since only the
-best possible is preserved, and everything less good is continually
-being rejected.</p>
-
-<p>Before I go on to expound in detail the phenomena which we refer
-to natural selection, I must briefly state that Darwin did not ascribe
-to natural selection by any means all the changes which have taken
-place in organisms in the course of time. On the one hand, he
-ascribed a not inconsiderable importance to the correlated variations
-we have already mentioned; still more, however, he relied on the
-direct influence of altered conditions of life, whether these consist
-in climatic and other changes in the environment, or in the assumption
-of new habits, and the increased or diminished use of individual
-parts and organs thereby induced. He recognized the principle
-so strongly emphasized by Lamarck, of use and disuse as a cause
-of heritable increase or decrease of the exercised or neglected part,
-though he did so with a certain reserve. I shall return later to these
-factors of modification, and shall then attempt to show that these
-too are to be referred to processes of selection, which are, however, of
-a different order from the phenomena which the Darwin-Wallace
-principle of natural selection serves to interpret. But, in the first
-instance, it appears to me to be necessary to show how far the
-Darwin-Wallace interpretation will suffice, and in the next lectures
-we shall occupy ourselves with this question exclusively.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_57"></a>[Pg 57]</span></p>
-
-<h2 class="nobreak" id="LECTURE_IV">LECTURE IV</h2>
-</div>
-
-<p class="c">THE COLORATION OF ANIMALS AND ITS RELATION
-TO THE PROCESSES OF SELECTION</p>
-
-<div class="blockquot">
-
-<p>Biological significance of colours&mdash;Protective colours of eggs&mdash;Animals of the
-snow-region&mdash;Animals of the desert&mdash;Transparent animals&mdash;Green animals&mdash;Nocturnal
-animals&mdash;Double colour-adaptation&mdash;Protective marking of caterpillars&mdash;Warning
-markings&mdash;Dimorphism of colouring in caterpillars&mdash;Shunting back of
-colouring in ontogeny&mdash;'Sympathetic' colouring in diurnal Lepidoptera&mdash;In nocturnal
-Lepidoptera&mdash;Theoretical considerations&mdash;The influence of illumination in the production
-of protective colouring, <i>Tropidoderus</i>&mdash;Harmony of protective colouring in
-minute details&mdash;<i>Notodonta</i>&mdash;Objections&mdash;Imitation of Strange objects, <i>Xylina</i>&mdash;Leaf-butterflies,
-<i>Kallima</i>&mdash;<i>Hebomoja</i>&mdash;Nocturnal Lepidoptera with leaf-markings&mdash;Orthoptera
-resembling leaves&mdash;Caterpillars of the Geometridæ.</p></div>
-
-
-<p><span class="smcap">We</span> have seen what Darwin meant by natural selection, and
-we understand that this process really implies a transformation of
-organisms by slow degrees, in the direction of adaptive fitness&mdash;a
-transformation which must ensue as necessarily as when a human
-selector, prompted by conscious intention, tries to improve an animal
-in a particular direction, by always selecting the 'fittest' animals
-for breeding. In nature, too, there is selection, because in every
-generation the majority succumb in the struggle for life, while on
-an average those which survive, attain to reproductive maturity, and
-transmit their characters to their descendants, are those which are
-best adapted to the conditions of their life&mdash;that is, which possess
-those variations of most advantage in overcoming the dangers of life.
-Since individuals are always variable in some degree, since their
-variations can be inherited by their progeny, and since the continually
-repeated elimination of the majority of those descendants
-is a fact, the inference from these premisses must be correct; there
-must be a 'natural selection' in the direction of a gradually increasing
-fitness and effectiveness of the forms of life.</p>
-
-<p>We cannot, however, directly observe this process of natural
-selection; it goes on too slowly, and our powers of observation are
-neither comprehensive nor fine enough. How could we set about
-investigating the millions of individuals which constitute the numerical
-strength of a species on a given area, to find out whether they possess
-some variable character in a definite percentage, and whether this
-percentage increases in the course of decades or centuries? And<span class="pagenum"><a id="Page_58"></a>[Pg 58]</span>
-there is, furthermore, the difficulty of estimating the biological
-importance of any variation that may occur. Even in cases where
-we know its significance quite well in a general way, we cannot
-estimate its relative value in reference to the variation of some other
-character, though that other may also be quite intelligible. Later
-on, we shall speak of protective colouring, and in so doing we shall
-discuss the caterpillars of one of the Sphingidæ, which occur in two
-protective colours, some being brown, others green. From the greater
-frequency of the brown form we may conclude that brown is here
-a better adaptation than green, but how could we infer this from
-the character itself, or from our merely approximate knowledge of
-the mode of life of the species, its habits, and the dangers which
-threaten it? A direct estimation of the relative protective value of
-the two colours is altogether out of the question. The survival of the
-fittest cannot be proved in nature, simply because we are not in
-a position to decide, <i>a priori</i>, what the fittest is. For this reason
-I was forced to try to make the process of natural selection clear by
-means of imagined examples, rather than observed ones.</p>
-
-<p>But though we cannot directly follow the uninterrupted process
-of natural selection which is going on under natural conditions, there
-is another kind of proof for this hypothesis, besides that which
-consists in logically deducing a process from correct premisses; I should
-like to call this the practical proof. If a hypothesis can be made
-to explain a great number of otherwise unintelligible facts, it thereby
-gains a high degree of probability, and this is increased when there
-are no facts to be found which are in contradiction to it.</p>
-
-<p>Both of these criteria are fulfilled by the selection-hypothesis,
-and indeed the phenomena which may be explained by it, and are
-intelligible in no other way, present themselves to us in such enormous
-numbers, that there can be no doubt whatever as to the correctness of
-the principle; all that can be still disputed is, how far it reaches.</p>
-
-<p>Let us now turn our attention to this practical way of proving
-the theory by the facts which it serves to interpret, beginning with
-a consideration of the external appearance of organisms, their colour
-and form.</p>
-
-
-<p class="c"><i>The Colour and Form of Organisms.</i></p>
-
-<p>Erasmus Darwin had in many cases already rightly recognized
-the biological significance of the colouring of an animal species, and
-we may be sure that many of the numerous good observers of earlier
-times had similar ideas. I can even state definitely that Rösel von
-Rosenhof, the famous miniature-painter and naturalist of Nürnberg<span class="pagenum"><a id="Page_59"></a>[Pg 59]</span>
-in the middle of the eighteenth century, recognized clearly, and gave
-beautiful descriptions of what we now call colour-adaptation. It is
-true that he gave them only as isolated instances, and was far from
-recognizing the phenomenon of colour-adaptation in general, or even
-from inquiring into its causes. From the time of Linné, the endeavour
-to establish new species overshadowed all the finer observation of
-life-habits and inter-relations, and, later on, after Blumenbach,
-Kielmeyer, Cuvier, and others, the eager investigation of the internal
-structure of animals also tended to divert attention from these
-œcological relations. In systematic zoology, colour ranked only as
-a diagnostic character of subordinate value, because it is often not
-very stable, and indeed is sometimes very variable; it was therefore
-found preferable to keep to such relatively stable differences as are to
-be found in the form, size, and number of parts.</p>
-
-<p>Charles Darwin was the first to redirect attention to the fact
-that the colouring of animals is anything but an unimportant matter;
-that, on the contrary, in many cases it is of use to the animal, e.g. in
-making it inconspicuous; a green insect is not readily seen on green
-leaves, nor a grey-brown one on the bark of a tree.</p>
-
-<p>It is plain that the origin of such a so-called 'sympathetic'
-coloration, harmonizing with the usual environment of the animal,
-can be easily interpreted in terms of the principle of selection; and it
-is equally evident that it cannot be explained by the Lamarckian
-principle of transformation. Through the accumulation of slight
-useful variations in colour, it is quite possible for a green or a brown
-insect to arise from a previous colour, but a grey or a brown insect
-could not possibly have become a green one simply by getting into
-the habit of sitting on a green leaf; and still less can the will of the
-animal or any kind of activity have brought the change about.
-Even if the animal had any idea that it would be very useful to it to
-be coloured green, now that it had got into the habit of sitting on
-a leaf, it could not have done anything towards attaining the desirable
-green colour. Quite recently the possibility of a kind of colour-photography
-on the skin of the animal has been suggested, but there
-are many species whose colouring is in contrast to their environment,
-so that the skin in these cases does not act as a photographic plate,
-and it would, therefore, have to be explained how it comes to pass
-that it functions as such in the sympathetically coloured animals.
-I do not ask for proof of the chemical composition of the stuff which
-is supposed to be sensitive to light. Whether this be iodide of silver
-or something quite different, the question remains the same: how
-comes it that it has only appeared in animals to which a sympathetic<span class="pagenum"><a id="Page_60"></a>[Pg 60]</span>
-colouring is advantageous in the struggle for life? And the answer,
-from our point of view, must read: it has arisen through natural
-selection in those species to which a sympathetic colouring is useful.
-Thus even if the supposition that sympathetic colouring is due to
-automatic photography on the part of the skin were correct, we
-should still have to regard it as an outcome of natural selection; but
-it is not correct&mdash;at least in general&mdash;as the above objection shows,
-and as will be further apparent from many of the phenomena of
-colour-adaptation which I shall now adduce.</p>
-
-<p>To explain sympathetic coloration, then, we must assume, with
-Darwin and Wallace, a process of selection due to the fact that, as
-changes took place in the course of time in the colouring of the
-surroundings, those individuals on an average most easily escaped
-the persecution of their enemies which diverged least in colour from
-their surroundings, and so, in the course of generations, an ever
-greater harmony with this colouring was established. Variations
-in colouring crop up everywhere, and as soon as these reached such
-a degree as to afford their possessors a more effective protection than
-the colouring of their fellows, then natural selection of necessity
-stepped in, and would only cease to act when the harmony with the
-environment had become complete, or, at least, so nearly so that any
-increase of it could not heighten the deception.</p>
-
-<p>Of course, it is presupposed in the working out this selective
-process that the species has enemies which see. This is the case,
-however, with most animals living on the earth or in the water,
-unless they are of microscopic minuteness. Many animals, too, are
-subject to persecution not only in their adult state, but at almost
-every period of their life, and so, in general, we should expect that
-many of them would have attained at each stage that coloration
-of body that would render them least liable to discovery by their
-enemies.</p>
-
-<p>And this is in reality the case: numerous animals are protected
-in some measure by so-called sympathetic colouring, from the egg to
-the adult state.</p>
-
-<p>Let us begin with the egg, and of course there is no need to
-speak of any eggs except those which are laid. Of these many are
-simply white in colour, e.g. the eggs of many birds, snakes, and
-lizards, and this seems to contradict our prediction; but these eggs
-are either hidden in earth, compost, or sand, as in the case of the
-reptiles, or they are laid in dome-shaped nests, or concealed in holes
-in trees, as in many birds; thus they require no protective colouring.</p>
-
-<p>In other cases, however, numerous eggs, especially of insects and<span class="pagenum"><a id="Page_61"></a>[Pg 61]</span>
-birds, possess a colouring which makes it very difficult to distinguish
-them from their usual surroundings. Our large green grasshopper
-(<i>Locusta viridissima</i>) lays its eggs in the earth, and they are brown,
-exactly like the earth which surrounds them. They are enough in
-themselves to refute the hypothesis that sympathetic colouring has
-arisen through self-photography, for these eggs lie in total darkness
-in the ground. Insect-eggs which are laid on the bark of trees are
-often grey-brown or whitish like it, and the eggs of the humming-bird
-hawk-moth (<i>Macroglossa stellatarum</i>), which are attached singly to the
-leaves of the bedstraw, have the same beautiful light-green colour as
-these leaves, and, in point of fact, green is a predominant colour of
-the eggs in a very large number of insects.</p>
-
-<p>But the eggs of many birds, too, exhibit 'sympathetic' colouring;
-thus the curlew (<i>Numenius arquata</i>) has green eggs, which are laid in
-the grass; but the red grouse (<i>Lagopus scoticus</i>) lays blackish-brown
-eggs, exactly of the colour of the surrounding moor-soil; and it has
-been observed that they remain uncovered for twelve days, for the
-hen lays only one egg daily, and does not begin to brood until the
-whole number of twelve is complete. Herein lies the reason of
-the colour-adaptation, which the eggs would not have required, if
-they had always been covered by the brooding bird.</p>
-
-<p>The eggs of birds are frequently not of one colour only; those of
-the Alpine ptarmigan (<i>Lagopus alpinus</i>), for instance, are ochre-yellow
-with brown and red-brown dots, resembling the nest, which is carelessly
-constructed of dry parts of plants. Sometimes this mingling of
-colours reaches an astonishing degree of resemblance to surroundings,
-as in the golden plover (<i>Charadrius pluvialis</i>), whose eggs, like those
-of the peewit (<i>Vanellus cristatus</i>), are laid among stones and grasses,
-not in a true nest, but in a flat depression in the sand, and, protected
-by a motley speckling with streaking of white, yellow, grey and
-brown, are excellently concealed. Perhaps the eggs of the sandpipers
-and gulls are even better protected, for their colouring is a mingling
-of yellow, brown, and grey, which imitates the sand in which they are
-laid so perfectly, that one may easily tread on them before becoming
-aware of them.</p>
-
-<p>But let us now turn from eggs to adult animals. Darwin first
-pointed out that the fauna of great regions may exhibit one and the
-same ground-colouring, as is the case in the Arctic zone and in the
-deserts. The most diverse inhabitants of these regions show quite
-similar coloration, namely, that which harmonizes with the dominant
-colour of the region itself. It is not only the persecuted animals,
-which need protection, that are sympathetically coloured in these<span class="pagenum"><a id="Page_62"></a>[Pg 62]</span>
-cases, the persecutors themselves are likewise adapted, and this need
-not surprise us, when we remember that the very existence of a beast
-of prey depends on its being able to gain possession of its victims,
-and that therefore it must be of the greatest use to it to contrast as
-little as possible with its surroundings, and thus be able to steal on
-its quarry unperceived. Those that are best adapted in colour will
-secure the most abundant food, and will reproduce most prolifically;
-and they will thus have a better prospect of transmitting their usual
-colouring to their offspring. The Polar bear would starve if he were
-brown or grey, like his relatives; among the ice and snow of the
-Polar regions his victims, the seals, would see him coming from afar.</p>
-
-<p>In the Arctic zone the adaptation of the colouring of the animals
-to the white of the surroundings is particularly striking. Most of
-the mammals there are pure white, or approximately white, at least
-during the long winter; and it is easily understood that they must be
-so if they are to survive in the midst of the snow and ice,&mdash;both
-beasts of prey and their victims. For the latter the sympathetic
-colouring is of 'protective' value; for the former, of 'aggressive'
-value (Poulton). Thus we find not only the Polar hare and the
-snow-bunting white, but also the Arctic fox, the Polar bear, and
-the great snowy owl; and though the brown sable is an exception,
-that is intelligible enough, for he lives on trees, and is best concealed
-when he cowers close to the dark trunk and branches. For him
-there would be no advantage in being white, and therefore he has not
-become so.</p>
-
-<p>Desert animals are also almost all sympathetically coloured, that
-is, they are of a peculiarly sandy yellow, or yellowish-brown, or
-clayey-yellow, or a mixture of all these colours; and here again the
-beasts of prey and their victims are similarly coloured. The lion
-must be almost invisible from a short distance, when he steals along
-towards his prey, crouching close to the ground; but the camel too, the
-various species of antelope, the giraffe, all the smaller mammals, and
-also the horned viper (<i>Vipera cerastes</i>), the Egyptian spectacled snake
-(<i>Naja haje</i>), many lizards, geckos, and the great Varanus, numerous
-birds, not a few insects, especially locusts, show the colours of the
-desert. It is true that the birds often have very conspicuous colours,
-such as white on breast and under parts, but the upper surface is
-coloured like the desert, and conceals them from pursuers whenever
-they cower close to the ground. It has even been observed that
-a locust of the genus <i>Tryxalis</i> is of a light sand-colour in the sandy
-part of the Libyan desert, but dark brown in its rocky parts, thus
-illustrating a double adaptation in the same species.</p>
-
-<p><span class="pagenum"><a id="Page_63"></a>[Pg 63]</span></p>
-
-<p>Another group, which agrees in colour with the general surroundings,
-is that of the 'glass-animals,' as they have been called,
-though perhaps 'crystal animals' is a better term. A great number
-of simple free-swimming marine forms, and a few fresh-water ones,
-are quite colourless, and perfectly transparent, or have at most
-a bluish or greenish tinge, and on this account they are quite
-invisible as long as they remain in the water. In our lakes there lives
-a little crustacean about a centimetre in length, of the order of
-water-fleas (<i>Leptodora hyalina</i>), a mighty hunter among the smallest
-animals, which swims forward jerkily with its long swimming-appendages,
-and widely spreads its six pairs of claws, armed with
-thorny bristles, like a weir basket, to seize its prey. We may have
-dozens of these in a glass of water without being able to see a single
-one, even when we hold the glass against the light, for the creatures
-are crystal-clear and transparent, and have exactly the same refractive
-power as the water. It requires a very sharp scrutiny and
-a knowledge of the animals to be able to detect in the water little
-yellowish stripes, which are the stomachs of the animals filled with
-food in process of digestion, for which, as we can readily understand,
-invisibility cannot very well be arranged. If the water be then
-strained through a fine cloth, a little gelatine-like mass of the bodies
-of the <i>Leptodora</i> will remain on the sieve.</p>
-
-<p>A great many of the lower marine animals are equally transparent,
-and as clear as water; most of the lower Medusæ, the ctenophores,
-various molluscs, the barrel-shaped Salpæ, worms, many crustaceans
-of quite different orders, and above all an enormous number of larvæ
-of the most diverse animal groups. I can remember seeing the sea
-at the shore at Mentone so full of Salpæ, that in every glass of sea-water
-drawn at random there were many of them, and sometimes a glass
-held a positive animal soup. But one did not see them in the glass of
-water, and only those who knew what to look for recognized them by
-the bluish intestinal sac that lies posteriorly in the invisible body.
-But when the water was poured off through a fine net, there remained
-on the filter a large mass of a crystalline gelatinous substance.</p>
-
-<p>It is obvious that this must serve as a protective arrangement,
-for the animals are not seen by their pursuers; but it is not an
-<i>absolute</i> protection, for they have many pursuers who do not wait till
-they see their prey, but are almost constantly snapping the mouth
-open and shut, leaving it to chance to bring them their prey. <i>No
-protective arrangement, however, affords absolute security</i>; it protects
-against some enemies, perhaps against many, but never against all.</p>
-
-<p>But now let us turn to a group of a different colouring, the green<span class="pagenum"><a id="Page_64"></a>[Pg 64]</span>
-animals. We are familiar with our big grass-green grasshopper, and
-we know how easily it is overlooked when it sits quietly on a high
-grass-stem, surrounded by grasses and herbage; the light grass-green
-of its whole body protects it most effectively from discovery: for
-myself, at least, I must confess that in a flowery meadow I have
-stood right in front of one, and have looked close to it for a long
-time without detecting it. In the same way countless insects of the
-most diverse groups&mdash;bugs, dipterous flies, sawflies, butterflies&mdash;and
-especially the larvæ (caterpillars) of the last, are of the same green as
-the plants on which they live, and this again applies to the predaceous
-species, as well as the species preyed upon. Thus the rapacious
-praying-mantis (<i>Mantis religiosa</i>) is as green as the grass in which it
-lurks motionless for its victim&mdash;a dragonfly, a fly, or a butterfly.</p>
-
-<p>There are also green spiders, green amphibians like the edible
-frog, and especially the tree-frog, green reptiles like lizards and the
-tree-snakes of tropical forests. It is always animals which live
-among green that are green in colour.</p>
-
-<p>We may wonder, for a moment, why there are so few green
-birds, since they spend so much of their time among the green leaves.
-But this paucity of green birds is only true of temperate climates.
-In Germany we have only the green woodpecker, the siskin, and
-a few other little birds, and even these are not of a bright green, but
-are rather greyish-green. The explanation lies in the long winter,
-when the trees are leafless. In the evergreen forests of the tropics
-there are numerous green birds belonging to very diverse families.</p>
-
-<p>Yet another group with a common colour-adaptation deserves
-mention&mdash;the beasts of the night. They are all more or less grey,
-brown, yellowish, or a mixture of these colours, and it is obvious that,
-in the duskiness of night, they must blend better with their environment
-on this account. White mice and white rats cannot exist under
-natural conditions, since they are conspicuous in the night, and the
-same would be true of white bats, nightjars, and owls; but all of these
-have a coloration suited to nocturnal habits.</p>
-
-<p>A very remarkable fact is that in many animals the colour-adaptation
-is a double one. Thus the Arctic fox is white only in
-winter, while in summer he is greyish-brown; the ermine changes
-in the same way, and the great white snowy owl of the Arctic regions
-has in summer a grey-brown variegated plumage. Many animals
-which are subject to persecution also change colour with the
-seasons, like the mountain hare (<i>Lepus variabilis</i>), which is brown
-in summer and pure white in winter, the Lapland lemming, and
-the ptarmigan (<i>Lagopus alpinus</i>), which do the same. It has been<span class="pagenum"><a id="Page_65"></a>[Pg 65]</span>
-doubted whether natural selection can explain this double coloration,
-but I do not know where the difficulty lies, and there is certainly no
-other principle whose aid we can evoke. The mountain hare must
-have had some sort of colour before it attained to seasonal dimorphism.
-Let us assume that it was brown, that the climate became
-colder and the winter longer, then those hares would have most
-chance of surviving which became lighter in winter, and so a white
-race was formed. Poulton has shown that the whiteness is due to
-the fact that the dark hairs of the summer coat grow white as they
-lengthen at the beginning of winter, and the abundance of new hairs
-which complete the winter coat are from the first white throughout.
-If the white hairs were to persist throughout the summer it would
-be very disadvantageous to their wearer; so a double selection must
-take place, in summer the individuals which remain white, in
-winter those which remain brown, being most frequently eliminated,
-so that only those would be left which were brown in summer and
-white in winter. This double selection would be favoured by the fact
-that there would be, in any case, a change of fur at the beginning of
-summer; the winter hairs fall out and the fur becomes thinner. The
-process does not differ essentially from that which takes place in any
-species when two or more parts or characters, which are not directly
-connected, have to be changed, such as, for instance, colour and
-fertility. The struggle for existence will in this case be favourable,
-on the one hand, to the advantageously coloured, and on the other to
-the most fertile, and though the two characters may at first only
-occur separately, they will soon be united by free crossing, until
-ultimately only those individuals will occur which are at once the
-most favourably coloured and the most fertile. So in this case there
-remain only those which are brown in summer and white in winter.</p>
-
-<p>We must ascribe to the influence of the processes of selection the
-exact regulation of the duration of the winter and summer dress,
-which has been carefully studied in the case of the variable hare.
-In the high Alps it remains white for six or seven months, in the
-south of Norway for eight months, in Northern Norway for nine
-months, and in Northern Greenland it never loses its white coat at all,
-as there the snow, even in summer, melts only in some places and for
-a short time. But apart from concealment there is certainly another
-adaptation involved here&mdash;namely, the growth of the hair as a
-protection against the cold. From an old experiment made in 1835
-by Captain J. Ross, and recently brought to light again by Poulton,
-we learn that a captive lemming kept in a room in winter did not
-change colour until it was exposed to the cold. The constitution of<span class="pagenum"><a id="Page_66"></a>[Pg 66]</span>
-animals which become white in winter is thus so organized that the
-setting in of cold weather acts as a stimulus which incites the skin
-to the production of white hairs. This predisposition also we must
-refer to the influence of natural selection, since it must have been
-very useful to the species that the winter coat should grow just when
-it was necessary as a protection against cold. This explains at the
-same time why the predisposition to respond to the stimulus of cold
-by a growth of winter fur finds expression earlier in those colonies of
-Arctic animals, such as the hare, which live in Lapland, than in those
-which live in the south of Norway.</p>
-
-<p>But that it is not the <i>direct</i> influence of cold which colours the
-hair of a furred animal white we can see from our common hare
-(<i>Lepus timidus</i>), which, in spite of the winter's cold, does not become
-white, but retains its brown coat, and not less so from the mountain
-hare (<i>Lepus variabilis</i>), which in the south of Sweden also remains
-brown, although the winter there may be exceedingly cold. But as the
-covering of the ground with snow is not so uninterrupted there as in
-the higher North, a white coat would be not a better protection
-than a brown one, but a worse. The white colouring of Arctic animals
-is therefore not directly due to the influence of the climate, as has
-often been maintained, but is due to it indirectly, that is, through the
-operation of natural selection. I have tried to make this clear by
-means of this example, so that we may not have to repeat it in
-considering those which are to follow.</p>
-
-<hr class="tb" />
-
-<p>But all attempts at any other explanation are even more decidedly
-excluded when we turn our attention to more complicated cases of colour-adaptation,
-which are not confined to the simple, general coloration, but
-are helped by markings and colour-patterns, that is, by schemes of colour.</p>
-
-<p>Thus numerous caterpillars exhibit definite lines and spots on
-their ground-colouring, which, in one way or another, aid in protecting
-them from their enemies.</p>
-
-<div class="figright" id="f2">
-<img src="images/fig2.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 2.</span> Longitudinally
-striped<br /> caterpillar of a Satyrid.<br />
-After Rösel.</p>
-</div>
-
-<p>The green grass-eating caterpillar of many of our <i>Satyridæ</i>
-has two or more darker or lighter lines running down the sides of its
-body, which make it much less conspicuous among the grasses on
-which it feeds than if it were a uniform green mass (Fig. 2). Not
-infrequently the colour and form present a remarkably close resemblance
-to the inflorescences or fruit-ears of the grasses. Caterpillars
-marked thus are never found on the leaves of trees, where they
-would immediately catch the eye. It is true that longitudinal
-striping often occurs on caterpillars which live on other plants
-besides grass, but as these other plants grow among the grasses the<span class="pagenum"><a id="Page_67"></a>[Pg 67]</span>
-protective efficacy is just the same. This is the case with the Pieridæ
-(Garden Whites).</p>
-
-<p>All the caterpillars of our Sphingidæ, on the other hand, which
-live on bushes and trees, have on the sides of the segments light
-oblique stripes, seven in number, which are
-disposed to the longitudinal axis of the body
-at the same angle as the lateral veins of a
-leaf of their food-plant have to the mid-rib.
-It cannot of course be said that the caterpillar
-thereby gains the appearance of a leaf,
-indeed, if one sees it apart from its food-plant
-it does not look in the least like a leaf,
-but among the leaves of a bush or tree this
-marking secures it in a high degree from discovery.
-Thus the caterpillar of the eyed hawk-moth
-(<i>Smerinthus ocellatus</i>), when it is sitting
-among the crowded foliage of a willow, is often
-very difficult to find, because its large green
-body does not appear as a single green spot, but
-is divided by the oblique lateral stripes into sections
-like the half of a willow leaf, so that even
-a searching glance is led astray, there being
-nothing to focus attention on the animal as
-distinguished from its surroundings (Fig. 3).
-As a boy I often had the interesting experience
-of overlooking a caterpillar which was sitting
-just before me, until after a time I chanced to hit upon the exact spot
-in the field of vision.</p>
-
-<div class="figright" id="f3">
-<img src="images/fig3.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 3.</span> Full-grown caterpillar of the Eyed<br />
-Hawk-moth, <i>Smerinthus ocellatus</i>. <i>sb</i>, the subdorsal<br />
-stripe.</p>
-</div>
-
-
-<p>In the majority of these caterpillars with oblique stripes, the
-likeness to the half of a leaf is heightened by the fact that the
-light oblique row is accompanied
-by a broader
-coloured band, suggesting
-the shade of the leaf's
-mid-rib. The caterpillar
-of <i>Sphinx ligustri</i> has a
-lilac band, and that of
-<i>Sphinx atropos</i> a blue one.
-In both cases it is difficult to believe that such striking colours can secure
-the animals from discovery, yet among the blending shadows of the
-leaf-complex of their food-plant they greatly increase their resemblance
-to a leaf-surface. Of the death's-head caterpillar (<i>Sphinx<span class="pagenum"><a id="Page_68"></a>[Pg 68]</span>
-atropos</i>) this sounds almost incredible, for this form is chiefly a bright
-golden yellow, and the narrow white oblique stripes have sky-blue
-borders becoming darker towards the under side; but it must not be forgotten
-that the potato is not the true food-plant of the species, for it lives,
-in its true home in Africa, and also in the south of Spain, on wild
-solanaceous plants, which, we are informed by Noll, have precisely
-these colours&mdash;golden-yellow and blue in the blossom, the fruit, and in
-part also in the leaves and stem. There the caterpillars sit the whole
-day long on the plants, while with us they have formed the habit of feeding
-only in the twilight and at night, and concealing themselves in the
-earth by day, a habit that is found in other caterpillars also, and
-which we must again ascribe to a process of natural selection.</p>
-
-<div class="figcenter" id="f4">
-<img src="images/fig4.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 4.</span> Full-grown caterpillar of the Elephant Hawk-moth (<i>Chærocampa elpenor</i>) in its<br />
-"terrifying attitude."</p>
-</div>
-
-
-
-<p>Some caterpillars exhibit other, more complex markings, which
-do not protect them by rendering them difficult to detect, but by
-terrifying the enemy who has discovered them, and warning him
-away. Such terrifying or aggressive colours are to be found, for
-instance, in the caterpillars of the Sphingid genus <i>Chærocampa</i> in
-the form of large eye-like spots, which occur in pairs close together
-on the fourth and fifth segments of the animal. Children and those
-unfamiliar with animals take these for true eyes; and as the
-caterpillar, when it is threatened by an enemy, draws in the head and
-anterior segments, so that the fourth one is greatly distended, the
-eye-spots seem to stand on a thick head (Fig. 4), and it cannot be
-wondered at that the smaller birds, lizards, and other enemies are
-so terrified that they refrain from attacking. Even hens hesitate to
-seize such a caterpillar in its defiant attitude, and I once looked on
-for a long time in a hen-coop while one hen after another rushed to
-pick up a caterpillar I had placed there, but, when close to it, hastily
-drew back the head already prepared to strike. Even a gallant
-cock was a long time in making up his mind to attack the terrible
-beast, and drew back repeatedly before he at length ventured to strike
-a deadly blow with his bill. After the first stroke the caterpillar, of
-course, was lost. Thus even this disguise is only a <i>relative</i> protection,
-effective only against smaller enemies. But that these are really
-frightened away, I had once an opportunity of observing, when I put<span class="pagenum"><a id="Page_69"></a>[Pg 69]</span>
-a caterpillar of the common elephant hawk-moth (<i>Chærocampa
-elpenor</i>) in the feeding-trough of a hencoop, and a sparrow flew down
-to feed from the trough. It descended at first with its back to
-the caterpillar and fed cheerily. But when by chance it turned round,
-and spied the caterpillar, it scurried hastily away.</p>
-
-<div class="figcenter" id="f5">
-<img src="images/fig5.jpg" alt=""/>
-<p class="caption center"><span class="smcap">Fig. 5.</span> The Eyed Hawk-moth in its 'terrifying attitude.'</p>
-</div>
-
-
-
-<p>Among Lepidoptera, too, eye-spots often occur on the wings, and
-to some extent, at least, they have in this case also the significance
-of warning marks. Take, for instance, the large blue and black
-eye-spots on the posterior wings of the eyed hawk-moth (<i>Smerinthus
-ocellatus</i>). When the insect is sitting quietly the two spots are
-not visible, as they are covered by the anterior wings, but as soon as the
-creature is alarmed it spreads all four wings, and now both eyes stand
-boldly out on the red posterior wings and alarm the assailant, as they
-give the impression of the head of a much larger animal (see Fig. 5).
-There are also eye-like spots which have not this significance and
-effect, as, for instance, the 'eye-spots' on the train-feathers of the
-peacock and the Argus pheasant, or the little eye-like spots on the
-under surface of many diurnal butterflies. In the first case, it is
-a matter of decoration; in the second, perhaps of the mimicry of dewdrops,
-which increases still further the resemblance to a withered
-leaf; but there are undoubtedly many cases in which the eye-spots
-serve as means of frightening off enemies, and these cases are especially
-common among butterflies.</p>
-
-
-
-<p>Such warning marks are in no way contradictory to the
-sympathetic colouring of the rest of the body, and indeed we usually
-find them in combination with it. In some cases the eye-spot, though
-very conspicuous, is covered, as in the eyed hawk-moth, when at rest,<span class="pagenum"><a id="Page_70"></a>[Pg 70]</span>
-by the sympathetically coloured parts&mdash;in this instance the anterior
-wings. In other cases eye-spots of considerable size lie clearly
-exposed, but exhibit the same sympathetic colours as the whole of the
-rest of the wing-surface. In this case they do not interfere with the
-protective influence of general colouring, because they are only visible
-from a very short distance. This is the case in the large <i>Caligo</i>
-species of South America, which only fly for a short time in the early
-morning and in the evening, remaining concealed throughout the day
-in dark shadowy places, where the mingled colouring of brown, grey,
-yellow, and black on the under surfaces of the wings prevents their
-being recognized from a distance as butterflies at all. But even the
-best sympathetic colouring is not
-an absolute protection, and when
-the insect is discovered by an
-enemy near at hand, the terrifying
-mark, a large deep-black spot
-on the posterior wing, comes into
-effect, and scares the assailant
-away.</p>
-
-<div class="figleft" id="f6">
-<img src="images/fig6.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 6.</span> Under surface of the wings of
-<i>Caligo</i>.</p>
-</div>
-
-<p>In such cases the sympathetic
-colouring was probably the first
-to arise, and the eye-spot was
-developed later by a new process
-of selection, brought about by the
-necessity of protecting the species
-more effectively than by mere inconspicuousness
-alone. In many
-cases it can be proved that the
-power of scaring off an enemy
-did not begin with the formation
-of the eye-spot, but with the development
-of a new instinct. When the caterpillar of <i>Chærocampa
-elpenor</i> is attacked it immediately assumes the defiant attitude described
-above, but the same striking attitude is assumed by the caterpillars
-of the allied American genus <i>Darapsa</i>, as I learn from an old
-illustration by Abbot and Smith, although this form possesses
-no eye-spots (<a href="#f7">Fig. 7</a>). Thus, then, metaphorically speaking, the caterpillar
-at first attempted to scare off its enemy by a terrifying attitude
-alone, and it was only subsequently, in the course of the phyletic evolution,
-that the eye-spots were added, in the elephant hawk-moths
-and other species, to heighten the terrifying effect. But that
-the eye-spot did not make its appearance suddenly is proved by several<span class="pagenum"><a id="Page_71"></a>[Pg 71]</span>
-American species of <i>Smerinthus</i>, in which they are much less perfectly
-developed than in the European species. In these Sphingidæ, too, the
-defiant attitude was evolved earlier than the eye-spots, as we may see
-from our poplar hawk-moth (<i>Smerinthus populi</i>), which, when alarmed,
-spreads out all four wings in the same peculiar manner which in the
-eyed hawk-moth (<i>Smerinthus ocellatus</i>) displays the eye-spots; it
-strikes about with its wings as if to scare off the enemy, an effect which
-will certainly be more surely achieved if, at the same time, a pair of
-eyes becomes suddenly visible.</p>
-
-<div class="figright" id="f7">
-<img src="images/fig7.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 7.</span> Caterpillar of a North American<br />
-<i>Darapsa</i> in its "terrifying attitude" (after<br />
-Abbot and Smith).</p>
-</div>
-
-<p>Sympathetically coloured caterpillars are, however, by no means
-the only ones; there are some with such striking, glaring colours that,
-far from rendering their possessors inconspicuous, they make them
-visible from a long way off; but this apparent contradiction of the
-theory of the colour-adaptation of animals that require protection has
-been explained by the acuteness of Alfred Russel Wallace. We know
-that among insects, and also among caterpillars, there are many which
-have a repulsive taste. In any case, certain caterpillars are
-rejected by many birds and lizards.
-Such species are, therefore, relatively
-safe from being devoured.
-If they were protectively coloured,
-or if, moreover, they resembled
-caterpillars with an agreeable
-taste, they would gain little advantage
-from their unpalatability;
-for the birds would at first take them for eatable, and would only discover
-their repulsiveness on attempting to eat them. But a caterpillar which
-has received a single stroke from a bird's bill is doomed to death. It
-must therefore be of the greatest advantage for unpalatable caterpillars,
-and unpalatable animals generally, to be in their colouring as conspicuously
-distinguishable as possible from the edible species. Hence,
-then, the glaring colours, which we can now refer without any further
-difficulty to the process of natural selection, for every individual of an
-ill-tasting species that is more conspicuously coloured than its fellows
-must have an advantage over them, and must have a better chance of
-surviving, because it will be less easily mistaken for a member of an
-edible species.</p>
-
-<p>I should like to discuss one other phenomenon, which is well
-calculated to give us a deeper insight into the transformation processes
-of organisms&mdash;I refer to the remarkable dimorphism of colour
-which occurs in many of the species of caterpillar just described.</p>
-
-<p>The caterpillar of the convolvulus hawk-moth (<i>Sphinx convolvuli</i>)<span class="pagenum"><a id="Page_72"></a>[Pg 72]</span>
-is in its full-grown stage green, like the wild convolvulus on which it
-lives, or brown like the ground on which its food-plant grows. It thus
-shows a double adaptation, each of which is capable of protecting it to a
-certain extent, and we might think to the <i>same</i> extent. But that is
-not so, the brown colouring is a more effective protection than the
-green, as we may learn from two facts. In the first place, the four
-young stages of the caterpillar are green, and it only becomes brown
-in the last stage, though sometimes even then it remains green. This
-shows that the brown is a relatively modern adaptation, and it could
-not have arisen had it not been better than the original green. In the
-second place, the green-coloured caterpillars of the convolvulus hawk-moth
-are nowadays much less numerous than the brown ones, and this
-implies that the latter survive oftener in the struggle for existence.
-We have here an interesting case of an easily recognizable process of
-selection still going on between the old green and the newer brown
-variety.</p>
-
-<p>It is hardly necessary to ask why the brown colour should in this
-case be a better protection than the green, for it is obvious that such
-a large green body as that of the full-grown convolvulus-caterpillar
-would be but badly concealed among the little leaves of the convolvulus
-plant in spite of its green colour; while the brown caterpillar,
-on the brown soil, with its pebbles, hollows, and irregular shadows, is
-excellently protected, especially if it passes the day concealed in the
-ground, as is actually the case.</p>
-
-<p>Our view is materially strengthened by the fact that the same
-phenomenon of double colouring occurs in several allied species of
-Sphingidæ, but in a manner which shows us that we have to do with
-a similar process of transformation, only at a more advanced stage.
-The caterpillar of <i>Chærocampa elpenor</i> (<a href="#f4">Fig. 4</a>) shows the same state
-of things as that of the convolvulus hawk-moth; it is brown or green,
-and the green form is the less common. But in the two other European
-species of <i>Chærocampa</i> the full-grown caterpillar is always brown,
-and indeed it becomes brown in the fourth stage, instead of, like
-<i>Chærocampa elpenor</i>, only in the fifth and last. Another indigenous
-sphingid species, <i>Deilephila vespertilio</i>, only remains green during the
-first two stages, and assumes in the third stage the grey-brown colour
-which it afterwards retains. The dark colour has obviously prevailed
-among the full-grown caterpillars for a considerable length of time,
-for it is in this, the largest and most conspicuous stage, that the
-change of colour must have been most necessary, and consequently
-the process of selection must have begun in it, and only after the more
-protective brown became general would it have extended to the next<span class="pagenum"><a id="Page_73"></a>[Pg 73]</span>
-stage below, if it were of use there too, and, later on, to still earlier
-stages in the life-history.</p>
-
-<p>One might be inclined to ascribe this shunting back of a new
-character from the later to the earlier stages of development to purely
-internal forces, which brought it about of necessity, and quite independently
-of whether the extension of the character was useful or
-injurious. We shall come back to this later, and try to find out how
-far this is the case, but in the meantime we may regard at least so
-much as established, that this shunting back does not take place
-everywhere and without limits, but that natural selection calls a halt
-as soon as its effect would be injurious.</p>
-
-<div class="figcenter" id="f8">
-<img src="images/fig8.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 8.</span> Caterpillar of the Buckthorn Hawk-moth, <i>Deilephila hippophaës</i>. <i>A</i>, Stage
-III. <i>B</i>, Stage V. <i>r</i>, ring-spots.</p>
-</div>
-
-<p>There could be no continuance of insect-metamorphosis if every
-character of the final stage had to be shunted back to the one next
-below, for then, for instance, the characters of the butterfly must, in
-the course of the phyletic evolution, be carried back to the pupa and
-larva. But even in the larval stage alone it can be seen that this
-carrying back is kept within well-defined limits. Thus, for instance,
-in the dimorphic caterpillars of the Sphingidæ the brown of the full-grown
-stage never comes so far down as the earliest stages, for the
-little caterpillars are all green, like the leaves and stems on which
-they sit. On the other hand, there are species in which the green
-persists, as apparently the most advantageous colour. Thus in the
-buckthorn hawk-moth (<i>Deilephila hippophaës</i>) (Fig. 8), which lives in
-the warm valleys of the Alps, and especially in Valais, the caterpillars
-are grey-green in all stages, and are exactly of the shade of the lower
-surface of the buckthorn leaves; they possess no oblique lines, for
-these would not make them more like the leaves, as the full-grown
-caterpillars are much bigger than an individual leaf of buckthorn,<span class="pagenum"><a id="Page_74"></a>[Pg 74]</span>
-on which, moreover, the lateral veins are not very conspicuous.
-Nevertheless the caterpillar enjoys very fair security, as it does not
-feed through the day, but only in twilight and at night; it passes the
-daytime concealed in the dry leaves and earth about the base of the
-bush. Its resemblance to the leaves is very great, and is increased by
-the fact that it bears on the last segment a comparatively large
-orange-coloured spot (<i>r</i>), exactly the colour of the buckthorn berry,
-which ripens just at the time that the caterpillar attains its full
-growth.</p>
-
-<p>But butterflies are as much persecuted, and have as much need
-of protection, as caterpillars, and among them, too, we find many
-instances of protective colouring, which are the more interesting in
-that they occur, as a rule, only on such parts of the body as remain
-visible when the insect is at rest, which is exactly what we should
-expect if the coloration has been wrought out in the course of
-natural selection. But it is well known that the resting position of
-diurnal Lepidoptera is quite different from that of the nocturnal forms,
-and is not even the same among all families, and in accordance with
-this we find the sympathetic colouring occurs on quite different areas
-in the different families.</p>
-
-<p>The reason why the butterflies only require to be protected by
-their colour in the sleeping or resting position is that no colour whatever
-could make a flying butterfly invisible to its enemies, because
-the background against which its body shows is continually changing
-during its flight, and, moreover, the movement alone is enough to
-betray it, even if it is of a dull colour.</p>
-
-<p>Thus, in general, only those parts of a butterfly's wing that are
-invisible at rest could safely bear bright or conspicuous colour, while
-the visible portions had to acquire sympathetic coloration through
-natural selection.</p>
-
-<p>As the diurnal butterflies, when at rest, turn their wings upward
-and bring them together, it is only the under side which is
-sympathetically coloured, and that only as far as it is visible, that
-is, the whole of the posterior wing, and as much of the anterior one
-as is not covered by it. Many diurnal butterflies, when at rest, fold
-the anterior wing so far back that only its tip remains visible, and in
-such cases only this tip is protectively coloured, while in other forms,
-which have not this habit, almost the whole surface of the wing is
-sympathetically coloured.</p>
-
-<p>A very simple protective colouring is exhibited by our 'lemon
-butterfly' (<i>Rhodocera rhamni</i>), in which the under surface is a
-whitish yellow, which protects the insect well when it settles on<span class="pagenum"><a id="Page_75"></a>[Pg 75]</span>
-the dry leaves on the ground in the light woods which it is fond of
-frequenting.</p>
-
-<p>Our gayest diurnal butterflies, the species of <i>Vanessa</i>, all have the
-under surface of a dusky colour, sometimes passing into a blackish
-brown, as in the peacock-butterfly, <i>Vanessa</i> (<i>v. io</i>), sometimes more
-into greyish brown, or brown-yellow, or reddish brown. They are
-never simple colours, but always consist of mixtures of different
-colour-tones&mdash;indeed, there is often a complex mingling of many
-colours, as grey, brown, black, white, green, blue, yellow, and red,
-made up of dots, strokes, spots, and rings, into a wonderful and very
-constant pattern, which, taken as a whole, has the effect of being
-uniform, and harmonizes with the soil, or with the highway, on
-which the species loves to settle, with much greater accuracy than
-a monochrome grey or brown would do. When the 'painted lady'
-(<i>Vanessa cardui</i>) settles on the ground it is hardly distinguishable
-from it, and this species in particular has a preference for settling on
-the ground. Other species of <i>Vanessa</i>, such as the peacock and the
-Camberwell beauty (<i>Vanessa antiopa</i>), are underneath of a dark
-blackish grey, or even black; when resting they press themselves
-into the darkest corners and crevices, and are thus most effectively
-secured from discovery.</p>
-
-<p>Many diurnal Lepidoptera, on the other hand, especially the wood-butterflies
-of the family Satyridæ, have the habit of resting on the
-trunks of trees, as <i>Satyrus proserpina</i> does on the great beech-trunks
-of the forest clearings. These large butterflies, coloured conspicuously
-on the upper surface in deep velvety black and white, are marked on
-the under surface exactly to match the whitish bark of the great
-beech, covered over with white, grey, blackish-brown, and yellow spots,
-and the butterfly whose flight one has just been carefully following
-disappears as it suddenly alights on such a tree-trunk. As I have
-already stated, the protective colour only extends over as much of the
-insect as is seen when it is at rest. As the anterior wings are folded
-far back between the posterior ones, the protective colouring is limited
-to the whole surface of the posterior wing, and the tip of the anterior
-one, as far as that is visible in the resting attitude; the protectively
-coloured area is somewhat sharply bounded, and it is often of very
-different extent in quite nearly allied species, according to whether
-the species folds the anterior wing far back or not. Thus in our
-common small tortoiseshell-butterfly (<i>Vanessa urticæ</i>) the protective
-area is considerably wider than in the large tortoiseshell (<i>Vanessa
-polychloros</i>), much as the two resemble each other in other details.</p>
-
-<p>This harmony between the wing tips and the posterior wings is<span class="pagenum"><a id="Page_76"></a>[Pg 76]</span>
-nowhere wanting, where the under side is protectively coloured at all,
-but in many cases the protective colouring spreads over almost the
-whole of the anterior wings, and these are then not folded far back
-when at rest, as will be seen later in the so-called leaf-butterflies.</p>
-
-<p>There is one genus of diurnal butterflies which seems to contradict
-the law that all the surface that is visible in the resting position
-exhibits the protective coloration&mdash;the South American wood-butterflies
-of the genus <i>Ageronia</i>. They have on the upper surface
-a very complicated bark-like pattern of mingled grey on grey, and
-this confirms the usual rule, for we know that these butterflies&mdash;a
-striking exception among all the other diurnal forms&mdash;settle with
-outspread wings on the trunk of a tree in exactly the same attitude
-as many of the nocturnal Lepidoptera of the family of the
-Loopers or Geometridæ, in which the upper surface is also deceptively
-like the bark of the tree on which they rest.</p>
-
-<div class="figcenter" id="f9">
-<img src="images/fig9.jpg" alt=""/>
-<p class="caption center"><span class="smcap">Fig. 9.</span> <i>Hebomoja glaucippe</i>, from India; under surface. <i>A</i>, in flight. <i>B</i>, in resting
-attitude.</p>
-</div>
-
-<p>In all the nocturnal Lepidoptera it is the <i>upper</i> side of the wing
-which is sympathetically coloured, if protective coloration has been
-developed at all. In all the Sphingidæ, many 'Owls' and Bombycidæ,
-the anterior wings are grey banded with darker zigzag lines,
-and mottled with many shades of black, grey, yellow, red, and even
-violet. As the anterior wings cover the body and the posterior wings
-like a roof, they make the resting insect very inconspicuous when
-it has settled on wooden fences, trunks of trees, or even old timber.
-When bright colours&mdash;red, yellow, or blue&mdash;occur in these moths<span class="pagenum"><a id="Page_77"></a>[Pg 77]</span>
-it is always on the posterior wings, which are covered when at rest.
-This can best be observed in the species of the genus <i>Catocala</i>.</p>
-
-<p>Let us now, however, interrupt our survey of the facts for
-a moment, and let us inquire whether all the cases of protective
-colouring in Lepidoptera we have considered can be referred to natural
-selection, or whether it is not conceivable that other causes may have
-evoked them.</p>
-
-<div class="figcenter" id="f10">
-<img src="images/fig10.jpg" alt=""/>
-<p class="caption center"><span class="smcap">Fig. 10.</span> <i>Xylina vetusta</i>, after Rösel. <i>A</i>, in flight. <i>B</i>, at rest.</p>
-</div>
-
-<p>The first thing to be said is that the Lamarckian principle of
-the inherited effects of use and disuse cannot here be taken into
-account, as the colours of the surface of the body do not exercise any
-active function at all; their effect is due simply to their presence,
-and it is for them quite indifferent whether and how often they have
-opportunity to protect their bearers from enemies, or whether no
-enemies ever chance to appear. It has frequently been suggested,
-too, that these colorations are associated with the differences in the
-strength of the illumination to which the different parts and surfaces
-are exposed. But this again is untenable, as is proved even by the
-dimorphism frequently occurring in caterpillars, for the green and the
-brown individuals are exposed to precisely the same light; and still
-more clearly by the sympathetic colouring, which is so exactly
-defined and yet so different on the under surface of the diurnal
-butterflies. Yet there are isolated cases in which it seems as if the
-direct influence of the light had brought about certain striking
-differences in the colouring of the parts of an insect, and I shall
-describe perhaps the prettiest of these cases, to which Brunner von
-Wattenwyl directed attention. It concerns one of the Orthoptera of
-Australia, a Phasmid, <i>Tropidoderus childreni</i>, Grey, which has
-a general colouring of leaf-green, but with singular deviations from
-it on certain areas of the body. In this insect the anterior wings
-which form the wing covers or elytra (<a href="#f11">Fig. 11</a>, <i>V</i>) are so short that they
-scarcely cover the half of the long abdomen. Their place is taken by
-the anterior margin of the posterior wing (<i>H. horn</i>), which is hard and
-horny like the elytra, and in the resting position protects the whole
-abdomen. All these covering parts are grass-green, except at the
-places where they overlap; on these areas they have a faded look, and<span class="pagenum"><a id="Page_78"></a>[Pg 78]</span>
-are yellowish instead of green. Brunner says of this: 'The phenomenon
-gives the impression that the more brilliant colour is
-a character due to daylight. If several sheets of white paper of
-unequal dimensions be placed one above the other, ... and exposed
-to the sun, after a short time silhouettes of the smaller sheets will
-appear on the larger ones, either in a lighter or in a darker colour.
-Probably this "fading" of the covered parts in the Phasmid also
-belongs to this "category of photographs."' This seems convincing,
-but analogous phenomena in other insects prevent our regarding the
-pretty comparison with the photograph as a sufficient explanation.
-If it were a question of a diurnal butterfly, such an assumption would
-have to be rejected on this ground alone, that the wing colouring is
-developed in the pupa, and appears perfect and unalterable as soon
-as the perfect insect emerges. But in the pupa the position of the
-wings is exactly the reverse of that seen in the resting attitude of
-a butterfly, that is, the protectively coloured under side of the wing
-is not turned towards the light but away from it. Moreover, in the
-pupa the anterior wings cover the posterior ones completely, no
-matter what the wing position may be later in the perfect insect.
-Furthermore, the thick and often darkly coloured sheath of the pupa
-prevents the light having any effect, and not a few species pass their
-pupal stage in such dark places&mdash;for instance, under stones, as in the
-case of many 'Blues'&mdash;that the light can hardly reach them. And if
-the light did exercise an influence, how could it produce such diverse
-coloration as the protective colours of diurnal butterflies, on the one
-side dark, even to blackness, on the other side, yellow, reddish, and
-even white and pure green; and how should the same rays of light
-call forth complicated colour patterns on one and the same surface,
-for instance, the white, sprinkled with green, of the Aurora butterfly
-(<i>Anthocharis cardaminis</i>)? Finally, we have only to remember that
-numerous nocturnal Lepidoptera pass through their pupa stage underground,
-although they exhibit brilliant as well as protective colours
-in the most appropriate distribution, to reject once for all the
-hypothesis that the influence of light plays any decisive rôle in determining
-the distribution of the colours on the wings of Lepidoptera.</p>
-
-<p>But it is otherwise with <i>Tropidoderus</i>. In this case the wings
-grow gradually during the slow growth of the animal, which takes
-place in full light, and the wings of the young insect probably lie one
-above the other, in exactly the same position, and cover the same
-places as in the full-grown form; we might, therefore, from the facts
-of the case, admit the possibility that the yellow of the covered
-portions is due to the exclusion of light.</p>
-
-<p><span class="pagenum"><a id="Page_79"></a>[Pg 79]</span></p>
-
-<div class="figcenter" id ="f11">
-<img src="images/fig11.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 11.</span> <i>Tropidoderus childreni</i>, after Brunner von Wattenwyl, in flying pose.
-<i>V</i> anterior wing. <i>H. häut</i>, membranous part of posterior wing. <i>H. horn</i>, horny portion.</p>
-</div>
-
-<p>But as soon as the conditions that obtain among Lepidoptera are
-also taken into consideration we recognize the insufficiency of the
-interpretation suggested, for among butterflies we have precisely the
-same phenomenon&mdash;sharp limitation of the protective colouring to
-the parts visible in the resting position, a fact which, in the case of the
-said butterflies, admits of no other interpretation than that of natural
-selection. Let us therefore see if we cannot, in the case of <i>Tropidoderus</i>,
-arrive at some better understanding of the phenomenon than
-that implied in the theory of direct light-influence. Obviously, the
-yellow parts of the animal do not require to be green, since they are not
-visible in the sitting position, and the locust in flight could not by
-any device be made invisible. It therefore only remains to be
-explained why the yellow parts are not colourless, and why they are
-not also green. We cannot at present answer with any confidence;
-it is possible that the colouring matter which causes the green only
-becomes green under the influence of direct sunlight, and otherwise
-remains yellow; it is possible, too, that, as in Lepidoptera (see
-Fig. 9), the full protective colour is only developed by natural
-selection in the places which are visible in the sitting position, and<span class="pagenum"><a id="Page_80"></a>[Pg 80]</span>
-that the covered parts take on any indifferent colour, which might
-be readily afforded by the metabolism of the insect. But this much
-is certain, that the covered parts would be green, if that were
-advantageous to the survival of the species, just as the under surface
-of the wings of some diurnal butterflies is green. Had it been
-required, the green colour would have resulted in the course of
-natural selection, just as it has resulted in the most different parts
-of the most diverse insects, even in those whose development takes
-place entirely removed from the influence of light. Therein lies the
-difference between our interpretation and that of Brunner von
-Wattenwyl: without natural selection no explanation of this case
-is possible.</p>
-
-<div class="figcenter" id="f12">
-<img src="images/fig12.jpg" alt=""/>
-<p class="caption center"><span class="smcap">Fig. 12.</span> <i>Notodonta camelina</i>, after Rösel. <i>A</i>, in flight. <i>B</i>, at rest.</p>
-</div>
-
-<p>Hitherto I have spoken only of the diurnal butterflies in which
-the anterior wings show an extension of the protective colouring
-which marks the whole surface of the posterior wings, and it was
-always the tips of the anterior wings that were thus coloured. But
-among the nocturnal Lepidoptera there are corresponding cases, in
-which a little tip of the posterior wing forms the continuation of
-the protective surface of the anterior wing. Some species of
-<i>Notodonta</i> and allied genera show in the posterior corner of the
-otherwise whitish posterior wings a little grey spot, and a hair tuft
-which in colour, and&mdash;when it is big enough&mdash;in marking, exactly
-resembles the protectively coloured anterior wings (Fig. 12). The
-'why' is at once clear, when one looks at the insect in the resting
-position, for only this little corner of the wing projects beyond the
-covering anterior wing. This has been regarded as telling against
-natural selection, for such a little spot could not possibly, by its
-colour, turn the scale as to the life or death of the individual, and
-so could not be selected. But one might say the same of the
-tip of the anterior wing in the diurnal forms, although there
-the protective surface is larger, often much larger. But who is to
-decide how large an exposed, unprotected spot must be in order to
-attract the attention of an enemy on the look out for food? Or<span class="pagenum"><a id="Page_81"></a>[Pg 81]</span>
-who can prove that the best and most familiar protective colouring
-really protects its possessors? What if, after all, it is all a game,
-a joke, which the Creator is playing with us poor mortals? Did not
-a trustworthy observer recently watch carefully, and see how a pair
-of sparrows daily cleared a wooden fence on which moths of the
-genus <i>Catocala</i> and other species of nocturnal Lepidoptera, excellently
-furnished with protective colours, were wont to settle by day? They
-did their work thoroughly, and hardly overlooked a single individual.
-But who has a right to see anything more in this than&mdash;what surely
-goes without saying&mdash;that the best protective colouring is not an
-absolute protection, and never preserves all from destruction, but
-always only some, and it may be very few.</p>
-
-<p>How else could there be such a high ratio of elimination, and
-such a constancy in the number of individuals of a species on any
-unchanging area? These sparrows had simply made full use of
-an experience, probably acquired by chance to begin with, and their
-vision had become sharpened for this particular species on the almost
-similarly coloured wooden fence, just as that of the expert butterfly
-collector does. It certainly does not follow from this that the
-protective colouring was useless, nor can we regard the harmony
-between the protruding tip of the anterior or posterior wing and the
-large protectively coloured surface of the covering wing as of no
-importance. On the contrary, if the tips were white or conspicuously
-coloured like the rest of the posterior wing, they would
-assuredly attract the sharp eye of hungry enemies to the spot, and so
-betray the victim. Instead of this, the spot in question is not only
-dark, but, in the case of <i>Notodonta</i>, is furnished with a tuft of hairs,
-which, in the insect's resting position (<a href="#f12">Fig. 12</a>, <i>B</i>), lies on the back,
-and looks like a dark, somewhat curved projecting tooth, in front
-of which there stands another, quite similar, which arises from the
-anterior wing, and behind there are other seven, rather smaller,
-dark teeth of the same kind, springing from the outer edge of the
-anterior wing. Taken altogether, they mimic the dentated edge of
-a withered leaf, and thus, in spite of their diverse origins, form
-a unified picture, and one with a considerable protective value. How
-is it possible to doubt that each of these hair-tufts has arisen under
-the influence of natural selection, and that its absence or imperfect
-development might result in the discovery and elimination of the
-insect concerned?</p>
-
-<p>These cases seem to me particularly beautiful proofs of the
-productive efficiency of selection. The wing is protected just
-as far as it protrudes from beneath the other&mdash;not a millimetre<span class="pagenum"><a id="Page_82"></a>[Pg 82]</span>
-further! How should it be otherwise, when the colouring of the
-parts just beside these is indifferent for the species, so that any
-variations in these parts in the direction of protective colouring
-never survive to be transmitted and accumulated?</p>
-
-<p>It is precisely this restriction to what is absolutely necessary
-that is the surest sign, here and elsewhere, that the character in
-question has been brought about by natural selection. And if this is
-the only possible, and at the same time quite sufficient explanation of
-the remarkably well-defined colour deliminations in all Lepidoptera,
-there can be no reason why we should try to drag in any other factor
-to explain the case of <i>Tropidoderus</i>, the less so as here again selection
-alone can account for the green of the exposed surfaces; and furthermore,
-the modification, common in other Phasmidæ, of the most anterior
-green stripe of the posterior wing into a firm cover protecting the
-soft abdomen, also points to natural selection; the cover-wings
-proper have here become too short, and so the edge of the posterior
-wing has been modified into a hard rib, which protects the soft body
-of the insect (<a href="#f11">Fig. 11</a>, <i>H. horn</i>). No differences in illumination,
-and no <i>direct</i> effect of any external influence whatever could have
-brought that about.</p>
-
-<p>How much more I might adduce in this connexion! The
-manifold diversity of colour and form adaptation is so great
-among insects, to which protection from their enemies is so necessary,
-and especially among butterflies, that I should never come to an
-end if I were to try to give even an approximate idea of it. Let
-us, therefore, turn now from such cases to a higher&mdash;the highest&mdash;grade
-of adaptation, that in which there is not only a mimicry of
-special and complex coloration, but in which the whole animal has
-become like some external object, and is thereby secured from
-discovery.</p>
-
-<p>We must first consider the case of our lappet moth (<i>Gastropacha
-quercifolia</i>), which in its copper-red colour and in the remarkable
-shape and dentated edges of its wings, and finally in the quite
-extraordinary clucking-hen-like attitude of the wings when at rest,
-greatly resembles some dry oak-leaves lying one above the other.</p>
-
-<p>Not unlike this is a 'shark' moth found in this country, <i>Xylina
-obsoleta</i>, which, as the name indicates, looks when at rest like
-a broken bit of half-rotten wood (<a href="#f10">Fig. 10</a>, p. 77). It 'feigns death,'
-as we commonly say, that is, it draws the legs and antennæ close to the
-body, and does not move; indeed, one may lift it up and throw it
-on the ground without its betraying by a single twitch that it lives.
-Only after it has been left undisturbed for some time does it show<span class="pagenum"><a id="Page_83"></a>[Pg 83]</span>
-signs of life again, and makes off hastily, to find a better hiding-place.
-The colouring of this moth is so curiously mingled&mdash;brown,
-whitish, black, and yellow&mdash;and traced with acute-angled
-lines and curves, that one cannot distinguish it at sight from a bit of
-rotten wood. I experienced that
-myself once when, passing a
-hedge, I thought I saw a
-<i>Xylina</i> sitting on the ground,
-and picked it up to examine it.
-I thought it was a bit of wood,
-and, disappointed, I threw it
-down again on the grass, but
-then I felt uncertain, and picked
-it up once more&mdash;to find that it
-was a moth after all<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a>!</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> Rösel says in this connexion: 'The marvellous form of this Papilio preserves it
-from injuries, for, when he hangs freely on a trunk of a tree, he would be taken ten
-times sooner for a piece of bark than for a living creature. By day, too, he is so little
-sensitive, that if he be thrown down from his resting-place he falls to the ground as
-if lifeless, and remains lying motionless. One may also throw him into the air, or
-turn him about, and he will rarely give a sign of life. I have impaled many of them
-on needles, without seeing any sign of sensitiveness on their part. This is the more
-remarkable that these birds (sic), after they have submitted to all the torment and misery
-one can inflict on them, without showing any sign of feeling, will, whenever they are
-left in peace and have no further disturbances to fear, quickly creep off to a dark
-corner and attempt to conceal themselves from future attacks.'&mdash;<i>Insektenbelustigungen</i>,
-Nürnberg, 1746, vol. i. p. 52.</p>
-
-</div>
-
-<p>This case of <i>Xylina</i> is hardly
-less remarkable, and its likeness
-to the mimicked object is scarcely
-less wonderful than that of the
-often discussed mimicry of a leaf,
-with stalk, midrib, and lateral
-veins, by many of the forest
-butterflies of South America and
-India.</p>
-
-<div class="figright" id="f13">
-<img src="images/fig13.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 13.</span> <i>Kallima paralecta</i>, from India,<br />
-right under side of the butterfly at rest.<br />
-<i>K</i>, head. <i>Lt</i>, maxillary palps. <i>B</i>, limbs. <i>V</i>,<br />
-anterior wing. <i>H</i>, posterior wing. <i>St</i>, 'tail'<br />
-of the latter, corresponding to the stalk of<br />
-the leaf. <i>gl</i><sup>1</sup> and <i>gl</i><sup>2</sup>, transparent spots. <i>Aufl</i>,<br />
-eye-spots. <i>Sch</i>, mould-spots.</p>
-</div>
-
-<p>The best known of these is
-the Indian <i>Kallima paralecta</i>,
-which, when it settles, is deceptively
-like a dead leaf, or rather
-like a dry or a half-withered
-one, on which brown alternates
-with red, and on which there
-are one or two translucent spots,
-without scales, presumably representing dewdrops. The upper surface
-of this butterfly is simply marked, but gorgeously coloured&mdash;blue-black<span class="pagenum"><a id="Page_84"></a>[Pg 84]</span>
-with a reddish yellow, or bluish white band&mdash;and quite constant. The
-under surface, on the other hand, although it always resembles a dead
-leaf, shows very varied ground colours, being sometimes greyish, sometimes
-yellowish, or reddish yellow, or even greenish. Often it shows the
-lateral veining of the leaf quite as distinctly as in Fig. 13, but often
-quite indistinctly, and the black, mouldy spots (<i>Sch</i>) of our figure may
-be more strongly marked, or they may be absent. It would seem as
-if the mimicry of different kinds of leaves was here aimed at&mdash;so to
-speak&mdash;just as in the case of the varied and numerous species of the
-South American genus <i>Anæa</i>, which usually live in the woods, and
-are all more or less leaf-like, but each species is like a different leaf,
-or like a leaf in a different condition, dry, moist, or decomposing. It
-is simply astounding to see this diversity of leaf mimicry, and the
-extraordinary faithfulness with which the impression of the leaf
-is reproduced. But it is by no means always the venation which
-causes the resemblance, for this is often inconspicuous; the high
-degree of deceptiveness is due to the silvery-clear yellow, dark
-yellow, red-brown to dark black-brown ground-colouring, which
-is never quite uniform, and over which there usually spreads
-a whitish ripple, combined with the remarkable imitation of the
-sheen of many leaves. The upper side of this butterfly is almost
-always conspicuously decorated with violet, dark blue or red, but
-always without any relation to the under surface. Not in all, but
-in many of the species of this genus, we find the round, translucent
-mirrors on the wing already mentioned in the case of <i>Kallima</i>, and
-in some species quite remarkable means are made use of to make the
-resemblance to a leaf thoroughly deceptive. Thus <i>Anæa polyxo</i>,
-when sitting, looks like a leaf out of the edge of which a caterpillar
-has eaten a little piece; in reality there is nothing missing from the
-wing, but on the front margin of the anterior wing a semicircular
-spot of a bright, soft, yellow colour stands out so sharply from the
-rest of the chestnut-brown wing surface, that it has the effect
-of a hole in the leaf.</p>
-
-<div class="figright" id="f14">
-<img src="images/fig14.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 14.</span> <i>Cœnophlebia archidona</i>, from Bolivia, in its<br />
-resting attitude. <i>mr</i>, midrib of the apparent leaf.<br />
-<i>st</i>, the apparent stalk.</p>
-</div>
-
-<p>A modern opponent of the selection theory (Eimer) has suggested
-that the marking of the lateral veins, and other resemblances to
-a leaf in <i>Kallima</i>, represent nothing more than the pattern which
-was present in any case, inherited from ancestors, and which in
-the course of time arranged itself in a particular manner according
-to internal developmental laws. Not selection&mdash;that is, adaptation
-to surroundings&mdash;but the internal developmental impulse has brought
-about the resemblance to the leaf. It is astonishing how a preconceived
-idea can blind a man and weaken his judgment! It goes
-<span class="pagenum"><a id="Page_85"></a>[Pg 85]</span>without saying that the adaptations do not start from a <i>tabula rasa</i>,
-but from what is already present; of course, natural selection makes
-use of the markings inherited from ancestors; it takes what already
-exists, and alters or extends it as suits best. Thus it is easy to prove
-that the clear mirrors (<a href="#f13">Fig. 13</a>, <i>gl</i><sup>1</sup> and <i>gl</i><sup>2</sup>) on the wings of <i>Kallima</i>
-have arisen from a modification of the nuclei of eye-spots, just as
-the dark mould-spots which often occur, frequently develop in
-association with the inherited eye-spots; not always however, for
-many such accumulations of black scales occur in spots on which
-there has never been an eye-spot. Thus, too, the 'midribs' of the
-butterfly have in part
-arisen from a gradual
-displacing, extending, and
-altering of the direction
-of inherited stripes as,
-for instance, is clearly
-recognizable in the posterior
-wing of Fig. 13, but
-sometimes they are new
-formations. But the veining
-of a leaf is never
-found on the wing of
-any butterfly of a species
-which has not the habit
-of resting among leaves,
-or which has not had it
-at one time, and it never
-corresponds to the natural
-marking of any genus
-which does not live in
-forests. This impression
-of leaf-venation has obviously
-arisen from quite different patterns of markings, and it has been
-reached now by one way, now by another. We can see this from
-the fact that, in different butterflies, it lies in quite different positions
-on the wing. In the <i>Kallima</i> species the stalk of the leaf lies in the
-tail of the posterior wing, the tip of the midrib lies near the tip of
-the wing; in <i>Cœnophlebia archidona</i> it is exactly reversed, the tip
-of the anterior wing (Fig. 14) is prolonged, and forms the stalk, while
-a broad, dark, stripe, the midrib (<i>mr</i>), runs from there across the
-middle of both wings, and seems to give off two or three lateral ribs
-running outwards. If it be asked whether this butterfly always sits<span class="pagenum"><a id="Page_86"></a>[Pg 86]</span>
-down so artistically that the 'upward turning leaf-stalk is in juxtaposition
-to a twig,' we may answer that a bird flying fast is not
-likely to look to see whether every leaf in the profusion of foliage
-in the primitive forests is properly fastened to its stalk or not, any
-more than we should do in the case of a painted bush, on which many
-a leaf has the appearance of floating in the air, just as in nature, or in
-its faithful copy, the photograph.</p>
-
-<div class="figleft" id="f15">
-<img src="images/fig15.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 15.</span> <i>Cærois chorinæus</i>, from the lower Amazon, in its<br />
-resting attitude. <i>V</i>, anterior wing. <i>H</i>, posterior wing. <i>mr</i>,<br />
-midrib of the apparent leaf. <i>sr</i>, lateral veins. <i>st</i>, hint of<br />
-a leaf-stalk.</p>
-</div>
-
-<p>Quite different from the leaf-marking either of <i>Cœnophlebia</i>
-or <i>Kallima</i> is that of one of the Satyrides of the lower Amazon
-valley, <i>Cærois chorinæus</i> (Fig. 15). If one spreads this butterfly
-out in the usual
-way it does not
-look in the least
-like a leaf, and one
-only sees a number
-of curiously placed
-disconnected stripes
-on the under surface
-of the wing.
-But if the wings
-be folded together
-to correspond with
-the sitting position
-of the butterfly,
-there appears the
-figure of a leaf,
-of which, however,
-only half is present,
-and whose midrib
-(<i>mr</i>) runs obliquely
-forward from the
-inner angle of the
-posterior wing. Here, again, it is not difficult to guess that this
-straight stripe has arisen, by displacement and straightening, from
-a curved line inherited from some remote ancestor, and it is these
-precise changes which are the work of the adaptive processes of
-natural selection. The same applies to the lateral ribs (<i>sr</i>), which are
-here four in number.</p>
-
-<p>But even the division of the wing surface by a single dark line,
-such as that which crosses the middle of the posterior wing of
-<i>Hebomoja</i> (<a href="#f9">Fig. 9</a>), an Indian butterfly, heightens not inconsiderably
-the resemblance of the resting butterfly to a leaf, a resemblance which<span class="pagenum"><a id="Page_87"></a>[Pg 87]</span>
-has already been shown in the form and colour. Indeed, even the
-sharp division of the wing surface into a darker inner and a lighter
-outer portion, which occurs in many species of <i>Anæa</i>, gives a very
-vivid impression of a leaf crossed by a midrib.</p>
-
-<p>It is not without a purpose that I have lingered so long over the
-leaf-butterflies. I wished to make it clear that we have by no means
-to do with a few exceptional cases, but with a great number, in all of
-which resemblance to a leaf has been aimed at, although it has been
-attained in varying degrees, and by very diverse ways. Whoever
-surveys this wealth of fact must certainly receive the impression,
-that, wherever it was advantageous to the existence of the species,
-the evolution of such a deceptive resemblance has also been possible.
-In any case one cannot but be convinced that it is not a case of
-chance resemblance, as some naturalists
-have recently tried to
-maintain.</p>
-
-<p>But I have not yet quite
-finished my outline-survey of the
-facts, for I must not omit to mention
-that, in the evergreen tropical
-forests, there are also large nocturnal
-Lepidoptera, which mimic
-leaves, sometimes green ones, sometimes
-brown, dead ones.</p>
-
-<div class="figright" id="f16">
-<img src="images/fig16.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 16.</span> <i>Phyllodes ornata</i>, from Assam.<br />
-Upper surface with leaf-like marking only<br />
-on the anterior wing, which is the only<br />
-part visible when at rest; 2/3 nat. size.</p>
-</div>
-
-<p>Fig. 16 gives a good picture,
-reduced to two-thirds, of such a
-species, <i>Phyllodes ornata</i>, from
-Assam. The posterior wings are conspicuously coloured in deep black
-and yellow; in the resting position they are covered by the anterior
-wings, and these are red-brown with black markings which precisely
-and clearly mimic the ribs of a leaf. The midrib begins near the tip
-of the anterior wing, but breaks off half-way across the wing at two
-silvery white spots, similar to those in many of the diurnal forms, which
-also mimic decaying leaves. Three pairs of side veins go off backwards
-and forwards with remarkable regularity from the midrib, almost at the
-same angle, and parallel to one another, and three more are indicated
-by vague shading. Then the midrib begins again in the internal half
-of the wing, though only represented by a broad shading. The whole
-suggests two torn, rotten leaves, one partly covering the other; and
-the deception will certainly be perfect when the moth rests on the
-ground or among decaying leaves.</p>
-
-<p>That all these extremely favourable protective colorations find<span class="pagenum"><a id="Page_88"></a>[Pg 88]</span>
-their explanation in the slow and gradually cumulative effects of
-natural selection cannot be disputed; it is beyond doubt that they
-cannot be explained, so far as we know, in any other way.</p>
-
-<p>If, however, it were possible for a species of butterfly living in
-the forest and among leaves to become, through natural selection,
-in any degree, and in a continually increasing degree, like a leaf,
-surely many insects living in the woods, and especially in the tropical
-woods, would also have followed such an advantageous path of
-variation&mdash;at least, so we should be inclined to think. And this is
-indeed the case; numerous insects, of different orders, if they are
-as large as a leaf, have taken on the colour, form, and usually also
-the markings, of a leaf. Thus green and also decaying and dead
-leaves are most realistically imitated by many tropical Locustidæ.
-Besides <i>Tropidoderus</i>, figured on <a href="#f11">p. 79</a>, a <i>Pterochroa</i> of South Brazil
-affords a particularly fine illustration of this, for not only does the
-ground-colour, brown or green, harmonize with that of a dead or
-fresh leaf, but, at the same time, all sorts of details are marked on the
-insect, which help to heighten the deceptive impression. Even the
-outline of the wings is leaf-like, and leaf-veins are marked on the wing-covers
-with the most beautiful distinctness, and finally there is,
-especially in the light-green individuals, a spot at the wing tip which,
-by means of a mixture of brown, yellow, reddish, and violet colour-tones,
-mimics a decaying spot with astonishing fidelity. Here, again,
-the origin of this special adaptation can be clearly recognized, for the
-vaguely concentric arrangement of the colours indicates that, in
-the ancestors of the species, an eye-spot had occurred on this area,
-of the same kind as we still see on the posterior wing, which is
-covered in the resting position. Thus we can again look back on the
-history of the species and conclude that the dissolution and degeneration
-of the eye-spot began at the time when the leaf resemblance was
-evolved, and this was probably caused by some change of habitat,
-which we can now no longer guess at.</p>
-
-<p>Many species of leaf-like Orthoptera, both in the Old and New
-World, have tough, green, parchment-like wing-covers which bear a
-remarkable resemblance to the thick Magnolia-like leaves of tropical
-plants. Along with these we must also mention the 'walking leaf,'
-which has been well known for centuries. In its case, not the wing-covers
-alone, but the head and thorax, and even the legs, are of the
-colour and shape of a leaf.</p>
-
-<p>The stick-insects, too, must not remain unnoticed; those quaint
-inhabitants of warm countries, whose elongated brown body looks like
-a knotted twig, and whose long legs, likewise stick-like, are stretched<span class="pagenum"><a id="Page_89"></a>[Pg 89]</span>
-out irregularly at different angles to the body, and usually remain
-motionless when the insect is resting. These creatures are vegetarian,
-and generally keep so still, that even the naturalist who is on
-the look-out for them may easily overlook them. Even such an
-experienced student of insects as Alfred Russel Wallace was deceived,
-for a native of the Phillipines once brought him a specimen as a
-'walking-stick' insect, which he rejected, saying that this time it was
-no animal but really a twig, until the native showed him that it was
-an insect whose likeness to a twig was increased by the fact that it
-bore on its back a ragged green growth, which looked exactly like
-a liverwort (<i>Jungermannia</i>), which occurs on the twigs of the trees
-in that region.</p>
-
-<p>We must also notice here the thorn-bugs, which are numerous on
-the prickly shrubs of tropical deserts and plateaux, especially in
-Mexico. These bear on the relatively very small body two or three
-large spines, which make them look like a part of the thorny bush on
-which they sit. But this masking by mimicry of thorns is not
-confined to insects, it is seen in lizards as well, notably in <i>Moloch
-horridus</i>, a lizard that lives in the Australian bush, and is covered all
-over with thorn-like scales.</p>
-
-<p>These examples should be enough to show that mimicry of
-the usual surroundings on the part of animals which are in need
-of protection, or are wont to lurk on the watch for their prey, are not
-isolated exceptions, chance resemblances, or, as they used to be called,
-'freaks of nature,' but that, on the contrary, they are the rule, depending
-on natural causes, and always occurring when these causes are
-operative. That such protective resemblances seem to be much more
-frequent in warmer climates than with us is probably a fallacy due
-to the fact that the number of species (especially of insects) is very
-much greater there, and that many insect types have their representatives
-of considerable size of body, which not only makes them
-more conspicuous <i>to us</i>, but makes some protective device in relation
-to their enemies or victims much more necessary.</p>
-
-<p>But we must here take account of one more example which
-occurs in our fauna in many modifications: the caterpillars of
-Geometridæ. Many of these soft and easily injured caterpillars
-resemble closely, in colour and shade, the bark of the tree or shrub on
-which they live (Fig. 17). At the same time they have the habit,
-when at rest, of stretching themselves out straight and stiff, so that
-they stand out free, at an acute angle from the branch, thus seeming
-like one of its lateral twigs. In many species the resemblance is
-heightened by the extraordinary pose of the head (<i>K</i>) and of the<span class="pagenum"><a id="Page_90"></a>[Pg 90]</span>
-claw-like feet (<i>F</i>), which, partly pressed close to the head, partly
-standing out from it, give the anterior end of the caterpillar the
-appearance of two terminal buds, while various little pointed, knotlike
-warts, scattered over the body, represent the sleeping buds of the
-little twig. Who has not at one time or other taken such a caterpillar
-for a little branch, and not inexpert observers only, but even
-trained naturalists? Many a time I have not been able to make
-quite sure of what I had before me until I touched it!</p>
-
-<div class="figcenter" id="f17">
-<img src="images/fig17.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 17.</span> Caterpillar of <i>Selenia tetralunaria</i>, seated on a birch twig. <i>K</i>, head. <i>F</i>, feet.
-<i>m</i>, tubercle, resembling a 'sleeping bud'; nat. size.</p>
-</div>
-
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_91"></a>[Pg 91]</span></p>
-
-<h2 class="nobreak" id="LECTURE_V">LECTURE V</h2>
-</div>
-
-<p class="c">TRUE MIMICRY</p>
-
-<div class="blockquot">
-<p>Mimicry: its discovery by Bates&mdash;Heliconiidæ and Pieridæ&mdash;Danaides&mdash;<i>Papilio
-merope</i> and its five females&mdash;The females lead the way&mdash;Species with mimicry in both
-sexes&mdash;Objections&mdash;Enemies of butterflies&mdash;The immunity of the models&mdash;Poisonousness
-of the food-plants of immune species&mdash;Several mimics of the same immune species&mdash;Persecuted
-species of the same genus resemble quite different models&mdash;<i>Elymnias</i>&mdash;Degree
-of resemblance&mdash;Differences between the caterpillars of the model and the copy&mdash;The
-same resemblance arrived at by different ways&mdash;Transparent-winged butterflies&mdash;The
-gradually increasing resemblance points to causes operating mechanically&mdash;Rarity
-of the mimetic species&mdash;Danger to the existence of the species not a necessary
-condition of mimetic transformation&mdash;<i>Papilio meriones</i> and <i>Papilio merope</i>&mdash;Comparison
-with the dimorphic caterpillars&mdash;<i>Papilio turnus</i>&mdash;'Mimicry rings' of immune species&mdash;<i>Danais
-erippus</i> and <i>Limenitis archippus</i>&mdash;Marked divergence of mimetic species from their
-nearest relatives&mdash;Mimicry in other insects&mdash;Imitators of ants and bees.</p></div>
-
-
-<p><span class="smcap">Let</span> us now turn to the most remarkable of all protective form- and
-colour-adaptations, the so-called Mimicry, including all cases of
-the imitation of one animal by another, which we came to know first
-through Bates, and to a fuller understanding of which A. R. Wallace
-and Fritz Müller have especially contributed.</p>
-
-<p>While the English naturalist, Bates<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a>, was collecting and observing
-on the banks of the Amazons&mdash;as he did for twelve years&mdash;it
-sometimes occurred that, among a swarm of those gaily coloured,
-quaintly shaped butterflies, the Heliconiidæ (<a href="#f21">Pl. II</a>, Fig. 13), he
-caught one which, on closer examination, proved to be essentially
-different from its numerous companions. It was certainly like them
-both in colour and form, but it belonged to quite a different family of
-butterflies, that of the Pieridæ or Whites (<a href="#f21">Pl. II</a>, Fig. 19). These
-whites with the colours of the Heliconiidæ always occurred singly in
-swarms of the latter form, and Bates found that, in the different
-districts of the Amazon, they always resembled in a striking manner
-the species of Heliconiidæ there prevalent. Many of them had been
-previously known to entomologists, and because they diverged so far
-from the usual type of the Pieridæ, especially in the form of the
-wing, the name Dysmorphia, the 'mis-shapen,' had been given to
-them, although the meaning of this 'mis-shapenness' long remained<span class="pagenum"><a id="Page_92"></a>[Pg 92]</span>
-a mystery. The French Lepidopterist, Boisduval, went a step further
-when he pointed out as something remarkable that nature sometimes
-makes several species of quite different families exactly alike, and
-called attention to three African butterflies, of which we shall have to
-speak later in detail. But even he was too much fettered by the old
-views of the immutability of species to arrive at a correct interpretation.
-Thus it was reserved for Bates to take the decisive step.
-Observing that the Heliconiidæ occurred frequently, and usually in
-large swarms, he concluded that they must have few enemies, and as
-he never saw the numerous insectivorous birds and insects hunting
-them, he further concluded that they must have something disagreeable
-which secured them from the attacks of these predaceous forms.
-On the other hand, he found that the heliconid-like Whites were
-always rare, and he took this as a sign that they were much persecuted,
-and that they must, therefore, be palatable tit-bits for the
-insectivores. If it were possible, then, that a species of Whites with
-the usual white colour of the family should give rise to variations,
-which would make them in any degree resemble the Heliconiidæ,
-which are secure from persecution, and if, in addition, those that
-exhibited the profitable variation attached themselves to swarms of
-the mimicked form, then these variants would be to a certain extent
-secured from attack, and more and more so in proportion as the
-resemblance to the protected model increased. The great likeness of
-these Whites to the Heliconiidæ, Bates further argued, would depend
-on a process of selection, based on the fact that, in each generation,
-those individuals would on the average survive for reproduction
-which were a little more like the model than the rest, and thus the
-resemblance, doubtless slight to begin with, would gradually reach its
-present degree of perfection.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> <i>Contributions to an Insect Fauna of the Amazon Valley</i>, Trans. Linn. Soc., Vol.
-XXIII, 1862.</p>
-
-</div>
-
-<p>Bates's hypotheses have been subsequently confirmed in the most
-striking way. The Heliconiidæ do possess a disagreeable taste and
-odour, and are utterly rejected by birds, lizards, and other animals.
-It has been directly observed that puff-birds, species of <i>Trogon</i>, and
-other insectivorous birds, looking down from the tops of trees in
-search of food, allowed to pass unheeded the swarms of gaily coloured
-Heliconiidæ which were fluttering among the leaves, and experiments
-with various insectivorous animals yielded the same result:
-<i>the Heliconiidæ are immune</i>. We can, therefore, not only understand
-that it must be advantageous to resemble them, we can also
-appreciate many of their peculiar characters, such as their gay
-coloration, which must serve as a sign of their disagreeable taste, and
-their slow, fluttering flight, as well as their habit of flocking together,<span class="pagenum"><a id="Page_93"></a>[Pg 93]</span>
-which must make it easier for the birds to recognize them as
-uneatable. Everything which marks out these unpalatable morsels,
-and makes them more readily recognizable, must be to their advantage,
-and therefore must have been favoured by natural selection
-(<a href="#f21">Pl. II</a>, Fig. 13).</p>
-
-<p>In the same way, every increase of resemblance on the part of the
-mimics would increase their chances of escaping notice, and any one
-who is accustomed to observe butterflies in nature can well understand
-that even very slight resemblances may have formed the
-beginning of the selection process; perhaps even a mere variation in the
-manner of flight, combined with the habit of associating with the
-swarms of Heliconiidæ. I myself have many times been momentarily
-deceived in our own woods by a White of unusually majestic flight, so
-that I took it for an <i>Apatura</i> or a <i>Limenitis</i>. If, therefore, individual
-Whites occurred here and there in the Amazon valley, which flew
-somewhat after the manner of the Heliconiidæ, and associated with
-them, they might possibly have attained a certain degree of security
-through that alone, and it would be greatly increased if at the same
-time they varied somewhat in colour in the direction of their
-companions.</p>
-
-<p>In any case there can be no doubt whatever that in these cases
-a real transformation of the species in colour and marking, and
-perhaps often, too, in form of wing, has taken place, and that within
-comparatively modern times&mdash;let us say during the distribution of a
-species which required protection over a large continent, or since the
-last breaking up of an immune species into local species. Various
-facts prove this; above all, the circumstance that it is often only the
-females which exhibit this protective mimicry; and that one and the
-same species may mimic a different immune species in different areas,
-but always the one occurring abundantly in that area, and so on.</p>
-
-<p>Definite examples will make this clearer, and I will only say in
-advance that, since the discovery of Bates, numerous cases of mimicry
-in butterflies have been found, not only in South America, but in all
-tropical countries which have a rich Lepidopteran fauna. And it is
-not only between the Heliconiidæ and the Pieridæ that such relations
-have been evolved; many much-persecuted, unprotected species of
-different families everywhere mimic species which are rejected on
-account of their nauseous taste, and these, too, belonging to different
-families. The Heliconiidæ are a purely American group, but in the
-Old World and in Australia their place is taken by the three great
-families of Danaides, Euplœides, and Acræides, since, as it seems, they
-all taste unpleasantly, and are rejected by all, or at least by most, of<span class="pagenum"><a id="Page_94"></a>[Pg 94]</span>
-the insectivorous birds. Numerous species of the genus <i>Danais</i>
-(<a href="#f19">Pl. I</a>, Fig. 8), <i>Amauris</i> (<a href="#f19">Pl. I</a>, Fig. 5), <i>Euplœa</i> (<a href="#f22">Pl. III</a>, Fig. 25, 27),
-and <i>Acræa</i> (<a href="#f21">Pl. II</a>, Fig. 2), and also many species of <i>Papilio</i> and
-other genera, enjoy the advantage of unpleasant taste, if not even of
-poisonousness; they are, therefore, secure from pursuit, and are, in
-consequence, much mimicked by palatable butterflies.</p>
-
-<p>As a further example, I now select a diurnal butterfly from
-Africa, <i>Papilio merope</i> Cramer<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a>, which was shown by Trimen in
-1868 to be mimetic. The species has a wide distribution, for, if we
-except slight local differences in the marking of the male, its range
-extends over the greater part of Africa, from Abyssinia to the Cape,
-and from East Africa to the Gold Coast.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> The West African form of <i>Papilio merope</i> has been quite recently distinguished
-from the southern form and regarded as a distinct species, the latter being now called
-<i>Papilio cenea</i>. The differences in the males are very slight&mdash;somewhat shorter wings,
-shorter wing-tail, and so on&mdash;differences which seem relatively unimportant in comparison
-with the differences between the males and the females.</p>
-
-</div>
-
-<p>The male is a beautiful large butterfly, yellowish white, with a
-touch of black, and with little tails to the posterior wings (<a href="#f19">Pl. I</a>, Fig. 1),
-like our own swallowtail. A very nearly related species occurs in
-Madagascar, and there the female is similarly coloured, though it may
-be distinguished by having a little more black on the wing. On the
-mainland of Africa, however, the females of <i>Papilio merope</i> are so
-different in colour and form of wing that it would be difficult to
-believe them of the same species as the male had not both sexes more
-than once been reared from the eggs of one mother. The females
-(<a href="#f19">Pl. I</a>, Fig. 6) in South Africa imitate a species of <i>Amauris</i>, <i>A. echeria</i>
-(<a href="#f19">Pl. I</a>, Fig. 7), of a dark ground-colour with white, or brownish-white,
-mirrors and spots, and they resemble it most deceptively. But what
-makes the case more interesting in its theoretical aspect is that
-<i>Danais echeria</i> of Cape Colony is markedly different from <i>Danais
-echeria</i> of Natal, and the female of <i>Papilio merope</i> has followed those two
-local varieties, and has likewise a Cape and a Natal local form. Even
-this is not all, for in Cape Colony there are two other females of
-<i>Papilio merope</i>. One of them has a yellow ground-colour, and resembles
-<i>Danais chrysippus</i>, which is extremely abundant there (<a href="#f19">Pl. I</a>, Fig. 3);
-the other is entirely different (<a href="#f19">Pl. I</a>, Fig. 4), for it closely mimics another
-Danaid occurring in the same districts of Africa, and also immune,
-<i>Amauris niavius</i> (<a href="#f19">Pl. I</a>, Fig. 5), not only in the beautiful pure white
-and deep black of the wing surface, but also in the distribution of
-these colours to form a pattern.</p>
-
-<p>We have thus in Africa four different females of <i>Papilio merope</i>,
-each of which mimics a protected species of Danaid. They are not always<span class="pagenum"><a id="Page_95"></a>[Pg 95]</span>
-locally separate, so that each is exclusively restricted to a particular
-region, for their areas of distribution often overlap, and, at the Cape
-for instance, one male form and three different forms of female have
-been reared from one set of eggs. In addition, we have the fact that
-between the two local forms of <i>Danais echeria</i> transition forms occur,
-and that the mimetic females of <i>Papilio merope</i> show the same transition
-forms locally, and we must admit that all these facts harmonize most
-beautifully with the selection interpretation, but defy any other. And
-that the last doubt may be dispelled, nature has preserved <i>the primitive
-female form</i> on the continent of Africa&mdash;namely, in Abyssinia,
-where, along with the mimetic females, there are others which are
-tailed like the males (<a href="#f19">Pl. I</a>, Fig. 1), and are like them in form and
-colour, a few minor differences excepted.</p>
-
-<p>Thus we have in <i>Papilio merope</i> a species which, in the course
-of its distribution through Africa, has scarcely varied at all in the
-male sex, but in the female has almost everywhere lost the outward
-appearance of a <i>Papilio</i>, and has assumed that of a Danaid, which is
-protected by being unpalatable, and not even everywhere the
-appearance of the same species, but in each place that of the
-prevailing one, and sometimes of several in one region. These
-females thus show at the present day a polymorphism which consists
-of four chief mimetic forms, to which has to be added the primitive
-form&mdash;that resembling the male. This has survived in Abyssinia
-alone, and even there it is not the only one, but occurs along with
-some of the mimetic forms.</p>
-
-<p>To the question why only the females are mimetic in this and
-other cases, Darwin and Wallace have answered that the females are
-more in need of protection. In the first place, the males among
-butterflies are considerably in the majority, and, secondly, the females
-must live longer in order to be able to lay their eggs. Moreover, the
-females, which are loaded with numerous eggs, are heavier in flight,
-and during the whole period of egg-laying&mdash;that is, for a considerable
-time&mdash;they are exposed to the attacks of numerous enemies. Whether
-one of the abundant males is devoured sooner or later is immaterial to
-the persistence of the species, since one male is sufficient to fertilize
-several females. The death of a single female, on the other hand,
-implies a loss of several hundred descendants to the species. It
-is, therefore, intelligible that, in species already somewhat rare, the
-female must first of all be protected; that is to say, that all variations
-tending in the direction of her protection would give rise to a
-process of selection resulting in an augmentation of the protective
-characters.</p>
-
-<p><span class="pagenum"><a id="Page_96"></a>[Pg 96]</span></p>
-
-<p>But there are also butterflies in which both sexes mimic a
-protected model. Thus many imitators of the unpalatable Acræides
-(<a href="#f21">Pl. II</a>, Fig. 21) resemble the model in both sexes, and of the South
-American Whites which mimic the Heliconiidæ there are some
-which have the appearance of the Heliconiidæ even in the male sex
-(<a href="#f21">Pl. II</a>, Fig. 18, 19), while others look like ordinary Whites (for
-instance, <i>Archonias potamea</i>). But in many of these species, which
-are mimetic in the female sex, we find also in the male some indications
-of the mimetic colouring, but in the first instance only on the under
-surface. Thus the females of <i>Perhybris pyrrha</i> (<a href="#f21">Pl. II</a>, Fig. 17) resemble
-in their black, yellow, and orange-red colour-pattern the immune
-American Danaid, <i>Lycorea halia</i> (<a href="#f21">Pl. II</a>, Fig. 12), but their mates are,
-on the upper surface, like our common Whites, though they already
-show on the under surface the orange-red transverse stripes of the
-<i>Lycorea</i> (<a href="#f21">Pl. II</a>, Fig. 16). In other mimetic species of Whites a similar
-beginning is even more faintly hinted at, and in others, again, the
-upper surface of the male is also provided with protective colours, and
-only a single white spot on the posterior, or sometimes even on the
-anterior wing as well, shows the original white of the Pieridæ (<a href="#f18">Fig. 18</a>).</p>
-
-<p>I do not know how any one can put any other construction on
-these facts than that the females first assumed the protective colouring,
-and that the males followed later, and more slowly. Whether
-this is due to inheritance on the female side, and thus ensues as
-a mechanical necessity, in virtue of laws of inheritance still unknown
-to us, or whether it arose because there was a certain advantage
-in protection to the males&mdash;though not such a marked one&mdash;and that
-these, therefore, followed independently along the same path of
-evolution as the females, has yet to be investigated. Personally,
-I incline to the latter view, because there are protected mimetic
-species, in which the female mimics one immune model, and the male
-another, quite different from the female's. A case in point is that of
-an Indian butterfly, <i>Euripus haliterses</i>, and also <i>Hypolimnas scopas</i>,
-in the latter of which the male resembles the male of <i>Euplœa
-pyrgion</i>, and the female is like the somewhat different female of the
-same protected species. The Indian <i>Papilio paradoxus</i>, too, seems to
-show the independence of the processes of mimetic adaptation, for the
-male is like the blue male of the immune <i>Euplœa binotata</i> (<a href="#f22">Pl. III</a>,
-Fig. 25), while the female resembles the radially-striped female of
-<i>Euplœa midamus</i> (<a href="#f22">Pl. III</a>, Fig. 27), and this double adaptation is
-repeated in another of the persecuted butterflies, <i>Elymnias leucocyma</i>
-(<a href="#f22">Pl. III</a>, Fig. 26, 28).</p>
-
-<p>Many objections have been made to the interpretation of mimicry<span class="pagenum"><a id="Page_97"></a>[Pg 97]</span>
-by selection. It has been asserted that butterflies are exposed to
-injury from birds only to an inconsiderable extent, not sufficient to
-account for such an intense and persistent process of selection,
-because they are not very welcome morsels, on account of the large
-and uneatable wings and the relatively small body. Doubt has also
-been raised as to the immunity of the models, which has not been
-proved in many of the species in regard to which it is assumed.
-Finally, it is maintained that the advantage which resemblance to an
-immune model brings is not proved, but is purely hypothetical; and
-that it is probable that the birds do not distinguish the colours
-and markings of the flying butterflies at all, but are at the most
-only deceived by resemblances in their manner of flight.</p>
-
-<p>The last objection contains a certain amount of truth, inasmuch
-as the manner of flight always plays a part in the mimicry of
-a strange species. We shall see later how much the instincts of
-a species contribute to the deception in all cases of protective colouring.
-It is, therefore, not improbable that, in many cases, the imitation
-of the flight of an immune species, and a gradually increasing
-familiarity with the habitats of the same immune species, preceded the
-modification of the colour. Indeed, the slow flight of immune species
-(Heliconiidæ) has been unanimously emphasized by observers, as
-a factor in facilitating the recognition of the butterflies by the sharp-sighted
-birds.</p>
-
-<p>That it was not only in earlier ages of the world's history that
-butterflies were much persecuted, as some have supposed, but that
-they are so still, seems to me indisputable in view of the observations
-of the last quarter of a century. Even in this country, where both
-butterflies and insect-eating birds are being more and more crowded
-out through cultivation, a considerable number of butterflies in flight
-fall victims to the birds. Kennel gives observations on this point
-in regard to the white-throat; Caspari for the swallows. The latter
-let about a hundred little tortoiseshell butterflies (<i>Vanessa antiopa</i>)
-fly from his window, 'but not ten of them reached the neighbouring
-wood,' all the rest being eaten by swallows, 'which congregated in
-numbers in front of his window.' Kathariner observed, in the
-highlands of Asia Minor, a flock of bee-eaters (<i>Merops</i>) which caught
-in flight and swallowed a great many individuals of a very beautiful
-diurnal butterfly (<i>Thais cerisyi</i>).</p>
-
-<p>Finally, Pastor Slevogt has collected much evidence to show that
-our indigenous butterflies have a great deal to suffer in the way
-of persecution from birds. And in regard to tropical countries, the
-chase of butterflies by insectivorous birds has long been known.<span class="pagenum"><a id="Page_98"></a>[Pg 98]</span>
-Thus Pöppig says that in the primitive forests one can easily
-recognize the place which has been selected by one of the Jacamars
-(Galbulidæ) as its favourite resting-place, for the wings of the largest
-and most beautiful butterflies, whose bodies alone are eaten, lie on
-the ground in a circle for a distance of several paces. We owe direct
-observations on the hunting of insects by birds of the primitive forest
-especially to Dr. Hahnel, who found many opportunities for observation
-in the course of his enthusiastic collecting journeys in Central
-and South America. He writes: 'No other family of butterflies
-suffered so much from birds as the Pieridæ (Whites), and these freebooters
-often snapped away the prettiest and freshest specimens
-from quite close to me. Every time I was amazed anew at the
-unfailing security of their flight, and I gladly paid for the spectacle
-by the loss of a few specimens.' Of the pursuit of one of the large
-<i>Caligo</i> species, whose leaf-like under surface, marked with eye-spots,
-I have already described, (<a href="#f6">Fig. 6</a>, p. 70), he says: 'With incredible
-skill this fairly large insect avoided every blow of the bill of the bird
-which followed it in close chase, and saved itself by flying from one
-shrub to another, till at last it was lost to sight in the thickest tangle
-of branches, and the exhausted bird gave up further attempts at
-pursuit.'</p>
-
-<p>But, in addition to the birds, the butterflies of the primitive
-forest have to dread the persecution of other insects, especially of the
-large predaceous dragon-flies, which throw themselves upon them in
-the midst of their flight. Hahnel often saw a specimen of the large,
-beautiful, blue <i>Morpho cisseis</i>, which was fluttering peacefully about
-the crown of a tree, suddenly shoot head downwards, 'like an ox
-with horns lowered, and then reascended apparently with difficulty,
-after it had torn itself free from its sudden assailant, whose jaws
-left distinct short scars.'</p>
-
-<p>In addition to birds and predatory insects the butterflies are
-persecuted by the whole army of lizards. In order to entice the
-butterflies, Hahnel laid bait in the wood, 'sugar-cane, little sweet
-bananas, and such like.' Various kinds of butterfly settled on it,
-'Satyrides, Ageroniæ, <i>Adelpha</i> and other Nymphalidæ.' He saw
-that they 'were persistently stalked and attacked by greedy lizards,
-which, in spite of their plump figure and uncouth gait, showed
-themselves able to spring suddenly out and snatch their prey with
-great adroitness. It is, however, very wonderful to see the agility
-such a persecuted insect displays in evading the repeated attacks of
-these marauders.' Thus on one occasion an <i>Adelpha</i> was driven off
-a dozen times from the exposed bait by a lizard, which pounced upon<span class="pagenum"><a id="Page_99"></a>[Pg 99]</span>
-it, but it always settled down for a short time on a leaf, and soon
-returned to its repast, whereupon the enemy 'instantaneously rushed
-upon it in a fury, until at last he was obliged to give in,' abandoning
-the attempt to catch a creature so adept in retreat.</p>
-
-<p>Many butterflies assemble at midday on sandbanks in the middle
-of the river, in order to drink, and there, too, the lizards are always
-lurking about. Hahnel gives a pretty and undoubtedly accurate
-description of the protective value of the long tail borne by many of
-the sail-like Papilios at the end of the posterior wing; they 'quite
-obviously' afford protection against the lizards, 'which, after
-snapping, often find themselves obliged to be content with the tail
-alone, while the rest of the animal flies away practically uninjured.'</p>
-
-<p>Not only is the great persecution of the butterflies a fact, the
-immunity of the known species, which are models for mimicry, is
-also certain. For numerous species, at any rate, this has now been
-established. First of all&mdash;as has already been said&mdash;this is true of
-the Heliconiidæ, in regard to which Wallace long ago showed that,
-if the thorax be pressed, they exude a yellowish juice of unpleasant
-smell. This is probably the blood of the insect, but that does not
-hinder the repulsive odour of the living butterfly being perceptible
-at a distance of 'several paces,' as Seitz observed in <i>Heliconius
-besei</i>.</p>
-
-<p>Repeated experiments have been made, which have shown that
-such butterflies are rejected not only by the insectivorous birds of
-the primitive forest, but also by tame turkeys, pheasants and partridges,
-usually so greedy. Hahnel has recently repeated these experiments
-in Brazil with hens, and he obtained the same result.
-The hens, 'which otherwise devoured all butterflies eagerly,' rejected
-all Ithomidæ, Heliconiidæ, the white Papilios, as also some of
-the gaily coloured Heliconiid-like moths which fly by day, such
-as <i>Esthema bicolor</i> and <i>Pericopis lycorea</i>. Obviously, the gay or
-conspicuous colour of these Lepidoptera acts as a warning signal of
-their unpalatability, and protects them from attempts on the part
-of the birds to investigate their flavour. Hence we find that the
-under surface of these insects is coloured like the upper. Even the
-numbers of these species which fly about indicates that they must
-be little decimated, and, in point of fact, we never find the wings of
-Heliconiidæ lying on the ground in the forests of South America,
-while those of the Nymphalidæ and other butterflies are by no means
-uncommonly seen as the remains of birds' meals.</p>
-
-<p>There is just as little room for doubt, as in the case of the
-Heliconiidæ and their allies, that the Danaidæ, Acræidæ, and the<span class="pagenum"><a id="Page_100"></a>[Pg 100]</span>
-Euplœidæ in the tropical regions of the Old World enjoy a certain
-immunity on account of their repulsive odour and taste. Here, too,
-observation and experiment have shown that birds, lizards, and
-predaceous insects leave the butterflies of these families unmolested.
-I need only mention the observation of Trimen that, under an acacia
-much visited by butterflies, on which Mantides&mdash;the so-called
-praying-insects&mdash;caught and devoured large numbers, the wings of an
-<i>Acræa</i> or a <i>Danais</i> were never found. These unpalatable butterflies
-also possess a motley or at least striking dress, recognizable from
-afar, and alike on both surfaces; and they also have a slow flight, by
-which they are readily recognized. They, too, usually assemble in
-large swarms, and both sexes are alike, or resemble each other
-closely in colouring, or at least they are both equally conspicuous.
-But even these cases do not complete the list of butterflies which are
-protected by their unpalatability; among the otherwise much-persecuted
-and therefore palatable Pieridæ (Whites) there is an
-Asiatic genus, <i>Delias</i>, which in all probability belongs to the immune
-butterflies, as their gaily coloured under surface indicates, and among
-the nocturnal Lepidoptera of different countries and families there are
-isolated generations which are very gaily and conspicuously coloured,
-and which are rejected by birds, their unpleasant odour being
-perceptible at a distance of several feet (Chalcosiidæ and Eusemiidæ).
-The latter no longer fly under cover of night, like their relatives, but
-have assumed diurnal habits.</p>
-
-<p>It is to be supposed that the repulsiveness of such 'unpalatable'
-butterflies is associated with the food-plant on which the caterpillar
-lives. Acrid, nauseous, astringent, and actually poisonous substances
-are produced in many plants, and we shall see later that this is to their
-own advantage; these substances pass into the insect, and they do so
-probably in part unaltered, in part certainly altered, but still they are
-protective, perhaps even in an increased degree. This is borne out by
-the fact that many caterpillars of immune butterflies live on more or
-less poisonous plants: the Acræidæ and Heliconiidæ on Passiflores,
-which contain nauseous substances; the Danaidæ on the poisonous
-Asclepiadæ, which are rich in milky juice or latex; the Euplœæ on
-the poisonous species of <i>Ficus</i>, the Neotropinæ on the Solanaceæ, and
-so on. But there are many genera, rich in species, and distributed
-over the whole earth, the caterpillars of which live on plants of very
-various families and characters, and of these the majority of species
-are palatable, though a few are repulsive in taste and odour, and
-therefore immune. This is the case in the genus <i>Papilio</i>. As far
-back as the sixties Wallace discovered that there were immune<span class="pagenum"><a id="Page_101"></a>[Pg 101]</span>
-species of <i>Papilio</i>, and that these were mimicked by other species.
-Later it was shown that these immune species live chiefly on
-poisonous plants (in the wide sense), on various Aristolochiæ; and
-Haase has recently grouped these together as poison-eaters
-(Aristolochia-butterflies or Pharmacophagæ). They are distinguished
-by a conspicuous red on the body. In some of them, as
-in <i>Papilio philoxenus</i>, a repulsive odour as of decomposing urine has
-been detected in the living animal.</p>
-
-<p>We see, then, that the much-persecuted and easily injured butterflies
-make use of a poisonous substance (in the widest sense), prepared
-in the plant for its own protection, and, wherever their own
-metabolism makes it possible, they use it to protect themselves.
-We need not wonder, therefore, that so many butterflies are immune,
-nor that among the numerous palatable species a small proportion
-have endeavoured to become like the protected species,
-as far as natural selection was able to bring such a resemblance
-about.</p>
-
-<p>There is hardly any adaptation phenomenon so widely distributed
-and diverse in its manifestations, which has been at the
-same time so much observed and followed out into all its details, as
-Mimicry; and it must surely be regarded as a justification of the
-validity of interpreting it in terms of Natural Selection that all
-the observed phenomena tally so beautifully with the deductions
-from the theory. I at least know of no facts which contradict the
-theory, but of many which might have been predicted from it.</p>
-
-<p>For instance, it might have been predicted from the theory alone
-that an immune species would often have several mimics, as, in point
-of fact, is frequently the case, and it would be easy to give numerous
-examples of this. Thus the two Danaids of South and Central
-Africa, <i>Amauris echeria</i> and <i>Amauris niavius</i>, are mimicked, not
-only by the two female forms of <i>Papilio merope</i>, as we have already
-described in detail, but the latter is also mimicked by Nymphalid,
-which requires protection, <i>Diadema anthedon</i>, and the former by two
-diurnal butterflies of different families, <i>Diadema nuina</i> and <i>Papilio
-echerioides</i>.</p>
-
-<p>Similarly, the black-and-red coloured <i>Heliconius melpomene</i> in
-Brazil is mimicked both by the female of a White (<i>Archonias
-teuthamis</i>), and by a <i>Papilio</i>, which has received the name of
-<i>P. euterpinus</i> on account of this resemblance. Thus, too, the
-immune <i>Methona psidii</i>, Cr. of Brazil, with its half-transparent
-wings marked with black bands, has five mimics, belonging to five
-different genera, and one of these is not a true diurnal butterfly at all,<span class="pagenum"><a id="Page_102"></a>[Pg 102]</span>
-but one of the day-flying species of the genus <i>Castnia</i>, whose
-systematic position is doubtful.</p>
-
-<div class="figleft" id="f18">
-<img src="images/fig18.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 18.</span> Upper surfaces of <i>A</i>, <i>Acræa<br />
-egina</i>, from the Gold Coast, immune.<br />
-<i>B</i>, <i>Papilio ridleyanus</i>, from Gaboon, not<br />
-immune. <i>C</i>, <i>Pseudacræa boisduvalii</i>,<br />
-from the Gold Coast, not immune.</p>
-</div>
-
-<p>The West African immune Acræid, <i>Acræa gea</i> (<a href="#f21">Pl. II</a>, Fig. 21),
-is deceptively mimicked, both as to the
-narrow, long shape of the wing and
-its blackish-brown and white mottled
-markings, by a Nymphalid, <i>Pseudacræa
-hirce</i>, by the female of a Papilio
-(<i>P. cynorta</i>) whose mate is quite different,
-and by the female of a Satyrid
-(<i>Elymnias phegea</i>) (<a href="#f21">Pl. II</a>, Fig. 20). In
-the <i>Papilio</i> the resemblance extends to
-the peculiar pitch-black shining spot
-on the under side of the base of the
-posterior wing, and all three are like
-the model on both surfaces, and therefore
-in flight as well as in the resting
-attitude.</p>
-
-<p>On the same West African coast
-occurs the strange greyish-black <i>Acræa
-egina</i>, with brick-red spots and bands,
-and coal-black dots (Fig. 18, <i>A</i>). This
-immune species is deceptively mimicked
-in its native country by two other
-butterflies&mdash;a Nymphalid, <i>Pseudacræa
-boisduvalii</i> (Fig. 18, <i>C</i>), and by a female
-<i>Papilio</i> (<i>P. ridleyanus</i>) (Fig. 18, <i>B</i>), by
-the latter not so exactly as by the
-former, but quite sufficiently to be confused
-with its model in flight.</p>
-
-<p>It would have been less easy to
-predict with certainty from the theory
-that, conversely, the different species
-of a genus which stood in need of
-protection would be able to mimic
-quite different immune models, for who
-would have ventured to prophesy how
-far the capacity of a species for variation
-might go, and how many different
-kinds of coloration it was able to
-assume? But the facts teach us that there is a wide range of possibility
-in this respect.</p>
-<p><span class="pagenum"><a id="Page_103"></a>[Pg 103]</span></p>
-
-<p>Most interesting in this respect is, perhaps, the Asiatic-African
-genus <i>Elymnias</i>, a Satyrid whose numerous (over thirty) species all
-seem to be in need of protection, for many of them mimic immune
-butterflies, while the rest are inconspicuous and are provided with
-protective colouring on the under surface. On Plates II and III
-some of the former are depicted beside their models. The single
-African species (<i>Elymnias phegea</i>) (<a href="#f21">Pl. II</a>, Fig. 20) mimics, as has
-been already mentioned, the prevalent <i>Acræa gea</i> (<a href="#f21">Pl. II</a>, Fig. 21).
-Many of the Asiatic Elymniidæ are mimics of the immune Euplœæ,
-especially the dark-brown species with steel-blue shimmer, such as
-<i>E. patna</i> in India, <i>E. beza</i> in Borneo, and <i>E. penanga</i> in Borneo. In
-Amboina there flies an <i>E. vitellia</i>, the female of which mimics
-accurately the plain, light-brown, inconspicuous <i>Euplœa climena</i>
-which occurs there. The male of <i>Elymnias leucocyma</i> (<a href="#f22">Pl. III</a>,
-Fig. 26) resembles the brown and blue shimmering <i>Euplœa binotata</i>
-(<a href="#f22">Pl. III</a>, Fig. 25), while the female mimics the dusky, radially-striped
-female of <i>Euplœa midamus</i> (<a href="#f22">Pl. III</a>, Figs. 27 and 28): the male of
-<i>Elymnias cassiphone</i> resembles the blackish-brown and deep-blue
-iridescent <i>Euplœa claudia</i>, while the female is like the female of
-<i>Euplœa midamus</i>. A number of species of <i>Elymnias</i> copy Danaids:
-thus both sexes of <i>E. lais</i> are like <i>Danais vulgaris</i> (<a href="#f22">Pl. III</a>, Figs. 29
-and 30), and <i>E. ceryx</i> and <i>E. timandra</i> are like another similar
-Danaid, <i>D. tytia</i>. The female only of <i>E. undularis</i> of Ceylon
-mimics the brown-yellow <i>D. genutia</i> (<a href="#f21">Pl. II</a>, Fig. 22) in general
-appearance, though not minutely, while the male (<a href="#f21">Pl. II</a>, Fig. 24)
-seems to attempt an imitation of the blue EuplϾ. A rare form,
-not often represented in collections, <i>Elymnias künstleri</i>, bears a
-striking resemblance to the Danaid, <i>Ideopsis daos</i> Boisd., with its
-white wings spotted with black, while three species mimic the
-probably immune Pierid genus <i>Delias</i>, especially on the under
-surface, which is decorated with yellow and red. Perhaps the one
-which has diverged farthest from the original type is <i>Elymnias
-agondas</i> Boisd. (<a href="#f21">Pl. II</a>, Fig. 32) of the Papua region and the island of
-Waigeu, for it bears two large blue eye-spots on the posterior wings,
-and thus, especially in the case of the almost white female, closely
-resembles <i>Tenaris bioculatus</i> (<a href="#f22">Pl. III</a>, Fig. 31). There are thus seven
-or eight types of marking and colouring differing from one another,
-and belonging to six different genera and a much greater number of
-species, which are mimicked by this one genus <i>Elymnias</i>.</p>
-
-<p>It is most interesting to note how these mimetic species give
-up, more or less, the original sympathetic colouring of the under
-surface, and use in establishing their mimicry the marking elements<span class="pagenum"><a id="Page_104"></a>[Pg 104]</span>
-which were originally directed towards concealment. According to
-the beautiful observations of Erich Haase on this genus <i>Elymnias</i>,
-the ground-colouring on the under surface must have been 'a grey,
-darkly mottled protective one,' as still occurs, for instance, in several
-mimetic species, such as <i>Elymnias lais</i> (<a href="#f22">Pl. III</a>, Fig. 30). This leaf-colouring
-disappears more and more the more perfect the mimicry
-of the model becomes, until, finally, the model is repeated on the
-under surface also. Compare, for instance, Figs. 30 and 32. From
-this we may conclude that a dress which makes Lepidoptera appear
-unpalatable morsels is a more effective protection than resemblance
-to a leaf. That might indeed be deduced even from the theory, for
-resemblance to a leaf never protects <i>absolutely</i>, and does so, in any
-case, only during rest, while apparent unpalatability repels assailants
-at all times.</p>
-
-<p>Those unversed in butterfly lore usually ask, when these mimetic
-relations are expounded to them, how we know that copies which are
-so like their models really belong to a different genus, or even family.
-There are certainly cases in which model and copy resemble each
-other so closely that even a zoologist cannot tell one from the other
-without close examination, as, for instance, in the case of certain
-transparent-winged Heliconiidæ of Brazil (Ithomiides) and their
-mimics belonging to the family of Whites. But even in such cases
-the likeness only extends as far as is theoretically requisite, that is,
-only to those characters that make the butterfly appear to the eye
-of its pursuer like another species, known to it to be unpalatable.
-The likeness does not extend to details, which can only be seen with
-a magnifying-glass or a microscope, and above all, it does not extend
-to the caterpillar, pupa, or egg. Thus, in the case cited, we may be
-certain that the caterpillar of <i>Ithomia</i> is quite different from that
-of the mimicking White, since the former will be, in structure, of the
-type of <i>Ithomia</i> caterpillar, and the other of the usual type of
-Whites. As yet, indeed, these two species are not known in their
-caterpillar stages, but other cases are known. A species belonging
-to the same genus as our indigenous 'kingfishers' (<i>Limenitis populi</i>),
-a diurnal butterfly of North America, <i>Limenitis archippus</i> (<a href="#f19">Pl. I</a>,
-Fig. 9), strongly resembles the brown-yellow, immune <i>Danais erippus</i>
-(<a href="#f19">Pl. I</a>, Fig. 8), while the caterpillars of both species are quite different,
-that of <i>Danais erippus</i> possessing the remarkable, soft and flexible
-horn-like processes of the Danaid caterpillars (<a href="#f19">Pl. I</a>, Fig. 10<i>a</i>), while
-the caterpillar of <i>Limenitis archippus</i> (<a href="#f19">Pl. I</a>, Fig. 11<i>a</i>) is at once
-recognizable by its blunt, club-shaped and spinose papillæ as a <i>Limenitis</i>
-caterpillar. The adaptation of the butterfly to its protected model has<span class="pagenum"><a id="Page_105"></a>[Pg 105]</span>
-thus exercised no influence upon the caterpillar. Nor has it affected
-the pupa, which in both cases exhibits the very different and quite
-characteristic form of the <i>Danais</i> pupa and the <i>Limenitis</i> pupa
-respectively (<a href="#f19">Pl. I</a>, Fig. 10<i>b</i>, and 11<i>b</i>).</p>
-
-<p>But even in the butterfly itself nothing is altered, except what
-increases the resemblance to the model. All else has remained unchanged,
-above all, the venation of the wings. Since the painstaking
-and valuable work of Herrich-Schäfer the venation has been made
-the basis of the whole systematic arrangement of butterflies, and it
-enables us, in point of fact, to distinguish with precision, not the
-families alone, but often even the genera, for the course of the veins
-in the different species of a single genus is the same, and that is true
-for the mimetic species as well as for others. Thus the Danaid-like
-<i>Limenitis</i> has the usual <i>Limenitis</i> venation, of the kind seen in our
-own indigenous species of <i>Limenitis</i>, and the already described
-<i>Elymnias</i> species of the African and Indian forests and grassy
-plains have always the venation characteristic of this genus, whether
-they be protected only by sympathetic colouring or imitate an
-immune <i>Euplœa</i>, a <i>Danais</i>, an <i>Acræa</i>, or a <i>Tenaris</i>. However
-much the contour of the wing may vary, the venation is unaffected,
-and we can distinguish model from copy by this means alone, so that,
-even when there is the closest resemblance, no doubt is possible. In
-its theoretical aspect this constancy of venation is obviously important,
-for as nothing about the organism is incapable of variation, the veining
-of the wings might have varied, as indeed it has varied from genus
-to genus in the course of the phylogenetic history; but as changes in
-venation could not be detected by the butterflies' enemies, however
-sharp-sighted, there has been no reason in these cases for variation
-in this respect.</p>
-
-<p>In this connexion Poulton has brought forward interesting facts
-showing that the mimics of one model, belonging to different genera,
-often secure the same effect in quite different ways. Thus the glass-like
-transparency of the wings in the Heliconiidæ of the genus
-<i>Methona</i> depends on a considerable reduction of the size of the scales,
-which ordinarily cover both sides of the wing as thickly as the tiles
-on a roof, and produce the colour. In another quite similar species,
-also transparent-winged, the Danaid <i>Ituna ilione</i>, the transparency
-is due to the absence of most of the scales, and in a third mimic,
-<i>Castnia linus</i>, var. <i>heliconoides</i>, the scales are not altered either in
-size or number, but have become absolutely unpigmented and transparent.
-In a fourth mimic, a Pierid, <i>Dismorphia crise</i>, the scales
-have not decreased in number, but have become quite minute, while<span class="pagenum"><a id="Page_106"></a>[Pg 106]</span>
-in a fifth case, the nocturnal <i>Hyelosia heliconoides</i> Swains., the same
-thing has happened as in <i>Castnia</i>, but the scales are also fewer in
-number. Thus in each of the mimics the changes which have taken
-place in the scales are quite different, but they bring about the same
-effect, the glass-like transparency of the wings, on which the resemblance
-to the model depends: what we have before us is, therefore,
-not a similarity of variation, but only an appearance of similarity in
-external features.</p>
-
-<p>In the face of such facts there can be no further question of the
-often repeated objection, that the resemblance of model and copy
-depend on the similarity of external influences upon species living
-in the same latitude, even if that were not already sufficiently refuted
-by the frequent restriction of the mimicry to the female. And that
-mimicry should be a mere matter of chance is negatived even by the
-single fact that model and copy always live in the same area, and that
-the local varieties of the model are faithfully followed by the mimic.
-An interesting example of this is furnished by <i>Elymnias undularis</i>,
-already mentioned, for in this case the female (<a href="#f21">Pl. II</a>, Fig. 23) mimics
-the brown-yellow <i>Danais plexippus</i> (<a href="#f21">Pl. II</a>, Fig. 22), not wherever
-<i>E. undularis</i> occurs, but only in Ceylon and British India. In Burmah,
-where another Danaid, <i>D. hegesippus</i>, is common, it mimics that; and
-in Malacca it does not copy a Danaid at all, but resembles the male
-of its own species, which in India is very different from it, since
-there the female mimics one of the blue iridescent EuplϾ (<a href="#f22">Pl. III</a>,
-Fig. 24). It cannot therefore be a matter of 'chance,' and we should
-have to give up all attempt at a scientific interpretation if we were not
-prepared to accept that of natural selection. Even the interference of
-a purposeful Power can hardly be seriously considered in this case,
-even by those who are inclined to such a view, for the <i>gradual</i>
-approximation to the model, which is a matter of course in a process
-of evolution, could only appear, if referred to the benevolent intelligence
-of a Creator, as an unworthy trick, designed to lead humanity
-astray in its strivings after knowledge. On the other hand, this
-gradual increase of resemblance, which becomes apparent when we
-compare several mimetic species&mdash;this carrying over, step by step,
-from the female to the male&mdash;and many other facts point to the
-working of natural forces according to law, and, if there is to be
-found anywhere in living nature a complicated process of self-regulation,
-it certainly lies before us here, clearer and less open to
-objections than almost anywhere else. I do not mean to say, however,
-that we can verify it statistically in detail, as has been demanded by
-the fanatical opponents of natural selection. A direct testing of<span class="pagenum"><a id="Page_107"></a>[Pg 107]</span>
-natural selection is, as has been already shown, nowhere possible:
-we can never exactly estimate how great the advantage is which
-a species requiring protection derives from a slight increase in the
-resemblance to an immune model; and I for one do not know how we
-could even definitely prove that a certain species needed a greater
-degree of protection than it had previously enjoyed in order to ensure
-its persistence in the struggle. It would be necessary to know the
-total number of individuals living on a certain area for many generations.
-If it appeared that there was a progressive diminution in the
-number of individuals, we should be justified in concluding that the
-species had not an adequate power of persistence, and that it therefore
-required a more effective protection. But it is impossible for us to
-collect such exact data for any species living under natural conditions,
-although we can often say approximately that a species is progressively
-decreasing in numbers. Even this, however, we can usually do only
-in cases which are influenced directly or indirectly by the interference
-of Man in nature, and in which the falling off in the species occurs so
-rapidly that there is no time for the slow counteractive influence
-of natural selection. We shall see later that in this way many species
-have been eliminated even within historic times.</p>
-
-<p>I have just spoken of the 'need of protection,' and I have a few
-remarks to add on that subject. It is a mistake to believe that every
-'rare' species, that is, one represented by few individuals, is already
-in process of disappearing. It is not the absolute number of individuals
-that determines the survival of a species, but the fact of the number
-remaining the same. It is equally mistaken to suppose that an
-amelioration of the conditions of existence for any species by natural
-selection is possible only when its persistence is already threatened;
-that is, when the number of individuals (the 'normal number') is
-steadily decreasing. On the contrary, it is of the essence of natural
-selection that every favourable variation which crops up is, <i>ceteris
-paribus</i>, preserved, and becomes the common possession of the species,
-quite independently of whether this improvement is absolutely
-necessary to its preservation or not. In the latter case it will simply
-become a commoner species instead of a rare one; and every species is,
-so to speak, striving to become common and widely distributed, since
-every advantageous variation that can possibly be produced is
-accumulated and made the common property of the species. But
-this has its limits, not only in the constitution and the structure
-of each species, but also in the external conditions of its life. If
-a species of butterfly be restricted, in the caterpillar stage, to a single,
-rare species of plant, its normal number will be, and must remain,<span class="pagenum"><a id="Page_108"></a>[Pg 108]</span>
-a small one. But if there arise within it a variation in the food-instinct
-whereby a second and it may be a commoner plant becomes
-available, then the normal number of the species will rise, and perhaps
-the original number of individuals may be more than doubled. It is,
-however, by no means necessary to assume that the species was
-previously in process of decadence; on the contrary its normal number
-may have remained quite constant.</p>
-
-<p>So, in the case of the mimetic butterflies, we do not need to
-assume that they all previously required protection in the sense that
-they would have become extinct had they not assumed a likeness to
-an immune species. We may indeed conclude, on other grounds, that
-it was the rarer species which increased their number of individuals
-by the mimetic protection, and in doing so they certainly enhanced
-at the same time their chance of survival as a species. In the
-more abundant species mimetic resemblance to species whose unpalatability
-rendered them immune could not have been evolved, as it
-would have been disadvantageous, not only for the model, but for the
-mimicking species itself, while in species less rich in individuals, such
-resemblance would necessarily have a protective value, no matter
-whether the species was in danger of extinction or not. The process
-of selection must have started simply because the mimetic individuals
-survived more frequently than the others, and the mimetic resemblance
-must have gone on increasing as long as the increase brought
-with it a more effective protection. It is, therefore, a fallacious
-objection to say that a species, whose existence was threatened,
-would, considering the slowness of the process of selection, have
-died out altogether before it could have acquired effective protection
-by mimicking an immune species. The assumption is false&mdash;the
-widespread, hazy idea that the process of natural selection can only
-begin when the existence of the species is threatened. On the
-contrary, every species utilizes every possibility of improvement; and
-every improvement for which variation supplies the necessary
-material is possible. The augmentation of the profitable variations
-follows as a necessity from the more frequent survival of the best-adapted
-individuals, and this 'more frequent survival' will be not
-only a relative one, due to the fact that the better adapted individuals
-will be less decimated, it will also be absolute, because more
-individuals of the species will survive than before. Of this <i>Papilio
-merope</i> may serve as an example; in Madagascar it now flies about
-only slightly varied from the original form, var. <i>meriones</i>. Here,
-therefore, the species is maintained, without the aid of mimetic
-protection. We do not know if the reason for this lies in the absence<span class="pagenum"><a id="Page_109"></a>[Pg 109]</span>
-of an immune model, or in the non-appearance of suitable mimetic
-variants, or in other conditions; but we know that without mimicry
-the species holds its own against its enemies. But if, in Abyssinia,
-a female of this butterfly exhibited variations which would make her
-resemble, in any degree, the unpalatable <i>Danias chrysippus</i>, these
-mimetic variants would be less decimated than the original form of
-female, and would, therefore, gain stability, and gradually increase
-both in mimetic resemblance and in the number of individuals. But
-is this any reason why the original form of the female should
-diminish in numbers? In itself, certainly not; the red mimetic
-females could increase in number without causing any decrease of the
-yellow ones, for the red are in no way in conflict with the yellow,
-and we must not think of the number of individuals as so fixed for
-each species that it cannot increase. On the contrary, it <i>must</i>
-increase, as soon as the conditions of existence are permanently
-improved, and this happens, in this case, through the mimetic protection
-of the red female. We can thus easily understand how mimetic
-and non-mimetic females can live side by side in Abyssinia.</p>
-
-<p>In all the rest of Africa, however, there are only mimetic females
-of <i>Papilio merope</i>, and none of the colour of the male; these last,
-therefore, have been crowded out by the mimetic form, not actively,
-but through the more frequent survival of the mimetic form, so that
-those like the male became gradually rarer, and finally died out&mdash;that
-is, ceased to occur. The matter is not so simple as it seems, and
-we shall best understand it by thinking of the dimorphism of the
-caterpillars of our hawk-moths, which we discussed before, in which
-the green form in the full-grown caterpillar is less well protected than
-the brown. In many species the brown form has crowded out the
-green, in others brown and green occur side by side, but the green is
-less abundant, and in some species very rare. This must be regarded
-as the simple result of the circumstance that a higher percentage of
-the green than of the brown caterpillars fall victims to enemies, and
-thus, in the course of generations, the green form becomes slowly
-but steadily rarer. This will be the case even if the newer and
-better adaptation raises the number of individuals (the 'normal
-number') in the species, for this increase must always be a limited
-one, even if it be very great, which is hardly likely in this case. For
-the normal number is not determined by the mortality at one stage,
-but by that at all the stages of life taken together. Thus a normal
-number always persists, notwithstanding the improved conditions for
-the species, and, on this assumption, the form under less favourable conditions
-cannot permanently hold its own with that under better con<span class="pagenum"><a id="Page_110"></a>[Pg 110]</span>ditions,
-but must gradually disappear. We can understand, then, that
-the primitive form of the <i>Papilio merope</i> female may persist even for
-a long time side by side with the mimetic form in certain habitats.
-It is, probably, not a mere chance, that this should have happened
-just in Abyssinia, for, in that region, the mimetic female is still
-tailed&mdash;that is, she has not yet reached the highest degree of resemblance
-to her immune model. In the whole of the rest of Africa the
-process of the transformation of the female has already reached its
-highest point, and on the east and west coasts, as well as in South
-Africa, the primitive form of the species is now represented only by
-the male.</p>
-
-<p>The gradual dying out of the less favourably conditioned forms
-of a species is a law which follows as a logical necessity from the
-essence of the process of selection, but its reality may be inferred
-from the phenomena themselves. On it depends, as far at least as
-adaptations are concerned, the transformation of species.</p>
-
-<p>A beautiful example of the crowding out of a less favoured form
-of a species by a more favoured one is afforded by a butterfly of
-North America, of which the two female forms have long been
-known, although the reason for their dimorphism was not understood.
-A yellow butterfly, <i>Papilio turnus</i>, not unlike our swallow-tail, has
-yellow females in the north and east of the United States, but black
-ones in the south and west. There was much guessing as to what
-the cause of this striking phenomenon might be, and it was for
-a time thought that this difference was directly due to the influence
-of climate, and, later, the black form of female was regarded as
-protectively coloured, because of the supposed greater persecution
-by birds in the south, since the female would be less easily recognized
-if of a dark colour, and would thus be better protected. This
-last explanation could hardly be looked upon as satisfactory, for
-a black butterfly in flight would be very easily seen by sharp-sighted
-birds; indeed, against a light background, it would be even more
-readily seen than a light one.</p>
-
-<p>Since we have acquired a more exact knowledge of the immune
-species of <i>Papilio</i> this case has become clear to us. For on those
-stretches of country on which the black female of <i>Papilio turnus</i>
-lives there occurs another <i>Papilio</i> which is black in both sexes,
-<i>Papilio philenor</i>, and this is one of those species which are protected
-by their unpleasant taste and odour. Here, therefore, we have a case
-of mimicry, the female of <i>Papilio turnus</i> imitates the immune <i>Papilio
-philenor</i>, and thereby secures protection for itself; but as the immune
-model only occurs in the southern half of the distribution of <i>Papilio
-<span class="pagenum"><a id="Page_111"></a>[Pg 111]</span>turnus</i> a somewhat sharp separation of the two forms of female
-has been evolved; the black, mimetic form, being the most fit,
-has completely crowded the primitive yellow form out of the area
-inhabited by <i>Papilio philenor</i>, while beyond this area, to the north
-and west, the yellow form alone prevails. The extensive and careful
-studies of Edwards have shown that the two forms occur together
-only in a very narrow transition region.</p>
-
-<p>We thus see that the facts, wherever we scrutinize them carefully,
-harmonize with the theory. Of course we can only penetrate to
-a certain depth with the theory of selection, and we are still far
-from having reached the fundamental causes of the phenomena.
-Indeed, our understanding must in the meantime stop short before the
-causes of variations and their accumulation, but up to that point the
-theory gives us clearness, and discloses the causal connexion of
-phenomena in the most beautiful way. Although we do not yet
-understand how the southern female <i>Papilio turnus</i> was able to
-produce the advantageous black, we do see why a black variation,
-when it did occur, should increase and be strengthened, until it
-crowded out the yellow form from the area of the immune model,
-and we are able in a general way to refer the whole complicated
-phenomena of mimicry to their proximate causes.</p>
-
-<p>This is true also of other phenomena which have had no part in
-establishing the theory, since attention was only directed to them
-later, and it is true even of some which, at first sight, seem to contradict
-the theory altogether. To this class belongs, for instance, the
-phenomenon that immune species not unfrequently mimic each other,
-as was first observed among the Heliconiid-like butterflies of South
-America. In four different families, the Danaidæ, the Neotropidæ, the
-Heliconiidæ, and the Acræidæ, there are species, distributed over
-the same area, which resemble each other in their conspicuous
-colouring and marking, and also in the peculiar shape of the wings.
-After what has been said one might be inclined to regard one of
-these species as the unpalatable model and the others as the palatable
-mimics, but they are all unpalatable, and are not eaten by birds.
-The puzzle of this apparent contradiction was solved by Fritz Müller<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a>,
-who pointed out that the aversion to non-edible butterflies is not
-innate in birds, but must be acquired. Each young bird has to learn
-from experience which victim is good to eat, and which bad. If
-every inedible species had its particular and distinctive colour-dress
-a considerable number of individuals of each species would fall victims
-to the experiments of young birds in each generation, for a butterfly<span class="pagenum"><a id="Page_112"></a>[Pg 112]</span>
-which has once been pecked at, or squeezed by the bill of a bird,
-is doomed to die. But if two inedible species which resemble each
-other inhabit the same area they will be regarded by the birds as
-one and the same, and if five or more inedible species resemble each
-other all five will present the same appearance to the bird, and it
-will not require to repeat on the other four the experience of
-unpalatability it has gained from one. Thus the total of five species
-will be no more severely decimated by the young birds than each of
-them would have been if it had occurred alone; the same number
-of victims of experiment, which are necessary every year in the
-education of the young birds, will, when all five species look alike,
-be divided among the whole 'mimicry ring,' as we may say. The
-advantage of the resemblance is thus obvious, and we can understand
-why a process of selection should develop among such inedible
-species which should result in their being readily mistaken for one
-another; we can understand why, in the neighbourhood of Fritz
-Müller's home, Blumenau, in the province of Santa Catarina in South
-Brazil, the Danaidæ, species of <i>Lycorea</i>; the Heliconiidæ, <i>Heliconius
-eucrate</i> and <i>Eueides isabella</i>; and the Neotropinæ, <i>Mechanitis
-lysimnia</i> and species of <i>Melinæa</i>, should all exhibit the same colours,
-brown, black and yellow, in a similar pattern, on similarly shaped
-wings. The agreement is by no means perfect in detail, but it can
-be noticed in all parts of South America inhabited by species of these
-genera, and the same differences which distinguish, for instance, the
-two species of <i>Heliconius</i> flying in two different regions, also
-distinguish the two species of <i>Eueides</i> and the two species of
-<i>Mechanitis</i>. In Honduras we find the same mutually protective
-company of inedible genera as in Santa Catarina, but represented
-by other species, which all differ from the species in Santa Catarina
-in the same characters, as, for instance, that they have two instead
-of one pale yellow cross-stripe on the anterior wings. The species
-are: <i>Lycorea atergatis</i>, <i>Heliconius telchinia</i>, <i>Eueides dynastes</i>,
-<i>Mechanitis doryssus</i>, and <i>Melinæa imitata</i><a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a>. In the environs of
-Bahia this mimicry ring consists of the following species: <i>Heliconius
-eucrate</i>, <i>Lycorea halia</i>, <i>Mechanitis lysimnia</i>, and <i>Melinæa ethra</i>, as
-figured on <a href="#f21">Pl. II</a>, Fig. 12, iv, and such a mutual assurance society
-has always one or other edible species as mimic. The larger the
-mimetic assurance company is, the less harm can mimics do to it.
-In the case figured it is two Pieridæ already known to us that have
-fairly well assumed the Heliconiid guise, namely, <i>Dismorphia astynome</i>
-<span class="pagenum"><a id="Page_113"></a>[Pg 113]</span>(<a href="#f21">Pl. II</a>, Figs. 18 and 19) and <i>Perhybris pyrrha</i> (<a href="#f21">Pl. II</a>, Figs. 16 and 17).
-In the latter of these the male still has, on the upper surface, just
-the appearance of one of our common Garden-whites, while the female
-is coloured quite like the Heliconiidæ, but without having lost the
-form of wing of the Whites. The larger the mimetic company is
-the greater will be the protection afforded to its palatable mimics,
-since they will be the more rarely seized by way of experiment. It
-is, of course, obvious that in this kind of mimicry&mdash;that is, in the
-imitation of an unpalatable and rejected species for protection&mdash;it
-is presupposed as a general postulate that the edible mimics are
-considerably in the minority, as Darwin showed; for if it were otherwise
-their enemies would soon discover that among the apparently
-unpalatable species there were some which were pleasant to taste.
-Here, too, the facts bear out the theory, although exceptions can
-easily be imagined, and do seem to occur.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> <i>Kosmos</i>, vol. v, 1881, p. 260 onwards.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> According to Poulton's report in <i>Nature</i>, July 6, 1889, of 'Sykes, Natural Selection
-in the Lepidoptera,' <i>Trans. Manchester Microscop. Soc.</i> 1897, p. 54.</p>
-
-</div>
-
-<hr class="full" />
-
-<div class="figcenter" id="f19">
-<img src="images/fig19.jpg" alt=""/>
-</div>
-
-<p class="c xxlarge">PLATE I</p>
-
-<div class="figcenter">
-<img src="images/fig20.jpg" alt=""/>
-</div>
-
-<table>
-
-<tr><td class="tdr"><span class="half">FIG</span></td>
-    <td class="tdl"></td></tr>
-
-<tr><td class="tdr">1.</td>
-    <td class="tdl"><span class="smcap">Papilio merope, male, Africa.</span></td></tr>
-
-<tr><td class="tdr">2.</td>
-    <td class="tdl"><span class="smcap">The same species, one form of mimetic female.</span></td></tr>
-
-<tr><td class="tdr">3.</td>
-    <td class="tdl"><span class="smcap">Danais chrysippus, Africa, immune model of Fig. 2.</span></td></tr>
-
-<tr><td class="tdr">4.</td>
-    <td class="tdl"><span class="smcap">Papilio merope, second form of mimetic female, S. Africa.</span></td></tr>
-
-<tr><td class="tdr">5.</td>
-    <td class="tdl"><span class="smcap">Amauris niavius, S. Africa, immune model of Fig 4.</span></td></tr>
-
-<tr><td class="tdr">6.</td>
-    <td class="tdl"><span class="smcap">Papilio merope, third form of mimetic female, S. Africa.</span></td></tr>
-
-<tr><td class="tdr">7.</td>
-    <td class="tdl"><span class="smcap">Amauris echeria, S. Africa, immune model of Fig. 6.</span></td></tr>
-
-<tr><td class="tdr">8.</td>
-    <td class="tdl"><span class="smcap">Danais erippus, immune model of Fig. 9, Central N. America.</span></td></tr>
-
-<tr><td class="tdr">9.</td>
-    <td class="tdl"><span class="smcap">Limenitis archippus, Central N. America, mimics the foregoing species.</span></td></tr>
-
-<tr><td class="tdr">10.</td>
-    <td class="tdl"><span class="smcap">Danais erippus</span>, (<i>a</i>) <span class="allsmcap">CATERPILLAR</span>, (<i>b</i>) <span class="allsmcap">PUPA.</span></td></tr>
-
-<tr><td class="tdr">11.</td>
-    <td class="tdl"><span class="smcap">Limenitis archippus</span>, (<i>a</i>) <span class="allsmcap">CATERPILLAR</span>, (<i>b</i>) <span class="allsmcap">PUPA.</span></td></tr>
-</table>
-
-
-<p class="l2 more"><i>To face Plate I</i></p>
-
-<hr class="full" />
-
-<p>This comparative rarity is true of the imitators of the Heliconiidæ
-and their great mimicry ring of unpalatable species, and is very
-general. Thus, for instance, there is a series of palatable mimics of the
-beautiful blue <i>EuplϾ</i> of the Indo-Malayan region (<a href="#f22">Pl. III</a>, Figs. 25
-and 27), but each of these mimics is rare compared with the hosts of the
-blue unpalatable company, for these immune butterflies also occur in
-many species, all similar to <i>Euplœa midamus</i> or <i>binotata</i> (<a href="#f21">Pl. II</a>,
-Figs. 1 and 3); and the same applies to the mimics of the Indo-Malayan
-Danaidæ. There are a great many <i>Danais</i> species, all of them
-resembling <i>Danais vulgaris</i> (<a href="#f22">Pl. III</a>, Fig. 20), which, when they occur
-together, form an inedible ring, and this ring is imitated by a whole
-series of edible species, each of which is comparatively rare. And there
-are no fewer than six species of <i>Papilio</i> which resemble these Danaids
-to the point of being easily mistaken for them, while another
-rare <i>Papilio</i> effectively copies the iridescence of the blue <i>EuplϾ</i>&mdash;a
-coloration so unusual in the genus that the species has received the
-name of <i>Papilio paradoxus</i>.</p>
-
-<p>But even in single species of butterflies immune through
-unpalatability there is usually a great abundance of individuals.
-Thus <i>Danais chrysippus</i>, which is distributed over the whole of
-Africa, is a very common butterfly wherever it can live at all; and
-in North America, in which country there are only two widely
-distributed species of <i>Danais</i>, these often occur in enormous numbers.
-The beautiful large <i>Danais erippus</i> Cramer (<a href="#f19">Pl. I</a>, Fig. 8), is
-distributed over almost all America, and in many places is not only
-frequent, but occurs in great swarms. Usually it peoples the broad,
-open stretches of the western prairies of the United States, but when<span class="pagenum"><a id="Page_114"></a>[Pg 114]</span>
-violent winds blow, as they do there in September especially, the
-insects are driven together into the small wooded spots of the prairie,
-and then they cover the trees in incredibly large crowds, often so
-thickly that the leaves are entirely hidden, and the trees look brown
-instead of green. Millions of butterflies go to make up such swarms,
-which have been observed in many parts of the United States, even
-quite in the East, in New Jersey, and elsewhere.</p>
-
-<p>Considering this extraordinary abundance of the immune species,
-it is not surprising that its palatable copy, <i>Limenitis archippus</i>
-(<a href="#f19">Pl. I</a>, Fig. 9), should also be widely distributed in North America,
-and in many places it is not rare, but even abundant. The enormous
-majority of <i>Danais erippus</i> will protect the species which resembles
-it so closely, even though it is not rare. Any doubt as to this being
-a case of mimicry disappears in face of the fact that, in Florida, there
-flies a second very similar but much darker brown North American
-<i>Danais</i>, and that it is accompanied there by an equally dark variety
-of <i>Limenitis archippus</i> (<i>L. eros</i>).</p>
-
-<p>To prove the correctness of the hypothesis of an actual process
-of selection&mdash;which we assume in our interpretation of mimicry&mdash;I
-mean the assumption that the disguise of the species seeking
-protection really deceives the enemy, and thus actually affords protection,
-I need only cite the evidence of an acute and experienced
-entomologist who was himself deceived by it. Seitz<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a>, to whom we
-owe many valuable biological observations on butterflies, relates that,
-while he was collecting in the neighbourhood of the town of Bahia,
-he was surrounded by swarms of <i>Catopsiliæ</i>, similar to our lemon
-butterfly, especially the common <i>Catopsilia argante</i>, but he took no
-notice of these, as he 'had already collected as many of them as he
-wanted.' It was only when he saw a pair <i>in copula</i> that he caught
-them in his net. But to his extreme surprise he found that he had
-not caught a <i>Catopsilia</i>, but a butterfly of the family Nymphalidæ,
-one of those <i>Anææ</i> whose numerous species are distributed over
-South America. These <i>Anææ</i> are dark, or beautifully bright on the
-upper surface, but on the under side are leaf-coloured, and one of
-them bears the name <i>Anæa opalina</i>, because it is quite clear and
-pale, and of opal-like brilliance. The captive was nearly related to
-<span class="pagenum"><a id="Page_115"></a>[Pg 115]</span>this species. Seitz was so much surprised by the discovery that the
-male, which had quickly detached itself from the female, escaped
-him, and he could only make out that, 'as it flew away, it unfolded
-dark wings, which certainly bore little resemblance to those of the
-lemon butterfly.' In the hope of securing more of this rare booty he
-then hunted only for <i>Catopsilia argante</i>, without however securing
-another coveted specimen&mdash;he caught no more <i>AnϾ</i>, which shows
-that in this case, too, the mimetic species was much rarer.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_6" href="#FNanchor_6" class="label">[6]</a> In citing this observation of Seitz, I do not mean to assert that there is true
-mimicry between <i>Anæa opalina</i>, or its allied species in Bahia, and the <i>Catopsilia</i>, though
-I regard this as extremely probable, because of the marked dimorphism between the
-male and the female, in conjunction with the very striking resemblance of the female
-to the <i>Catopsilia</i>. The example was given only to show how very deceptive such
-resemblances may be. To assert with confidence that it is a case of mimicry we
-should require to know that <i>Catopsilia</i> is immune, and on that point we have as yet no
-information.</p>
-
-</div>
-
-<hr class="full" />
-
-<div class="figcenter" id="f21">
-<img src="images/fig21.jpg" alt=""/>
-</div>
-
-<p class="c xxlarge">PLATE II</p>
-
-<div class="figcenter">
-<img src="images/fig20.jpg" alt=""/>
-</div>
-
-<table>
-
-<tr><td class="tdr"><span class="half">FIG.</span></td>
-    <td class="tdl"></td></tr>
-
-<tr><td class="tdrt">12-15</td>
-    <td class="tdl"><span class="allsmcap">REPRESENT A 'MIMICRY-RING' COMPOSED OF FOUR IMMUNE<br />
-                     SPECIES BELONGING TO THREE DIFFERENT FAMILIES AND FOUR<br />
-                     DIFFERENT GENERA.</span></td></tr>
-
-<tr><td class="tdr">12.</td>
-    <td class="tdl"><span class="smcap">Heliconius eucrate, Bahia.</span></td></tr>
-
-<tr><td class="tdr">13.</td>
-    <td class="tdl"><span class="smcap">Lycorea halia, Bahia.</span></td></tr>
-
-<tr><td class="tdr">14.</td>
-    <td class="tdl"><span class="smcap">Mechanitis lysimnia, Bahia.</span></td></tr>
-
-<tr><td class="tdr">15.</td>
-    <td class="tdl"><span class="smcap">Melinæa ethra, Bahia.</span></td></tr>
-
-<tr><td class="tdrt">16, 17.</td>
-    <td class="tdl"><span class="smcap">Perhybris pyrrha, male and female, S. American<br />
-                    'Whites' (Pieridæ). The female mimics an immune<br />
-                     Heliconiid, while the male shows only an indication<br />
-                     of the mimetic colouring on the under surface.</span></td></tr>
-
-<tr><td class="tdrt">18, 19.</td>
-    <td class="tdl"><span class="smcap">Dismorphia Astynome, male and female, also belonging<br />
-                     to the family of 'Whites,' and mimicking immune<br />
-                     Heliconiids; a white spot on the posterior wing of<br />
-                     the male is all that remains of the original 'White'<br />
-                     coloration.</span></td></tr>
-
-<tr><td class="tdrt">20.</td>
-    <td class="tdl"><span class="smcap">Elymnias phegea, W. Africa, of the family Satyrides,<br />
-mimics the foregoing species.</span></td></tr>
-
-<tr><td class="tdr">21.</td>
-    <td class="tdl"><span class="smcap">Acræa gea, an immune W. African species.</span></td></tr>
-
-<tr><td class="tdr">22.</td>
-    <td class="tdl"><span class="smcap">Danais genutia, an immune Danaid from Ceylon.</span></td></tr>
-
-<tr><td class="tdrt">23.</td>
-    <td class="tdl"><span class="smcap">Plymnias undularis, female, one of the mimics of Fig. 22.<br />
-The male, which is quite different, is figured on<br />
-Plate III (Fig. 24).</span></td></tr>
-</table>
-
-<p class="l3 more"><i>To face Plate II</i></p>
-
-<hr class="full" />
-
-<p>We see, then, that the need for protection in butterflies has
-a great influence on their external appearance, especially as regards
-their colour and marking. First, because the resting insect frequently
-has the visible surfaces sympathetically coloured, and also, because
-there are numerous species, indeed whole families, which contain
-nauseous, perhaps even actually poisonous, juices, and these have been
-subject to a double process of selection, directed towards the increase
-of the nauseousness, and at the same time towards acquiring as
-conspicuous a dress as possible. Thus the whole surface of these
-butterflies became gaily coloured, and often&mdash;as in many of the
-tropical nocturnal Lepidoptera which fly by day, the Agaristidæ,
-Euschemidæ, and Glaucopidæ&mdash;quite glaringly bright. We thus
-understand the striking or at least readily recognizable colours of
-the Heliconiidæ, the Euplœæ, the Danaidæ, and the Acræidæ.
-Finally, the unpalatable species influence many others which are
-edible, since the latter strive to resemble an immune species; and how
-considerable the variations and colour transformations thus induced
-can be is shown by the Whites of the genus <i>Perhybris</i> (<a href="#f21">Pl. II</a>, Figs. 16
-and 17) and <i>Archonias</i>, in which the male has wholly or partially
-retained the primitive dress of the Whites, and in which, side by side
-with wholly mimetic species, other species occur in which both sexes
-exhibit the garb of the Whites unaltered. Such cases tell decidedly
-against the often expressed view that mimetic species must have had
-from the outset a great resemblance to the model; they show rather
-that very great deviations in form, but more especially in colour, have
-been brought about solely by the necessity for mimetic adaptation, and
-that they have come about only slowly and step by step, as the
-different grades of resemblance to the model in different species of
-the same genus clearly show.</p>
-
-<p>Lepidoptera are by no means the only insects which exhibit the
-phenomenon of mimicry, nor are insects the only animals in which it
-occurs; and unpleasant taste and odour are not the only protective
-characters; there are many others, as, for instance, among insects, the
-hardness of the chitinous cuticle.</p>
-
-<p><span class="pagenum"><a id="Page_116"></a>[Pg 116]</span></p>
-
-<p>One of the most beautiful examples of mimicry was discovered
-by Gerstäcker, not in free nature, but in the entomological collection
-at Berlin. There he found beside a green, metallic weevil-beetle, one of
-the Pachyrhynchidæ from the Philippines, two other insects with the
-same metallic sheen and very similar form of body. They had been
-put in beside the weevil as duplicates, but more careful observation
-showed that they were delicate Gryllidæ, which mimicked the hard
-beetles so deceptively that even the practised eye of the entomologist
-was misled by them. Later on it was shown that these Gryllids live
-in the Philippines beside the weevils, and even on the same leaves with
-them, and that the beetles are protected from the attacks of birds and
-other enemies by the extraordinary hardness of their cuticle. The
-case is especially remarkable because in general the Gryllidæ have no
-metallic shimmer, and the form of body must have been considerably
-altered to make them resemble the beetle. The usually broad head of
-the Gryllids is in this case narrower, the usually flat wing-covers are
-arched and pear-shaped, and the legs have become quite beetle-like.
-The security enjoyed by the weevil must be very perfect, for it is
-mimicked by three other species of beetle in the Philippines.</p>
-
-<p>Animals can also be protected from attack by the possession of
-dangerous weapons. To this class belong insects with poisonous
-stings, like the bees, wasps, and ants, and in some degree also the
-ichneumon-flies. We cannot wonder, therefore, that these dreaded
-species find imitators. In this case it is not of so much importance
-that the copy should be rarer than the model, for anything that looks
-like a dangerous insect will be avoided, since close investigation is in
-this case attended with danger. So we find that hornets, wasps, and
-bees are frequently imitated by other insects, by beetles, flies, and
-butterflies; and these must derive a certain advantage, even when the
-resemblance is only a general one. Many Longicorns, which visit
-flowers, are striped black and yellow, like a wasp, and so are many
-flies, like the species of <i>Syrphus</i>, and so on. The Longicorn <i>Necydalis
-major</i> bears a strong resemblance to a large ichneumon-fly; it
-has the same long-drawn-out body, the same swellings on the femur
-and tibia, the curved antennæ, the glossy brown colour, and its wing-covers
-are quite short, leaving the wings free, so that the deception is
-very complete.</p>
-
-<p>Bees, too, are sometimes so well imitated that they are hardly
-to be distinguished from their mimics, not in flight only, but also
-when visiting flowers. The best and commonest mimic of our honey-bee
-is a perfectly harmless fly of the same size and colour, the drone-fly
-(<i>Eristalis tenax</i>). The two are often to be seen together on the
-<span class="pagenum"><a id="Page_117"></a>[Pg 117]</span>same flowering shrub, as, for instance, in autumn, on the Japanese
-buckwheat of our gardens (<i>Polygonum sieboldii</i>), both busily seeking
-for honey. I once noticed a boy catching the flies with a net in
-order to imprison them, but a bee stung him severely in the finger.
-He immediately abandoned the chase, and gave up the flies, perceiving
-the dangers of confusion. So the animal enemies of <i>Eristalis</i> will
-often prefer to leave it in peace rather than run the risk of
-being stung.</p>
-
-<hr class="full" />
-
-<div class="figcenter" id="f22">
-<img src="images/fig22.jpg" alt=""/>
-</div>
-
-<p class="c xxlarge">PLATE III</p>
-
-<div class="figcenter">
-<img src="images/fig20.jpg" alt=""/>
-</div>
-
-<table>
-
-<tr><td class="tdr"><span class="half">FIG.</span></td>
-    <td class="tdl"></td></tr>
-
-<tr><td class="tdrt">24.</td>
-    <td class="tdl"><span class="smcap">Elymnias undularis, male of the species of which the<br />
-mimetic female is depicted in Fig. 23.</span></td></tr>
-
-<tr><td class="tdr">25.</td>
-    <td class="tdl"><span class="smcap">Euplœa binotata, immune Indian species, mimicked by</span></td></tr>
-
-<tr><td class="tdr">26.</td>
-    <td class="tdl"><span class="smcap">Elymnias leucocyma, male, of which</span></td></tr>
-
-<tr><td class="tdr">27.</td>
-    <td class="tdl"><span class="smcap">Euplœa midamus.</span></td></tr>
-
-<tr><td class="tdr">28.</td>
-    <td class="tdl"><span class="allsmcap">THE FEMALE MIMICS FAIRLY CLOSELY</span></td></tr>
-
-<tr><td class="tdr">29.</td>
-    <td class="tdl"><span class="smcap">Danais vulgaris, immune Indian Danaid.</span></td></tr>
-
-<tr><td class="tdrt">30.</td>
-    <td class="tdl"><span class="smcap">Elymnias lais, mimetic of the foregoing species, but only<br />
-                     on the upper surface. The lower surface retains the<br />
-                     original protective colouring representing a decaying<br />
-                     leaf.</span></td></tr>
-
-<tr><td class="tdr">31.</td>
-    <td class="tdl"><span class="smcap">Tenaris bioculatus, from the Papua region.</span></td></tr>
-
-<tr><td class="tdrt">32.</td>
-    <td class="tdl"><span class="smcap">Elymnias agondas, mimics the foregoing species from the<br />
-same locality.</span></td></tr>
-</table>
-
-<p class="l3 more"><i>To face Plate III</i></p>
-
-<hr class="full" />
-
-<p>There is still another relation between two species which can be
-induced by mimicry&mdash;namely, parasitism, when, for instance, the
-so-called cuckoo-bees and parasitic humble-bees deceptively resemble
-in colour, arrangement of hair, and form of body, the species into
-whose nests they smuggle their eggs, to have them brought up at the
-expense of the bee or humble-bee in question. In the same way,
-among the numerous parasites of ant nests, there are some which
-copy the ants themselves, and so secure themselves from molestation,
-although they devour the ants' eggs and pupæ. Thus, among the
-hosts of South American driver-ants (<i>Eciton prædator</i>) there lives
-a predaceous beetle of the family Staphylinæ, which has received
-the name <i>Mimeciton</i> because it resembles the ant in form and in
-the nature of the external surface, though not in colour, which is to
-be explained by the fact that this ant has no compound eyes, and is
-therefore almost blind, or at any rate cannot see colours.</p>
-
-<p>I should never come to an end were I to attempt to exhibit the
-great wealth of observations now available in regard to mimicry.
-But this at least may be added, that isolated cases of mimicry have
-been found even among Vertebrates. Thus, according to Wallace,
-the red-and-black striped poisonous coral snake of South America
-(<i>Elaps</i>) is most realistically imitated by a non-poisonous snake
-(<i>Erythrolampus</i>) of the same region. Among birds, Wallace cites
-a few cases which may be regarded as mimicry, but none are known
-among mammals, which is not to be wondered at when we consider
-how very much less numerous in individuals the species are which
-live together on one area, and how much less likely it is that two
-species should be, to begin with, so near each other in size, habit, and
-form that the process of natural selection could bring about a
-deceptive degree of resemblance. Without doubt it is among insects
-that the conditions for mimicry are especially favourable, partly
-because of the enormous number of species which live together and
-have interrelations on the same area, even in our latitudes and much
-more so in the tropics, and also because of their usually great
-fecundity, and their rapid multiplication, both of which are factors<span class="pagenum"><a id="Page_118"></a>[Pg 118]</span>
-favourable to starting and continuing the processes of natural
-selection. Furthermore, we have to take into account the hosts of
-enemies which depend wholly or in great part on insects for food,
-and destroy them in enormous numbers, eliminating them in inverse
-proportion to the perfection of their adaptation. Finally, there is
-the extreme susceptibility of many insects to injury. This makes
-it very desirable that they should have some disguise sufficient to protect
-them from even the first attempt at an attack, since that would in
-many cases prove fatal.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_119"></a>[Pg 119]</span></p>
-
-<h2 class="nobreak" id="LECTURE_VI">LECTURE VI</h2>
-</div>
-
-<p class="c">PROTECTIVE ADAPTATIONS IN PLANTS</p>
-
-<div class="blockquot">
-
-<p>Protection against large animals&mdash;Poisons&mdash;Ethereal oils&mdash;Spines and thorns&mdash;Sharp
-and stinging-hairs&mdash;Felt-hairs&mdash;Position of the thorns: buckthorn&mdash;Tragacanth
-shrub&mdash;Prigana scrub&mdash;Alpine shrubs&mdash;Protection against small enemies&mdash;Chemical
-substances&mdash;Mechanical protective arrangements&mdash;Raphides&mdash;Conclusion.</p></div>
-
-
-<p><span class="smcap">We</span> have seen in how many different ways animals are able to
-adapt themselves to the conditions of life, both protectively and
-aggressively; how they approximate in their colour to that of their
-surroundings so that they harmonize with it; how they copy lifeless
-objects, or parts of plants, leaves, or twigs, or even mimic, in form
-and colour, other animals which are in some way protected. When
-we consider that by far the greater number of species find protection
-in some degree through their colouring, and often through their form,
-and when, at the same time, we remember how different this colouring
-is in nearly related species, and even within the same species (dimorphism),
-we can scarcely avoid the impression that the forms of life
-are made of a plastic material, which, like the sculptor's clay, can
-be kneaded at will into almost any desired form.</p>
-
-<p>This impression is corroborated when we turn our attention to
-plants, and consider the different ways in which they are able to
-protect themselves from the attacks of animals.</p>
-
-<p>That plants stand in need of some protection is obvious enough,
-since their leaves and other green parts contain much nourishment,
-and an endless army of animals, large and small, depends upon these
-alone for sustenance. Indeed, the existence of animals depends
-altogether on the occurrence of plants, for carnivorous and saprophytic
-animals could only arise after vegetarian forms had been already in
-existence. But if the green parts of the plants were left defenceless
-at the mercy of the multitude of herbivorous animals, it would not be
-long before they were exterminated from the face of the earth, for the
-animals would devour unsparingly whatever was within their reach,
-and, as their increase does not depend on their ratio of elimination
-alone, but also on their fertility, and on their rapidity of multiplication,
-they would go on increasing in numbers at the expense of the<span class="pagenum"><a id="Page_120"></a>[Pg 120]</span>
-superabundant nourishment until the plants on which they depended
-were themselves consumed.</p>
-
-<p>When we inquire into the means whereby plants evade such
-a fate we are astonished at the endless diversity of the devices
-employed.</p>
-
-<p>Let us consider first of all the menace to plants from the larger
-herbivores, from elephants and cattle down to the hare and the roe-deer;
-we find that many plants are protected by poisons, which
-develop in the sap of their stems, leaves, roots, and fruits. The juicy
-and beautifully leaved Belladonna (<i>Atropa belladonna</i>) is never
-touched by roe-deer, stags, or other herbivores, and the same is true of
-the thorn-apple (<i>Datura stramonium</i>), the henbane (<i>Hyoscyamus
-niger</i>), the spotted hemlock (<i>Conium maculatum</i>), the danewort of
-our woods (<i>Sambucus ebulus</i>), and many others; they all contain a
-poison. Like the unpalatable butterflies, these unpalatable plants are
-also furnished with a warning sign of their undesirability, namely,
-a disagreeable odour, perceptible even by man, which scares off animals
-from touching them. The development of this through natural
-selection presents no very serious difficulty.</p>
-
-<p>But, strangely enough, there are not a few poisonous plants in
-which we, at least, are unable to detect any such warning sign.
-Among these are the blue aconite (<i>Aconitum</i>), the black hellebore
-(<i>Helleborus niger</i>), the meadow-saffron (<i>Colchicum autumnale</i>), species
-of Gentian, of spurge (<i>Euphorbia</i>), and others. Yet these are avoided
-by deer, roe-deer, chamois, hares, and marmots, and our cattle, horses,
-and sheep also usually leave them untouched. A case has, however,
-been reported from the valley of the Aur, on the lower Rhine, which
-seems to contradict this. On the rocky grass-slopes of the valley the
-poisonous hellebore (<i>Helleborus viridis</i>) grows in great abundance, and
-the sheep of that region, which were wont to graze on the slopes,
-avoided these plants. But some sheep from another part were imported
-into the valley, and these ate the hellebore, with the result that
-many died. If these poisonous plants, then, were furnished with a
-warning sign such as a disagreeable odour, not perceptible to us, we
-should have to assume that the imported sheep had a less acute sense
-of smell than the others, which is not impossible in domesticated
-animals. If there were no such warning sign, then it must have been
-not an instinct but a continuous <i>tradition</i> which prevented the native
-sheep from touching the inedible plants.</p>
-
-<p>A more naïve interpretation of nature than that of our day
-would have regarded the fragrant ethereal oils developed in the seeds
-of many plants, as in those of fennel, cummin, and other Umbelliferous<span class="pagenum"><a id="Page_121"></a>[Pg 121]</span>
-plants, as a peculiarity designed for the use and profit of man. But
-these ethereal substances are obviously a means of protection against
-the depredations of seed-eating birds, for a sparrow which was allowed
-to eat three or four seeds of cummin died very soon afterwards.</p>
-
-<p>Many plants produce bitter substances in their green parts, and
-so secure at least some measure of protection, as is the case with the
-majority of mosses, the ferns, and species of <i>Plantago</i> and <i>Linaria</i>.
-Others, again, deposit silicic acid in their cell-walls, or develop in
-addition a very thick epidermis, so that they afford at the best an
-unpleasant food, e.g. many grasses, the horse-tails, the rhododendron,
-and the bilberry. Others, again (<i>Alchemilla vulgaris</i>), have cup-shaped
-leaves, which retain rain and dew for a long time, and this protects
-them from grazing animals, which are unwilling to touch wet grass
-and plants.</p>
-
-<p>Especially widely distributed and diverse is the protection of
-plants by sharp thorns and spines. It is extremely interesting to
-note in how many different and advantageous ways this armature
-is disposed.</p>
-
-<p>Obvious at once is the fact that thorns and spines only occur on
-those parts which are naturally exposed to attack. Thus we find
-them particularly strong in young plants, and on the lower parts of
-older ones. The holly, for instance, has crenate, spinose leaves only
-to the height to which grazing animals can reach; beyond that the
-leaves are smooth-edged and spineless, like those of the camelia. It
-is almost the same with some wild pear-trees, which are quite covered
-with thorns as long as they are low, but afterwards grow a thornless
-crown.</p>
-
-<p>Similarly, low bushes, when they are armed with thorns or the
-like at all, are covered with them all over, like the rose-bush.</p>
-
-<p>When the leaves of a plant are spinose the spines are disposed on
-the parts usually attacked; and thus we understand why the
-enormous floating leaves of <i>Victoria regia</i> should have on their
-under surface long, pointed spines which, especially at the upturned
-margin, attain a length of several inches; it is from water animals&mdash;water
-snails&mdash;that danger threatens them.</p>
-
-<p>Thorns are developed in the most diverse ways. In many of the
-bushes on the coast of the Mediterranean true leaves are wanting
-altogether, the green branches and twigs being themselves the
-assimilating parts, and these are so stiff and rigid, so like some
-kind of thorn, that they suffice to scare off any greedy herbivore.
-Among our own bushes the Broom (<i>Spartium scoparium</i>) may be
-taken as an example of this class.</p>
-
-<p><span class="pagenum"><a id="Page_122"></a>[Pg 122]</span></p>
-
-<p>In other cases the spines are found on the leaves themselves, but
-there is great diversity in their mode of arrangement. In many
-tropical plants, such as the Yucca and the Aloe, the point of the long,
-reed-shaped leaf is transformed into a spine, and this is the case in
-many of our native grasses. Kerner von Marilaun notes that, in the
-Southern Alps, two such grasses, <i>Festuca alpestris</i> and <i>Nardus
-stricta</i>, occur frequently in certain localities, and they prick the
-muzzles of the cattle so badly that they return bleeding from
-the pasture. This prevents these Alpine runs from being made full
-use of, so the grasses are as far as possible extirpated by man, and,
-curiously enough, also by the cattle themselves, for they seize the
-grass at the base of the tuft with their teeth, pull it out, and let it
-fall, so that it withers. Kerner saw thousands of such pieces of turf
-which had been pulled up by the cattle lying dried and bleached by
-the sun on some of the Alpine grazing grounds in the Tyrolese
-Stubaithal.</p>
-
-<p>Again, in many plants the whole leaf-edge is transformed into
-a spiny wall, which may be enlarged by indentations and lobate
-projections, as in the holly, and, in a much higher degree, in the
-thistles (<i>Carduus</i>), in <i>Eryngium</i>, in <i>Acanthus</i>, and in many Solanaceæ.
-Often, too, there are barbed hooks on the leaf-edge, which work like
-a saw; or the leaf-edge, though without spines, may be made sharp
-by deposits of silicic acid, as in the sedges, whose sharp edges are
-moved to and fro in the mouths of ruminants, and thus injure the
-mucous membrane. The hook-bristles of the fig-cactus (<i>Opuntia</i>),
-which, though small, are abundantly provided with barbs, must also
-be mentioned; for they are to be found in great numbers surrounding
-the buds of these plants, and most effectively protect them from being
-eaten away by animals (Fig. 19).</p>
-
-<p>To this category, too, belong the short, prickly bristles of the
-rough-leaved plants, which cover the whole plant as with an overcoat
-of sharp needles; of these we may mention the adder's tongue
-(<i>Echium vulgare</i>), the comfrey (<i>Symphytum officinale</i>), and the borage
-(<i>Borago officinalis</i>).</p>
-
-<p>Very well known are the stinging-hairs of the Urticaceæ, long
-hairs (Fig. 20) with an elastic base, but with glass-like, brittle,
-rounded heads, which break off at the lightest touch, whereupon the
-sharp point of the broken hair penetrates the skin of the creature
-which has touched it, and the poisonous contents of the hair are
-poured into the wound. Even our large stinging-nettle (<i>Urtica
-dioica</i>) can cause intense irritation, and evoke the 'nettle-rash,'
-named after it, on the human skin; but there are many tropical<span class="pagenum"><a id="Page_123"></a>[Pg 123]</span>
-species of nettle, e.g. <i>Urtica stimulata</i> in Java, and others, which have
-an effect similar to that of snake-poison and produce tetanoid spasms,
-and so on. In addition to formic acid these hairs contain an
-undefined ferment, a so-called Enzyme. It need scarcely be said
-that these stinging-hairs must have much more severe effects on
-the mucous membrane of the mouth of grazing animals than on the
-human skin, and that they are therefore an excellent protection for
-the plants. As a matter of fact we never find our nettle patches
-eaten away, and even the donkey, which eats thistles freely, turns
-away from the stinging-nettle. But even these stinging-hairs, like
-all other protective devices, do not afford an <i>absolute</i> protection. The
-caterpillars of several of our diurnal butterflies feed exclusively on
-the stinging-nettle, and they eat up the leaves, stinging-hairs and all.
-This is the case with five species of the genus <i>Vanessa</i>, namely:
-<i>Vanessa io</i>, the 'peacock,' <i>Vanessa urticæ</i>, the small tortoiseshell,
-<i>Vanessa prorsa</i>, <i>Vanessa C. album</i>, the C. butterfly, and <i>Vanessa
-atalanta</i>, the admiral.</p>
-
-<div class="figleft" id="f23">
-<img src="images/fig23.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 19.</span> Barbed bristles<br />
-of <i>Opuntia rafinesquii</i>; enlarged.</p>
-</div>
-
-<div class="figright" id="f24">
-<img src="images/fig24.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 20.</span> Vertical section through<br />
-a piece of a leaf of the Stinging-nettle<br />
-<i>(Urtica dioica</i>), bearing two stinging-hairs;<br />
-magnified 85 times; adapted<br />
-from Kerner and Haberlandt.</p>
-</div>
-
-<p>We are all familiar with our mulleins (<i>Verbascum</i>), those<span class="pagenum"><a id="Page_124"></a>[Pg 124]</span>
-beautiful flower-spikes with the thick, soft felt leaves, which grow
-on stony or sandy soil. Harmless as they look, they are much
-disliked by animals as food, for the thick hairy felt which covers
-them breaks up in the mouth, and sticks in the folds of the mucous
-membrane, causing burning sensations and other discomforts. They,
-too, are therefore spared by grazing animals, but they have smaller
-enemies, like the caterpillars of the genus <i>Cucullia</i>, which, however,
-never completely destroy them, but only eat large holes in their leaves.</p>
-
-<p>Let us now consider in somewhat greater detail the true thorns,
-the most conspicuous protection of many plants. It is very remarkable
-that these are always so placed, and so regulated as to their length and
-character, as to afford protection to the most important and the most
-exposed parts of the plant. Thus many bushes, which would otherwise
-be in danger of being completely devoured by cattle, are stiff
-with thorns which are nothing else than pointed,
-hard twigs without, or with very little foliage.
-Among these are the sloes, the buckthorn
-(<i>Rhamnus</i>), the sea-buckthorn (<i>Hippophäe</i>), and
-the barberry (<i>Berberis</i>). In the last-named
-three thorns arise in a group, and protect
-the young bud from danger in three directions
-(Fig. 21).</p>
-
-<div class="figleft" id="f25">
-<img src="images/fig25.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 21.</span> A piece of a<br />
-twig of Barberry (<i>Berberis<br />
-vulgaris</i>) in spring; after<br />
-Kerner.</p>
-</div>
-
-<p>The fine-leaved mimosas of the tropics have
-similar but very long and sharp thorns, and
-their leaves are movable and sensitive, so that,
-when they are touched, they shut up and draw
-back behind the rampart of stiff thorns, which
-are just of the right length to protect them.</p>
-
-<p>In many thorny bushes only the young shoots of each spring
-remain green through the summer, and in autumn they become
-transformed into thorns, under whose protection the shoots of the
-following spring will develop. Sometimes, too, the leaf-stalks
-are modified in the course of the summer into thorns, as in
-Tragacanth (<i>Astragalus tragacantha</i>). In this case the young leaves
-are protected by a circle of thorns, consisting of the leaf-stalks of the
-preceding year which have not fallen off (<a href="#f26">Fig. 22</a>, <i>A</i>, <i>B</i>, <i>C</i>).</p>
-
-<p>I should have to go on for a long time with my exposition, even
-if I were to confine attention to the essential facts; we shall, therefore,
-only recall the well-known phenomenon of the Cactuses, in
-which the leaves are entirely transformed into spines, which may
-attain a length of eight centimetres, while the fleshy stem alone
-represents the green&mdash;that is, the assimilating parts of the plant.<span class="pagenum"><a id="Page_125"></a>[Pg 125]</span>
-The species of Cactus are almost the only plants which grow on the
-stony, hard, and hot plateaux of Mexico, and they are protected from
-desiccation by the thickness of their epidermis. But, enticing as is
-the food promised by the juicy stem, animals rarely venture to
-approach them, and it is only when tortured by thirst that horses
-and asses occasionally knock off the spines with their hoofs, and so
-reach the soft tissues rich in water. For this attempt, however, as
-Alexander von Humboldt pointed out, they often suffer, as the sharp
-spines are apt to pierce the hoof. In any case, the cactuses are
-effectively protected from the danger of extermination by grazing
-animals.</p>
-
-<div class="figcenter" id="f26">
-<img src="images/fig26.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 22.</span> Tragacanth (<i>Astragalus tragacantha</i>). <i>A</i>, two spring shoots. <i>B</i>, a single
-leaf, from which the three uppermost leaflets have fallen off. <i>C</i>, leaf midrib,
-from which all the leaflets have fallen off. After Kerner.</p>
-</div>
-
-<p>It must certainly strike every one that many districts, especially
-those which are dry, hot, and stony, are conspicuously rich in thorny
-plants, and it has often been supposed that the production of thorns
-must be a direct result of these peculiar conditions of life; indeed,
-the hard, thorny habit of many of these plants has even been
-regarded as a protection against desiccation. This, however, is contradicted
-by all those thorny plants which, like the cactuses, possess
-tissues extremely rich in sap, and in which desiccation is prevented,
-not by the thorns, but by the thick epidermis. The only satisfactory
-explanation is that afforded in terms of natural selection. In such
-hot, and at the same time dry regions, the plant-growth is often very<span class="pagenum"><a id="Page_126"></a>[Pg 126]</span>
-scanty, and the food available for the grazing animals is, at least at
-times, very scarce; on this account, if the plants are to survive there
-at all, they must be armed with the most perfect means of protection
-possible against the attacks of hungry and thirsty animals. The
-struggle for existence in relation to such enemies is much more severe
-than in more luxuriant regions, and the protection by thorns has
-been developed to the highest possible pitch of perfection; species
-which were unable to develop this protection died out altogether.
-Hence the cactuses of Mexico, and the many thorny bushes and
-shrubs of the hot, and, in the summer, dried-up stony coast-lands of
-the Mediterranean in Spain, Corsica, Africa, and other countries.
-This so-called 'Prigana scrub' embraces a number of species, whose
-nearest relatives in our climate are not provided with spines, as, for
-instance, <i>Genista hispanica</i>, <i>Onobrychis cornuta</i>, <i>Sonchus cervicornus</i>,
-<i>Euphorbia spinosa</i>, <i>Stachys spinosa</i>, and others.</p>
-
-<p>Why do so few thorny plants grow on the rich and well-watered
-Alpine pastures? Probably because there is to be found there a rich
-and luxuriant plant-growth which can never be wholly exterminated
-by the grazing of animals, so that an individual species would not,
-by developing thorns, have gained any advantage in the way of
-increased capacity for existence.</p>
-
-<p>But these Alpine grazing grounds serve well to illustrate how
-great may be the advantage which protective devices give to a species.
-Much to the annoyance of the herdsmen, who endeavour to extirpate
-them as far as possible, enormous masses of rhododendrons often
-cover whole stretches, because their hard silicious leaves cannot be
-eaten, and many other plants despised of cattle flourish and increase
-on the grazing runs, like the repulsively bitter, large <i>Gentiana
-asclepiadea</i>, the malodorous <i>Aposeris fœtida</i>, and various ferns of
-disagreeable taste.</p>
-
-<p>The advantage derived by plants from the possession of any
-kind of protective device against grazing animals is perhaps best of
-all seen in the 'shrubbery,' which on every Alp is to be found in the
-immediate neighbourhood of the herdsman's hut. There, where the
-cattle daily assemble, and where the soil is continually being richly
-manured by them, we always find a large, luxuriantly growing
-company of the poisonous aconite, the bitter goosefoot (<i>Chenopodium
-bonus henricus</i>), the stinging-nettle, the thistle (<i>Cirsium spinosissimum</i>),
-the ill-smelling <i>Atriplex</i>, and some other inedible species,
-while the palatable herbs are gradually exterminated by the cattle
-which daily gather round the hut (Kerner).</p>
-
-<p>To sum up. We have seen that there is among plants an<span class="pagenum"><a id="Page_127"></a>[Pg 127]</span>
-extraordinary diversity of protective adaptations, which secures them
-from extermination by the larger herbivores.</p>
-
-<p>Since all useful contrivances, or, as we say, all adaptations, are
-capable of interpretation in terms of the process of selection, we must
-refer this great array of the most diverse protective devices to
-natural selection; and again, as among animals, we receive the
-impression that the organism is, to a certain extent, really capable
-of producing every variation necessary to its maintenance. Literally
-speaking, this would not be correct, but at any rate the number of
-adaptations possible to each form of life must be an enormous one,
-so great, indeed, that ultimately every species does secure protection
-for itself in some manner and in some degree, whether it be by the
-production of a poison or a nauseous substance within itself, or by
-surrounding itself with thorns or spines. And if it be, in a certain
-sense, a matter of 'chance' whether a plant has taken to one method
-of defence or to another, according as its innate constitution favoured
-the production of one rather than of any other, yet it would not be
-easy to prove, even in the case of the purely chemical means of
-protection, that these would have occurred in the same distribution
-and concentration as a necessary result of the metabolism of the
-plant, even if they had not been useful and consequently augmented
-by selection. But in the case of the mechanical means of protection
-this mode of explanation fails as utterly as that of the direct effect of
-the conditions of life. Why the holly should have spinose leaves
-beneath and smooth ones above can never be deduced from the
-constitution of the species.</p>
-
-<p>While the protective adaptations of plants against the larger
-herbivores always point to natural selection, our appreciation of the
-adaptability of plants, and at the same time of the potency of natural
-selection, will be strengthened still more if we turn our attention
-for a little to the arrangements which prevent the extermination of
-plants by the lower and small animals.</p>
-
-<p>It might indeed be supposed that extermination by these could
-hardly be an imminent danger, but if we think of the cockchafer
-blight, or of the destruction of whole woods by the caterpillar of the
-'white nun,' or even of the destruction of several successive plantings
-of young salad plants which the snails often cause in our gardens,
-it cannot be doubted that all plants would be exterminated by insects
-and snails alone unless they were protected against them in some
-degree.</p>
-
-<p>We owe our detailed knowledge of the means by which plants
-protect themselves against the menace of the greedy and prolific<span class="pagenum"><a id="Page_128"></a>[Pg 128]</span>
-snails to the beautiful investigations of Stahl, Professor of Botany in
-the University of Jena.</p>
-
-<p>In this case, too, both chemical and mechanical means are made
-use of. The minute quantity of tannic acid which is contained in the
-leaves of the clover prevents many snails from eating them, as, for
-instance, the garden snail (<i>Helix hortensis</i>). If the leaves be soaked
-so as to wash out the tannin the snail readily accepts them as food.
-It is true that the small, whitish field-slug (<i>Limax agrestis</i>) does
-not object to the presence of the tannin, and eats the fresh leaves of
-the clover; indeed, there is no such thing as absolute protection.
-In discussing the herbivorous mammals I have already mentioned
-that many trees and shrubs, mosses and ferns are effectively protected
-by the large amount of tannin they contain; this protection is
-effective also against snails, for all these plants are fairly free from
-their attacks; and the same is true of many other tannin-containing
-plants, species of saxifrage and sedum, the strawberry, many water-plants,
-like the pond-weeds (<i>Potamogeton</i>), the horn-nut (<i>Trapa</i>), the
-mare's tail (<i>Hippuris</i>). All these plants are only eaten by snails
-in case of necessity, or in the washed-out state.</p>
-
-<p>In other plants protection is gained by means of some acid,
-especially oxalic acid, like the wood-sorrel (<i>Oxalis acetosella</i>), the
-sorrel (<i>Rumex</i>), and the species of Begonia. When Stahl smeared
-slices of carrot, which is a favourite food of snails, with a weak
-(one per cent.) solution of oxalate of potassium, they were refused
-by the snails, and this is not surprising when we remember that even
-the external skin of the snail is very sensitive, and the mucous
-membrane of the mouth is not likely to be less so.</p>
-
-<p>Similarly, many plants develop ethereal oils in the hairs which
-cover them, as in the herb-Robert (<i>Geranium robertianum</i>). Even
-the almost omnivorous field-slug (<i>Limax agrestis</i>) does not touch
-this plant, and if it be placed upon it, escapes with all dispatch from
-the ethereal oil, which burns its naked skin, by covering itself with
-mucus and letting itself down to the ground by a thread. The
-mints (<i>Mentha</i>) and the dittany (<i>Dictamnus albus</i>) also produce
-such oils.</p>
-
-<p>Among chemical means of protection must be named the pure
-bitter stuffs, such as are found in the species of gentian, the milkwort
-(<i>Polygala amara</i>), and in many other plants, and also the curious
-'oil-bodies' of the liverworts.</p>
-
-<p>But some plants also defend themselves against the attacks of
-snails by mechanical means.</p>
-
-<p>First there are the various kinds of bristle arrangements, which<span class="pagenum"><a id="Page_129"></a>[Pg 129]</span>
-prevent the snails from creeping up the stalks. We never find the
-comfrey (<i>Symphytum officinale</i>) of our meadows eaten by snails, for
-it is thickly covered over with stiff bristles, which are most disagreeable
-to the snail, and the stinging-nettle (<i>Urtica dioica</i>) is
-similarly protected by bristle hairs, while, as we have already
-seen, its stinging-hairs secure immunity from the attacks of larger
-animals.</p>
-
-<p>And although it is true that the majority of plants do not
-prevent the snails from creeping up their stalks, yet they do not
-serve them in any great degree as food, since the green parts often
-offer resistance to mastication and digestion. Thus the lime encrustations
-which cover the stoneworts (<i>Chara</i>) prevent snails from
-eating them. If the lime be dissolved by means of acids, and the
-plants then offered to the snails, they will eat them greedily. The
-same is true of the silicifying of the cell-walls, so widely distributed
-among mosses and grasses, and when this occurs in a high degree it
-forms an effective protection even against the large herbivores. Our
-slightly siliceous grasses are secure from snails, and that it is really
-the presence of the silicic acid which deters them from an otherwise
-welcome kind of food is proved by Stahl's experiment of growing
-maize in pure water, and so obtaining plants poor in silica. These
-were devoured without ceremony by the snails.</p>
-
-<p>Of the many other protective peculiarities which make it difficult
-for snails to eat plants I shall only recall the so-called 'Raphides,'
-those microscopic crystal-like needles of oxalate of lime, pointed at
-both ends, which lie close together in the tissues of many plants.
-Cuckoo pint (<i>Arum maculatum</i>), the narcissi, the snowdrops (<i>Leucojum</i>),
-the squill (<i>Scilla</i>), and the asparagus contain them, and all
-these plants are spared by snails obviously because during mastication
-they are unpleasantly affected by the raphides. Even the
-voracious field-slug rejects these.</p>
-
-<p>Of course it cannot be said that these raphides protect against
-all other enemies. They are effective against rodents and ruminants,
-and also against locusts, but a number of caterpillars seek out by
-preference just those plants which contain raphides. Thus certain
-caterpillars of the Sphingidæ feed on species of <i>Galium</i> and <i>Epilobium</i>,
-the leaves of the vine, and the wild balsam (<i>Impatiens</i>).
-The caterpillar of <i>Chærocampa elpenor</i>, which especially prefers
-<i>Vitis</i> and <i>Epilobium</i>, has transferred its affections to the fuchsias
-in our gardens, which came from South America; the butterfly not
-infrequently lays its eggs on these plants, and the caterpillars devour
-them readily; but the fuchsias may also contain raphides.</p>
-
-<p><span class="pagenum"><a id="Page_130"></a>[Pg 130]</span></p>
-
-<p>We may say, indeed, that almost all wild Phanerogams are
-protected in some degree against snails, and this almost suggests the
-question: What then is left for the snails to feed on if everything is
-thus armed against them? But, in the first place, there remain our
-cultivated plants, which, like the garden lettuce (<i>Lactuca</i>), are quite
-without defence; and secondly, the snails often eat the plants only
-after they have been rooted up and lie rotting on the ground, that
-is, when the protective ingredient has been dissolved out by the rain;
-finally, no means of protection, as I have often said already, is absolute
-or effective against all snails. Many of these are, as Stahl calls
-them, 'specialists.' Thus, the large slug of our woods eats the
-poisonous fungi which are rejected by other snails, and in the same
-way there are many other specialists which, however, are not likely
-to eliminate unaided the plants to which they have adapted themselves.
-There are certainly also omnivorous forms, like the field-slug
-(<i>Limax agrestis</i>), to which we have referred so often, and <i>Arion
-empiricorum</i>, the red slug, but just because these eat so many kinds
-of plant they are less dangerous to any one species.</p>
-
-<p>These manifold devices for protecting plants against the depredations
-of snails afford another proof that innumerable details in the
-organization of plants, as of animals, must be referred to natural
-selection, since they are capable of interpretation in no other way. If
-these protective devices were to be found only in isolated plants, we
-might perhaps talk of 'chance'; we might refer them to the inborn
-constitution of the plant, which made the production of bristles, or
-bitter stuffs, or the deposition of silicic acid a necessity, and which
-'happened' to make the plants distasteful to certain snails. But as it
-appears that all plants are protected against snails, one in this way,
-another in that, this objection cannot be sustained. Furthermore,
-some of the beautiful experiments made by Stahl to prove the protective
-effect of these devices showed, at the same time, that they
-were not in themselves indispensable to the existence of the plant;
-maize, for instance, develops a plant perfectly capable of life, even
-though silicic acid be withheld, and the acid is, therefore, not an
-element essential to its constitution, but a means of protection against
-voracious animals. The clearest proof of this is afforded by plants
-like the lettuce (<i>Lactuca</i>), which formed protective stuffs in the wild
-state, but have lost them altogether under cultivation, through disuse,
-as we shall see more precisely later on. As the eyes of animals which
-live in darkness have degenerated, so the plants which have been
-taken under the protection of man have lost their natural means
-of defence, because these were no longer necessary to the maintenance<span class="pagenum"><a id="Page_131"></a>[Pg 131]</span>
-of the species. Even the protective bitter substances (tannin-compounds)
-are not essential to the constitution of the genus <i>Lactuca</i>;
-their formation may be discontinued without the plant being otherwise
-affected. And in this case it is not a question of the withdrawal
-of something which has to be taken in from outside, it is the non-development
-of what is purely a product of the internal metabolism.</p>
-
-<p>The adaptations of plants against snails are instructive in another
-way, namely, in their extraordinary diversity. Here again we see
-how great is the plasticity of organic forms, and how precisely,
-though in many very different ways, they adapt themselves to the
-conditions of their life, in this case the weaknesses of their greedy
-enemies, and all to attain the same end, the security of their existence
-as a species. We see at the same time that innumerable minute
-details in the structure and character of a species, which may appear
-unimportant, may yet have their definite uses&mdash;hairs, bristles, and
-raphides, as well as bitter substances, ethereal oils, acids, and tannin-compounds.
-But we must, of course, have minute and exhaustive
-investigations, like those of Stahl, in regard to the biological relations
-of these peculiarities before their utility can become clear to us.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_132"></a>[Pg 132]</span></p>
-
-<h2 class="nobreak" id="LECTURE_VII">LECTURE VII</h2>
-</div>
-
-<p class="c">CARNIVOROUS PLANTS</p>
-
-<div class="blockquot">
-
-<p>Introduction&mdash;The Bladderworts or Utriculariæ&mdash;Pitcher-plants, Nepenthes&mdash;The
-Toothwort, Lathræa&mdash;The Butterwort, Pinguicula&mdash;The Sundew, Drosera&mdash;The Flytrap&mdash;Aldrovandia&mdash;Conclusions.</p></div>
-
-
-<p><span class="smcap">That</span> the principle of selection dominates, to a large extent at
-least, all the structural characters of plants, and moulds these in direct
-relation to the prospects of greater success which may be offered in
-the vicissitudes of the life-conditions of a single species or group
-of species, is nowhere more apparent than in the case of the so-called
-'insectivorous' or 'carnivorous' plants. Here again it was Charles
-Darwin who led the way, for while many plants had long been
-known on the sticky leaves of which insects were often caught
-and killed, it had occurred to no one to regard this as of any special
-use for the plant, much less to look on the peculiar dispositions of
-such leaves as especially determined for this purpose. Darwin was
-the first to show that there is no small number of plants&mdash;we now
-know about 500&mdash;which secure only a portion of their nutritive
-material by the usual method of assimilation, and gain another and
-smaller portion by dissolving and utilizing animal protoplasm,
-especially nitrogenous muscle substance. The correctness of this
-interpretation was at first disputed, but Darwin showed that pieces
-of muscle, or any nitrogenous organic substance, were really dissolved
-by the relevant parts of the plant, and were afterwards absorbed. It
-can therefore no longer be doubted that the remarkable contrivances
-by which animals are laid hold of by plants&mdash;are in a certain sense
-caught and killed&mdash;have arisen with reference to this particular end;
-or, to speak less metaphorically, that existing structural and functional
-peculiarities in a plant which caused animals to be held fast were
-of advantage to the nutrition of the plant, and were therefore
-augmented and perfected by natural selection. That this was possible
-is obvious from the number of insectivorous plants which now live
-upon the earth, and that these processes of selection ran their courses
-quite independently of one another, and even that they started from
-different parts of the plant, is shown by the diversity of the con<span class="pagenum"><a id="Page_133"></a>[Pg 133]</span>trivances
-which occur in plants of several different families. A few
-of these I wish to discuss in some detail.</p>
-
-<div class="figcenter" id="f27">
-<img src="images/fig27.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 23.</span> <i>Utricularia grafiana</i>, after Kerner. <i>A</i>, a plant in its natural position,
-floating in the water. <i>FA</i>, traps. <i>B</i>, a trap enlarged four times. <i>sz</i>, suctorial cells.
-<i>kl</i>, valve, which closes the entrance to the trap. <i>C</i>, suctorial cells on the internal wall
-of the trap, enlarged 250 times.</p>
-</div>
-
-<p>The marshes of European countries, and also those of warmer lands,
-often contain bladderworts, or Utriculariæ (Fig. 23)&mdash;floating water-plants,
-without roots, and with horizontally spread, long-drawn-out,
-tendril-like shoots, in part thickly covered with whorls of delicate,
-needle-shaped leaves, in part bearing sparse leaves of quite peculiar
-structure. These are stalked, hollow bladders (Fig. 23 <i>A</i>, <i>FA</i>), with
-quite a narrow entrance at the apex, which is closed, as far as larger
-animals are concerned, by projecting bristle-like hairs (<i>B</i>). Small
-animals, such as water-fleas (<i>Daphnia</i>), species of <i>Cyclops</i>, and
-Ostracods, can swim in between the bristles, and they then come
-in contact with a valve which opens easily inwards (<i>B</i>, <i>kl</i>) and allows
-them to penetrate into the interior of the trap. Once inside they are<span class="pagenum"><a id="Page_134"></a>[Pg 134]</span>
-captives, for the valve does not open outwards; therefore they soon
-die and decompose, and are then taken up by special absorptive cells
-(<i>B</i>, <i>C</i>, <i>sz</i>) and utilized as nourishment for the plants. In this way
-the Utriculariæ catch numerous little crustaceans and insect larvæ,
-which slip into their traps, presumably for concealment.</p>
-
-<div class="figleft" id="f28">
-<img src="images/fig28.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 24.</span> Pitcher of <i>Nepenthes villosa</i>, after<br />
-Kerner. <i>St</i>, stalk of the leaf. <i>Spr</i>, its apex.<br />
-<i>Fk</i>, the pitcher. <i>R</i>, the margin beset with<br />
-incurved spines.</p>
-</div>
-
-<p>Another example is found in the marsh plants of the genus
-<i>Nepenthes</i>, some species of which
-live as climbers on the outskirts
-of tropical forests, climbing up the
-trees and letting their long, thin
-tendrils hang downwards, often
-over ponds and stagnant pools,
-where swarms of small flying
-insects abound. These plants
-have developed exceedingly remarkable
-contrivances for catching
-insects and using them as
-food (Fig. 24). The long stalks
-(<i>St</i>) of their leaves (<i>Spr</i>) are
-first bent downwards, then they
-suddenly turn sharply upwards,
-and the upturned portion is modified
-into a pitcher-like structure,
-in the bottom of which a fluid
-gathers, acid in taste, containing
-pepsin, and therefore a digestive
-fluid. Nitrogenous substances,
-such as flesh, dissolve in this fluid,
-and insects which fall into the
-pitcher from the rim are killed
-and dissolved. There are many
-species of <i>Nepenthes</i>, but not all
-of them possess the trap-structure
-in equal perfection, so that we
-are able, to some extent, to follow
-the course of its evolution, from
-a broad leaf-stalk, somewhat bent
-over at the edges, to the marvellous closed pitcher shown by <i>Nepenthes
-villosa</i> (Fig. 24) of Borneo. In this species the pitchers attain a length
-of fifty centimetres, and are beautifully coloured, resembling in that
-respect, as well as in their form, the tobacco-pipe-like flowers of the
-tropical Aristolochiæ. When we come to discuss the origin of flowers,<span class="pagenum"><a id="Page_135"></a>[Pg 135]</span>
-we shall see that the bright, conspicuous colour possesses a very considerable
-value in attracting insects; and in the case of the pitcher-plant,
-too, the gorgeous colour probably allures insects to settle on the rim of
-the pitcher, and they are tempted to dally the longer since it secretes
-honey. But the thick, swollen rim of the pitcher is as smooth as if it
-were made of polished wax, and resembles the petals of those
-magnificent large orchids, the Stanhopeæ; the inner surface of the
-pitcher below the margin is also smooth, so that insects which creep
-about seeking honey are apt to slip and fall to the bottom. Even if
-many of them are not at once killed by the digestive fluid, but are able
-to climb up the smooth wall again, they cannot escape, for beneath
-the swollen rim, which projects inwards, there is a circle of strong
-bristles or teeth, with the points directed downwards, which, like
-thorns, prevent the captive's escape. Thus the pitchers of <i>Nepenthes</i>
-secure and digest a large number of insects, and we can easily understand
-that the plant acquires a considerable amount of valuable
-nourishment in this way, for ready-made protoplasm is a convenient
-food to which the plant has to do but little in order to convert it into
-its own particular kind of living matter.</p>
-
-<p>The toothwort (<i>Lathræa squamaria</i>) must also be briefly noticed
-here, because it does not catch insects through the medium either of
-air or of water, but through the earth. As is well known, this plant
-is parasitic on the roots of various foliage-trees. It is of a pale
-yellowish colour, and has no green assimilating parts. For such
-a plant it must be of particular value to be able to catch animals and
-to use them as food. To this end the short, pale leaves, which
-surround the creeping, underground stem in the form of closely
-appressed scales, have been modified into snares for minute animals.
-The leaves have their upper parts recurved downwards, and the edges
-have grown together, so that only a small opening is left at the base,
-and this leads into a system of tunnels. Aphides, rotifers, bear-animalcules,
-but especially springtails (Podurids), creep into these
-hollow leaves, are held fast by a sticky secretion, and are dissolved
-and absorbed.</p>
-
-<p>Another example, also indigenous, is that graceful marsh plant,
-the butterwort (<i>Pinguicula vulgaris</i>), whose broad, tongue-shaped
-leaves, arranged in the form of a rosette, have been modified into an
-insect trap by the turning up of their edges, while the middle is
-deepened into a longitudinal groove (Fig. 25). The whole upper
-surface of the leaf is covered with an enormous number of little
-mushroom-shaped glands (<i>B</i>, <i>C</i>, <i>Dr</i>), which secrete a viscid slime.
-Insects which settle on the leaf stick fast, and as the glands continue<span class="pagenum"><a id="Page_136"></a>[Pg 136]</span>
-to pour out more and more slime, while at the same time the edges of
-the leaf, stimulated by the struggling of the insect, curl over still
-farther, the victims are drowned in the slime, and ultimately
-absorbed; for this secretion is so powerful that even fragments of
-cartilage are dissolved by it in forty-eight hours. Midges and mayflies
-in particular fall victims to this plant, which is common in
-marshy places both in mountain and plain.</p>
-
-<div class="figcenter" id="f29">
-<img src="images/fig29.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 25.</span> Butterwort (<i>Pinguicula vulgaris</i>). <i>A</i>, the entire plant, showing the
-incurved margins of the leaves and some insects caught by the secretion. <i>B</i>, cross-section
-through a leaf, enlarged 50 times. <i>r</i>, the margin. <i>Dr</i>, <i>Dr</i><sup>l</sup>, two kinds of
-glands. <i>C</i>, a portion of the leaf-surface, magnified 180 times.</p>
-</div>
-
-<p>We must also mention the sundew (<i>Drosera rotundifolia</i>), which
-takes its name from the seeming dewdrops that sparkle in the sun on
-the leaves, or rather on the rounded extremities of long and rather
-thick cilia-like hairs which cover the whole upper surface of the leaf.
-In reality the apparent dewdrops consist of a sticky, clear, viscid
-slime, which is secreted by the glandular ends of the pin-shaped hairs
-or 'tentacles.' Insects which settle on the leaf are caught by the
-slime, and in this case also an acid, pepsin-containing fluid is secreted,
-which gradually effects the digestion of the soluble parts of the insect.
-It is especially noteworthy that it is not only those tentacles which<span class="pagenum"><a id="Page_137"></a>[Pg 137]</span>
-are in contact with the insect that take part in its digestion and
-absorption, for all the others gradually alter their position from the
-moment when any nitrogenous body, be it a fragment of flesh or an
-insect, touches any of them. All begin to curve slowly towards the
-stimulating object (Fig. 27), so that, after one to three hours, all the
-tentacles have their heads towards it, and collectively pour out their
-digestive juice upon it.</p>
-
-<div class="figleft" id="f30">
-<img src="images/fig30.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 26.</span> The Sundew (<i>Drosera rotundifolia</i>),
-after Kerner.</p>
-</div>
-
-<div class="figcenter" id="f31">
-<img src="images/fig31.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 27.</span> A leaf of the Sundew,
-with half of the tentacles curved
-in upon a captured insect; enlarged
-4 times.</p>
-</div>
-
-<p>The sundew grows in marshes, as, for instance, those of the Black
-Forest, and also on the moss-covered ridges there, and it is easy to
-observe that a leaf often shows not merely a single gnat, midge, or
-little dragon-fly, but several, sometimes as many as a dozen. In this
-case, again, the value of the arrangement from the point of view of
-nourishment can be no inconsiderable one.</p>
-
-<p>In the case of the sundew we are obviously face to face with an
-exceedingly complex adaptation, for not only is there a secretion
-of the peculiar digestive juices, which occur only in carnivorous
-plants, but the secreting tentacles are actively motile. That the
-tentacles more remote from the captive may be excited to curve
-towards it, it is necessary that the stimulus exerted by it on the
-heads of the tentacles connected with it be conveyed to the base, and<span class="pagenum"><a id="Page_138"></a>[Pg 138]</span>
-thence to the tips of the other tentacles, for they curve throughout
-their whole length. The utility of the contrivance is obvious, but
-that an arrangement so divergent from the ordinary dispositions of
-plants could be brought about points to the length of time that the
-processes of natural selection must
-have gone on, preserving every new
-little variation, and adding it to the
-rest.</p>
-
-<div class="figleft" id="f32">
-<img src="images/fig32.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 28.</span> Leaf of Venus Fly-trap<br />
-(<i>Dionæa muscipula</i>), after Kerner. <i>A</i>,<br />
-leaf-blade (<i>Spr</i>) open. <i>St</i>, leaf-stalk.<br />
-<i>Stch</i>, sensitive hairs. <i>B</i>, vertical section<br />
-through the closed leaf-blade.</p>
-</div>
-
-<div class="figleft" id="f33">
-<img src="images/fig33.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 29.</span> <i>Aldrovandia vesiculosa</i>, a branch with the<br />
-traps <i>FA</i>.</p>
-</div>
-
-<p>Two plants remain to be noticed
-in conclusion, both possessing movable,
-closing traps for catching animals.
-The so-called Venus fly-trap (<i>Dionæa
-muscipula</i>) is a marsh plant of North
-America, the leaves of which, like
-those of <i>Pinguicula</i> and <i>Drosera</i>, are
-arranged in a rosette on the ground.
-The individual leaf has a spatula-like
-stalk and a blade in two halves (Fig.
-28, <i>A</i>), each edged with long and
-strong spinous processes, directed obliquely
-inwards. The halves of the
-blade, when the necessary stimulus
-is applied to the surface, can close
-together in a very short time, from 10 to 30 seconds. The two
-rows of marginal spines then cross, as the interlocking fingers of
-the hands do, and thus form a cage out of which the imprisoned
-insect cannot escape. The appropriate stimulus to set the mechanism
-in motion is a light
-touch, while a more
-violent shock, or strong
-pressure, or a current
-of air, does not cause
-the trap to close. But
-if a fly comes to creep
-about on the leaf, and
-in doing so touches one
-of six short jointed hairs rising erect from a minute cushion of cells,
-then the leaf closes, quickly indeed, but at the same time so gently
-and imperceptibly that the fly is unaware of danger and does not
-try to escape. Then numerous purple mucous glands begin to
-surround the victim with pepsin-containing, acid, digestive juice
-which gradually dissolves it.</p>
-
-<p><span class="pagenum"><a id="Page_139"></a>[Pg 139]</span></p>
-
-<p>One of the water-plants of Southern Europe, <i>Aldrovandia
-vesiculosa</i>, which is also to be found in swamps on the northern ridge
-of the Alps, possesses, in addition to the capturing and digesting
-apparatus proper, an active motile apparatus, which is set in motion
-through sensitive hairs. When I found the plant for the first time
-in a swamp at Lindau, on the Lake of Constance, I took it at first
-sight for an <i>Utricularia</i>, for the two plants resemble each other
-in external appearance (cf. Figs. <a href="#f26">22</a> and 29), but the modification
-of the leaves into traps is quite different. On both halves of the leaf-blade
-there are numerous bristles (Fig. 30, <i>A</i>), and the lightest touch
-on these by a little water animal acts as a releasing stimulus to the
-motile elements of the leaf (<i>Stch</i>). As in the Venus fly-trap, the
-two halves of the leaf close together somewhat quickly, but quite
-quietly, and the animal is caught. Fig. 30 shows a section of one
-of these traps in its closed state. The captive animals cannot escape,
-because the margins of the leaf shut quite tightly on one another,
-and are beset with little teeth. Numerous little glands (<i>Dr</i>) secrete
-a digestive juice, and after some days, or even weeks, the insoluble
-remains of the minute animals may be found inside the trap.</p>
-
-<div class="figcenter" id="f34">
-<img src="images/fig34.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 30.</span> <i>Aldrovandia</i>: its trap apparatus. <i>A</i>, open. <i>St</i>, stalk
-of the leaf. <i>Spr</i>, blade of the leaf. <i>Stch</i>, sensitive bristles. <i>Dr</i>, glands.
-<i>B</i>, closed, a cross-section.</p>
-</div>
-
-<p>Many more cases of animal-catching plants might be adduced,
-but it is far from my intention to try to describe all the existing
-contrivances; those already mentioned may suffice to give an idea of
-the diversity and of the detailed effectiveness of these adaptations.
-They amplify&mdash;so it seems to me&mdash;our conception of the scope of
-natural selection, by showing us that adaptations may arise which<span class="pagenum"><a id="Page_140"></a>[Pg 140]</span>
-are quite foreign to the original mode of life of the organism in
-question, and stand, indeed, in apparent contradiction to its fundamental
-physiological processes. It is hardly necessary to enter into
-a special argument to show that they can only have been brought
-about in the course of natural selection, since every other interpretation
-of their occurrence fails. Neither climatic nor any other external
-direct influence could have effected these modifications of the parts of
-plants, which are all so different, yet all so well suited to their
-purpose; they are different even in plants growing quite close
-together, like the sundew and the butterwort. The Lamarckian
-principle of use and disuse hardly enters into the question at all, since
-plants do not possess a will, and we can hardly speak of 'chance'
-where we have to do with such complex and diversely combined
-transformations. A process of selection actually operative in each of
-these cases can easily be thought out, and I shall leave it to my
-readers themselves to do this, and shall only indicate that we have
-to do with increasing elaboration in two different directions: first,
-improvements in the ability to utilize animal substances which
-happened to stick to the leaves, and second, an increase in the
-probability of animals sticking to the leaves, and so becoming
-available. Thus there arose, on the one hand, dissolving and
-digestive juices, and arrangements for absorption; and, on the other
-hand, viscid slime, and traps of various kinds to secure the animals,
-as well as honey and bright colours to attract them.</p>
-
-<p>But it is not merely transformations in the form of the stems and
-leaves which have come about; there are also important physiological
-changes. The sensitiveness to stimulus of various parts of the leaf is
-greatly increased, to a certain extent in the butterwort, the edges of
-whose leaves turn inwards in response to stimulus, still more in the
-sundew, in which the stimulus is conveyed from the tentacles touched
-to all the others, but most wonderfully of all in the Venus fly-trap
-and <i>Aldrovandia</i>, whose sensitive hairs so transmit the stimulus that
-the whole leaf is affected by it, and is set in motion, in a manner
-quite comparable to the effects of a nerve-stimulus in animals.</p>
-
-<p>Thus the case of carnivorous or insectivorous plants shows us that,
-in the course of natural selection, quite new organs can be produced
-in a plant by a thoroughgoing transformation of old ones, as, for
-instance, the pitchers of <i>Nepenthes</i>, and that, furthermore, even the
-physiological capacities of the plant may be changed in the most far-reaching
-manner, increasing and varying until they come to resemble
-the functions of the animal body.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_141"></a>[Pg 141]</span></p>
-
-<h2 class="nobreak" id="LECTURE_VIII">LECTURE VIII</h2>
-</div>
-
-<p class="c">THE INSTINCTS OF ANIMALS</p>
-
-<div class="blockquot">
-
-<p>The robber-wasp&mdash;Statement of the problem&mdash;Material basis of instincts&mdash;Instincts
-are not 'inherited habits'&mdash;Instinct of self-preservation&mdash;Fugitive instinct: death-feigning&mdash;Masking
-of crabs&mdash;Nutritive instinct&mdash;Monophagous caterpillars&mdash;Diverse
-modes of acquiring food: May-flies, sea-cucumbers, fishes that snare&mdash;'Aberration' of
-instinct&mdash;Change of instinct during metamorphosis: Eristalis, Sitaris&mdash;Imperfection
-of adaptation points to origin through natural selection&mdash;Instinct and will&mdash;Instincts
-and protective coloration&mdash;Leisurely flight of Heliconiidæ&mdash;Rapid flight of Papilionidæ&mdash;Instincts
-which act only once in a lifetime&mdash;Pupation of butterflies&mdash;Pupation of the
-Longicorns&mdash;Pupation of the silk-moth&mdash;The emperor moth&mdash;The cocoons of Atlas&mdash;Oviposition
-of butterflies.</p></div>
-
-
-<p><span class="smcap">We</span> have hitherto considered animals with especial regard to the
-variation and re-adaptation of morphological characters, e.g. modifications
-of form and colour; and we have now to ask whether their
-behaviour also is to be referred as to its origin, in whole or in part, to
-the principle of selection. All around us we can see that animals
-know how to use their parts or organs in a purposeful manner: the
-duckling swims at once upon the water; the chicken which has just
-been hatched from the egg pecks at the seeds lying on the ground; the
-butterfly but newly emerged from the pupa, as soon as its wings
-have dried and hardened, knows how to use them in flight; and the
-predatory wasp requires no instruction to recognize her victim, a
-particular caterpillar, a grasshopper, or some other definite insect;
-she knows how to attack it, to paralyse it by stings, and then hesitates
-not a moment as to what she has to do next; she drags it to her nest,
-deposits it in one of the cells already prepared for her future brood,
-lays a single egg upon it, and roofs the cell carefully over. It is only
-because all these complex acts are so precisely performed, as precisely
-as if the wasp knew why she performed them, that the species is able
-to maintain its existence, for only thus can the rearing of the next
-generation be secured. Out of the egg there slips a little larva, which
-at once makes for the paralysed victim, feeds upon it, and grows
-thereby, then, within the shelter of the closed cell, passes through the
-pupa stage and is transformed into a perfect wasp. Many species
-of these predatory wasps do not lay the egg directly beside or upon
-their prey, but lest its movements should endanger their offspring,<span class="pagenum"><a id="Page_142"></a>[Pg 142]</span>
-they hang the egg above it by a silken thread. It is thus in security,
-and the young larva, too, when it appears, can withdraw to its safely
-swinging resting-place as soon as danger threatens from the convulsive
-struggles of the unfortunate victim at whose body it is
-gnawing.</p>
-
-<p>Every animal has a great many such 'instincts,' which lead it,
-indeed force it, to act appropriately towards an end, without having
-any consciousness of that end. For how should the butterfly know
-what flying is, or that it possessed the power of flight at all, or who
-could have shown the predatory wasp, when she wakened from the
-pupa sleep to quite a new kind of life, all that she had to do in order
-to procure food for herself and to secure shelter and nourishment
-for the brood which was still enclosed within her ovary? Since
-species have developed from other species, these regulators of the
-body, the instincts, cannot have been the same in earlier times; they
-must have evolved out of the instincts of ancestors, and the questions
-we have to ask are: By what factors? In what way? Has the
-principle of selection been operative here too, or can we refer instincts
-to the inherited effects of use and disuse?</p>
-
-<p>Before I enter upon this question it is necessary to consider for
-a little the physiological basis of instinct. We can distinguish three
-kinds of actions: purely reflex, purely instinctive, and purely conscious
-actions. In the case of the first, we see most clearly that they
-depend on an existing mechanism, for they follow of necessity on
-a particular stimulus, and cannot always be suppressed. Bright light
-striking our eye makes the pupil narrower by a contraction of the
-iris, and in the same way our eyelids close if a finger be thrust
-suddenly towards them. We know, too, the principle of these reflex
-mechanisms; they depend on nerve connexions. Sensory nerves are
-so connected in the nerve-centres with motor nerves, that a stimulus
-affecting the former at the periphery of the body, as at the eye, is
-carried to certain nerve-cells of the brain, and from these it excites to
-activity certain motor centres, so that definite movements are set up.
-It is rarely only one muscle that is thus excited to activity, there are
-usually several, and here we have the transition to instinctive action,
-which consists in a longer or shorter series of actions, that is, of motor
-combinations. These, too, are originally, at least, set a-going by
-a sense impression, an external stimulus which affects a sensory nerve
-exactly in the same way as in the reflex mechanism, and this stimulus
-is carried to a particular group of sensory nerve-cells in the central
-nervous organ, and from these transmitted by very fine inter-connexions
-to motor centres. There are extraordinarily complex<span class="pagenum"><a id="Page_143"></a>[Pg 143]</span>
-instinctive actions, and in these the completion of one action is
-obviously the stimulus to the second, the completion of the second
-to the third, and so on, until the entire chain of inter-dependent
-movements which make up the whole performance has been completed.</p>
-
-<p>Instincts have thus a material basis in the cells and fibres of the
-nervous system, and through variations in the connexions and
-irritability of these nervous parts they too can be modified, like any
-of the other characters of the body, such as form and colour.</p>
-
-<p>Conscious actions depend directly on the will, and they have
-a close connexion with instinctive actions in as far as these also can
-be controlled by the will, that is, can be set a-going or inhibited, and
-also, on the other hand, in as far as purely voluntary actions may
-become instinctive through frequent repetition. The first case is
-illustrated, for instance, when the suckling of a child at the mother's
-breast is continued into the second year of life, as not infrequently
-happens in the southern countries of Europe. Such a child knows
-exactly why it wants the breast, and its action is a conscious one,
-while the newborn child seeks about with the mouth instinctively,
-and when it has found what it sought performs the somewhat complex
-sucking movements automatically. The second case is illustrated,
-when, for instance, we have made a habit of winding up a watch
-on going to bed, and do it when we happen to change our clothes
-through the day, although it is then purposeless and would have been
-omitted if the action had required a conscious effort of will. One can
-often observe on oneself in how short a time a conscious action may
-become instinctive. I once sent my keyless watch to a watchmaker
-for repairs, and received from him for the time an ordinary watch,
-which had to be wound with a key, which key I kept for safety in
-my purse. At the end of eight days I got back my own watch, and
-on undressing the first night I found myself 'instinctively' taking
-my purse from my pocket in order to get the key, which, as I very
-well knew, I no longer needed. And that a long series of complex
-movements, originally performed only consciously, may be gone
-through instinctively, is shown by the fact that pieces of music
-which have been learnt by heart can often be played without mistake
-from beginning to end while the player is thinking of quite other
-things. The complex instinctive actions of animals are performed in
-quite a similar manner.</p>
-
-<p>There is thus no sharp boundary line between reflex and instinctive
-actions, nor between instinctive and conscious actions, but
-one passes over into the other, and the thought suggests itself, that in<span class="pagenum"><a id="Page_144"></a>[Pg 144]</span>
-the phyletic development also transitions from one kind of action
-to the other must have taken place. As long as one believes the
-Lamarckian principle to be really operative one can suppose that
-actions, which were originally dependent on the will, when they were
-often repeated, became instinctive, or, in other words, that instincts,
-many of them at least, are inherited habits.</p>
-
-<p>I shall endeavour to show later on that this assumption, plausible
-as it seems at first sight, cannot be correct; but in the meantime
-I must confine myself to saying that there are a great number of
-instincts which must be referred to the process of selection, and that
-the rest can be similarly interpreted in their essentials at least.</p>
-
-<p>The instinct of self-preservation is universally distributed, and it
-is exhibited in many animals by flight from their enemies. The hare
-flees from the fox and from men; the bird flies away at the approach
-of the cat; the butterfly flies from even the shadow of the net spread
-to catch it. These might be regarded as purely conscious actions, and
-in the case of the hare and the bird experience and will have undoubtedly
-some part in them, but even in these the basis of the action
-is an organic impulse; this, and not reflection, causes the animal to
-flee at sight of an enemy. In the butterfly, indeed, this must be
-purely instinctive, since it is done with the same precision immediately
-on leaving the pupa state, before the animal has had any experience.
-But even in the case of the hare and the bird, taking to flight would
-in most cases come too late if reflection were necessary first; if it is
-to be effective it must take place as instantaneously as the shutting of
-the lids when danger threatens the eye.</p>
-
-<p>The hermit-crab (<a href="#f38">Fig. 34</a>, p. 163), which conceals its soft
-abdomen in an empty mollusc shell, and drags that about with it on
-the floor of the sea, withdraws with lightning-like rapidity into its
-house as soon as any suspicious movement catches its eye, and it is
-very difficult to grip one of its legs with the forceps in time to draw
-it out of its shell. The same is the case with the so-called Serpulids,
-worms of the genus <i>Serpula</i>, and its allies; it is not easy to seize
-them, because, however quick one is with the forceps, their instinct
-of fugitive self-preservation acts more quickly still, and they shoot
-back into their protecting tubes before one has had time to grasp
-them. But this impulse to flee from enemies, though it seems almost
-a matter of course, is by no means common to all animals, for in quite
-a large number the instinct of self-preservation finds expression in an
-exactly contrary manner, namely, in the so-called 'death-feigning,' that
-is, remaining absolutely motionless in a definite position precisely prescribed
-to the animal by its instinct. In speaking of protective colouring,<span class="pagenum"><a id="Page_145"></a>[Pg 145]</span>
-I drew attention to the 'wood-moth' (<i>Xylina</i>), which resembles a
-broken fragment of half-decayed wood so deceptively, and I pointed
-out that the colour-resemblance to wood would be in itself of but
-little use to the insect if it were not combined with the instinct to
-remain motionless in danger, to 'feign death.' The antennæ and legs
-are drawn close to the body, so that they rather heighten the disguise,
-and, instead of running away, the insect does not move a muscle
-until the danger is past. This instinct must have evolved hand in
-hand with the resemblance to a piece of wood, and, just as we sought
-to interpret the latter from the fact that the moths which most
-resembled the wood had always the best chance of surviving, so we
-maintain that those moths would profit most by their resemblance
-which drew in their legs and antennæ closely and lay most perfectly
-still. Thus the brain-mechanism, which effected the keeping still
-whenever the senses announced danger, would be more and more
-firmly established and perfected in the course of selection.</p>
-
-<p>Even nearly related animals may have quite different instincts
-which secure them against danger. Thus in the group of pocket
-crabs (Notopoda) there are some species which run away when
-danger threatens, but others which anticipate the risk of discovery
-by masking themselves to a certain extent. With the last pair of
-legs they hold over themselves a large piece of sponge, which then
-grows till it often leaves only the limbs and frontal region uncovered.
-Of course there can be no question of consciousness in what the crab
-does, as is proved by the fact that these crabs will, in case of necessity,
-take a transparent piece of glass instead of the sponge; but the
-impulse to cover themselves with something is strong in them, and
-finds expression not only when they see a really protective substance,
-but even when they see one which is transparent and therefore
-wholly useless for the purpose. Crabs from which the sponge has
-been taken away wander about until they find another; the impulse
-is thus set up not only by the sight of the sponge or of a stone, but
-also by the feeling that their back is uncovered. The large spider-crab
-of the Mediterranean (<i>Maja squinado</i>) effects its disguise in
-a somewhat different manner. It has peculiar hooked bristles on the
-back, and on these it hooks little bunches of seaweed, often many of
-them, so that it is entirely covered and looks like a bunch of wrack
-rather than like an animal. Here again a bodily variation has gone
-hand in hand with the development of the instinct to cover itself:
-the bristles of the back have become hooked. Many instincts are
-accompanied by structural modifications, and in the crabs which cover
-themselves with sponge or stone this is the case, for the last pair of<span class="pagenum"><a id="Page_146"></a>[Pg 146]</span>
-thoracic legs is turned towards the back, instead of being set at the
-side of the body, as is usual among crabs. They are thus enabled
-to hold the sponge much better and more permanently, and as this is
-advantageous we may well ascribe the change to natural selection.</p>
-
-<p>Let us now turn our attention to another category of instincts,
-the most common and most indispensable of all, those which lead to
-the seeking and devouring of food.</p>
-
-<p>The chicken just emerged from the egg picks up the seeds thrown
-to it with no experience of what eating is, or what can be made to
-serve it as food; its instinct for food expresses itself in picking up,
-and it is awakened or stimulated to action by sight of the seeds. As
-Lloyd Morgan in his excellent book on <i>Habit and Instinct</i> well says,
-'It does not pick at the seeds because instinct says to it that this
-is something to be picked up and tested, but because it cannot do
-anything else.'</p>
-
-<p>In the same way the instinct to seek for food wakes in the kitten
-at the sight of a mouse. I once set before a kitten which had never
-seen a mouse a living one in a trap. The kitten became greatly
-excited, and when I opened the trap and let the mouse run away she
-overtook and caught it in a few bounds. The instinct in this case
-does not express itself as in the chicken by the rapid lowering of the
-head and seizing the food, but in quite a different combination of
-movements, in running after and grasping the fleeing victim. But
-that is not all that is included in the instinctive action in the case of
-the cat, for there is also the whole wild and gruesome play, the
-familiar letting go and catching again, the passionate growling of
-satisfaction which, in its wildness, reminds us much more of a blood-thirsty
-tiger than of a tame domestic animal.</p>
-
-<p>As the egg-laying instinct of the female butterfly is excited only
-by the sight and odour of a particular plant, so also is the food
-instinct of the caterpillar. If we put a silkworm caterpillar (<i>Bombyx
-mori</i>) just out of the egg upon a mulberry leaf it will soon begin to
-gnaw at it; but put it on a beech leaf or on that of any other indigenous
-tree, shrub, or herb and it will not touch it, but simply die of
-hunger. And yet it could quite well eat many of these leaves, and
-thrive on them too, but the smell and perhaps also the sight of them
-is not the appropriate stimulus to liberate the instinct of eating.
-There are many species of caterpillar which are 'monophagous,' that
-is to say, restricted to a single species of plant in a country. One may
-ask how such a restriction of the liberating stimulus to a single
-species could have been brought about by natural selection, since it
-could not possibly be advantageous to be so much restricted in food.<span class="pagenum"><a id="Page_147"></a>[Pg 147]</span>
-The answer to this will be found in the following facts. On the
-Belladonna plant there lives a little beetle whose feeding instinct is
-aroused by this plant alone. But as <i>Atropa belladonna</i> is avoided
-entirely by other animals on account of its poisonousness, this beetle
-is, so to speak, sole proprietor of the Belladonna; no other species
-disputes its food, and in this there must assuredly be a great
-advantage, as soon as the other instincts, above all that of egg-laying,
-are so regulated as to secure that the larva shall have access to its
-food-plant; and this is the case. The monophagy of many caterpillars
-is to be understood in the same way; it is an adaptation to
-a plant otherwise little sought after, and it is combined with a more
-or less complete loss of sensitiveness to the stimulus of other plants.
-The establishment of such a specialized food-instinct depends on its
-utility, and has resulted from the preference given, through natural
-selection, to those individuals in which the food-instinct responded to
-the stimulus of the smallest possible number of plants, and at the
-same time to those which showed themselves best adapted to a plant
-especially favourable to their kind, whose food-instinct was not only
-most strongly excited by this one plant, but also whose stomach and
-general metabolism made the best use of it. So we understand why
-so many caterpillars live on poisonous plants, not only some of our
-indigenous Sphingidæ, like <i>Deilephila euphorbiæ</i>, but whole groups
-of tropical Papilionidæ, Danaides, Acræides, and Heliconiidæ. With
-this again is connected the poisonousness or nauseousness of these
-butterflies.</p>
-
-<p>How diversely the instinct to procure food may be developed in
-one and the same group of animals is shown by the fact that not
-infrequently plant-eating, saphrophytic, and flesh-eating animals
-occur in a single group of organisms, as, for instance, in the order of
-water-fleas or Daphnidae, or in the class of Infusorians. Many
-species find their food by making an eddy in the water, which brings
-a stream towards the mouth, and with it all sorts of vegetable or
-dead particles; others live by preying upon other animals like
-themselves.</p>
-
-<p>But even when the food-instinct in all the species of a group
-is directed towards living prey, the procuring of it may be achieved
-by means of quite different instincts. Such finer graduations of
-the food-instinct are found not infrequently within quite small groups
-of animals, as in the Ephemeridæ or Day-flies. All their larvæ live
-by preying on other animals, but those of one family, represented by
-the genus <i>Chloëon</i>, seek to secure their victims by agility and
-speed, while the larvæ of the second family, with the typical<span class="pagenum"><a id="Page_148"></a>[Pg 148]</span>
-genus <i>Baëtis</i>, have the instinct to press their smooth broad bodies,
-with large-eyed head, close to the brook pebbles on which they sit.
-They are exactly like these in colour, and thus they lurk almost
-invisible, until a victim comes within their reach, when they throw
-themselves upon it with a bound. The third group, with the typical
-genus <i>Ephemera</i>, follows its instinct to dig deep tubes in the mud
-at the bottom of the water, and to lurk in these for their prey. We
-have thus within this small group of Day-flies three distinct
-modifications of the food-instinct, which differ essentially from one
-another, are made up of quite
-different combinations of actions,
-and, consequently, must have their
-foundation in essentially different
-directive brain-mechanisms. All
-these cases have only one feature
-in common; the animals all throw
-themselves upon their prey as soon
-as they are near enough.</p>
-
-<div class="figleft" id="f35">
-<img src="images/fig35.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 31.</span> Sea-cucumber (<i>Cucumaria</i>), with<br />
-expanded tentacles (<i>a</i>), and protruded<br />
-tube-feet (<i>b</i>); after Ludwig.</p>
-</div>
-
-<p>But even this common feature
-is not everywhere part of the
-food-instinct. The sea-cucumber
-(<i>Cucumaria</i>) (Fig. 31), according
-to the observations made on it
-by Eisig in the Aquarium of the
-Zoological Station at Naples, gets
-its food in the following manner.
-The animal sits half or entirely
-erect on a projecting piece of rock
-and unfolds its ten tree-like tentacles
-which surround the mouth.
-These are branched, and have quite
-the effect of little tufts of seaweed.
-They are probably taken for such by many minute animals;
-for larvæ of all kinds, Infusorians, Rotifers and worms settle down
-on them. But the sea-cucumber bends inwards first one tentacle and
-then another, so slowly as barely to be noticeable, brings the point to
-its mouth, lets it glide gradually deeper into the gullet, until the
-whole tentacle is within, and after a time draws it out again equally
-slowly and unfolds it anew. Obviously it wipes the tentacle inside
-the gullet, and retains everything living that was upon it. This
-performance is repeated continually, day and night, and it is usually
-the only externally visible sign of life which the animal displays.</p>
-
-<p><span class="pagenum"><a id="Page_149"></a>[Pg 149]</span></p>
-
-<p>This remarkable instinct is associated with a structural peculiarity,
-for without the arborescent tentacles the fishing would not be nearly
-so successful. Other sea-cucumbers or Holothurians have different
-tentacles, and use them in quite a different manner, filling the mouth
-with mud by means of them.</p>
-
-<p>Very frequently, indeed, there are visible structural changes
-associated with the modified food-instinct. Most predatory fishes
-chase their prey, like the pike, the perch, and the shark, but there are
-also lurkers, and these show in addition to the lurking instinct certain
-definite bodily adaptations, without which this instinct could not have
-such full play.</p>
-
-<p>Thus in a marine fish known as the 'star-gazer' (<i>Uranoscopus</i>)
-the eyes are situated not on the sides but on the top of the head, and
-the mouth is also directed upwards. Its instinct leads it to bury
-itself in the sand so that only the eyes are uncovered. It lies in wait
-in this way until a suitable victim comes within reach, and then snaps
-at it with a sudden movement. But it also possesses a decoying organ,
-a soft worm-shaped flap, which it protrudes from the mouth as soon
-as little fishes draw near. They make for this bait, and are thus
-caught.</p>
-
-<p>Such ingenious fishing, which is quite suggestive of the human
-method of catching trout with artificial bait, occurs in many predatory
-fishes; but in every case the fish acts instinctively, without reflection,
-on becoming aware of approaching prey. The suitability of the
-action to the end does not depend upon consciousness of the end, or
-upon reflection, but is a purely mechanical action, performed in response
-to the stimulus of a sense-impression.</p>
-
-<p>This is best shown by the fact that the instincts may lead their
-possessors astray, which always happens when an animal is transferred
-to an unnatural situation, to which its instincts are not adapted, so to
-speak. The mole-cricket, which is in the habit of escaping pursuit by
-burrowing in the earth, makes violent motions with the forelegs, even
-if it be placed upon a plate of glass into which it could not possibly
-burrow; an ant-lion (<i>Myrmeleo</i>), whose instinct impels it to bore into
-loose sand by pushing backwards with the abdomen, goes backwards
-on a plate of glass as soon as danger threatens, and endeavours, with
-the utmost exertions, to bore into it. It knows no other mode of
-flight, and its intellect is much too weak to suggest any novel mode.
-Even the mode of escape most universal among animals, that of
-simply running away, does not occur to it; it acts as it must in
-accordance with its inborn instinct; it cannot do otherwise.</p>
-
-<p>The change of instincts in the different stages of development of<span class="pagenum"><a id="Page_150"></a>[Pg 150]</span>
-one and the same animal have always seemed to me very remarkable;
-for instance, the change of the food-instinct in the caterpillar and the
-butterfly, where the food-instinct is liberated in the caterpillar by the
-leaf of a particular plant, but in the butterfly by the sight and
-fragrance of a flower, the nectar of which it sucks. In this case
-everything is different in the two stages of development, the whole
-apparatus for seeking and taking food, as well as the nerve-mechanism
-which determines these modes of action. And how far apart often
-are the stimuli which liberate the instinct! The larva of the flower-visiting,
-honey-sucking <i>Eristalis tenax</i> is the ugly, white, so-called
-rat-tailed larva, well described by Réaumur, which lives swimming in
-liquid manure, and feeds on that! What complete and far-reaching
-changes, not only in the visible structure, but also in the finer
-nervous mechanisms, which we cannot yet verify, must have taken
-place in the vicissitudes of time and circumstance during the life-history
-of this insect!</p>
-
-<div class="figcenter" id="f36">
-<img src="images/fig36.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 32.</span> Metamorphosis of <i>Sitaris humeralis</i>, an oil-beetle, after Fabre.
-<i>a</i>, first larval form, much enlarged. <i>b</i>, second larval form. <i>c</i>, resting stage of
-this larva (so-called 'pseudo-pupa'). <i>d</i>, third larval form. <i>e</i>, pupa.</p>
-</div>
-
-<p>Not the food-instinct alone, but the instinct of self-preservation,
-of mode of motion, in short, every kind of instinct, may vary in the
-course of an individual life. Let us follow the somewhat complex
-life-history of a beetle of the family of the Blister-beetles or
-Cantharides, as we learnt it first from Fabre. The female of the
-red-shouldered bee-beetle (<i>Sitaris humeralis</i>) lays its eggs on the
-ground in the neighbourhood of the underground nest of a honey-gathering
-burrowing-bee (<i>Anthophora</i>). The larvæ, when they emerge,
-are agile, six-legged, and furnished with a horny head and biting
-mouth-parts, as well as with a tail-fork for springing (Fig. 32, <i>a</i>).<span class="pagenum"><a id="Page_151"></a>[Pg 151]</span>
-The little animals have at first no food-instinct, or at least none
-manifests itself, but they run about, and as soon as they see a bee of
-the genus <i>Anthophora</i> they spring upon it and hide themselves in its
-thick, hairy coat. If they have been fortunate the bee is a female,
-who founds a new colony and builds cells, in each of which she
-deposits some honey and lays an egg upon it. As soon as this has
-been done the <i>Sitaris</i> larva leaves its hiding-place, bites the egg of
-the bee open, and gradually eats up the contents. Then it moults,
-and takes the form of a grub with minute feet and imperfect masticating
-organs; the tail-fork, too, is lost, for all these parts are now
-useless, since it can obtain liquid nourishment without further change
-of place, from the honey in the cell, in exactly the quantity necessary
-to its growth. Then it spends the winter in a hardened, pupa-like
-skin, and it is not till the next year (the third), after another short
-larval stage (<i>d</i>) and subsequent true pupahood (<i>e</i>), that the fully-formed
-beetle emerges. This again possesses biting mouth-parts, and
-eats leaves, and has legs to run with and wings to fly with.</p>
-
-<p>In this beetle, then, the food-instinct changes three times in the
-course of its life; first the egg of the bee is the liberating stimulus,
-then the honey, and finally leaves. The instinct of moving about
-varies likewise, expressing itself first in running and jumping and in
-catching on, then in lying still within the cell, and, lastly, in flying
-and running about on bushes and trees.</p>
-
-<p>We can well understand that, in the course of innumerable
-generations and species of insects, the various stages of development
-would, by means of selection, become more and more different from
-each other, both structurally and in their instincts, as they adapted
-themselves better to different conditions of life; and thus ultimately
-instincts frequently and markedly divergent have been developed
-in the successive stages of life. No other interpretation is possible;
-through natural selection alone can we understand even the
-principle of such adaptations. An animal can thus very well be
-compared to a machine which is so arranged that it works correctly
-under all ordinary circumstances, that is to say, it performs all the
-actions necessary to the preservation of the individual and of its
-kind. The parts of the machine are fitted together in the best
-possible way, and work on each other so ingeniously that, under
-normal circumstances, a result suited to the end is achieved. We
-have seen how precisely the liberating stimulus for an action may be
-defined, and this secures a far-reaching specialization of instincts.
-But as every machine can work only with the material for which
-it was constructed, so the instinct can only call forth an action<span class="pagenum"><a id="Page_152"></a>[Pg 152]</span>
-effectively adjusted to its end when the animal is under natural
-conditions. Its specialization has its limits, and in this lies the
-reason of its limited purposiveness. For instance, if the larva of
-<i>Sitaris</i> were not impelled by the sight of every bee to spring on
-it and cling to it, but only by the <i>females</i>, then many of them would
-be saved from the fate that awaits them if they attach themselves to
-male bees, which make no nest, or even to other flying insects, in
-which case also there is no possibility of further development. But both
-these things happen, although the latter has not yet, to my knowledge,
-been recorded of <i>Sitaris</i>, but only of its relative, the larva of <i>Melöe</i>.</p>
-
-<p>'Instinct goes astray,' it is often said; but in truth it does not
-go astray, but is only not so highly specialized in relation to the
-liberating stimulus of the action as seems to us necessary for perfect
-purposiveness. But in this very imperfection there lies, as it seems to
-me, another proof that we have to do with the results of a process of
-selection, for it is of the very nature of these never to be perfect, but
-only relatively perfect, that is to say, just as perfect as is necessary
-to the maintenance of the species. At the moment at which this
-grade of perfection is reached every possibility of a further
-increase in the effectiveness of adjustment to the end ceases, because
-it would then no longer directly further the end. Why, for instance,
-should the liberating stimulus in this case be more highly specialized,
-since enough of the <i>Sitaris</i> larvæ already succeed in attaching themselves
-to female bees? It is not for nothing that the beetles of this
-family are so prolific; what is lacking in the perfection of the instinct
-is made up for by the multitude of young larvæ. A single female of
-the oil-beetle (<i>Melöe</i>) lays several hundred eggs.</p>
-
-<p>In speaking of the animal as a machine, it must be added that it
-is a machine which can be altered in varying degrees, which can
-be regulated to work at high or low pressure, slowly or quickly, finely
-or roughly. This regulating is the work of the intelligence, the limited
-'thinking-power,' which must be ascribed to the higher animals in
-a very considerable degree, but which in the lower animals becomes
-less and less apparent, until finally it is unrecognizable. That
-instinctive actions can be modified or inhibited by intelligence and
-will is proved by any trained beast of prey which masters its
-hunger and the impulse to snap at the piece of flesh held before
-it, because it knows that if it does not control itself painful blows
-will be the consequence. In a later lecture I shall return to the connexion
-between will and instinct; all that concerns us here is to
-regard instincts as the outcome of the processes of selection, and as an
-indirect proof of the reality of these.</p>
-
-<p><span class="pagenum"><a id="Page_153"></a>[Pg 153]</span></p>
-
-<p>From what I have already said at least so much must be clear,
-that nothing, in principle, stands in the way of referring instincts to
-selection, since their very essence is their adaptation to an end, and
-such purposive changes are precisely those that are preserved in the
-struggle for existence. It might, however, be supposed that in all
-this the principle of use and disuse also had a share, and that without
-it no changes in instincts could have come about.</p>
-
-<p>There are, however, numerous instincts in considering which this
-can be entirely excluded.</p>
-
-<p>At an earlier stage we discussed in detail the protective colourings
-which secure insects, and especially butterflies, from extermination by
-their numerous enemies, and it was mentioned that this was always
-accompanied by corresponding instincts, without which the protective
-colouring and the deceptive form would have profited nothing,
-or at any rate not nearly so much. If the caterpillar of the
-<i>Catocala sponsa</i>, which resembles the bark of an oak so deceptively,
-did not possess at the same time the instinct to creep away from the
-leaves and hide in the clefts of the bark on the trunk of the oak-tree,
-its disguise would be of very little use to it; and if the predatory and
-grass-coloured praying mantis was not impelled by instinct to lie in
-wait among the grass for its prey, instead of pursuing it, it would
-rarely succeed in seizing any of its victims, because of its somewhat
-leisurely mode of movement. This adaptation of the instincts to the
-protective colouring is carried into the most minute and apparently
-trifling details. Thus different observers have established the fact
-that the nauseous, sometimes even poisonous, butterflies, which are
-distinguished by their glaring or sharply contrasted colour-pattern,
-are all slow fliers. This is the case with the Danaides and Euplœides
-of the Old World and the Heliconiides of the New; many of their
-mimetic imitators also fly slowly.</p>
-
-<p>If we inquire how this instinct of fluttering, careless flight has
-come to be, we may leave habit as <i>primum movens</i> out of the
-question altogether, for there are no external conditions which could
-have induced the butterfly to take to slower flight than its ancestors
-exhibited. That it is now advantageous for it&mdash;since it acts as a
-signal of its nauseousness&mdash;to be as clearly seen and recognized as
-possible can exercise no direct influence on its manner of flight, since it
-knows nothing about it. Even if we assume that individual variations
-cropped up which had an instinct for slower flight, there would
-still, without selection, be no reason why this variation in particular
-should multiply, still less why the originally slight slowing of the
-flight should become more marked in the course of generations. On<span class="pagenum"><a id="Page_154"></a>[Pg 154]</span>
-the contrary, the butterflies fly a great deal, just as all other diurnal
-butterflies do; they exert their power of flight as long as the sun shines,
-and if the exercise of one generation influences the next, they ought to
-become gradually more capable of rapid flight. In this case exactly the
-opposite takes place to what is ascribed to the Lamarckian principle;
-more constant use must here have brought about a diminution of the
-activity of the relevant parts. It is quite otherwise when we look at
-it from the point of view of selection. The variants which cropped
-up by chance with slower flight survived because they were most
-easily recognized and avoided; they are the most frequent survivors;
-they leave descendants which inherit the slower flight-instinct, and this
-goes on increasing in them as long as the increase carries any
-advantage with it. As soon as this ceases to be the case the variation
-comes to a standstill, for it is adapted to the average of the conditions
-at a given time.</p>
-
-<p>We may picture to ourselves the thousand kinds of regulations of
-animal movements through instinct as having come about in a similar
-way; in the majority of cases we <i>must</i> picture it thus. For it is only
-in the case of those with high intelligence that we can ask whether
-the animal did not by deliberation help in establishing the purposive
-variation in its movements. Among insects in any case this could only
-be taken into account to a very limited extent, although I do not
-dispute that the more intelligent among them may learn, and may
-make experiments, and can modify their actions accordingly. But in
-fleeing from an enemy experience has nothing to do with it, for the
-first time it is caught it pays the penalty with its life. Without care,
-and with no idea of the dangers surrounding them on all sides, the
-butterflies float about, guided only by their instinct, which, however,
-is so exactly adapted to the conditions of their life that a sufficient
-number of them to preserve the species always happily escapes all the
-many dangers. I may remind you of Hahnel's case of the butterfly,
-already mentioned, which escaped the agile lizard by flying rapidly
-up from the sweet bait, but settled again upon it without fear
-immediately afterwards, to fly from the lizard as before, and did so
-several times in succession. We usually judge such actions far too
-much from the human standpoint; the butterfly does not wish to
-escape the death which threatens it; it knows nothing about death;
-it is not with it as it was with Dr. Hahnel himself, who when he was
-once in danger from a jaguar in a thicket was so affected by the
-thought of the death he had happily escaped that he never cared to
-pass the place again, but made a long circuit to his home. The
-butterfly does not act according to reflection and imagination; it flies<span class="pagenum"><a id="Page_155"></a>[Pg 155]</span>
-up with lightning-like rapidity when the lizard rushes at it, because
-this rapid movement, which it <i>sees</i>, acts as the stimulus which liberates
-the flight-instinct, and this works so promptly that in most cases the
-insect is rescued from danger. Its disposition, however, is not otherwise
-affected by its narrow escape, and it obeys anew the food-instinct
-which impels it to settle again on the bait, until the flight-instinct is
-again set a-going by the visual impression of the re-advance of the
-lizard. It is the plaything of its instincts, a machine which works
-exactly as it must. That it is only sense-impressions and not conceptions
-which here liberate the actions can be well seen in the case of
-shy species of butterfly like our purple emperor (<i>Apatura iris</i>), which
-flies up like lightning from the moist wood-paths on which it loves to
-settle as soon as any rapidly moving visual image, even if it be
-only a shadow, strikes its eyes. For this reason the collector tries to
-approach it so as not to throw his shadow before him, for then the
-insect lets the advancing enemy get quite close, and only flies up when
-the net is quickly thrust towards it. In all probability the eye of
-this insect is particularly well adapted for perceiving movements, and
-certainly the flight-instinct reacts very promptly to such visual impressions,
-and we can understand that it must have been so regulated
-if, as we assume, the regulation came about through processes of
-selection, for the enemies of the butterflies, such as birds, dragon-flies,
-and lizards shoot quickly out on their prey, and therefore those
-butterflies must always have survived whose instinct impelled them
-to take to flight most quickly.</p>
-
-<p>In this, then, as in a thousand other cases, the instinct of flight,
-or indeed any other mode of movement, cannot be interpreted as an
-'inherited habit,' because there is no evidence of the possession of
-that degree of intelligence which could have induced the variation in
-the previous habit, that is, in manner of movement. The same is true
-of animals of low intelligence in regard to all the other instincts,
-which otherwise might seem to be explicable in terms of the
-Lamarckian principle.</p>
-
-<p>In addition, there is a whole large group of instincts in regard
-to which the idea of the Lamarckian principle cannot be entertained,
-as I showed years ago, and it consists of all those instincts which are
-only exercised once in the course of a lifetime. These cannot
-possibly depend on practice in an individual lifetime, and transmission
-of the results of this exercise to the following generation;
-they can therefore only be interpreted in terms of selection, unless
-we are to give up all attempts at a scientific interpretation, and simply
-accept them as 'marvels.'</p>
-
-<p><span class="pagenum"><a id="Page_156"></a>[Pg 156]</span></p>
-
-<p>To this class belong all the diverse instincts by which insects protect
-themselves against attack during the pupa stage. Even the way in
-which the caterpillars of many diurnal butterflies hang themselves up
-in pupation is not by any means a very simple instinctive action.
-The caterpillar first spins, in a suitable place, a small round disk
-of silk threads, to which it then attaches the posterior end of its
-body, so securely that it cannot be easily torn away. More complicated
-still is the securing of the pupa when it does not hang freely,
-but is to remain pressed against a wall or a tree, as is the case in the
-Papilionidæ and the Pieridæ. In this case the caterpillar must, in
-addition to the usual cradle, spin a thread of silk, in an ingenious
-way, diagonally across the thorax, so that it may cross about the
-middle of the wing rudiments, and not be too loose, lest the pupa fall
-out, yet not too tight, lest the thread cut too deeply into the wing
-rudiments and hinder their development. When one remembers that
-it is the caterpillar that does all this, before it has taken the form
-of the pupa, and that it must all be adapted to the pupa's form, we
-are amazed at the extraordinary exactness with which instinct
-prescribes all the individual movements which make the whole of the
-complex performance effective. And yet, as each caterpillar only
-accomplishes this performance once in its life, it could at no time
-in the development of the species have become a habit in the case of
-any individual caterpillar, and it cannot therefore be an 'inherited
-habit.'</p>
-
-<p>But however diverse are the methods of securing the safety of
-the pupæ in the different families of butterflies, they must all be
-referred back to a single root, if the butterfly pedigree can be traced
-back to a single ancestral group. The caterpillar of the Sphingidæ
-does not creep up walls and trees when it is ready to enter on the
-pupa stage, as so many of the caterpillars of the diurnal butterflies do,
-but instead its instinct compels it to run about on the ground until it
-has found a spot which seems to it suited for boring into the earth,
-or, to speak less metaphorically, until it comes to a place which, from
-its nature, acts as a liberating stimulus to the instinct to burrow.
-Then it penetrates more or less deeply, according to the species, and
-makes a small chamber, which it lines with silken threads to prevent
-it collapsing; this done, it moults, and enters on the pupa stage.
-The exactness with which the individual movements are prescribed
-by instinct is seen in the way in which the size of the chamber is
-regulated so as to be exactly as large as is necessary to give the pupa
-room enough without leaving any superfluous free space. This is not
-so simple as it seems, and is not directly conditioned by the size of the<span class="pagenum"><a id="Page_157"></a>[Pg 157]</span>
-animal, for the caterpillar is longer and altogether of greater volume
-than the pupa. The same thing is seen in the stag-beetle (<i>Lucanus
-cervus</i>), the largest of our indigenous beetles, which gets its name
-from the powerful antler-like jaws which distinguish the male. It
-also undergoes its pupal metamorphosis in the earth, and makes
-a large hard ball of clay, hollow inside, and as smooth as if polished,
-and its cavity is exactly the size of the future pupa, or to speak more
-precisely, of the fully-formed beetle. For, as Rösel von Rosenhof in
-his day 'observed with amazement,' the balls in which the males lie
-have a much longer cavity than those built by the females, and for
-this reason, that when the fully-formed beetle emerges from the
-pupa it must, if it is a male, have room to stretch out its horns,
-which have till then lain upon the breast. 'For the beetles do not
-leave their dwelling-place until all their parts are sufficiently strong
-and properly hardened, and till the season has arrived in which they
-are wont to fly about.' The male larva thus makes a much longer
-pupa-house than the female larva, in anticipation, so to speak, of the
-enormous size of the jaws which will grow out later!</p>
-
-<p>Here the instinct has two modes of expression, according as the
-bodily parts are male or female. Here we have to do with an action
-which is performed once in a lifetime, and thus the possibility of
-any other explanation of the origin of this instinct than through
-natural selection is excluded.</p>
-
-<p>Not less significant is the case of the silk-cocoons. The cocoons
-spun by the silkworm are egg-shaped, and consist of a single thread
-many thousand yards in length, which is wound round the spinning
-caterpillar so that not a space is left uncovered. The web is firm,
-tough, and very difficult to tear; therefore we must grant that the
-pupa resting within will enjoy a very considerable degree of security
-against injury. But the moth must be able to get out, and that this
-may be possible the caterpillar is impelled by instinct to make its
-spinning movements such that the cocoon is eventually looser at the
-anterior end, so that the insect, when it is ready to emerge, can tear
-it asunder with its feet and make a way out for itself. For this very
-reason, because the silk must be torn and spoilt by the emerging
-insect, silk-breeders kill the pupating insect before it begins to make
-its way out.</p>
-
-<p>But there are species whose cocoons are provided from the very
-start with an outlet, for the caterpillar spins the silk round itself in
-such a way that a round opening is left. But this opening would
-be not only a convenient door for the butterfly to emerge by, but an
-equally convenient entrance for all its enemies. It is, therefore, closed<span class="pagenum"><a id="Page_158"></a>[Pg 158]</span>
-up. In the case of the 'emperor moth' (<i>Saturnia carpini</i>) this is
-effected by means of a circle of stiff bristles of silk on the inside
-(Fig. 33), the points of which bend outwards like those of a weir-basket
-(<i>r</i>); from the inside the emerging moth can easily push aside
-the bristles, while the threatening enemy from without is scared off
-by the stiff points of the bristles.</p>
-
-<div class="figleft" id="f37">
-<img src="images/fig37.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 33.</span> Cocoon of the Emperor Moth<br />
-(<i>Saturnia carpini</i>), after Rösel. <i>A</i>, enclosed<br />
-pupa. <i>B</i>, emerging moth. <i>r</i>, hedge of bristles.<br />
-<i>fl</i>, wings.</p>
-</div>
-
-<p>Such a cocoon is comparable to a work of art in which every
-part harmonizes with the rest, and all together are adapted as well as
-possible to their purpose. And yet it is all accomplished without the
-caterpillar having the remotest conception of what it is aiming at
-when it winds the endless silken thread about itself in the artistic
-and precisely prescribed coils. Nor has it any time for trying
-experiments or for learning; it must make all the complex bendings
-and turnings of the head which
-spins the thread, and of the
-anterior part of the body which
-guides the thread, quite exactly
-and correctly the first time if
-a good cocoon is to be produced.
-Here every possibility of interpreting
-this instinct as 'an
-inherited habit' is excluded, for
-each caterpillar becomes a pupa
-only once; and it is just as
-impossible to suppose that it can
-be directed by intelligence, since
-it can neither know that it
-is about to become a pupa, nor
-that, in the pupa stage, it will be in danger from enemies which will
-attempt to force their way into the cocoon, nor that the hedge of
-bristles will protect it from such enemies. Our only clue to an interpretation
-is in the slow process by which minute useful variations in
-the primitive instinct of spinning are accumulated through selection;
-and it is wonderful to see how exactly these spinning powers are
-adapted to the particular life-conditions of individual species.</p>
-
-<p>Thus there are several of the Saturnides whose enormous caterpillars
-live on large-leaved trees, and these make use of the large
-leaves to form a shelter for the pupa stage, spinning them together so
-that the cocoon is for the most part surrounded by leaf. But as the
-leaf might easily fall off with the weight of the pupa, they make
-the leaf-stalk fast to the twig from which it grows by binding the
-two firmly together with a broad, strong, closely-apposed silken band.<span class="pagenum"><a id="Page_159"></a>[Pg 159]</span>
-Seitz relates of the largest of all these spinners, the Chinese <i>Attacus
-atlas</i>, that this silk sheath 'is continued to the nearest strong branch,
-so that it is impossible with the hand to detach the leaves that conceal
-an Atlas-pupa from the tree.' To be sure, this pupa weighs
-about eleven grammes!</p>
-
-<p>Since instincts vary, as well as the visible parts of an animal,
-a fulcrum is afforded by means of which selection can bring about all
-these very special adaptations to given conditions, since it always
-preserves for breeding the best suited variations of an already
-existing instinct. Any other interpretation is once more excluded.</p>
-
-<p>The same may be said of insects and their egg-laying. This, too,
-is in many cases only performed once in a lifetime, and the insect
-dies before it has seen the fruit of its labour. Yet egg-laying is
-performed in the most effective manner, and with the most perfect
-security of result. It seems as if the insect knew, so to speak,
-exactly where, in what numbers, and how it should lay its eggs.
-Many Mayflies (Ephemeridæ) let their eggs fall all at once into the
-water in which the larvæ live; many Lepidoptera, such as <i>Macroglossa
-stellatarum</i>, lay their eggs singly, and on definite plants&mdash;the humming-bird
-hawk-moth, just referred to, on <i>Galium mollugo</i>; others, like
-<i>Melitæa cinxia</i>, lay their eggs in heaps on the leaves of the way-bread
-(<i>Plantago media</i>), or, like <i>Aglia tau</i>, on the bark of a large beech-tree.
-Nothing in these different modes of egg-laying is due to chance
-or caprice; all is determined and regulated by instinct, and all, as far
-as we can see, is as well adapted to its purpose as possible. When,
-for instance, <i>Macroglossa stellatarum</i> lays her eggs singly, or in twos
-or threes, on the green leaves of the food-plant, it thereby obviates
-the danger of scarcity of food for the comparatively large caterpillars,
-since not many of them could subsist together on a single plant of
-Galium, while <i>Aglia tau</i> can place several hundred eggs on the same
-beech-tree trunk without having to fear that its caterpillars will not
-find abundant nourishment. The precision with which the egg-laying
-instinct works is even greater in other species in which there are
-more special requirements, e.g. when the eggs have to be laid on the
-under side of the leaves, as in <i>Vanessa prorsa</i>, or where they have to
-be cemented together in a little pillar, so that they bear a deceptive
-resemblance to the green flower-buds of the food-plant (the stinging-nettle).</p>
-
-<p>It is certainly astonishing how exactly the stimulus in these
-cases is specialized to the liberation of the instinct. In general the
-smell of the food-plant of the caterpillar is enough for most butterflies,
-and this attracts the female ready to deposit its eggs, but<span class="pagenum"><a id="Page_160"></a>[Pg 160]</span>
-complete liberation of the instinct is only effected by the visual
-impression of the under side of the leaf. We cannot but be
-astonished that there is room for such finely graded nerve-mechanisms
-in the little brain of a butterfly, and yet it would be easy enough to
-adduce still more complex instincts connected with oviposition in
-insects. The large water-beetle, <i>Hydrophilus piceus</i>, lays its eggs on
-a floating raft made by itself; the gall-wasps must first pierce with
-their ovipositor into a particular part of a particular plant to be able
-to lay the eggs in the proper place, and this in no haphazard way but
-with great carefulness and in a perfectly definite manner. But there
-is no necessity to refer here to many or to the most complicated
-cases of egg-laying; I only wish to show that, even in the simple
-cases, such as that of the butterflies just referred to, there is
-a precisely regulated combination of actions which is executed
-mechanically, and which cannot be interpreted as inherited habit,
-because it never was a habit in any individual of any generation.</p>
-
-<p>It is thus placed beyond the possibility of doubt that very many
-instincts, at least, must depend on selection, and it would be useless
-to go further in this direction by extending our survey to other
-groups of instincts. I shall, however, return later on to the study of
-instincts, and, after we have become acquainted with the main
-features of the laws of inheritance, it will then be seen that, even
-among higher animals, instincts can never be interpreted in terms of
-the Lamarckian principle.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_161"></a>[Pg 161]</span></p>
-
-<h2 class="nobreak" id="LECTURE_IX">LECTURE IX</h2>
-</div>
-
-<p class="c">ORGANIC PARTNERSHIPS OR SYMBIOSIS</p>
-
-<div class="blockquot">
-
-<p>Hermit-crabs and sea-anemones&mdash;Hermit-crabs and hydroid polyps&mdash;Fishes and
-sea-anemones&mdash;Green fresh-water polyps&mdash;Green Amœba&mdash;Sea-anemones and yellow
-Algæ&mdash;Cecropia trees and ants&mdash;Lichens&mdash;Root fungi&mdash;Origin of Symbiosis&mdash;Nostoc
-and Azolla apparently contradict the origin through natural selection.</p></div>
-
-
-<p><span class="smcap">We</span> have already seen, by means of many examples, to what a
-great degree animals and plants are able to adapt themselves to new
-conditions of life; how animals imitate their surroundings in colour
-and form, how instincts have varied in all directions, how plants have
-made use of the chance of frequent contact with little animals to
-obtain nourishment from them, and have developed contrivances
-adapted for bringing as many of these as possible into their power
-and causing them to yield them the largest possible amount of food.
-A great many of these could only be interpreted in terms of natural
-selection, and in others it seemed at least very probable that selection
-was one of the factors in bringing them about.</p>
-
-<p>Particularly clear proof of the reality of natural selection is
-afforded by those cases where one form of life associates itself with
-a very different one so intimately that they are dependent on one
-another and cannot live without one another&mdash;at least in extreme
-cases&mdash;and that new organs, and, indeed, new dual organisms, are
-sometimes produced by this interdependence of life. This phenomenon&mdash;so-called
-'Symbiosis'&mdash;was discovered by two sharp-sighted
-botanists, Anton de Bary and Schwendener. But Symbiosis occurs
-not only between plants; it occurs also between plants and animals
-and between two species of animal, and we understand by it a life of
-partnership depending on mutual benefits, so that each of the two
-species affords some advantage to the other, and makes existence
-easier for it. In this respect Symbiosis differs from Parasitism, in
-which one species is simply preyed upon by another without receiving
-any benefit from it in return, and also from the more innocent
-Commensalism of Van Beneden, the table-companionship in which
-one species depends for its existence on the richly-spread table of
-another. Symbiosis is particularly interesting, because, in addition to
-extreme cases with marked adaptations, many occur which are of<span class="pagenum"><a id="Page_162"></a>[Pg 162]</span>
-great simplicity, and which seem to have brought about almost no
-change in the two associated species.</p>
-
-<p>We shall take our first examples from the Animal Kingdom.</p>
-
-<p>The partnership between certain sea-anemones (Actiniæ) and
-hermit-crabs (Paguridæ) had been noticed long before any particular
-attention was devoted to it. Many species of hermit-crab frequently
-carry a large sea-anemone about with them on the mollusc shell
-which they use as a protecting-house; indeed, two or three of these
-beautiful many-tentacled polyps are often attached to them, and this
-is not at all a matter of chance, but depends upon instinct on the part
-of both animals; they have the feeling of belonging to each other. If
-the sea-anemone be taken away from the hermit-crab and put in a
-distant part of the aquarium, the crab seeks about till it finds it, then
-seizes it with its claws and sets it on its house again. The instinct to
-cover itself with Actiniæ is so strong within it that it loads itself
-with as many of these friends as it can procure, sometimes with more
-than there is room for on the shell. The sea-anemone on its part
-calmly submits to the crab's manipulations&mdash;a fact very surprising to
-any one who is aware of the anemone's ordinarily extreme sensitiveness
-to contact, and knows how it immediately draws itself
-together on any attempt to detach it from the ground, and will often
-let itself be torn in pieces rather than give way. The mutual instincts
-of the two creatures are thus adapted to each other; but it does not
-at first sight seem as if any structural changes had taken place in
-favour of the partnership. This is true, indeed, as regards the hermit-crab,
-but not as regards the sea-anemone, although the nature of the
-adaptation on the sea-anemone's part only becomes apparent when
-the two animals are closely observed in their life together.</p>
-
-<p>We owe our understanding of this adaptive change in the sea-anemone,
-and, indeed, our knowledge of this whole case of Symbiosis,
-to the beautiful observations of Eisig. Starting from the hypothesis
-that the mutual relations could only be the outcome of natural
-selection, Eisig pointed out that this partnership must offer some
-advantage not to one partner only, but to both; otherwise it could not
-have arisen through selection. The advantage to the sea-anemone is
-obvious enough; since of itself it can only move very slowly, and is
-usually firmly fixed in one place, it is easy to see that it would be
-useful to it to be carried about on the floor of the sea by the hermit-crab,
-and to get its share of the hermit-crab's food. But the service
-yielded to the hermit-crab by the sea-anemone in return is not nearly
-so apparent. Eisig made an observation in the Zoological Station at
-Naples which solved this riddle. He saw an octopus attack the<span class="pagenum"><a id="Page_163"></a>[Pg 163]</span>
-hermit-crab and attempt to draw it out of its shell with the point of
-one of its eight arms. But before this had succeeded there sprang
-from the body of the sea-anemone a large number of thin worm-like
-threads which spread over the arm of the robber, who immediately let
-go his hold of the crustacean and troubled himself no further with it.
-These threads, called acontia, are thickly beset with stinging-cells,
-which must at least cause a violent smarting on the soft skin of the
-octopus. Thus we see that the Actinia instinctively defends its
-partner from attacks, and does it so effectively that we need not
-wonder how the instinct to provide itself with Actiniæ could have
-arisen in the hermit-crab. But the acontia seem to have been greatly
-strengthened in the course of the sea-anemone's association with
-hermit-crabs, for they do not occur in all forms, and they are most
-highly developed in those which live in Symbiosis with crustaceans.</p>
-
-<div class="figcenter" id="f38">
-<img src="images/fig38.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 34.</span> Hermit-crab (<i>E</i>), within a Gasteropod shell, on which a colony
-of <i>Podocoryne carnea</i> has established itself. From the common root-work (which
-is not clearly shown) there arise numerous nutritive polyps with tentacles (<i>np</i>),
-among which are smaller 'blastostyle' polyps with a circle of medusoid buds
-(<i>mk</i>), spine-like personæ (<i>stp</i>), and on the margin of the mollusc shell a row
-of defensive individuals (<i>wp</i>). <i>F</i>, antennæ. <i>Au</i>, eyes of the hermit-crab;
-slightly enlarged.</p>
-</div>
-
-<p>In this case the structural change, the transformation of the
-mesenteric filaments that occur in all Actiniæ into projectile acontia,
-is comparatively slight, but in another partnership between hermit-crabs
-and polyps the latter have undergone a much more marked
-adaptation. At Naples <i>Eupagurus prideauxii</i> is one of the commonest
-hermit-crabs. It lives at a depth of about a hundred feet,
-and is often brought to the Zoological Station by the fishermen in
-large quantities. Its borrowed mollusc shell often bears a little
-polyp, <i>Podocoryne carnea</i> (Fig. 34), which forms colonies of often<span class="pagenum"><a id="Page_164"></a>[Pg 164]</span>
-several hundred individuals, arising from a common root-work of
-stolons which covers the shell. The polyp colony is composed of
-different kinds of individuals or personæ, illustrating the principle of
-division of labour: it includes (1) nutritive persons (<i>np</i>) which possess
-a proboscis, mouth, and tentacles on their club-shaped bodies; (2)
-much smaller blastostyles (<i>bl</i>), that is to say, polyps with degenerate
-mouth and tentacles, which are wholly given over to the production of
-buds (<i>mk</i>), which then develop into sexual animals, little free-swimming
-medusoids; and (3) protective personæ in the form of hard spines
-(<i>stp</i>), beneath the shelter of which the soft polyps withdraw when the
-mollusc shell is rocked about on the sea-floor by the rolling of the
-waves. In addition to these three different kinds of individuals or
-personæ there are also (4) defensive polyps (<i>wp</i>) of long, thread-like
-shape, thickly set with stinging-cells, but possessing neither mouth
-nor tentacles. It might at first be thought that these are for the
-defence of the colony, but this is not so; the fact is that they rather
-serve for the direct defence of the hermit-crab. This is indicated by
-the position they occupy in the colony; they are not regularly distributed
-over the surface, but are ranged round the edge, and, indeed,
-only on the edge which surrounds the opening of the mollusc shell.
-Here these defensive polyps stand in close array, sometimes spirally
-contracted, sometimes hanging loosely down over the hermit-crab like
-a fringe. Their function, like the acontia of Actiniæ, is to defend the
-crab when an enemy tries to follow it within the shelter of its
-domicile. This can easily be demonstrated by drawing out the
-hermit-crab from the Gasteropod shell, and, when the colony has
-settled down again, seizing the shell with the forceps and drawing it
-slowly through the water. The water-stream which then flows upon
-the shell mimics the attack of an enemy, and immediately all the
-defensive polyps, as at a given signal, strike from above downwards,
-and repeat this three or four times; they are scaring off the supposed
-enemy.</p>
-
-<p>In this species of polyp a special form of individual has developed
-with a quite definite position in the colony, and furnished with a
-special instinct or reflex mechanism which is directly useful only to
-the crab, and has therefore, in a sense, arisen for its advantage. This
-can quite well be explained through natural selection, for indirectly
-these polyps are also of use to the colony, inasmuch as they protect
-their valuable partner, and thus render it possible for the hydroid
-colony to make the partnership of use to the hermit-crab as well as to
-itself.</p>
-
-<p>This mutual arrangement thus satisfies the requirement which,<span class="pagenum"><a id="Page_165"></a>[Pg 165]</span>
-from the selectionist point of view, must be made in regard to all
-that is new&mdash;that it must be useful to its possessor.</p>
-
-<p>If it be asked what service the hermit-crab renders to the polyp
-colony in return, the answer is that, as in the symbiosis with sea-anemones,
-the hermit-crab carries the polyps to their food, which
-is also its own. Hermit-crabs eat all sorts of animal food, living
-or dead, which they find on the sea-floor, and the remains of their
-meal fall to the share of the polyps. Once, without special intention,
-I laid a hermit-crab with its polyp colony in a flat vessel of sea-water
-beside a bright green living sponge. After some time the majority of
-the polyps had become bright green; they had filled themselves with
-the green cells of the sponge.</p>
-
-<p>I do not know how else we should picture to ourselves the origin
-of symbiotic instincts in such lowly animals except through the
-transmission and augmentation of variations in the instincts of the
-two partners&mdash;variations which made their possessors more capable
-of survival. Mollusc shells, ever since there were any, must
-have served as a foundation and point of attachment for polyp
-colonies; as a matter of fact, we find to-day on mollusc shells many
-kinds of polyp colonies which show no special adaptation to a life of
-partnership with hermit-crabs. From such indifferent associations
-a symbiotic one must gradually have been evolved in some instances,
-through the preservation and augmentation of every useful variation,
-both of instincts and reflex actions, as well as of form and structure.
-I shall not attempt to trace the course of this evolution in detail, but
-it is obvious that the development of defensive polyps, and of their
-instinct to defend the crustacean, can be interpreted neither as due
-to any direct influence nor as due to the effect of use, but only to the
-utility of this arrangement, the beginnings of which&mdash;polyps with
-stinging-cells&mdash;were already present. Their augmentation and
-perfecting must be referred entirely to natural selection. It is the
-same with adaptations which do not refer directly to the crustacean
-partner, but rather to the disposition of the polyps on the shell. The
-spinous personæ which protect the softer polyps from being crushed
-by being rolled about on the pebbles by the waves cannot possibly
-be regarded as the direct result of this crushing. But it is obvious
-that some such colonies must have had among their members some
-with a stronger external skeleton, and therefore less easily crushed
-than the rest, and this would lead to their more frequent survival.</p>
-
-<p>No adaptation seems to have taken place in the hermit-crab in this
-case, but that is probably only apparently the case; the probability
-is that it would not tolerate the presence of the polyp-colony on the<span class="pagenum"><a id="Page_166"></a>[Pg 166]</span>
-shell unless its instinct compelled it thereto, just as its instinct impels
-it to cover itself with sea-anemones, and fearlessly to grasp the
-dangerous animal, which, however, only shows its partner its softer
-side. Truly, such transformations of instinct are wonderful enough,
-but that they should have come about through intelligence is here
-quite inconceivable; there remains nothing but natural selection.</p>
-
-<p>A case in which no apparent corporeal adaptations have occurred,
-but which depends altogether on slight modifications of the instincts,
-is afforded by the well-known relations between ants and aphides.
-These two groups of insects live in a kind of symbiosis, although
-they are by no means inseparably connected with each other.
-Wherever strong colonies of aphides cover the young shoots of
-a plant, such as a stinging-nettle, a rose, or an elder, we almost always
-find ants which walk cautiously about among the plant-lice, often in
-great numbers, stopping now and again to stroke them with their
-antennæ, and then licking up the sweet juice from the intestine which
-they now give forth. Darwin showed by experiment that the
-aphides retain this juice if no ants are on the spot, and only give
-it off when ants are put beside them. Herein lies the proof that we
-have again to do with a case of modification of instincts. This
-juice is, of course, not the secretion of special glands, as it was still
-believed to be in Darwin's time, and it does not come from the
-so-called 'honey-tubes' situated on the back of the abdomen of the
-aphides; it is simply their excrement, which is liquid like their food,
-and the voiding of it has become instinctively connected with the
-presence of the friendly ants.</p>
-
-<p>That the aphides are not in any way afraid of the ants implies,
-in itself, a modification of their instinct, for these poisonous insects,
-prone to biting, are otherwise much dreaded in the insect world.
-Moreover, the aphides, harmless as they seem, are not quite without
-means of defence, although these are never used against the ants.
-Other animals which approach them they bespatter with the sticky,
-oily secretion prepared in the so-called 'honey-tubes' already noted,
-squirting it especially into the eyes of an assailant, so that the attack
-is abandoned.</p>
-
-<p>Of course the aphides have no idea wherein the utility of their
-friendship for the ants consists, but it is not difficult for us to
-discover it, since the ants, by their mere presence in the aphid
-colony, frighten and keep off their enemies. We see, then, that
-the conditions for a process of natural selection are here afforded: the
-instinct to be friendly to the ants is thoroughly useful, and the
-instinct of the ants to seek out the aphides, and, instead of devouring<span class="pagenum"><a id="Page_167"></a>[Pg 167]</span>
-them, to 'milk' them, is also advantageous; it must be an old
-acquisition, an instinct early developed, for in several species it has
-gone so far that the aphides are carried into the ants' nest, and are
-there (as one might say) kept and tended as domesticated animals.</p>
-
-<p>A pretty case of symbiosis between two animals is reported by
-Sluiter, and I mention it because it concerns a vertebrate animal,
-and intelligence has something to do with it. In the neighbourhood
-of Batavia there are frequently to be found on the coral reefs large
-yellow sea-anemones, with very numerous and comparatively long
-tentacles, and a little brightly-coloured fish, of the genus <i>Trachichthys</i>,
-makes use of these forests, beset with stinging-cells, to find security
-from its enemies. These appear to be numerous, for in an aquarium,
-at any rate, the little fish very soon falls a victim to one or other
-of them, unless he is supplied with the protective sea-anemones.
-When this is the case it swims blithely about among the tentacles,
-and the sea-anemone does not sting it; for there has been a modification
-of instinct on its part as well as on that of the fish. The
-advantage it gains from the fish is, that the latter brings large
-morsels of food&mdash;in the aquarium, pieces of meat&mdash;into the anemone's
-mouth. In doing so it tears away fibres for itself, and even if the
-Actinia has swallowed pieces too quickly, the fish pulls them half out
-of the gullet again, and only relinquishes them to be consumed by its
-partner when it has satisfied its own appetite. In this case, again,
-the modification of the instinct is the only adaptation which has been
-brought about by the symbiosis, and its origin seems difficult to
-understand. How can the fish have first formed the habit of putting
-its prey into the mouth of the anemone instead of eating it directly?
-Although in many cases it is difficult to guess at the beginnings of
-a process of selection, because they are scarcely discoverable in
-the subsequently accumulated variations, yet in this instance we may
-perhaps picture them to ourselves in this way: The fish was in
-the habit of letting fall pieces of food which could not be swallowed
-whole, and of diving down upon them repeatedly, to tear off
-a fragment each time. As the sea-floor in flat places is often covered
-with sea-anemones, these pieces would often sink down upon one,
-which would welcome it as a dainty, and set about swallowing it,
-slowly in its own fashion. The fish must then have found by
-experience that it could tear off little bits much more easily from
-a piece that was held firmly by the anemone than from one that was
-lying loose upon the ground, and this may have caused it to do
-intentionally what was at first done by chance. But the sea-anemone,
-suffering no harm from the fish&mdash;indeed, its association of<span class="pagenum"><a id="Page_168"></a>[Pg 168]</span>
-ideas, if I may use the expression, must rather have been little fishes
-and unexpected food&mdash;had no cause to shoot its microscopic arrows
-at it, and did not do so even when the fish concealed itself among
-the tentacles. This latter habit on the part of the fish would
-be developed into an instinct through natural selection, since the
-individuals that most frequently exhibited it would be the best
-protected, and therefore, on an average, the most likely to survive.
-Whether the benevolent attitude of the anemone towards the fish is
-to be regarded as the expression of an instinct is open to dispute, for
-it is quite conceivable that each individual sea-anemone is disposed to
-gentleness by the behaviour of the fish, and so the development of
-a special hereditary instinct was unnecessary, because without it each
-anemone reacted in the manner most likely to secure its own
-advantage<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a>.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> Since the above was written Plate has observed several similar cases in the Red
-Sea. A little fish lives along with the anemone, <i>Crambactis aurantiaca</i>, a foot in size, and
-not only conceals itself among its tentacles, but remains among them when the anemone
-draws them in. These fishes, therefore, must be immune against the stinging-cells of
-the sea-anemone; and in the same way another species of fish appears to be immune
-from the strong poison secreted by sea-urchins of the genus <i>Diadema</i> from the points
-of their spines, among which the fishes live. This relation certainly seems more like
-a one-sided adaptation on the part of the fishes than a true symbiosis, but in the cases
-observed by Sluiter the return service of the fishes seems to be regularly rendered.
-Here, as everywhere else in nature, there are transition stages, and a one-sided
-protective relation may gradually, under favourable circumstances, be transformed
-into a symbiosis.</p>
-
-</div>
-
-<p>The same may be true of the fish as far as laying its booty in
-the mouth of the anemone is concerned; there may be no inherited
-instinct in this; it may be an intelligent action, which is learnt anew
-in the lifetime of each individual.</p>
-
-<p>It might of course be objected to this interpretation that the
-beginning of the process, namely, the assumption that chance fragments
-from the food of the fish falling just on the anemone is very
-improbable; but I once observed that flat rocks washed over by the
-sea on the Mediterranean coast (not far from Ajaccio) were so thickly
-covered with green anemones that at first I took the green growth for
-some strange sea-grass new to me until I had pulled up a little tuft
-of the supposed plants and identified them as the soft tentacles of
-<i>Anthea cereus</i>. Anemones must be equally abundant in the tropical
-seas of Java, and a sinking fragment must often alight on the mouth
-of one of them.</p>
-
-<p>Much attention and keen discussion have in the last few decades
-been focussed on cases of symbiosis between unicellular Algæ and
-simple animals. A good example is our green fresh-water polyp,
-<i>Hydra viridis</i> (Fig. 35, <i>A</i>). Its beautiful colour is due to chlorophyll,
-and it was long a matter of surprise that animals should produce<span class="pagenum"><a id="Page_169"></a>[Pg 169]</span>
-chlorophyll, which is a characteristic and fundamental important substance
-of assimilating plants, until Geza Entz and M. Braun demonstrated
-that the green did not belong to the animal at all, but to
-unicellular green Algæ, so-called Zoochlorellæ, which are embedded
-in the endoderm cells of the polyps in great numbers (Fig. 35, <i>zchl</i>).
-As these algoid cells assimilate, and thus liberate oxygen, their presence
-is of advantage to the polyp. That&mdash;as was at first believed&mdash;they
-also yield nourishment to the polyp I consider very probable, notwithstanding
-the apparently opposed results of the experiments of so
-acute an observer as von Graff, for I have seen a large number of these
-animals thrive for months, and multiply rapidly by budding in pure
-water which contained no food of any kind. In favour of this view,
-too, are some observations, to be cited presently, on unicellular animals,
-in regard to whose nourishment by the zoochlorellæ living within them
-there can be no doubt at all.</p>
-
-<div class="figcenter" id="f39">
-<img src="images/fig39.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 35.</span> <i>Hydra viridis</i>, the Green Fresh-water Polyp. <i>A</i>, the entire animal, greatly
-enlarged. <i>M</i>, the mouth. <i>t</i>, tentacles. <i>sp</i>, testis. <i>ov</i>, ovary, both in the ectoderm. <i>Ei</i>, a
-ripe ovum, already green, in process of being extruded. After Leuckart and Nitsche.<br />
-<br />
-<i>B</i>, section of the body-wall, about the position of the ovary in <i>A</i>. <i>Eiz</i>, the ovum
-lying in the ectoderm (<i>ect</i>), in which zoochlorellæ (<i>zchl</i>), belonging to the endoderm
-(<i>ent</i>), have already migrated through the supporting middle lamella (<i>st</i>). <i>eik</i>, nucleus
-of ovum. After Hamann.
-</p>
-</div>
-
-<p>The little algæ on their part find a peaceful and relatively secure
-abode within the polyp, and they apparently do not occur outside of
-it, at least they do not now migrate from outside into the animal, but<span class="pagenum"><a id="Page_170"></a>[Pg 170]</span>
-are carried over as a heritable possession of the polyps from one
-generation to another, and in a very interesting manner, namely, by
-means of the eggs, and by these alone. As Hamann has shown, the
-zoochlorellæ migrate at the time when an egg is formed in the outer
-layer of the body of the polyp (Fig. 35) from the inner layer outwards,
-piercing through the supporting layer between them (st) and penetrating
-into the egg (<i>B</i>, <i>Eiz</i>). They make their way only into the egg,
-not into the sperm-cells, which in any case are too small to include
-them. Thus they are absent from no young polyp of this species,
-and it is easy to understand why earlier experimental attempts to
-rear colourless polyps from eggs could
-never succeed even in the purest water.</p>
-
-<div class="figleft" id="f40">
-<img src="images/fig40.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 36.</span> <i>A</i>, <i>Amœba viridis</i>. <i>k</i>, the<br />
-nucleus. <i>cv</i>, contractile vacuole. <i>zchl</i>,<br />
-the zoochlorellæ. <i>B</i>, a single zoochlorella<br />
-under high power. After<br />
-A. Gruber.</p>
-</div>
-
-<p>Quite similar green algæ live in
-symbiosis with unicellular animals, as,
-for instance, with an amœba (Fig. 36)
-and with an Infusorian of the genus
-<i>Bursaria</i>. In the Zoological Institute
-in Freiburg there is a living colony
-of a green amœba and a green
-<i>Bursaria</i>, both of which came from
-America, sent to us some years ago by
-Professor Wilder, of Chicago, inside
-a letter with dried <i>Sphagnum</i>, or
-bog-moss. The plants came from stagnant
-water in the Connecticut valley
-in Massachusetts. That in this case
-the zoochlorellæ are of use to the
-animals within which they live, not
-only by giving off oxygen, but also by
-yielding food-stuff, has been proved by A. Gruber, who bred the two
-green species for seven years in pure water which contained no trace
-of any kind of organic food for them. Nevertheless, they multiplied
-rapidly, and still form a green scum on the walls of the glass in which
-they are kept. They only die away when they are kept in the dark,
-where the algæ are unable to assimilate; then one green cell after
-another wanes and disappears, and, in consequence, their hosts also die
-from the double cause of lack of oxygen and lack of food.</p>
-
-<p>Even in this case the symbiotically united organisms have not
-remained unaltered. The algæ at least differ from others of their
-kind in their power of resistance to living animal protoplasm. They
-are not digested by it, and we may infer from this that they possess
-some sort of protective adaptation against the dissolving power of<span class="pagenum"><a id="Page_171"></a>[Pg 171]</span>
-animal digestive juices; they must, therefore, have undergone some
-variation, and adapted themselves to the new situation. Probably
-their cell-membrane has become impenetrable to the stuffs which
-would naturally digest them, an adaptation which could not be
-referred to direct effect or to use, but only to the accumulation of
-useful variations which cropped up&mdash;in other words, to natural selection.
-That any adaptive variation has taken place on the part of the
-host, whether polyp, amœba, or Infusorian, cannot be made out. None
-of these have altered their original mode of life; they do not depend
-on the nourishment afforded by the algæ, but feed on other animals,
-if these come in their way, and they live in water rich in oxygen like
-other species allied to them, and therefore are not altogether dependent
-on the algæ in this connexion; but they can no more help
-having their partners than the pig can help having Trichinæ in its
-muscles.</p>
-
-<p>Similar plant-cells, not green however, but yellow, called zooxanthellæ,
-live in great numbers in the endoderm of various sea-anemones
-and in the soft plasmic substance of many Radiolarians. In both these
-cases we must look for the benefit they confer on their host in the
-oxygen they give off, for, like the green zoochlorellæ, they break up
-carbonic acid gas in the light, and give off oxygen; they no longer
-occur, as far as is known, in a free state, but are always associated
-with the host, and they must therefore have altered in constitution,
-and have adapted themselves to the conditions of the symbiosis.</p>
-
-<p>Higher plants, too, sometimes have symbiotic relations with
-animals; the most remarkable and best-known example is the relation
-between ants and certain trees, in which the ants protect trees which
-afford them in return both a dwelling-place and food. We owe our
-knowledge of these cases to Thomas Belt and Fritz Müller, and more
-recently it has been materially increased by Schimper's researches.</p>
-
-<p>In the forests of South America there grow 'Imbauba,' or candelabra-trees,
-species of the genus <i>Cecropia</i>, which well deserve their
-name, for their bare branches stretch out like candelabra, and bear
-little bunches of leaves only at their tips. These leaves are menaced
-by the leaf-cutting ants of the genus <i>Œcodoma</i>, which attack
-numerous species of plants in these regions, often in tens of thousands,
-biting off the leaves, cutting them in pieces on the ground, and carrying
-them on their backs piece by piece to their nests. There they use
-them to make a kind of compost heap, on which fungi, to which the ants
-are very partial, readily grow. The candelabra-tree protects itself
-from these dangerous robbers, inasmuch as it has established an
-association with another ant (<i>Azteca instabilis</i>), which finds a safe<span class="pagenum"><a id="Page_172"></a>[Pg 172]</span>
-dwelling-place in its hollow, chambered stem (Fig. 37, <i>A</i>), and feeds
-on a brown sap which oozes from the inside. On the stem there are
-even little pits regularly arranged in definite places (<i>E</i>), through
-which the female of <i>Azteca</i> can easily bore her way into the interior.
-There she lays her eggs, and soon the whole interior of the trunk
-teems with ants, which come trooping out whenever the tree is
-shaken.</p>
-
-<div class="figleft" id="f41">
-<img src="images/fig41.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 37.</span> <i>A</i>, a piece of a twig of an<br />
-Imbauba-tree (<i>Cecropia adenopus</i>), with<br />
-the leaves cut off. At the leaf-bases are<br />
-the hair-cushions (<i>P</i>). <i>E</i>, the opening<br />
-for the associated ant (<i>Azteca instabilis</i>).<br />
-<i>B</i>, a piece of the hair-cushion with the<br />
-egg-shaped nutritive corpuscles (<i>nk</i>).<br />
-After Schimper.</p>
-</div>
-
-<p>This alone would not suffice to protect the tree against the leaf-cutting
-ants, for how should the Aztec ants living inside notice the presence
-of the lightly climbing leaf-cutters? But that is provided for, for
-the Aztecs also frequent the outside
-of the trunk, and just where attack
-would be most disastrous, namely,
-at the stalks of the young leaves.
-At these places there is a peculiar
-velvet-like cushion of hair (<i>P</i>), from
-which grow little stalked white
-papillæ (Fig. 37, <i>B</i>), which are rich
-in nourishment, and are not only
-eaten by the ants, but are harvested
-by them, being carried off into
-the ants' dwellings, presumably to
-feed their larvæ. In this case,
-then, a particular organ, offering
-special attraction to ants, has been
-developed by the plant at the
-places more especially threatened;
-while, as regards the ants, it is probable
-that only the instincts of
-feeding and habitat require to be
-modified, since courage and thirst
-for battle are present in all ants,
-almost any species being ready at any time to throw itself on any
-other which intrudes into its domain.</p>
-
-<p>It should be noted that not all the candelabra-trees live in
-symbiosis with ants, and so secure a means of defence against the
-leaf-cutters. Schimper found in the primitive forests of South
-America several species of <i>Cecropia</i> which never had ants in the
-chambers of their hollow stem. But these species did not exhibit the
-nutritive cushions at the base of the leaf-stalk; these contrivances for
-attracting and retaining the presence of partner ants were altogether
-absent. Indeed, only one species, <i>Cecropia peltata</i>, has produced these<span class="pagenum"><a id="Page_173"></a>[Pg 173]</span>
-peculiar structures, and, as they are of no <i>direct</i> use to the tree, we
-must say that it has produced them only for the ants. Here, again,
-natural selection must have gradually brought about the development
-of these nutritive cushions, though as yet we do not know what the
-beginnings of the process may have been. In no case can the origin of
-these cushions be referred to any direct influence of the environmental
-conditions.</p>
-
-<p>We may now pass to the association of two species of plants, of
-which the lichens furnish the best-known and probably most complete
-illustration. Till about twenty years ago the lichens, which in so
-many diverse forms clothe the bark of trees, the stones, and the rocks,
-were regarded as simple plants like the flowering plants, the ferns,
-or the mosses; and many lichenologists occupied themselves with the
-exact systematic distinction of about a thousand species, each of
-which could be as well and exactly classified, according to form,
-colour, habitat, and minute structure, as any other kind of plant.
-Then De Bary and Schwendener discovered that the lichens were
-made up of two kinds of plants, fungi and algæ, so intimately
-associated with and adapted to one another, that on coming together
-they always assume the same specific form.</p>
-
-<div class="figcenter" id="f42">
-<img src="images/fig42.jpg" alt=""/>
-<p class="caption center"><span class="smcap">Fig. 38.</span> A fragment of a Lichen (<i>Ephebe kerneri</i>), magnified 450 times.<br />
-<i>a</i>, the green alga-cells. <i>P</i>, the fungoid filaments. After Kerner.</p>
-</div>
-
-<p>The framework, and therefore the largest part, and the one
-which determines the form of a lichen, is due to the fungus (Fig. 38).
-Colourless threads of fungus ramify in a definite manner according
-to the species of fungus, and in the network of spaces left by this
-ramification green alga-cells (<i>a</i>) lie singly, or in rows, or groups.
-The fungus is propagated by multitudes of minute spores, which it<span class="pagenum"><a id="Page_174"></a>[Pg 174]</span>
-produces periodically, and these are disseminated in the air by the
-bursting of the sporangia and are carried away by the wind in
-the form of fine dust; the alga multiplies simply by continual
-division into two, but it also, like the whole lichen, can survive
-desiccation, and, after falling to pieces, is likewise carried through the
-air as microscopic dust.</p>
-
-<p>The partnership of the two plants rests on a basis of mutual
-benefit; the fungus, like all fungi, is without chlorophyll, and cannot
-therefore decompose carbonic acid gas or elaborate its own organic
-food-stuffs; it receives these from the alga. The alga has in the network
-of the fungus a safe shelter and basis of attachment, for the
-fungus is able to bore into the bark of trees and even into stones;
-besides which it absorbs water and salts, and supplies these to the
-partner alga. We here see the mutual advantage derived from
-the partnership, which is really an extremely intimate one. Fungus
-spores, sown by themselves, spring up and develop some branchings
-of fungoid hyphæ, a so-called mycelium, but without the requisite
-partner alga these remain weak and soon die away. The alga, on
-the other hand, can, in some cases, though not in all, survive without
-the fungus if the necessary conditions of its life be supplied to it, but
-it grows differently and more luxuriantly in association with the
-fungus.</p>
-
-<p>The same species of alga may be found associated with different
-species of fungi, and then each partnership forms a distinct species of
-lichen of definite and characteristic appearance; Stahl even succeeded
-in making new species of lichen artificially by bringing the spores of
-a lichen-forming fungus into contact with alga-cells, with which they
-had never been associated in free nature.</p>
-
-<p>The most remarkable feature of this remarkable association
-seems to me to be the formation of common reproductive bodies&mdash;an
-adaptation in face of which all doubt as to the theory of selection
-must disappear. Periodically there are developed in the substance of
-the lichen small corpuscles, the so-called soredia, each of which consists
-of one or more alga-cells surrounded and kept together by
-threads of the fungus. When they are developed in large numbers
-they form a floury dust over the maternal lichen, which 'breaks up'
-and leaves them, like the spores of the fungus, to be carried away by
-the wind. If these alight on favourable soil nothing more is needed
-than the external conditions of development, light, warmth, and water,
-to enable the lichen to spring up anew. The great advantage to the
-preservation of 'species' is obvious, for, when multiplication by
-the ordinary method occurs among lichens, the spores of the fungus,<span class="pagenum"><a id="Page_175"></a>[Pg 175]</span>
-even if they have fallen on good ground, can only develop into
-a new lichen if chance bring to them the proper partner alga.</p>
-
-<p>Obviously there must be, in the formation of the soredia, great
-advantage for the species, or rather 'for the two species,' for the
-fungi as well as the algæ benefit by the arrangement, which ensures
-the continuance of the partnership. It was not without reason,
-however, that the dual organism was so long regarded as a simple
-species in the natural history sense, <i>for that is what it really is</i>,
-although it has arisen in a manner quite different from the usual
-origin of species. As we know species which consist only of single
-cells, and others which consist of many cells, differentiated in different
-ways, and forming a cell-community or 'person,' and, finally, others
-which consist of a community of diversely differentiated personæ,
-making up a 'stock'; so in the lichens we see that even different
-species may combine to form a new physiological whole, a vital unit,
-an individual of the highest order. When, at the outset of these
-lectures, I said that the theory of evolution was now no longer
-a mere hypothesis, and that its general truth could no longer be
-doubted by any one acquainted with the facts available, I had in my
-mind, among other facts, especially that of symbiosis, and above all
-the case of the lichens.</p>
-
-<p>There are many other interesting cases of symbiosis between
-two different kinds of plants, and one side of the partnership is
-represented by fungi in a relatively large number of instances. The
-reason is not far to seek: fungi must always be dependent on other
-plants for their food; they must be parasitic, because they cannot
-themselves produce the organic substances they require. They must
-therefore associate themselves in some way with other organisms,
-living or dead, and as a general rule they simply prey upon their
-associate, sucking up its juices and killing it. But in not a few cases
-they can render services in return, and, as we have seen in the case of
-the lichens, symbiosis may then occur. Fungi in general have the
-power of discovering and absorbing the least trace of water in
-the soil, and with it they absorb the salts necessary to the plant, and
-in this, apparently, consists the service which they are able to render
-even to large plants fixed deep in the earth, such as shrubs and trees.
-The roots of many of our forest trees, e.g. beech, oak, fir, silver
-poplar, and bushes like broom, heaths, and rhododendrons, are thickly
-wrapped round with a network of fungoid threads, and the mutual
-relations just indicated exist between these and the plants in question
-(Fig. 39, <i>A</i> and <i>B</i>). The plants give to the fungi some contribution
-from the superfluity of their food-stuffs, and receive in return water<span class="pagenum"><a id="Page_176"></a>[Pg 176]</span>
-and salts, which are of value especially in times of drought. Perhaps
-there is some connexion between this and the fact that limes
-wither and lose their leaves so quickly during great summer-heat;
-these and many other of our trees possess no root-fungi or
-mycorhizæ.</p>
-
-<p>It is easy to understand, therefore, that genuine 'symbiosis' may
-have arisen from parasitism. But that this is not the only path that
-leads to symbiosis is shown by the cases of animal symbiosis we have
-already discussed.</p>
-
-<div class="figcenter" id="f43">
-<img src="images/fig43.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 39.</span> <i>A</i>, fragment of a Silver Poplar root, with an envelope of symbiotic
-fungoid filaments (mycelium); after Kerner. <i>B</i>, apex of a Beech root,
-with the closely enveloping mantle of mycelium; enlarged 480 times.</p>
-</div>
-
-<p>The partnership between polyps and hermit-crabs may have
-arisen from a one-sided commensalism, since polyps establishing
-themselves on mollusc shells which were often made use of by
-hermit-crabs would be better fed than those which settled down on
-stones. There are still species which make use of both modes of
-settlement. Then followed the adaptation of the crustacean to the
-polyp, for, first, those hermit-crabs would thrive best which tolerated
-the presence of the polyp; then those which sought its presence, that
-is to say, which gave a preference to shells covered with polyps; and,
-finally, those which would take no others, and even themselves fixed
-the sea-anemone upon it, if it chanced to be removed. Intelligence
-need not be taken into account in the matter at all, not even in the
-hermit-crab's case. We have only to recall the complex instincts,
-exercised only once in a lifetime, which compel the silkworm
-and the emperor moth to elaborate their effective cocoons. The
-elaboration of the spinning-instinct can only be due to natural
-selection, for the insect can have had no idea of the utility of its
-performance, and the same is true in the case of the sea-anemones or<span class="pagenum"><a id="Page_177"></a>[Pg 177]</span>
-the hydroid polyps and the hermit-crab. The sea-anemone is quite
-unconscious that it is defending its partner, the hermit-crab, when it
-lashes out its stinging acontia on any disturbance, and the hermit-crab
-is equally unaware that the sea-anemone is contributing to its
-safety; both animals act quite unconsciously, purely instinctively,
-and the origin of these instincts, on which the symbiosis is based,
-must be due, not to intelligent activities which have become habitual,
-but only to the survival of the fittest.</p>
-
-<p>According to the principle of natural selection nothing can arise
-but that which is of use directly or indirectly to its possessor.
-Nevertheless, there are cases in which it appears as if something had
-arisen, which was of no use to the species in which the variation
-appeared, but only to the species protected by it. This is the case
-in the remarkable symbiosis between algæ of the family Nostocaceæ
-and the floating, moss-like water-fern <i>Azolla</i>. This plant, in external
-appearance almost like duckweed, has on the under surface of its
-leaves a minute opening, leading into a relatively roomy hair-lined
-cavity, and in this cavity there is always, enclosed in jelly, a bluish
-green unicellular alga, <i>Anabæna</i>. The cavity is present in every
-leaf, and the alga is present in every cavity, making its way in from
-a deposit of alga-cells which is found on the incurved tip of every
-young shoot. As soon as a young leaf of <i>Azolla</i> unfolds from the
-bud it receives its <i>Anabæna</i> cells from this deposit, and no one has
-yet found either twigs or leaves which were free from the algæ. But
-no one has succeeded in discovering any benefit derived by the <i>Azolla</i>
-from this partnership.</p>
-
-<p>This looks like a contradiction of the theory of selection, but
-there remains the possibility that there is some benefit rendered to
-the <i>Azolla</i> by the alga, though we cannot see it as yet. There is
-also the possibility that the cavity is an organ which was of use
-to the plant at an earlier time, perhaps as an insect-trap, but has
-now lost its significance, and is utilized by the alga as a dwelling-place.
-This, however, is contradicted by the remarkable distribution
-of the four known species of <i>Azolla</i>. Two of these are widely
-distributed in America; the third lives in Australia, Asia, and Africa;
-the fourth in the region of the Nile: all four have cavities in their
-leaves, and in all these forms the cavity is inhabited by the same
-species of <i>Anabæna</i>. This indicates that the leaf-cavity and the
-partnership with the alga must have originated in remote antiquity;
-the symbiosis must date from a time before the four modern species
-of <i>Azolla</i> had split off from a single parent-species. But no rudimentary
-organ, that is to say, no organ not of use to the plant itself,<span class="pagenum"><a id="Page_178"></a>[Pg 178]</span>
-would have been preserved through such a vast period of time, as we
-shall see later, for useless organs disappear in the course of ages.
-As the cavity has not yet disappeared, we may assume with some
-probability that it is useful to the plant, whether by means of the
-<i>Anabæna</i>, or in some other unknown way. To draw an argument
-against the reality of the processes of selection from our lack of
-knowledge of what this advantage may be would be as unreasonable
-as if, notwithstanding our experience that stones sink in the water,
-we were to assume of a particular stone which we did not see sink,
-because it was hidden from our sight by bushes, that perhaps it had
-not sunk, but was capable of floating.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_179"></a>[Pg 179]</span></p>
-
-<h2 class="nobreak" id="LECTURE_X">LECTURE X</h2>
-</div>
-
-<p class="c">THE ORIGIN OF FLOWERS</p>
-
-<div class="blockquot">
-
-<p>Introduction&mdash;Precursors of Darwin&mdash;Pollination by wind&mdash;Arrangements in
-flowers for securing cross-fertilization&mdash;Salvia, Pedicularis&mdash;Flowers visited by flies&mdash;Aristolochia&mdash;Pinguicula&mdash;Daphne&mdash;Orchids&mdash;Flowers
-are built up of adaptations&mdash;Mouth-parts
-of insects&mdash;Proboscis of butterflies&mdash;Mouth-parts of the cockroach&mdash;Of the
-bee&mdash;Pollen baskets of bees&mdash;Origin of flowers&mdash;Attraction of insects by colour&mdash;Limitation
-of the area visited&mdash;Nägeli's objection to the theory of selection&mdash;Other
-interpretations excluded&mdash;<i>Viola calcarata</i>&mdash;Only those changes which are useful to their
-possessors have persisted&mdash;Deceptive flowers&mdash;Cypripedium&mdash;Pollinia of Orchis&mdash;The
-case of the Yucca-moth&mdash;The relative imperfection of the adaptations tells in favour of
-their origin through natural selection&mdash;Honey thieves.</p></div>
-
-
-<p><span class="smcap">When</span> one species is so intimately bound up with another that
-neither can live for any length of time except in partnership, that is
-certainly an example of far-reaching mutual adaptation, but there
-are innumerable cases of mutual adaptation, in which, although there
-is no common life in the same place, yet the first form of life is
-adjusted in relation to the peculiarities of the second, and the second
-to those of the first. One of the most beautiful, and, in regard to
-natural selection, the most instructive of these cases is illustrated by
-the relations between insects and the higher plants, relations which
-have grown out of the fact that many insects have formed the habit
-of visiting the flowers of the plants for the sake of the pollen. In
-this connexion the theory of selection has made the most unexpected
-and highly interesting disclosures, for it has informed us how the
-flowers have arisen.</p>
-
-<p>In earlier times the beauty, the splendour of colour, and the
-fragrance of flowers were regarded as phenomena created for the
-delight of mankind, or as an outcome of the infinite creative power
-of Mother Nature, who loves to run riot in form and colour. Without
-allowing our pleasure in all this manifold beauty to be spoilt, we
-must nowadays form quite a different conception of the way in which
-the flowers have been called into being. Although here, as everywhere
-else in Nature, we cannot go back to ultimate causes, yet we
-can show, on very satisfactory evidence, that the flowers illustrate
-the reaction of the plants to the visits of insects, and that they have
-been in large measure evoked by these visits. There might, indeed,<span class="pagenum"><a id="Page_180"></a>[Pg 180]</span>
-have been blossoms, but there would have been no flowers&mdash;that is to
-say, blossoms with large, coloured, outer parts, with fragrance, and with
-nectar inside, unless the blossoms had been sought out by insects
-during the long ages. Flowers are adaptations of the higher flowering
-plants to the visits of insects. There can be no doubt about that now,
-for&mdash;thanks to the numerous and very detailed studies of a small
-number of prominent workers&mdash;we need not only suppose it, we can
-prove it with all the certainty that can be desired. The mutual
-adaptations of insects and flowers afford one of the clearest examples
-of the mode of operation and the power of natural selection, and the
-case cannot therefore be omitted from lectures on the theory of
-descent.</p>
-
-<p>That bees and many other insects visit flowers for the sake of the
-nectar and pollen has been known to men from very early times.
-But this fact by itself would only explain why adaptations to flower-visiting
-have taken place in these insects to enable them, for instance,
-to reach the nectar out of deep corolla-tubes, or to load themselves
-with a great quantity of pollen, and to carry it to their hives, as
-happens in the case of the bees. But what causes the plants to
-produce nectar, and offer it to the insects, since it is of no use to
-themselves? And further, what induces them to make the pillage
-easier to the insects, by making their blossoms visible from afar
-through their brilliant colours, or by sending forth a stream of
-fragrance that, even during the night, guides their visitors towards
-them?</p>
-
-<p>As far back as the end of the eighteenth century a thoughtful
-and clear-sighted Berlin naturalist, Christian Konrad Sprengel, took
-a great step towards answering this question. In the year 1793 he
-published a paper entitled 'The Newly Discovered Secret of Nature
-in the Structure and Fertilization of Flowers<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a>,' in which he quite
-correctly recognized and interpreted a great many of the remarkable
-adaptations of flowers to the visits of insects. Unfortunately, the
-value of these discoveries was not appreciated in Sprengel's own
-time, and his work had to wait more than half a century for
-recognition.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> <i>Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung der Blumen</i>, Berlin,
-1793.</p>
-
-</div>
-
-<p>Sprengel was completely dominated by the idea of an all-wise
-Creator, who 'has not created even a single hair without intention,'
-and, guided by this idea, he endeavoured to penetrate into the
-significance of many little details in the structure of flowers. Thus
-he recognized that the hairs which cover the lower surface of the<span class="pagenum"><a id="Page_181"></a>[Pg 181]</span>
-petals of the wood-cranesbill (<i>Geranium sylvaticum</i>) protect the
-nectar of the flower from being diluted with rain, and he drew the
-conclusion, correct enough, though far removed from our modern
-ideas as regards the directly efficient cause, that the nectar was there
-for the insects.</p>
-
-<p>He was also impressed by the fact that the sky-blue corolla of
-the forget-me-not (<i>Myosotis palustris</i>) has a beautiful yellow ring
-round the entrance to the corolla-tube, and he interpreted this as
-a means by which insects were shown the way to the nectar which
-is concealed in the depths of the tube.</p>
-
-<div class="figright" id="f44">
-<img src="images/fig44.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 40.</span> <i>Potentilla verna</i>, after Hermann Müller. <i>A</i>,<br />
-seen from above. <i>Kbl</i>, sepals. <i>Bl</i>, petals. <i>Nt</i>, nectaries<br />
-near the base of the stamens. <i>B</i>, section through the<br />
-flower. <i>Gr</i>, stigma. <i>St</i>, stamen. <i>Nt</i>, nectary.</p>
-</div>
-
-<p>We now know that such 'honey-guides' are present in most of the
-flowers visited by insects, in the form of spots, lines, or other marking,
-usually of conspicuous colour, that is, of a colour contrasting with the
-ground colour of the
-flower. Thus, in species
-of Iris, regular paths of
-short hairs lead the way
-to the place where the
-nectar lies. In the
-spring potentilla (<i>Potentilla
-verna</i>) (Fig. 40) the
-yellow petals (<i>A</i>, <i>Bl</i>)
-become bright orange-red
-towards their bases,
-and this shows the way
-to the nectaries, which
-lie at the bases of the
-stamens (<i>st</i>), and are
-protected by hairs, the
-so-called 'nectar-covers' (<i>Saftdecke</i>) of Sprengel, from being washed
-by rain.</p>
-
-<p>The recognition of the honey-guides led Sprengel on to the idea
-that the general colouring of the flower effects on a large scale what
-the honey-guides do in a more detailed way&mdash;it attracts the attention
-of passing insects to where nectar is to be found; indeed, he went an
-important step further by recognizing that there are flowers which
-cannot fertilize themselves, in which the insect, in its search for
-honey, covers itself with pollen, which is then rubbed off on the
-stigma of the next flower visited, fertilization being thus effected.
-He demonstrated this not only for the Iris, but for many other
-flowers, and he drew the conclusion that 'Nature does not seem to
-have wished that any flower should be fertilized by its own pollen.'<span class="pagenum"><a id="Page_182"></a>[Pg 182]</span>
-How near Sprengel was to reaching a complete solution of the
-problem is now plain to us, for he even discovered that many flowers,
-such as <i>Hemerocallis fulva</i>, remained infertile if they were dusted
-with their own pollen.</p>
-
-<p>Even the numerous experiments of that admirable German
-botanist, C. F. Gärtner, although they advanced matters further,
-did not suffice to make the relations between insects and flowers
-thoroughly clear; for this the basis of the theory of Descent and
-Selection was necessary. Here, again, it was reserved for Charles
-Darwin to lead the way where both contemporaries and predecessors
-had been blindly groping. He recognized that, <i>in general</i>, self-fertilization
-is disadvantageous to plants; that they produce fewer
-seeds, and that these produce feebler plants, than when they are cross-fertilized;
-that, therefore, those flowers which are arranged to secure
-cross-fertilization have an advantage over those which are self-fertilized.
-In many species, as Sprengel had already pointed out,
-self-fertilization leads to actual infertility; only a few plants are as
-fertile with their own pollen as with that of another plant; and
-Darwin believed that, in all flowering plants, crossing with others of
-the same kind, at least from time to time, is necessary if they are not
-to degenerate.</p>
-
-<p>Thus the advantage which the flowers derive from the visits of
-insects lies in the fact that insects are instrumental in the cross-fertilization
-of the flowers, and we can now understand how the plant
-was able to vary in a manner favourable to the insect-visits, and to
-exhibit adaptations which serve exclusively to make these visits easier;
-we understand how it was possible that there should develop among
-flowers an endless number of contrivances which served solely to
-attract insects, and even how, for the same end, the insignificant
-blossoms of the oldest Phanerogams must have been transformed into
-real flowers.</p>
-
-<p>We must not imagine, however, that the obviously important
-crossing of plant-individuals, usually called 'cross-pollination,' can
-be effected only by means of insects. There were numerous plants
-in earlier times, and there is still a whole series in which cross-fertilization
-is effected through the air by the wind; these are the
-anemophilous or wind-pollinated Angiosperms.</p>
-
-<p>To these belong most of the catkin-bearers, such as hazel and
-birch, and also the grasses and sedges, the hemp and the hop, and so
-forth. In these plants there is no real flower, but only an inconspicuous
-blossom, without brightly-coloured outer envelopes, without
-fragrance or nectar; all of them have smooth pollen grains, which<span class="pagenum"><a id="Page_183"></a>[Pg 183]</span>
-easily separate into fine dust and are carried away by the wind until
-they fall, by chance, far from their place of origin, on the stigma of
-a female blossom.</p>
-
-<div class="figright" id="f45">
-<img src="images/fig45.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 41.</span> Flower of Meadow Sage (<i>Salvia pratensis</i>),<br />
-after H. Müller. <i>st´</i>, immature anthers concealed<br />
-in the 'helmet' of the flower. <i>st´´</i>, mature anther<br />
-lowered. <i>gr´</i>, immature stigma. <i>gr´´</i>, mature<br />
-stigma. <i>U</i>, the lower lip of the corolla, the<br />
-landing-stage for the bee.</p>
-</div>
-
-<p>By far the greater number of the phanerogams, however, especially
-all our indigenous 'flowers,' are, as a rule, fertilized by means
-of insects, and it is amazing to see in what diverse ways, often highly
-specialized, they have adapted themselves to the visits of insects.
-Thus there are flowers in which the nectar lies open to view, and
-these can be feasted on by all manner of insects; there are others in
-which the nectar is rather more concealed, but still easily found,
-and reached by insects with short mouth-parts, e.g. large flowers
-blooming by day and bearing much pollen, like the Magnolias.
-These have been called
-beetle-flowers, because they
-are visited especially by the
-honey-loving Longicorns.</p>
-
-<p>Other flowers blooming
-by day are especially adapted
-to fertilization by means of
-bees; they are always beautifully
-coloured, often blue;
-they are fragrant, and contain
-nectar deep down in the
-flower, where it can only be
-reached by the comparatively
-long proboscis of the bee.
-Different arrangements in the
-different flowers secure that
-the bee cannot enjoy the
-nectar without at the same
-time effecting the cross-pollination. Thus the stamens of the meadow
-sage (<i>Salvia pratensis</i>) are at first hidden within the helmet-shaped
-upper lip of the flower (Fig. 41, <i>st´</i>), but bear lower down on their stalk
-a short handle-like process, which turns the pollen-bearing anther
-downwards (<i>st´´</i>) as soon as it is pressed back by an intruding insect.
-The pollen-sacs then strike downwards on the back of the bee, and
-cover it with pollen. When the bee visits another more mature flower,
-the long style, which was at first hidden within the helmet, has bent
-downwards (<i>gr´´</i>), and now stands just in front of the entrance to the
-flower, so that the bee must rub off a part of the pollen covering its
-back on to the stigma, and fertilization is thus effected.</p>
-
-<p>There are other flowers which are specially disposed to suit the<span class="pagenum"><a id="Page_184"></a>[Pg 184]</span>
-visits of the humble-bees, as, for instance, <i>Pedicularis asplenifolia</i>,
-the fern-leaved louse-wort, a plant of the high Alps (Fig. 42). The
-first thing that strikes us about this plant is the thickly tufted hair
-covering on the calyx (<i>k</i>), which serves to keep off little wingless
-insects from the flower; then there is the strange left-sided twisting
-of the individual flowers, whose under lip allows only a strong insect
-like the humble-bee to gain access, towards the left, to the corolla-tube
-(<i>kr</i>), in the depths of which the nectar is concealed. While the
-humble-bee is sucking up the nectar it becomes dusted over with
-pollen from the anthers, which falls to dust at a touch, and when it
-insinuates itself into a second flower its powdered back comes first
-into contact with the stigma of the pistil (<i>gr</i>) which projects from
-the elongated bill-shaped under lip, dusting it over with the pollen
-of the first visited flower. Butterflies and smaller bees cannot rob
-this flower; it is strictly a humble-bee's flower.</p>
-
-<div class="figcenter" id="f46">
-<img src="images/fig46.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 42.</span> Alpine Lousewort (<i>Pedicularis asplenifolia</i>). <i>A</i>, flower seen from
-the left side, enlarged three times; the arrows show the path by which the
-humble-bee enters. <i>B</i>, the same flower, seen from the left, after removal of
-the calyx, the lower lip and the left half of the upper lip. <i>C</i>, ovary (<i>ov</i>),
-nectary (<i>n</i>), and base of style. <i>D</i>, tip of style, bearing the stigma. <i>E</i>, two
-anthers turned towards one another. <i>o</i>, upper lip. <i>u</i>, lower lip. <i>gr</i>, style.
-<i>st</i>, anthers. <i>kr</i>, corolla-tube. <i>k</i>, calyx.</p>
-</div>
-
-<p>There are not a few of such flowers adapted to a very restricted
-circle of visitors, and in all of them we find contrivances which close
-the entrance to all except what we may call the welcome insects;
-sometimes there are cushions of bristles which prevent little insects
-from creeping up from below, or it is the oblique position of the
-flower which prevents their getting in from the stem; sometimes it is
-the length and narrowness of the corolla-tube, or the deep and hidden
-situation of the nectar, which only allows intelligent insects to find
-the treasure.</p>
-
-<p><span class="pagenum"><a id="Page_185"></a>[Pg 185]</span></p>
-
-<p>Very remarkable are those flowers which are adapted to the visits
-of flies, for they correspond in several respects to the peculiarities of
-these insects. In the first place, flies are fond of decaying substances
-and the odours given off by these, and so the flowers which depend
-for their cross-fertilization on flies have taken on the dull and ugly
-colours of decay, and give out a disagreeable smell. But flies are also
-shy and restless, turning now hither, now thither, and cannot be
-reckoned among the 'constant' insect visitors, that is to say, they
-do not persistently visit the same species; it is, therefore, evident that
-they might easily carry away the pollen without any useful result
-ensuing. Moreover, their intelligence is of a low order, and they do
-not seek nectar with the perseverance shown by bees and humble-bees.
-It is not surprising, therefore, to find that many of the flowers adapted
-for the visits of flies are so constructed that they detain their visitors
-until they have done their duty, that is to say, until they have effected,
-or at least begun, the process of cross-pollination.</p>
-
-<div class="figleft" id="f47">
-<img src="images/fig47.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 43.</span> Flower of Birthwort (<i>Aristolochia<br />
-clematitis</i>) cut in half. <i>A</i>, before<br />
-pollination by small flies. <i>b</i>, the<br />
-bristles. <i>B</i>, after pollination. <i>P</i>, pollen<br />
-mass. <i>N</i>, stigma, <i>b</i>, the bristles.<br />
-<i>b´</i>, their remains. After H. Müller.</p>
-</div>
-
-<div class="figright" id="f48">
-<img src="images/fig48.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 44.</span> Alpine Butterwort<br />
-(<i>Pinguicula alpina</i>).<br />
-<i>A</i>, section through the flower.<br />
-<i>K</i>, calyx. <i>bh</i>, bristly prominences.<br />
-<i>sp</i>, spur. <i>st</i>, stamen.<br />
-<i>n</i>, stigma. <i>B</i>, stigma and<br />
-stamen more magnified.<br />
-After H. Müller.</p>
-</div>
-
-<p>Our birthwort (<i>Aristolochia clematitis</i>) and the Cuckoo-pint
-(<i>Arum maculatum</i>) are pit-fall flowers, whose long corolla-tubes have<span class="pagenum"><a id="Page_186"></a>[Pg 186]</span>
-an enlargement at the base, in which both pistil and stamens are contained.
-In the birthwort (Fig. 43) the narrow entrance-tube is thickly
-beset with stiff hairs (<i>A</i>, <i>b</i>), whose points are all directed towards
-the base. Little flies can creep down quite comfortably into the basal
-expansion, but once there they are kept imprisoned until the flower,
-in consequence of the pollination of the stigma, begins to wither, the first
-parts to go being these very bristles (<i>B</i>, <i>b´</i>), whose points, like a fish-weir,
-prevented the flies from creeping out. Other 'fly-flowers,' as for
-instance the Alpine butterwort (<i>Pinguicula alpina</i>) (Fig. 44), securely
-imprison the plump fly as soon as it has succeeded in forcing itself in
-far enough to reach, with its short proboscis, the nectar contained in
-the spur (<i>sp</i>) of the corolla. The backward-directed bristles hold it
-fast for some time, and it is only by hard pressing with the back
-against the anthers (<i>st</i>) lying above it, and against the stigma (<i>n</i>),
-that it ultimately succeeds in getting free, but it never does so without
-having either loaded itself with pollen, or rubbed off on the stigma
-the pollen it brought with it from another similar flower. The Alpine
-butterwort is protogynous, that is to say, the pistil ripens first, the
-pollen later, so that the possibility of self-fertilization is altogether
-excluded.</p>
-
-<p>It would be impossible to give even an approximate idea of the
-diversity of the contrivances for securing fertilization in flowers
-without spending many hours over them, for they are different in
-almost every flower, often widely so, and even in species of the same
-genus they are by no means always alike; for not infrequently one
-species is adapted to one circle of visitors, and its near relative to
-another. Thus the flower of the common Daphne (<i>Daphne mezereum</i>)
-(Fig. 45, <i>A</i> and <i>C</i>) is adapted to the visits of butterflies, bees, and hover-flies,
-while its nearest relative (<i>Daphne striata</i>) (Fig. 45, <i>B</i> and <i>D</i>) has
-a somewhat narrower and longer corolla-tube, so that only butterflies
-can feast upon it. This example shows that there are exclusively
-'butterfly flowers,' but specialization goes further, for there are flowers
-adapted to diurnal and others to nocturnal Lepidoptera. The former
-have usually bright, often red colours, and a pleasant aromatic
-fragrance, and in all of them the nectar lies at the bottom of a very
-narrow corolla-tube. To this class belong, for instance, the species of
-pink, many orchids, such as <i>Orchis ustulata</i>, and <i>Nigritella angustifolia</i>
-of the Alps, which smells strongly of vanilla; also the beautiful
-campion (<i>Lychnis diurna</i>) and the Alpine primrose (<i>Primula farinosa</i>).
-The flowers adapted to nocturnal Lepidoptera are characterized by
-pale, often white colour, and a strong and pleasant smell, which only
-begins to stream out after sunset, and indeed many of these flowers<span class="pagenum"><a id="Page_187"></a>[Pg 187]</span>
-are quite closed by day. This is the case with the large, white, scentless
-bindweed (<i>Convolvulus sepium</i>), which is chiefly visited and
-fertilized by the largest of our hawk-moths (<i>Sphinx convolvuli</i>). The
-pale soapwort (<i>Saponaria officinalis</i>) exhales a delicate fragrance
-which attracts the Sphingidæ from afar, and the sweet smell of the
-honeysuckle (<i>Lonicera periclymenum</i>) is well known, and has the same
-effect; an arbour of honeysuckle often attracts whole companies of our
-most beautiful Sphingidæ and Noctuidæ on warm June nights, to the
-great delight of the moth-collecting youth.</p>
-
-<div class="figcenter" id="f49">
-<img src="images/fig49.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 45.</span> <i>Daphne mezereum</i> (<i>A</i> and <i>C</i>) and Daphne striata (<i>B</i> and <i>D</i>). The
-former visited by butterflies, bees, and flies, the latter by butterflies only.
-<i>A</i> and <i>B</i>, vertical sections of the flowers. <i>St</i>, stamens. <i>Gr</i>, style. <i>n</i>, nectary.
-<i>C</i> and <i>D</i>, flowers seen from above. After H. Müller.</p>
-</div>
-
-<p>I cannot conclude this account of flower-adaptations without considering
-the orchids somewhat more in detail, for it is among them
-that we find the most far-reaching adaptations to the visits of insects.
-Among them, too, great diversity prevails, as is evident from the fact
-that Darwin devoted a whole book to the arrangements for fertilization
-in orchids, but the main features are very much the same in the
-majority. Figure 46 gives a representation of one of our commonest
-species (<i>Orchis mascula</i>), A shows the flower in side view, <i>B</i> as it
-appears from in front. The flower seems as it were to float on the
-end of the stalk (<i>st</i>), stretching out horizontally the spur (<i>sp</i>) which
-contains the nectar. Between the large, broad under lip (<i>U</i>), marked
-with a honey-guide (<i>sm</i>), and offering a convenient alighting surface,
-and the broad, cushion-like stigma (<i>n</i>) lies the entrance to the spur.
-Fertilization occurs in the following way:&mdash;The fly or bee, when it is
-in the act of pushing its proboscis into the nectar-containing spur,<span class="pagenum"><a id="Page_188"></a>[Pg 188]</span>
-knocks with its head against the so-called rostellum (<i>r</i>), a little beak-like
-process at the base of the stamens (<i>p</i>). The pollen masses are of
-very peculiar construction, not falling to dust, but forming little
-stalked clubs, with the pollen grains glued together, and so arranged
-that they spring off when the rostellum is touched and attach themselves
-to the head of the insect, as at <i>D</i> on the pencil (Fig. 46). When
-the bee has sucked up the nectar out of the spur, and then proceeds to
-penetrate into another flower of the same species, the pollinia have
-bent downwards on its forehead (<i>E</i>), and must unfailingly come in
-contact with the stigma of the second flower, to which they now
-remain attached, and effect its fertilization. What a long chain of
-purposeful arrangements in a single flower, and no interpretation of
-them is available except through natural selection!</p>
-
-<div class="figcenter" id="f50">
-<img src="images/fig50.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 46.</span> Common Orchis (<i>Orchis mascula</i>). <i>A</i>, flower in side view. <i>st</i>,
-stalk. <i>sp</i>, spur with the nectary (<i>n</i>). <i>ei</i>, entrance to the spur. <i>U</i>, lower lip.
-<i>B</i>, flower from in front. <i>p</i>, pollinia. <i>Sm</i>, honey-guide. <i>ei</i>, entrance to the
-nectar. <i>na</i>, stigma. <i>r</i>, rostellum. <i>U</i>, lower lip. <i>C</i>, vertical section through
-the rostellum (<i>r</i>), pollinium (<i>p</i>). <i>ei</i>, entrance. <i>D</i>, the pollinia removed and
-standing erect on the tip of a lead-pencil. <i>E</i>, the same, somewhat later,
-curved downwards.</p>
-</div>
-
-<p>And how diversely are these again modified in the different
-genera and species of orchids, of which one is adapted to the visits of
-butterflies exclusively, as <i>Orchis ustulata</i>, another to those of bees, as
-<i>Orchis morio</i>, and a third to those of flies, as <i>Ophrys muscifera</i>. These
-flowers are adapted to insect visits in the minutest details of the form
-of the petals, which are smooth, as if polished with wax, where insects
-are not intended to creep, but velvety or hairy where the path leads<span class="pagenum"><a id="Page_189"></a>[Pg 189]</span>
-to the nectar, and at the same time to the pollen and the stigma.
-And then there is the diversity in the form and colour of the 'honey-guides'
-on the 'alighting surface,' that is, the under lip of the flower,
-upon which the insect sits and holds fast, while it pushes its head as
-far as possible into the spur, so that its proboscis may reach the nectar
-lying deep within it! Even though we cannot pretend to guess at the
-significance of every curve and colour-spot in one of the great tropical
-orchids, such as <i>Stanhopea tigrina</i>, yet we may believe, with Sprengel,
-that all this has its significance, or has had it for the ancestors of the
-plant in question, and in fact that the flower is made up of nothing
-but adaptations, either actual or inherited from its ancestors, although
-sometimes perhaps no longer of functional importance.</p>
-
-<p>So far, then, we have illustrated the fact that there are hundreds
-and thousands of contrivances in flowers adapted solely to the visits
-of insects and to securing cross-fertilization, and these adaptations go
-so far that we might almost believe them to be the outcome of the
-most exact calculation and the most ingenious reflection. But they all
-admit of interpretation through natural selection, for all these details,
-which used to be looked upon as merely ornamental, are directly or
-indirectly of use to the species; directly, when, for instance, they concern
-the dusting of the insect with the pollen; indirectly, when they are
-a means of attracting visits.</p>
-
-<p>Moreover, the evidence of the operation of the processes of selection
-becomes absolutely convincing when we consider that, as in
-symbiosis, there are always two sets of adaptations taking place
-independently of one another&mdash;those of the flowers to the visits of
-the insects, and those of the insects to the habit of visiting the flowers.
-To understand this clearly we must turn our attention to the insects,
-and try to see in what way they have been changed by adapting
-themselves to the diet which the flowers afford.</p>
-
-<p>As is well known, several orders of insects possess mouth-parts
-which are suited for sucking up fluids, and these have evolved, through
-adaptation to a fluid diet, from the biting mouth-parts of the primitive
-insects which we see still surviving in several orders. Thus the
-Diptera may have gradually acquired the sucking proboscis which
-occurs in many of them by licking up decaying vegetable and animal
-matter, and by piercing into and sucking living animals. But even
-among the Diptera several families have more recently adapted themselves
-quite specially to a flower diet, to honey-sucking, like the
-hover-flies, the Syrphidæ,and the Bombyliidæ, whose long thin proboscis
-penetrates deep into narrow corolla-tubes, and is able to suck up the
-nectar from the very bottom. The transformation was not so impor<span class="pagenum"><a id="Page_190"></a>[Pg 190]</span>tant
-in this case, since the already existing sucking apparatus only
-required to be a little altered.</p>
-
-<p>Again, in the order Hemiptera (Bugs) the suctorial proboscis does
-not owe its origin to a diet of flowers, for no member of the group is
-now adapted to that mode of obtaining food.</p>
-
-<div class="figleft" id="f51">
-<img src="images/fig51.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 47.</span> Head of a Butterfly. <i>A</i>, seen from<br />
-in front. <i>au</i>, eyes. <i>la</i>, upper lip. <i>md</i>, rudiments<br />
-of the mandibles. <i>pm</i>, rudimentary<br />
-maxillary palps. <i>mx´</i>, the first maxillæ<br />
-modified into the suctorial proboscis. <i>pl</i>,<br />
-palps of labium or second maxillæ, cut off<br />
-at the root, remaining in <i>B</i>&mdash;which is a side<br />
-view. <i>at</i>, antennæ. Adapted from Savigny.</p>
-</div>
-
-<p>The proboscis of the Lepidoptera, on the other hand, depends
-entirely on adaptation to honey-sucking, and we may go the length
-of saying that the order of Lepidoptera would not exist if there were
-no flowers. This large and diverse insect-group is probably descended
-from the ancestors of the modern caddis-flies or Phryganidæ, whose
-weakly developed jaws were chiefly used for licking up the sugary
-juices of plants. But as flowering plants evolved the licking
-apparatus of the primitive butterflies
-developed more and more
-into a sucking organ, and was
-ultimately transformed into the
-long, spirally coiled suctorial proboscis
-as we see it in the modern
-butterflies (Fig. 47). It has taken
-some pains to trace this organ
-back to the biting mouth-parts of
-the primitive insects, for nearly
-everything about it has degenerated
-and become stunted except
-the maxillæ (<i>mx´</i>). Even the
-palps (<i>pm</i>) of these have become
-so small and inconspicuous in
-most of the Lepidoptera that it
-is only quite recently that remains
-of them have been recognized in a minute protuberance
-among the hairs. The mandibles (<i>md</i>) have quite degenerated,
-and even the under lip has disappeared, and only its palps are
-well developed (<i>B</i>, <i>pl</i>). But the first maxillæ (<i>mx´</i>), although
-very strong and long, are so extraordinarily altered in shape and
-structure that they diverge from the maxillæ of all other insects.
-They have become hollow, probe-like half-tubes, which fit together
-exactly, and thus form a closed sucking-tube of most complex
-construction, composed of many very small joints, after the fashion
-of a chain-saw, which are all moved by little muscles, and are subject
-to the will through nerves, and are also furnished with tactile
-and taste papillæ. Except this remarkable sucking proboscis there
-are no peculiarities in the body of the butterfly which might be<span class="pagenum"><a id="Page_191"></a>[Pg 191]</span>
-regarded as adaptations to flower-visiting, with a few isolated
-exceptions, of which one will be mentioned later. This is intelligible
-enough, for the butterfly has nothing more to seek from the flower
-beyond food for itself; it does not carry stores for offspring.</p>
-
-<p>The bees, however, do this, and accordingly we find that in them
-the adaptations to flower-visiting are not confined to the mouth-parts.</p>
-
-<p>As far as we can judge now, the flower-visiting bees are
-descended from insects which resembled the modern burrowing-wasps.
-Among these the females themselves live on nectar and
-pollen, and build cells in holes in the ground, and feed their brood.
-They do not feed them on honey, however,
-but on animals&mdash;on caterpillars, grasshoppers,
-and other insects, which they
-kill by a sting in the abdomen, or often
-only paralyse, so that the victim is
-brought into the cells of the nest alive
-but defenceless, and remains alive until
-the young larva of the wasp, which
-emerges from the egg, sets to work to
-devour it.</p>
-
-<div class="figright" id="f52">
-<img src="images/fig52.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 48.</span> Mouth-parts of the<br />
-Cockroach (<i>Periplaneta orientalis</i>),<br />
-after R. Hertwig. <i>la</i>, upper lip<br />
-or labrum. <i>md</i>, mandibles. <i>mx</i><sup>1</sup>,<br />
-first maxillæ, with <i>c</i>, cardo, <i>st</i>,<br />
-stipes, <i>li</i>, internal lobe or lacinia,<br />
-<i>le</i>, external lobe or galea, and <i>pm</i>,<br />
-the maxillary palp. <i>mx</i><sup>2</sup>, the<br />
-labium or second maxillæ, with<br />
-similar detailed parts.</p>
-</div>
-
-<p>Before I go on to explain the origin
-of the sucking proboscis of the bee from
-the biting mouth-parts of the primitive
-insects I must first briefly consider the
-latter.</p>
-
-<p>The biting mouth-parts of beetles,
-Neuroptera, and Orthoptera (Fig. 48),
-consist of three pairs of jaws, of which
-the first, the mandibles (<i>md</i>), are simply
-powerful pincers for seizing and tearing
-or chewing the food. They have no part in the development of the
-suctorial apparatus either in bees or in butterflies, so they may be left
-out of account. The two other pairs of jaws, the first and second
-maxillæ (<i>mx</i><sup>1</sup> and <i>mx</i><sup>2</sup>), are constructed exactly on the same type,
-having a jointed basal portion (<i>st</i>) bearing two lobes, an external (<i>le</i>)
-and an internal (<i>li</i>), and a feeler or palp, usually with several joints,
-directed outwards from the lobes (<i>pm</i> and <i>pl</i>). The second pair of
-maxillæ (<i>mx</i><sup>2</sup>) differs from the first chiefly in this, that the components
-of the pair meet in the median line of the body, and fuse
-more or less to form the so-called 'under lip' or labium. In the
-example given, the cockroach (<i>Periplaneta orientalis</i>), this fusion<span class="pagenum"><a id="Page_192"></a>[Pg 192]</span>
-is only partial, the lobes having remained separate (<i>le</i> and <i>li</i>); and the
-same is true of the bee, but in this case the inner lobes have grown
-into a long worm-like process which is thrust into the nectar in the
-act of sucking.</p>
-
-<div class="figleft" id="f53">
-<img src="images/fig53.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 49.</span> Head of the Bee. <i>Au</i>, compound<br />
-eyes. <i>au</i>, ocelli. <i>at</i>, antennæ. <i>la</i>, upper lip.<br />
-<i>md</i>, mandibles. <i>mx</i><sup>1</sup>, first maxillæ, with <i>pm</i>,<br />
-the rudimentary maxillary palp. <i>mx</i><sup>2</sup>, second<br />
-maxillæ with the internal lobes (<i>li</i>) fused to<br />
-form the 'tongue.' <i>le</i>, the external lobes of<br />
-the second maxillæ, known as 'paraglossæ.'<br />
-<i>pl</i>, labial palp.</p>
-</div>
-
-<p>Even the burrowing-wasps exhibit the beginnings of variation in
-this direction, for the under lip is somewhat lengthened and modified
-into a licking organ. The adaptation has not gone much further than
-this, even in one of the true
-flower-bees, <i>Prosopis</i>, which feeds
-its larvæ with pollen and honey,
-and it is only in the true honey-bee
-that the adaptation is complete
-(Fig. 49). Here the so-called
-'inner lobe' of the under lip (<i>li</i>) has
-elongated into the worm-shaped
-process already mentioned; it
-is thickly covered with short
-bristles, and is called the 'tongue'
-of the bee (<i>li</i>). The outer lobes
-of the under lip have degenerated
-into little leaf-like organs, the
-so-called accessory tongue or
-paraglossa (<i>le</i>), while the palps
-of the under lip (<i>pl</i>) have elongated
-to correspond with the
-tongue, and serve as a sensitive
-and probably also as a smelling
-organ, in contrast to the palps
-of the first maxillæ, which have
-shrunk to minute stumps (<i>pm</i>).
-The whole of the under lip, which
-has elongated even in its basal
-portions, forms, with the equally
-long first maxillæ, the proboscis of
-the bee. The first maxillæ are sheath-like half-tubes, closely apposed
-around the tongue, and form along with it the suctorial tube, through
-which the nectar is sucked up. Thus, of the three pairs of jaws
-in insects, only the first pair, the mandibles, have remained unaltered,
-obviously because the bee requires a biting-organ for eating pollen,
-for kneading wax, and for building cells.</p>
-
-<p>But bees do not only feast on nectar and pollen themselves,
-they carry these home as food for their larvæ. The form already<span class="pagenum"><a id="Page_193"></a>[Pg 193]</span>
-mentioned, <i>Prosopis</i>, takes up pollen and nectar in its mouth, and
-afterwards disgorges the pulp as food for its larvæ, but the rest of
-the true bees have special and much more effective collecting-organs,
-either a thick covering of hair on the abdomen, or along the whole
-length of the posterior legs, or finally, a highly developed collecting
-apparatus, such as that possessed by the honey-bee&mdash;the basket and
-brush on the hind leg. The former is a hollow on the outer surface
-of the tibia, the latter a considerable enlargement of the basal tarsal
-joint, which, at the same time, is covered on the inner surface with
-short bristles, arranged in transverse rows like a brush. The bee
-kneads the pollen into the basket, and one can often see bees flying
-back to the hive with a thick yellow ball of pollen on the hind leg.
-In those bees which collect on the abdomen, like <i>Osmia</i> and <i>Megachile</i>,
-the pollen mass forms a thick clump on the belly, and in the case
-of <i>Andrena</i> Sprengel observed long ago that it sometimes flew with
-a packet of pollen bigger than its own body on the hind leg.</p>
-
-<p>All these are contrivances which have gradually originated
-through the habit of carrying home pollen for the helpless larvæ shut
-up in the cells. They have developed differently in the various
-groups of bees, probably because the primary variations with which
-the process of selection began were different in the various ancestral
-forms.</p>
-
-<p>In the ancestors of those which carry pollen on the abdomen
-there was probably a thick covering of hair on the ventral surface of
-the body, which served as a starting-point for the selection, and,
-in consequence, the further course of the adaptation would be concerned
-solely with this hair-covered surface, while variations in other
-less hairy spots would remain un-utilized.</p>
-
-<p>After all this it will no longer seem a paradoxical statement that
-the existence of gaily coloured, diversely formed, and fragrant flowers
-is due to the visits of insects, and that, on the other hand, many
-insects have undergone essential transformations in their mouth-parts
-and otherwise as an adaptation to a flower diet, and that an entire
-order of insects with thousands of species&mdash;the Lepidoptera&mdash;would
-not be in existence at all if there had been no flowers. We must now
-attempt to show, in a more detailed way, how, by what steps, and
-under what conditions, our modern flowers have arisen from the
-earlier flowering plants. In this I follow closely the classic exposition
-which we owe to Hermann Müller.</p>
-
-<p>The ancestral forms of the modern higher plants, the so-called
-'primitive seed plants' or 'Archisperms,' were all anemophilous, as
-the Conifers and Cycads are still. Their smooth pollen-grains,<span class="pagenum"><a id="Page_194"></a>[Pg 194]</span>
-produced in enormous quantities, fell like clouds of dust into the air,
-were carried by the wind hither and thither, and some occasionally
-alighted on the stigma of a female flower. In these plants the sexes
-often occur separately on different trees or individuals, and there must
-be a certain advantage in this when the pollination is effected by the
-wind.</p>
-
-<p>The male flowers of the Archisperms would be visited by insects
-in remote ages, just as they are now; but the visitors came to feed
-upon the pollen, and did not render any service to the plant in
-return; they rather did it harm by reducing its store of pollen. If it
-was possible to cause the insect to benefit the plant at the same time
-as it was pillaging the pollen, by carrying some of it to female
-blossoms and thereby securing cross-fertilization, it would be of great
-advantage, for the plant would no longer require to produce such
-enormous quantities of pollen, and the fertilization would be much
-more certain than when it depended on the wind. It is obvious that
-the successful pollination of anemophilous plants implies good weather
-and a favourable wind.</p>
-
-<div class="figcenter" id="f54">
-<img src="images/fig54.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 50.</span> Flowers of the Willow (<i>Salix cinerea</i>); after H. Müller. <i>A</i>, the
-male. <i>B</i>, the female catkin. <i>C</i>, individual male flower; <i>n</i>, nectary. <i>D</i>, individual
-female flower; <i>n</i>, nectary. <i>E</i>, Poplar, an exceptional hermaphrodite
-flower.</p>
-</div>
-
-<p>It is plain that the utilization of the insect-visitors in fertilization
-might be secured in either of two ways; the female blossoms might
-also offer something attractive to the insects, or hermaphrodite flowers
-might be formed. As a matter of fact, both ways have been followed
-by Nature. An example of the former is the willow, the cross-fertilization
-of which was forced upon the insects by the development
-in both female and male blossoms of a nectary (Fig. 50, <i>C</i> and <i>D</i>), a
-little pit or basin in which nectar was secreted. The insects flew now
-to male and now to female willow-catkins, and in doing so they<span class="pagenum"><a id="Page_195"></a>[Pg 195]</span>
-carried to the stigma of the female blossom the pollen, which in this
-case was not dusty but sticky, so that it readily adhered to their
-bodies.</p>
-
-<p>The securing of cross-fertilization by the development of hermaphrodite
-flowers has, however, occurred much more frequently, and
-we can understand that this method secured the advantageous crossing
-much more perfectly, for the pollen had necessarily to be carried from
-blossom to blossom, while, in cases like that of the willow, countless
-male blossoms might be visited for nectar one after the other before
-the insect made up its mind to fly to a female blossom of the same
-species. The beginnings of the modification of the unisexual flowers
-in this direction may be seen in variations which occur even now,
-for we not infrequently find, in a male catkin, individual blossoms,
-which, in addition to the stamens, possess also a pistil with a stigma.
-(Fig. 50 <i>E</i> shows such an abnormal hermaphrodite flower from a
-poplar.)</p>
-
-<p>As soon as hermaphrodite flowers came into existence the
-struggle to attract insects began in a more intense degree. Every
-little improvement in this direction would form the starting-point of
-a process of selection, and would be carried on and increased to the
-highest possible pitch of perfection.</p>
-
-<p>It was probably the outer envelopes of the blossoms which first
-changed their original green into other colours, usually those which
-contrasted strongly with the green, and thus directed the attention of
-the insects to the flowers. Variations in the colour of ordinary leaves
-are always cropping up from time to time, whether it be that the
-green is transformed into yellow or that the chlorophyll disappears
-more or less completely and red or blue coloured juices take its place.
-Many insects can undoubtedly see colour, and are attracted by the
-size of coloured flowers, as Hermann Müller found by counting the
-visits of insects to two nearly related species of mallow, one of which,
-<i>Malva silvestris</i>, has very large bright rose-red flowers visible from
-afar, while the other, <i>Malva rotundifolia</i>, has very inconspicuous
-small pale-red flowers. To the former there were thirty-one different
-visitors, to the latter he could only make sure of four. The second
-species, as is to be expected, depends chiefly on self-fertilization.</p>
-
-<p>It has recently been disputed from various quarters that insects
-are attracted by the colours of the flowers, and these objections are
-based chiefly on experiments with artificial flowers. But when, for
-instance, Plateau, in the course of such experiments saw bees and
-butterflies first fly towards the artificial flowers, and then turn away
-and concern themselves no more about them, that only proves that<span class="pagenum"><a id="Page_196"></a>[Pg 196]</span>
-their sight is sharper than we have given them credit for; for though
-they may be deceived at a distance, they are not so when they are
-near; it is possible, too, that the sense of smell turns the scale<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a>.
-I have myself made similar experiments with diurnal butterflies,
-before which I placed a single artificial chrysanthemum midst a mass
-of natural flowers. It rarely happened indeed that a butterfly settled
-on the artificial flower; they usually flew first above it, but did not
-alight. Twice, however, I saw them alight on the artificial flower,
-and eagerly grope about with the proboscis for a few moments, then
-fly quickly away. They had visited the real chrysanthemums or
-horse-daisies with evident delight, and eagerly sucked up the honey
-from the many individual florets of every flower, and they now
-endeavoured to do the same in the artificial flower, and only desisted
-when the attempt proved unsuccessful. In this experiment the
-colours were of course only white and yellow; with red and blue it is
-probably more difficult to give the exact impression of the natural
-flower-colours; and in addition there is the absence of the delicate
-fragrance exhaled by the flower.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> The experiments of Plateau have since been criticized by Kienitz-Gerloff, who
-altogether denies their value (1903).</p>
-
-</div>
-
-<p>It must be allowed that the colour is certainly not the sole
-attraction to the flower; the fragrance helps in most cases, and even
-this is not the object of the insect's visits. The real object is the
-nectar, to which colour and fragrance only show the way. The
-development of fragrance and nectar must, like that of the colour,
-have been carried on and increased by processes of selection, which
-had their basis in the necessity for securing insect-visits, and as soon
-as these main qualities of the flower were established greater refinements
-would begin, and flower-forms would be evolved, which would
-diverge farther and farther, especially in shape, from the originally
-simple and regular form of the blossom.</p>
-
-<p>The reason for this must have lain chiefly in the fact that, after
-insect-visits in general were secured by a flower, it would be advantageous
-to exclude all insects which would pillage the nectar without
-rendering in return the service of cross-fertilization&mdash;all those, therefore,
-which were unsuited either because of their minute size or
-because of the inconstancy of their visits. Before the butterflies and
-the bees existed, the regularly formed flat flower with unconcealed
-nectar would be visited by a mixed company of caddis-flies, saw-flies,
-and ichneumon-flies. But as the nectar changed its place to the
-deeper recesses of the flower it was withdrawn from all but the more
-intelligent insects, and thus the circle of visitors was already narrowed<span class="pagenum"><a id="Page_197"></a>[Pg 197]</span>
-to some extent. But when in a particular species the petals fused
-into a short tube, all visitors were excluded whose mouth-parts were
-too short to reach the nectar; while among those which could reach it
-the process of proboscis-formation began; the under lip, or the first
-maxillæ, or both parts together, lengthened step for step with the
-corolla-tube of the flower, and thus from the caddis-flies came the
-butterflies, and from the ichneumon-flies the burrowing-wasps
-(<i>Sphegidæ</i>) and the bees.</p>
-
-<p>At first sight one might perhaps imagine that it would have been
-more advantageous to the flowers to attract a great many visitors,
-but this is obviously not the case. On the contrary, specialized
-flowers, accessible only to a few visitors, have a much greater certainty
-of being pollinated by them, because insects which only fly to a
-few species are more certain to visit these, and above all to visit many
-flowers of the same species one after another. Hermann Müller
-observed that, in four minutes, one of the humming-bird hawk-moths
-(<i>Macroglossa stellatarum</i>) visited 108 different flowers of the same
-species, the beautiful Alpine violet (<i>Viola calcarata</i>), one after the
-other, and it may have effected an equal number of pollinations in
-that short time.</p>
-
-<p>It was, therefore, a real advantage to the flowers to narrow their
-circle of visitors more and more by varying so that only the useful
-visitors could gain access to their nectar, and that the rest should be
-excluded. Thus there arose 'bee-flowers,' 'butterfly-flowers,' 'hawk-moth
-flowers,' and, indeed, in many cases, a species of flower has become
-so highly specialized that its fertilization can only be brought about by
-a single species of insect. This explains the remarkable adaptations
-of the orchids and the enormous length of the proboscis in certain
-butterflies. Even our own hawk-moths <i>Macroglossa stellatarum</i> and
-<i>Sphinx convolvuli</i> show an astonishing length of proboscis, which
-measures 8 cm. in the latter species. In <i>Macrosilia cluentius</i>, in
-Brazil, the proboscis is 20 cm. in length; and in Madagascar there
-grows an orchid with nectaries 30 cm. in length, filled with nectar to
-a depth of 2 cm., but the fertilizing hawk-moth is not yet known.</p>
-
-<p>Thus we may say that the flowers, by varying in one direction or
-another, have selected a definite circle of visitors, and, conversely, that
-particular insect-groups have selected particular flowers for themselves,
-for those transformations of the flowers were always most
-advantageous which secured to them the exclusive visits of their best
-crossing agents, and these transformations were, on the one hand, such
-as kept off unwelcome visitors, and, on the other hand, such as
-attracted the most suitable ones.</p>
-
-<p><span class="pagenum"><a id="Page_198"></a>[Pg 198]</span></p>
-
-<p>From the botanical point of view the assumption that flowers
-and flower-visiting insects have been adapted to each other by means
-of processes of selection has been regarded as untenable, because
-every variation in the flower presupposes a corresponding one in the
-insect. I should not have mentioned this objection had it not come
-from such a famous naturalist as Nägeli, and if it were not both
-interesting and useful in our present discussion. Nägeli maintained
-that selection could not, for instance, have effected a lengthening of
-the corolla-tube of a flower, because the proboscis of the insects must
-have lengthened <i>simultaneously</i> with it. If the corolla-tube had
-lengthened alone, without the proboscis of the butterfly being at
-the same time elongated, the flower would no longer be fertilized
-at all, and if the lengthening of the proboscis preceded that of the
-corolla-tube it would have no value for the butterfly, and could
-not therefore have been the object of a process of selection.</p>
-
-<p>This objection overlooks the facts that a species of plant and of
-butterfly consists not of one individual but of thousands or millions,
-and that these are not absolutely uniform, but in fact heterogeneous.
-It is precisely in this that the struggle for existence consists&mdash;that
-the individuals of every species differ from one another, and that
-some are better, others less well constituted. The elimination of the
-latter and the preferring of the former constitutes the process of
-selection, which always secures the fitter by continually rejecting the
-less fit. In the case we are considering, then, there would be, among
-the individuals of the plant-species concerned, flowers with a longer
-and flowers with a shorter corolla-tube, and among the butterflies
-some with a longer and some with a shorter proboscis. If among the
-flowers the longer ones were more certain to be cross-fertilized than
-the shorter ones, because hurtful visitors were better excluded, the
-longer ones would produce more and better seeds, and would transmit
-their character to more descendants; and if, among the butterflies,
-those with the longer proboscis had an advantage, because the nectar
-in the longer tubes would, so to speak, be reserved for them, and they
-would thus be better nourished than those with the shorter proboscis,
-the number of individuals with long proboscis must have increased from
-generation to generation. Thus the length of the corolla-tube and
-the length of the proboscis would go on increasing as long as there
-was any advantage in it for the flower, and both parties must of
-necessity have varied <i>pari passu</i>, since every lengthening of the
-corolla was accompanied by a preferring of the longest proboscis
-variation. The augmentation of the characters depended on, and
-could only have depended on, a guiding of the variations in the<span class="pagenum"><a id="Page_199"></a>[Pg 199]</span>
-direction of utility. But this is exactly what we call, after Darwin
-and Wallace, Natural Selection.</p>
-
-<p>We have, however, in the history of flowers, a means of
-demonstrating the reality of the processes of selection in two other
-ways. In the first place, it is obvious that no other interpretation
-can be given of such simultaneous mutual adaptations of two
-different kinds of organisms. If we were to postulate, as Nägeli, for
-instance, did, an intrinsic Power of Development in organisms, which
-produces and guides their variations, we should, as I have already
-said, be compelled also to take for granted a kind of pre-established
-harmony, such as Leibnitz assumed to account for the correlation of
-body and mind: plant and insect must always have been correspondingly
-altered so that they bore the same relation to each other
-as two clocks which were so exactly fashioned that they always kept
-time, though they did not influence each other. But the case would
-be more complicated than that of the clocks, because the changes
-which must have taken place on both sides were quite different, and
-yet at the same time such that they corresponded as exactly as Will
-and Action. The whole history of the earth and of the forms of life
-must, therefore, have been foreseen down to the smallest details, and
-embodied in the postulated Power of Development.</p>
-
-<p>But such an assumption could hardly lay claim to the rank of
-a scientific hypothesis. Although every grain of sand blown about
-by the wind on this earth could certainly only have fallen where it
-actually did fall, yet it is in the power of any of us to throw a handful
-of sand wherever it pleases us, and although even this act of
-throwing must have had its sufficient reason in us, yet no one could
-maintain that its direction and the places where the grains fell were
-predestined in the history of the earth. In other words: That which
-we call chance plays a part also in the evolution of organisms, and the
-assumption of a Power of Development, predestinating even in detail,
-is contradicted by the fact that species are transformed in accordance
-with the chance conditions of their life.</p>
-
-<p>This can be clearly demonstrated in the case of flowers. That
-the wild pansy (<i>Viola tricolor</i>), which lives in the plains and on
-mountains of moderate elevation, is fertilized by bees, and the nearly
-allied <i>Viola calcarata</i> of the High Alps by Lepidoptera, is readily
-intelligible, since bees are very abundant in the lower region, and
-make the fertilization of the species a certainty, while this is not so
-in the High Alps. There the Lepidoptera are greatly in the majority,
-as every one knows who has traversed the flower-decked meads of
-the High Alps in July, and has seen the hundreds and thousands<span class="pagenum"><a id="Page_200"></a>[Pg 200]</span>
-of butterflies and moths which fly from flower to flower. Thus the
-viola of the High Alps has become a 'butterfly-flower' by the development
-of its nectaries into a long spur, accessible only to the proboscis
-of a moth or butterfly. The chance which led certain individuals of
-the ancestral species to climb the Alps must also have supplied the
-incentive to the production of the changes adapted to the visits of
-the prevalent insect. The hypothesis of a predestinating Power
-of Development suffers utter shipwreck in face of facts like these.</p>
-
-<p>We have, furthermore, an excellent touchstone for the reality of
-the processes of selection in the <i>quality</i> of the variations in flowers
-and insects. Natural selection can only bring about those changes
-which are of use to the possessors themselves; we should therefore
-expect to find among flowers only such arrangements as are, directly or
-indirectly, of use to them, and, conversely, among insects only such as
-are useful to the insect.</p>
-
-<p>And this is what we actually do find. All the arrangements of
-the flowers&mdash;their colour, their form, their honey-guides, their hairy
-honey-paths (Iris), their fragrance, and their honey itself&mdash;are all
-indirectly useful to the plant itself, because they all co-operate in
-compelling the honey-seeking insect to effect the fertilization of the
-flower. This is most clearly seen in the case of the so-called
-'Deceptive' flowers, which attract insects by their size and beauty,
-their fragrance, and their resemblance to other flowers, and force
-their visitors to be the means of their cross-fertilization, although
-they contain no nectar at all. This is the case, according to Hermann
-Müller, with the most beautiful of our indigenous orchids, the lady's
-slipper (<i>Cypripedium calceolaris</i>). This flower is visited by bees of
-the genus <i>Andrena</i>, which creep into the large wooden-shoe-shaped
-under lip in the search for honey, only to find themselves prisoners,
-for they cannot get out, at least by the way they came in, because of
-the steep and smoothly polished walls of the flower. There is only
-one way for the bee; it must force itself under the stigma, which it
-can only do with great exertion, and not without being smeared with
-pollen, which it carries to the next flower into which it creeps. It
-can only leave this one in the same way, and thus the pollen is transferred
-to the stigma by a mechanical necessity.</p>
-
-<p>Such remarkable cases remind us in some ways of those cases
-of mimicry in which the deceptions have to be used with caution or
-they lose their effect. One might be disposed to imagine that such
-an intelligent insect as a bee would not be deceived by the lady's
-slipper more than once, and would not creep into a second flower
-after discovering that there was no nectar in the first. But this<span class="pagenum"><a id="Page_201"></a>[Pg 201]</span>
-conclusion is not correct, for the bees are well accustomed in many
-flowers to find that the nectar has already been taken by other bees;
-they could therefore not conclude from one unsuccessful visit that
-the <i>Cypripedium</i> did not produce nectar at all, but would try again
-in a second, a third, and a fourth flower. If these orchids had
-abundantly covered flower-spikes like many species of <i>Orchis</i>, and if
-the species were common, the bees would probably soon learn not to
-visit them, but the reverse is the case. There is usually only one or,
-at most, two open flowers on the lady's slipper, and the plant is rare,
-and probably occurs nowhere in large numbers.</p>
-
-<p>If we could find a flower in which the nectar lay open and accessible
-to all insects, and which did not require any service from them in
-return, the case could not be interpreted in terms of natural selection;
-but we do not know of any such case.</p>
-
-<div class="figright" id="f55">
-<img src="images/fig55.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 51.</span> The Yucca-moth (<i>Pronuba<br />
-yuccasella</i>). <i>M</i>, laying eggs in<br />
-the ovary of the Yucca flower.<br />
-<i>n</i>, the stigma. After Riley.</p>
-</div>
-
-<p>Conversely, too, there are no adaptations
-in the insects which are useful
-only to the flowers, and which are not
-of some use, directly or indirectly, to
-the insect itself. Bees and butterflies
-certainly carry the pollen from one
-flower to the stigma of another, but
-they are not impelled to do this by
-a special instinct; they are forced to do
-it by the structure of the flower, which
-has its stamens so placed and arranged
-that they must shake their pollen over
-the visitor, or it may be that the anthers
-are modified into stalked, viscid pollinia
-which spring off at a touch, and fix
-themselves, so to speak, on the insect's head. And even this is not all
-in the case of the orchis, for the insect would never of its own accord
-transfer these pollinia on to the stigma of the next flower; this is
-effected by the physical peculiarity which causes the pollinia, after
-a short time, to bend forwards on the insect's head.</p>
-
-<p>All this fits in as well as possible with the hypothesis: how
-could an instinct to carry pollen from one flower to the stigma of
-another have been developed in an insect through natural selection,
-since the insect itself has nothing to gain from this proceeding?
-Accordingly, we never find in the insect any pincers or any kind of
-grasping organ adapted for seizing and transmitting the pollen.</p>
-
-<p>There is, however, one very remarkable case in which this
-appears to be so, indeed really is so, and nevertheless it is not<span class="pagenum"><a id="Page_202"></a>[Pg 202]</span>
-contradictory to, but is corroborative of, the theory of selection. The
-excellent American entomologist, Riley, established by means of
-careful observations that the large white flowers of the Yucca are
-fertilized by a little moth which behaves in a manner otherwise
-unheard of among insects. Only the females visit the flowers, and
-they at once busy themselves collecting a large ball of pollen. To
-this end they have on the maxillary palps (Fig. 52, <i>C</i>, <i>mxp</i>) a long
-process (<i>si</i>), curved in the form of a sickle, and covered with hairs,
-which probably no other Lepidopteron possesses, with the help of
-which the moth very quickly sweeps together a ball of pollen, it
-may be three times the size of her own head. With this ball the
-insect flies to the next flower, and there she lays her egg, by means of
-an ovipositor otherwise unknown among Lepidoptera (Fig. 52, <i>A</i>, <i>op</i>),
-in the pods of the flower. Finally, she pushes the ball of pollen deep
-into the funnel-shaped stigmatic opening on the pistil (Fig. 51, <i>n</i>), and
-so effects the cross-fertilization. The ovules develop, and when the
-caterpillars emerge from the egg four to five days later they feed on
-these until they are ready to enter on the pupa stage. Each little
-caterpillar requires about eighteen or twenty seeds for its nourishment
-(Fig. 52, <i>B</i>, <i>r</i>).</p>
-
-<div class="figcenter" id="f56">
-<img src="images/fig56.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 52.</span> The fertilization of the Yucca. <i>A</i>, ovipositor of the Yucca-moth.
-<i>op</i>, its sheath. <i>sp</i>, its apex. <i>op</i><sup>1</sup>, the protruded oviduct. <i>B</i>, two ovaries of the
-Yucca, showing the holes by which the young moths escape, and (<i>r</i>) a
-caterpillar in the interior. <i>C</i>, head of the female moth, with the sickle-shaped
-process (<i>si</i>) on the maxillary palps for sweeping off the pollen and rolling it
-into a ball. <i>mx</i><sup>1</sup>, the proboscis. <i>au</i>, eye. <i>p</i><sup>1</sup> base of first leg. <i>D</i>, longitudinal
-section through an ovary of the Yucca, soon after the laying of two eggs (<i>ei</i>).
-<i>stk</i>, the canal made by the ovipositor.</p>
-</div>
-
-<p><span class="pagenum"><a id="Page_203"></a>[Pg 203]</span></p>
-
-<p>Here, then, we find an adaptation of certain parts of the moth's
-body in relation to the fertilization of the flower, but in this case
-it is as much in the interest of the moth as of the plant. By
-carrying the pollen to the stigma the moths secure the development
-of the ovules, which serve their offspring as food, so that we have
-here to do with a peculiar form of care for offspring, which is not
-more remarkable than many other kinds of brood-care in insects,
-such as ants, bees, Sphex-wasps, ichneumon-flies, and gall-flies.</p>
-
-<p>It might be objected that this case of the Yucca is not so much
-one of effecting fertilization as of parasitism; but the eggs, which are
-laid in the seed-pods, are very few, and the caterpillars which emerge
-from them only devour a very small proportion of the seeds, of which
-there may be about 200 (Fig. 52, <i>B</i>). Thus the plants also derive
-an advantage from the moth's procedure, for quite enough seeds are
-left. The form and position of the stamens and of the stigma seem
-to be as exactly adapted to the visits of the moth as the moth is to
-the transference of the pollen, for the Yucca can only be fertilized by
-this one moth, and sets no seed if the moth be absent. For this
-reason the species of Yucca cultivated in Europe remain sterile.</p>
-
-<p>Thus the apparent contradiction is explained, and the facts
-everywhere support the hypothesis that the adaptations between
-flowers and insects depend upon processes of selection.</p>
-
-<p>This origin is incontrovertibly proved, it seems to me, in another
-way, namely, by the merely <i>relative</i> perfection of the adaptations, or
-rather, by their relative imperfection.</p>
-
-<p>I have already pointed out that all adaptations which depend
-upon natural selection can only be relatively perfect, as follows from
-the nature of their efficient causes, for natural selection only operates
-as long as a further increase of the character concerned would be of
-advantage to the existence of the species. It cannot be operative
-beyond this point, because the existence of the species cannot be more
-perfectly secured in this direction, or, to speak more precisely, because
-further variations in the direction hitherto followed would no longer
-be improvements, even though they might appear so to us.</p>
-
-<p>Thus the corolla of many flowers is suited to the thick, hairy
-head and thorax of the bee, for to these only does the pollen adhere
-in sufficient quantity to fertilize the next flower; yet the same flowers
-are frequently visited by butterflies, and in many of them there has
-been no adaptation to prevent these useless visits. Obviously this is
-because preventive arrangements could only begin, according to our
-theory, when they were necessary to the preservation of the species;
-in this case, therefore, only when the pillaging visits of the butterflies<span class="pagenum"><a id="Page_204"></a>[Pg 204]</span>
-withdrew so many flowers from the influence of the effective
-pollinating visitor, the bee, that too few seeds were formed, and the
-survival of the species was threatened by the continual dwindling of
-the normal number. As long as the bees visit the flowers frequently
-enough to ensure the formation of the necessary number of seeds
-a process of selection could not set in; but should the bees find, for
-instance, that nearly all the flowers had been robbed of their nectar,
-and should therefore visit them less diligently, then every variation of
-the flower which made honey less accessible to the butterflies would
-become the objective of a process of selection.</p>
-
-<p>Everywhere we find similar imperfections of adaptation which
-indicate that they must depend on processes of selection. Thus
-numerous flowers are visited by insects other than those which
-pollinate them, and these bring them no advantage, but merely rob
-them of nectar and pollen; the most beautiful contrivances of many
-flowers, such as <i>Glycinia</i>, which are directed towards cross-fertilization
-by bees, are rendered of no effect because wood-bees and humble-bees
-bite holes into the nectaries from the outside, and so reach the nectar
-by the shortest way. I do not know whether bees in the native
-land of the <i>Glycinia</i> do the same thing, but in any case they can do
-no sensible injury to the species, since otherwise processes of selection
-would have set in which would have prevented the damage in some
-way or other, whether by the production of stinging-hairs, or hairs
-with a burning secretion, or in some other way. If the actual
-constitution of the plant made this impossible, the species would
-become less abundant and would gradually die out.</p>
-
-<p>Thus the relative imperfection of the flower-adaptations, which
-in general are so worthy of admiration, affords a further indication
-that their origin is due to processes of selection.</p>
-
-
-<p class="c">ADDITIONAL NOTE TO CHAPTER X.</p>
-
-<p>It has been remarked that the chapter on the Origin of Flowers
-in the German Edition contains no discussion and refutation of the
-objections which have up till recently been urged against the theory
-of flowers propounded by Darwin and Hermann Müller. I admit
-that this chapter seemed to be so harmonious and so well rounded,
-and at the same time so convincing as to the reality of the processes
-of selection, that the feeble objections to it, and the attempts of
-opponents to find another explanation of the phenomena, might well
-be disregarded in this book.</p>
-
-<p>However, the most important of these objections and counter-theories
-may here be briefly mentioned.</p>
-
-<p><span class="pagenum"><a id="Page_205"></a>[Pg 205]</span></p>
-
-<p>Plateau in Ghent was the first to collect <i>facts</i> which appeared to
-contradict the Darwinian theory of flowers; he observed that insects
-avoided <i>artificial</i> flowers, even when they were indistinguishable in
-colour from natural ones as far as our eyes could perceive, and he
-concluded from this that it is not the colour which guides the insects
-to the flowers, that they find the blossoms less by their sense of
-sight than by their sense of smell. But great caution is required in
-drawing conclusions from experiments of this kind. I once placed
-artificial marguerites (<i>Chrysanthemum leucanthemum</i>) among natural
-ones in a roomy frame in the open air, and for a considerable time
-I was unable to see any of the numerous butterflies (<i>Vanessa urticæ</i>),
-which were flying about the real chrysanthemums, settle on one of
-the artificial flowers. The insects often flew quite close to them without
-paying them the least attention, and I was inclined to conclude that
-they either perceived the difference at sight, or that they missed the
-odour of the natural flowers in the artificial ones. But in the course
-of a few days it happened twice in my presence that a butterfly
-settled on one of the artificial blooms and <i>persistently groped about
-with fully outstretched tube to find the entrance to the honey</i>. It was
-only after prolonged futile attempts that it desisted and flew away.
-That bees are guided by the eye in their visits to flowers has been
-shown by A. Forel, who cut off the whole proboscis, together with the
-antennæ, from humble-bees which were swarming eagerly about the
-flowers. He thus robbed them of the whole apparatus of smell, and
-nevertheless they flew down from a considerable height direct to the
-same flowers. An English observer, Mr. G. N. Bulman, has been led
-to believe, with Plateau, that it is a matter of entire indifference to
-the bees whether the flowers are blue, or red, or simply green in
-colour, if only they contain honey, and that therefore the bees could
-have played no part in the development of blue flowers, as Hermann
-Müller assumed they had, and that they could have no preference for
-blue or any other colour, as Sir John Lubbock and others had concluded
-from their experiments. This is correct in so far that bees
-feed as eagerly on the greenish blossoms of the lime-tree as they do
-on the deep-blue gentian of the Alpine meadows or the red blossoms
-of the Weigelia, the dog-roses of our gardens or the yellow buttercups
-(<i>Ranunculus</i>) of our meadows; they despise nothing that
-yields them honey. But it certainly does not follow from this
-that the bees may not, under certain circumstances, have exercised
-a selecting influence upon the fixation and intensification
-of a new colour-variety of a flower. This is less a question of
-a <i>colour-preference</i>, in the human sense, on the part of the<span class="pagenum"><a id="Page_206"></a>[Pg 206]</span>
-bees than of the <i>greater visibility</i> of the colour in question in the
-environment peculiar to the flower, and of the amount of rivalry
-the bees meet with from other insects in regard to the same flower. In
-individual cases this would be difficult to demonstrate, especially since
-we can form only an approximate idea of the insect's power of seeing
-colour, and cannot judge what the colours of the individual blossoms
-count for in the mosaic picture of a flowery meadow. Yet this is the
-important point, for, as soon as the bees perceive one colour more
-readily than another, the preponderance of this colour-variety over
-other variations is assured, since it will be more frequently visited.
-In the same way we cannot guess in individual cases why one species
-of flower should exhale perfume while a nearly related species does
-not. But when we remember that many flowers adapted for the
-visits of dipterous insects possess a nauseous carrion-like smell, by
-means of which they not only attract flies but scare off other
-insects, we can readily imagine cases in which it was of importance
-to a flower to be able to be easily found by bees without
-betraying itself by its pleasant fragrance to other less desirable
-visitors.</p>
-
-<p>Thus, therefore, we can understand the odourless but intensely
-blue species of gentian, if we may assume that its blue colour is more
-visible to bees than to other insects. If I were to elaborate in detail
-all the principles which here suggest themselves to me I should
-require to write a complete section, and I am unwilling to do this
-until I can bring forward a much larger number of new observations
-than I am at present in a position to do. All I wish to do
-here is to exhort doubters to modesty, and to remind them that these
-matters are exceedingly complex, and that we should be glad and
-grateful that expert observers like Darwin and Hermann Müller have
-given us some insight into the principles interconnecting the facts,
-instead of imagining whenever we meet with some little apparently
-contradictory fact, which may indeed be quite correct in itself,
-that the whole theory of the development of flowers through
-insects has been overthrown. Let us rather endeavour to understand
-such facts, and to arrange them in their places as stones of
-the new building.</p>
-
-<p>Often the contradiction is merely the result of the imperfect
-theoretical conceptions of its discoverer, as we have already shown in
-regard to Nägeli. Bulman, too, fancies he has proved that bees do
-not distinguish between the different varieties of a flower, but visit
-them indiscriminately with the same eagerness, thus causing intercrossing
-of all the varieties, and preventing any one from becoming<span class="pagenum"><a id="Page_207"></a>[Pg 207]</span>
-dominant. But are the varieties which we plant side by side in our
-gardens of the kind that are evolved by bees? That is to say, are
-their <i>differences such as will turn the scale for or against the visits of
-the bees</i>? If one were less, another more easily seen by the bees; or if
-one were more fragrant, or had a fragrance more agreeable to bees
-than the other, the result of the experiment would probably have
-been very different.</p>
-
-<p>One more objection has been made. It is said that the bees,
-although exclusively restricted, both themselves and their descendants,
-to a diet of flowers, are not so constant <i>to a particular flower</i> as the
-theory requires. They do indeed exhibit a 'considerable amount of
-constancy,' and often visit a large number of flowers of the same
-species in succession, but the theory requires that they should not
-only confine themselves to this one species, but to a <i>single variety</i> of
-this species. These views show that their authors have not penetrated
-far towards an understanding of the nature of selection. Nature does
-not operate with individual flowers, but with millions and myriads of
-them, and not with the flowers of a single spring, but with those of
-hundreds and thousands of years. How often a particular bee may
-carry pollen uselessly to a strange flower without thereby lowering
-the aggregate of seeds so far that the existence of the species seems
-imperilled, or how often she may fertilize the pistil of a useful variation
-with the pollen of the parent species, without interrupting or
-hindering the process of the evolution of the variety, no mortal can calculate,
-and what the theory requires can only be formulated in this way:
-The constancy of the bees in their visits to the flowers must be so great
-that, on an average, the quantity of seeds will be formed which suffices
-for the preservation of the species. And in regard to the transformation
-of a species, the attraction which the useful variety has for the bees
-must, on an average, be <i>somewhat stronger</i> than that of the parent species.
-As soon as this is the case the seeds of the variety will be formed in
-preponderant numbers, although they may not all be quite pure from
-the first, and by degrees, in the course of generations, the plants of the
-new variety will preponderate more and more over those of the parent
-form, and finally will alone remain. In the first case we have before
-our eyes the proof that, in spite of the imperfect constancy of the bees,
-a sufficient number of seeds is produced to secure the existence of the
-species. Or does Mr. Bulman conclude from the fact that the bees are
-<i>not absolutely constant</i> that flowers are not fertilized by bees at all?</p>
-
-<p>I cannot conclude this note without touching briefly upon what
-the opponents of the flower theory have contributed, and what
-explanation of the facts they are prepared to offer.</p>
-
-<p><span class="pagenum"><a id="Page_208"></a>[Pg 208]</span></p>
-
-<p>In his important work, <i>Mechanische-physiologische Theorie der
-Abstammungslehre</i>, published in 1884, Nägeli, as a convinced
-opponent of the theory of selection, attempted an explanation. He
-was quite aware that his assumption of an inward 'perfecting
-principle' would not suffice to explain the mutual adaptations of
-flowers and insects, and he refers the transformation of the first
-inconspicuous blossoms into flowers to the mechanical stimulus which
-the visiting insects exerted upon the parts of the blossom. By the
-pressure of their footsteps, the pushing and probing with their
-proboscis, they have, he says, transformed gradually, for instance, the
-little covering leaves at the base of a pollen vessel into large flower
-petals, caused the conversion of short flower-tubes into long ones,
-and of the pollen, once dry and dusty, into the firmly adhesive
-mass formed in the anther lobes of our modern flowers. The colour of
-the flowers depends, according to him, upon the influence of light, which
-certainly no more explains the yellow ring on a blue ground in the
-forget-me-not than it does the many other nectar-guides which show
-the insect the way to the honey. Nägeli works with the Lamarckian
-principle in the most daring way, and with the same <i>naïveté</i> as
-Lamarck himself in his time, that is, without offering any sort of
-explanation as to how the minute impression made, say by the foot
-or by the proboscis of an insect, upon a flower, is to be handed on to
-the flowers of succeeding generations. He treats the unending chain
-of generations as if it were a single individual, and operates with his
-'secular' stimulus, and with 'weak stimuli, lasting through countless
-generations,' as though they were a proved fact. But I have not
-even touched upon the question as to whether these 'stimuli' could
-produce the changes he ascribes to them, even if they were continually
-affecting the flower. How the scale-like covering leaves of the
-pollen vessels could become larger and petal-like through the treading
-of an insect's foot is as difficult to see as why a honey-tube should
-<i>become longer</i> because of the butterfly's honey-sucking: might it not
-just as well become <i>wider</i>, <i>narrower</i>, or even <i>shorter</i>? I see no convincing
-reason why it should become <i>longer</i>! And even if it did
-so, it would necessarily continue to lengthen as time went on, and
-this is not the case, for we find corolla-tubes of all possible lengths,
-but, <i>it is to be noted, always in harmony with the length of the
-proboscis of the visiting insect</i>. In a similar way Henslow has
-recently attempted to refer the origin of flowers to the mechanical
-stimulus exercised upon it by the visiting insects. 'An insect hanging
-to the lower petal of a flower elongates the same by its weight, and
-the lengthened petal is transmitted by heredity.'...'The irritation<span class="pagenum"><a id="Page_209"></a>[Pg 209]</span>
-caused by its feet in walking along the flower causes the appearance
-of colouring matter, and the colour is likewise transmitted.'...'As it
-probes for honey it causes a flow of sweet sap to that part, and this
-also becomes hereditary!'</p>
-
-<p>In this case, also, it is simply taken for granted that every
-little passing irritation not only produces a perceptible effect, but
-that this effect is transmissible. In a later lecture we shall have to
-discuss in detail the question of the inheritance of functional modifications.
-It is enough to say here that, if this kind of transmission
-really took place even in the case of such minute and transitory
-changes, there could be no dispute as to the correctness of the
-'Lamarckian principle,' since every fairly strong and lasting irritation
-could be demonstrated with certainty to produce an effect. When
-a butterfly, floating freely in the air, sucks honey from a tube, the
-irritation must be almost analogous to that caused by a comb lightly
-drawn by some one through our hair, and this is supposed to effect
-the gradual lengthening of the corolla-tube of the flower!</p>
-
-<p>The secretion of honey, too, depends upon the persistent irritation
-of the proboscis! Then 'deceptive flowers,' like the Cypripedium we
-have mentioned, could not exist at all, for they contain no honey,
-although the proboscis of the bee must cause the same irritation in
-them as in other orchids which do contain honey. This whole
-'theory' of direct effect is, moreover, only a crude and apparent interpretation,
-which explains the conditions only in so far as they can
-be seen from a distance; it fails as soon as they are more exactly
-examined; all the great differences in the position of the honey, its
-concealment from intelligent insects, its protection from rain by
-means of hairs, and against unwelcome guests by a sticky secretion,
-the development of a corolla-tube which corresponds in length
-to the length of the visiting insect's proboscis, the development of
-spurs on the flower, in short, all the numerous contrivances which
-have reference to cross-fertilization by insects remain quite unintelligible
-in the light of this theory&mdash;it is a mere <i>pis aller</i> explanation
-for those who continue to struggle against accepting the theory of
-selection.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_210"></a>[Pg 210]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XI">LECTURE XI</h2>
-</div>
-
-<p class="c">SEXUAL SELECTION</p>
-
-<div class="blockquot">
-
-<p>Decorative colouring of male butterflies and birds&mdash;Wallace's interpretation&mdash;Preponderance
-of males&mdash;Choice of the females&mdash;Sense by sight in butterflies&mdash;Attractive
-odours&mdash;Scent-scales&mdash;Fragrance of the females&mdash;The limits of natural
-and sexual selection not clearly defined&mdash;Odours of particular species&mdash;Odours of
-other animals at the breeding season&mdash;Song of the Cicadas, and of birds&mdash;Diversity
-of decoration successively acquired&mdash;Humming-birds&mdash;Substitution of other aids to
-wooing in place of personal decoration&mdash;Smelling organs of male insects and crabs&mdash;Contrivances
-for seizing and holding the female&mdash;Small size of certain males&mdash;Weapons
-of males used in struggle for the females&mdash;Turban eyes of Ephemerids&mdash;Hoods that
-can be inflated on the head of birds&mdash;Absence of secondary sexual characters in lower
-animals&mdash;Transference of male characters to the females&mdash;Lycæna&mdash;Parrots&mdash;Fashion
-operative in the phyletic modifications of colour&mdash;Pattern of markings on the upper
-surface of a butterfly's wing simpler than on the under side&mdash;Conclusion.</p></div>
-
-
-<p><span class="smcap">We</span> found in the process of Natural Selection an explanation
-of numerous effective adaptations in plants and animals, as regards
-form, colouring, and metabolism, of the most diverse weapons and
-protective devices, of the existence of those forms of blossoms which
-we call flowers, of instincts, and so on. The origin of the most
-characteristic parts of whole orders of insects can only be understood
-as adaptations to the environment brought about by means of natural
-selection. Impressed by this, we have now to ask whether <i>all</i> the
-transformations of organisms may not be referred to adaptation
-to the continually changing conditions of life? We shall return to
-this question later, but in the meantime we are far from being
-able to answer it in the affirmative, for there are undoubtedly
-a great many characters, at least in animals, which cannot have
-owed their origin to natural selection in the form in which we have
-studied it so far.</p>
-
-<p>How could the splendid plumage of the humming-birds, of the
-pheasants, of the parrots, the wonderful colour-patterns of so many
-diurnal butterflies, be referred to the process of natural selection,
-since all these characters can have no significance for their possessors
-in the struggle for existence? Or of what use in the struggle for
-existence could the possession of its gorgeous dress of feathers be to
-the bird of Paradise; or of what service is the azure blue iridescence
-of the <i>Morpho</i> of Brazil, which makes it conspicuous from a distance<span class="pagenum"><a id="Page_211"></a>[Pg 211]</span>
-when it plays about the crowns of the palm-trees? We might indeed
-suppose that they are warning signs of unpalatableness, like those of
-the Heliconiides or of the gaily coloured caterpillars, but, in the first
-place, these gay creatures are by no means inedible, and are indeed
-much persecuted; and, secondly, the females have quite different and
-very much darker and simpler colours. The gleaming splendour of
-all these birds of Paradise and humming-birds, as well as that of
-many butterflies, is found in the male sex only. The females of the
-birds just mentioned are dark in colour and without the sparkling
-decorative feathers of the males; they are plain&mdash;just like the
-females of many butterflies. Alfred Russel Wallace has suggested
-that the explanation of this lies in the greater need of the females for
-protection, since, as is well known, they usually perform the labours
-of brooding, and are thus frequently exposed to the attacks of
-enemies. It is undoubtedly true that the dark and inconspicuous
-colouring of many birds and butterflies depends on this need for
-protection, but this does not explain the brilliant colours of the males
-of these species. Or can it be that these require no explanation
-further than that they are, so to speak, a chance secondary outcome of
-the structural relations of the feathers and wing-scales respectively,
-which brought with it some other advantage not known to us?
-Perhaps something in the same way as the red colour of the blood in
-all vertebrates, from fishes upwards, cannot be useful on the ground
-that it appears red to us, but because it is the expression of the
-chemical constitution of the hæmoglobin, a body which is indispensable
-to the metabolism, which here has the secondary and
-intrinsically quite unimportant peculiarity of reflecting the red rays
-of light.</p>
-
-<p>No one can seriously believe this in regard to butterflies who
-knows that their colours are dependent on the scales which thickly
-cover the wings, and the significance of which, in part at least, is
-just to give this or that colour to the wing. They are degenerate or
-colourless among the transparent-winged butterflies, and their colour
-depends partly on pigment, partly on fluorescence and interference
-conditioned by the fine microscopical structure of a system of intercrossing
-lines on faintly coloured scales. The scales of our male 'blue'
-butterflies (<i>Lycæna</i>) only appear blue because of their structure, while
-the brown scales of their mates are due to a brown pigment. If the
-pigment be removed from the scales of the female by boiling
-with caustic potash, and they be then dried, they do not look blue like
-those of the male; the scales of the male, therefore, must possess
-something which those of the female do not.</p>
-
-<p><span class="pagenum"><a id="Page_212"></a>[Pg 212]</span></p>
-
-<p>Still less will any one be disposed to regard the marvellous
-splendour of the plumage of the male bird of Paradise, with its
-erectile collars&mdash;glistening like burnished metal&mdash;on the neck, breast
-or shoulders, with its tufts, with its specially decorative feathers
-standing singly out from the rest of the plumage, on head, wings, or
-tail, with its mane-like bunch of loose, pendulous feathers on the
-belly and on the sides, in short, with its extraordinary, diverse, and
-unique equipment of feathers, as a mere unintentional accessory effect
-of a feather dress designed for flight and protective warmth. Such
-conspicuous, diverse, and unusual specializations of plumage must
-have some other significance than that just indicated.</p>
-
-<p>Alfred Russel Wallace regards these distinctive features of the
-male as an expression of the greater vigour, and the more active
-metabolism of the males, but it is unproved that the vigour of the
-male birds is greater than that of the females, and it is not easy to see
-why a more active metabolism should be necessary for the production
-of strikingly bright colours than for that of a dark or protective
-colour. Moreover, there are brilliantly coloured females, both among
-birds and butterflies, and in nearly allied species the males may be
-either gorgeous or quite plain like the females.</p>
-
-<p>Darwin refers the origin of these secondary sexual characters to
-processes of selection quite analogous to those of ordinary natural
-selection, only that in this case it is not the maintenance of the
-species which is aimed at, but the attainment of reproduction by the
-single individual. The males are to some extent obliged to struggle
-for the possession of the females, and every little variation which
-enables a male to gain possession of a female more readily than his
-neighbour has for this reason a greater likelihood of being transmitted
-to descendants. Thus, attractive variations which once crop up
-will be transmitted to more and more numerous males of the species,
-and among these it will always be those possessing the character
-in question in the highest degree which will have the best chance
-of securing a mate, and so the character will continue to be
-augmented as long as variations in this direction appear.</p>
-
-<p>Two kinds of preliminary conditions, however, must be assumed.
-As the ordinary natural selection could never have operated but for
-the fact that in every generation a great many individuals, indeed the
-majority of them, perish before they have had time to reproduce,
-so the process of sexual selection could never have come into
-operation if every male were able ultimately to secure a mate, no
-matter what degree of attractiveness to the latter he possessed. If
-the numbers of males and females were equal, so that there was<span class="pagenum"><a id="Page_213"></a>[Pg 213]</span>
-always one female to one male, there could be no choice exercised
-either by male or female, for there would always remain individuals
-enough of both sexes, so that no male need remain unmated.</p>
-
-<p>But this is not the case: the proportions of the sexes are very
-rarely as 1 : 1; there is usually a preponderating number of males,
-more rarely of females. Among birds the males are usually in the
-majority, still more so among fishes; and among diurnal butterflies
-there are often a hundred males to one female (Bates), although there
-seem to be a few tropical Papilionidæ among which the females have
-rather the preponderance. Darwin called attention to the fact that
-one could infer the greater rarity of the females even from the pricelists
-of butterflies issued by the late Dr. Staudinger in connexion
-with his business, for the females in most species, except the very
-common ones, are priced much higher than the males, often twice as
-high. In the whole list of many thousands of species there are only
-eleven species of nocturnal Lepidoptera in which the males are dearer
-than the females.</p>
-
-<p>Among the Mayflies or Ephemerides, too, the males are in the
-majority; in many of them there are sixty males to one female:
-but there are other kinds of insects, such as the dragon-flies
-(Libellulidæ), in which the females are three or four times as
-numerous. There are also, it may be remembered, some kinds of
-insects, such as Aphides, which have become capable of parthenogenetic
-reproduction, and in which the males are becoming extinct,
-e.g. in the case of <i>Cerataphis</i> in British orchid-houses.</p>
-
-<p>The first postulate implied in 'sexual selection,' namely, that
-there be an unequal number of individuals in the two sexes, is therefore
-fulfilled in Nature; we have now to inquire whether the second
-condition postulated&mdash;the power of choice&mdash;may also be regarded
-as a reality.</p>
-
-<p>This point has been disputed from many sides, and even by one
-of the founders of the whole selection theory, Alfred Russel
-Wallace. This naturalist doubts whether a choice is exercised among
-birds by either sex in regard to pairing, and maintains that, even
-if there could be a choice, this could not have produced such differences
-in colour and character of the plumage, since that would
-presuppose the existence of similar taste in the females through
-many generations. In a similar way it has been doubted whether
-butterflies can be said to exercise any real power of sexual choice,
-whether a more beautiful male is as such preferred to a less beautiful
-suitor.</p>
-
-<p>It must be admitted that direct observation of choosing is<span class="pagenum"><a id="Page_214"></a>[Pg 214]</span>
-difficult, and that as yet there is very little that can be said with
-certainty on this point. But there are, after all, some precise
-observations on mammals and birds which prove that the female
-shows active inclination to, or disinclination for, a particular male. If
-we hold fast to this fact, and add to it that the distinctive markings
-of the males are wonderfully developed during the period of courtship,
-and are displayed before the females, and that they only appear
-in mammals, birds, amphibians, and fishes at the time of sexual
-maturity, it seems to me that there can be no doubt that they are
-intended to fascinate the females, and to induce them to yield themselves
-to the males. The opponents of the theory of sexual selection
-attach too much importance to isolated cases; they imagine that each
-female must make a choice between several males. But the theory of
-sexual selection does not demand this, any more than the theory
-of natural selection requires the assumption that every individual of
-a species which is better equipped for the struggle for existence must
-necessarily survive and attain to reproduction, or, conversely, that the
-less well equipped must necessarily perish.</p>
-
-<p>All that the theory requires is, that the selective and eliminative
-processes do, <i>on an average</i>, secure their ends, and in the same way the
-theory of sexual selection does not need the assumption that every
-female is in a position to exercise a scrupulous choice from among
-a troop of males, but only that, on an average, the males more
-agreeable to the females are selected, and those less agreeable rejected.
-If this is the case, it must result in the male characters most attractive
-to the females gaining preponderance, and becoming more and
-more firmly established in the species, increasing in intensity, and
-finally becoming a stable possession of all the males.</p>
-
-<p>When we go more into details we shall see that the <i>particular
-qualities</i> of the distinctive masculine characters are exactly such
-as they would be if they owed their existence to processes of selection;
-in other words, from this point of view the phenomena of
-the decorative sexual characters can be understood up to a certain
-point. It seems to me that we are bound to accept the process of
-sexual selection as really operative, and instead of throwing doubt
-upon it, because the choice of the females can rarely be directly
-established, we should rather deduce from the numerous sexual
-characters of the males, which have a significance only in relation
-to courtship, that the females of the species are sensitive to these
-distinguishing characters, and are really capable of exercising
-a choice.</p>
-
-<p>In my mind at least there remains no doubt that the 'sexual<span class="pagenum"><a id="Page_215"></a>[Pg 215]</span>
-selection' of Darwin is an important factor in the transformation of
-species, even if I only take into consideration those secondary sexual
-characters which are related to wooing. We shall see, however, that
-there are others in regard to whose origin through processes of
-selection doubt is still less legitimate, and from which, on this
-account, we can argue back to the courtship characters.</p>
-
-<p>The first beginning of transformation is not, even in ordinary
-natural selection, to be understood as due to selection, but is to be
-regarded as <i>a given variation</i> (the causes of which we shall discuss
-later on); it is only the increase of such incipient variations in
-a definite direction that can depend on natural selection, and they
-<i>must</i> depend on it in so far as the transformations are purposeful.
-Now, all secondary sexual characters can be recognized as useful,
-save only the decorative distinctions, although these also undoubtedly
-represent intensifications of originally unimportant variations. Are
-we then to regard these alone as the mere outcome of the internal
-impulsive forces of the organism, while in the case of the analogous
-sexual characters for tracking, catching, and holding the female, and
-so forth, the augmentation and the directing must be referred to
-processes of selection? But if there be any utility at all in the
-decorative sexual characters it can only lie in their greater attractiveness
-to the females, and it can only be of any account if the
-females have, in a certain sense, the power of choice. Independently,
-therefore, of direct observations as to the actual occurrence of choosing,
-we should be compelled by our chain of reasoning to assume that
-there was such a power of choice&mdash;and I shall immediately discuss it
-more precisely.</p>
-
-<p>If we consider the decorative, distinctive characters of the males
-more closely, we find that they are of very diverse kinds. The males
-of many animals are distinguished from the females chiefly by greater
-beauty of form, and especially of colour. This is the case in many
-birds, some amphibians, like the water-salamander, many fishes, many
-insects, and above all, in diurnal Lepidoptera. Especially among birds
-the dimorphism between the sexes is in obvious relation to the excess
-in the number of male individuals, or&mdash;what practically comes to the
-same thing&mdash;to polygamy. For when a male attaches to himself four
-or ten females the result is the same as if the number of female
-individuals were divided by four or by ten. Thus the fowls and
-pheasants, which are polygamous, are adorned by magnificent colours
-in the male sex, while the monogamous partridges and quails exhibit
-the same colouring in both sexes. Of course 'beautiful' is a relative
-term, and we must not simply assume that what seems beautiful to<span class="pagenum"><a id="Page_216"></a>[Pg 216]</span>
-us appears so to all animals; yet when we see that all the male birds
-which are beautifully decorative according to our taste&mdash;whether
-humming-birds, pheasants, birds of Paradise, or rock-cocks (<i>Rupicola
-crocea</i>)&mdash;unfold their 'feather-wheels, 'fans,' 'collars,' and so forth,
-before the eyes of the females in the breeding season, and display
-them in all their brilliance, we must conclude that, in these instances
-at least, human taste accords with that of the animals. That birds
-have sharp vision and distinguish colours is well known; it is
-not for nothing that the service berries and many other berries
-suitable for birds are red, the mistletoe berries white, in contrast
-to the evergreen foliage of this plant, the juniper berries black
-so that they stand out amid the snows of winter; in this
-direction, then, there is no difficulty in the way of sexual
-selection.</p>
-
-<p>Even among much lower animals, like the butterflies, there seems
-to me no reason for the assumption that they do not see the gorgeous
-colours and often very complicated markings, the bars and eye-spots,
-on the wings of their fellows of the same species. Of course if each
-facet of the insect eye contributed only a single visual impression,
-as Johannes Müller supposed, then even an eye with 12,000 facets
-would give but a rough and ill-defined picture of objects more than
-a few feet away, and I confess that for a long time I regarded this
-as an obstacle in the way of referring the sexual dimorphism of
-butterflies to processes of selection. But we now know, through
-Exner, that this is not the case; we know that each facet gives
-a little picture, and not an 'inverted' but an 'upright' one, and
-experiment with the excised insect eye has directly shown that it
-throws on a photographic plate a tolerably clear image of even
-distant objects, such as the frame of a window, a large letter painted
-on the window, or even a church tower visible through it.</p>
-
-<p>Furthermore, the structure of the eye allows of incomparably
-clearer vision of near objects, for in that case the eyes act like lenses,
-and reveal much more minute details than we ourselves are able to
-make out. Here again, therefore, there is no obstacle to the
-Darwinian hypothesis of a choice on the part of the females, for
-although it cannot be demonstrated from the structure of the eye
-itself that insects see colour, and that colours have a specially exciting
-influence on them, yet we can deduce this with certainty from the
-phenomena of their life. The butterflies fly to gaily coloured flowers,
-and as they find in them their food, the nectar of the flowers, we
-may take for granted that the sight of the colour of their food-providing
-plants is associated with an agreeable sensation, and this<span class="pagenum"><a id="Page_217"></a>[Pg 217]</span>
-is an indication that similar colours in their fellows may awaken
-similar agreeable sensations.</p>
-
-<div class="figcenter" id="f57">
-<img src="images/fig57.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 53.</span> Scent-scales of diurnal butterflies. <i>a</i>, of Pieris. <i>b</i>, of Argynnis
-paphia. <i>c</i>, of a Satyrid. <i>d</i>, of Lycæna. All highly magnified.</p>
-</div>
-
-<p>This conclusion is furthermore confirmed by the fact that, in the
-male sex, numerous species of butterfly possess another means of
-exciting the females, namely, by pleasant odours. Volatile ethereal
-oils are secreted by certain cells of the skin, and exhale into the
-air through specially constructed scales. Usually the apparatus for
-dispersing fragrance occurs on the wing in the form of the so-called
-scent-scales (<i>Duftschuppen</i>), peculiar modifications of the ordinary
-colour-scales of the wing, but sometimes they take the form of
-brush-like hair-tufts on the abdomen, and they are in all cases so
-arranged that the volatile perfume from the cells of the skin
-penetrates into them, and then evaporates through very thin spots
-on the surface of the scale, or through brush-like, expanded fringes
-on their tips. Many of these have long been known to entomologists,
-because their divergence in form from the ordinary scales attracted
-attention; and it was also observed that they never occurred on the
-females, but only on the males. Their significance, however, remained
-obscure until, by a happy chance, Fritz Müller, in his Brazilian
-garden, discovered the fact that there are butterflies which give off
-fragrance like a flower, and then close investigation revealed to him
-the connexion between this delicate odour and the so-called 'male
-scales.' One can convince oneself of the correctness of the observation
-even in some of our own butterflies by brushing the finger over the
-wing of a newly caught male Garden White (<i>Pieris napi</i>). The
-finger will be found covered with a white dust, the rubbed-off wing-scales,
-<span class="pagenum"><a id="Page_218"></a>[Pg 218]</span>and it will have a delicate perfume of lemon or balsam, thus
-proving that the fragrance adheres to the scales.</p>
-
-<div class="figleft" id="f58">
-<img src="images/fig58.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 54.</span> A portion of the upper surface<br />
-of the wing of a male 'blue' (<i>Lycæna<br />
-menalcas</i>); after Dr. F. Köhler. <i>bl</i>, ordinary<br />
-blue scales. <i>d</i>, scent-scales. Highly<br />
-magnified.</p>
-</div>
-
-<div class="figleft" id="f59">
-<img src="images/fig59.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 55.</span> <i>Zeuxidia wallacei</i>,<br />
-male, showing four tufts<br />
-of long, bristle-like, bright<br />
-yellow scent-scales (<i>d</i>) on<br />
-the upper surface of the<br />
-posterior wing.</p>
-</div>
-
-<p>In the last case, that is, among the Whites (Pieridæ) (Fig. 53, <i>a</i>),
-the scent-scales are distributed fairly regularly over the upper surface
-of the wing, and the same is true
-of our blue butterflies, the Lycænnidæ
-whose minute lute-shaped
-scales are shown singly in Fig.
-53, <i>d</i>, but in their natural position
-among the ordinary scales in Fig.
-54. In many other diurnal, and
-also in nocturnal Lepidoptera, the
-fragrant scales are united into tufts
-and localized in definite areas.
-They then often form fairly large spots, stripes, or brushes, which
-are easily visible to the naked eye. Thus the males of our
-various species of grass-butterflies (Satyridæ) have velvet-like black
-spots on the anterior wings, while the fritillary, <i>Argynnis paphia</i>,
-has coal-black stripes on four longitudinal
-ribs of the anterior wing which are absent
-in the females, and which are composed of
-hundreds of odoriferous scales. Certain large
-forest butterflies of South America, resembling
-our <i>Apatura</i>, bear in the middle of the
-gorgeous green shimmering posterior wing a
-thick expansible brush of long, bright yellow
-scent-scales, and a similar arrangement obtains
-in the beautiful violet butterfly of the Malay
-Islands, the <i>Zeuxidia wallacei</i> depicted in
-Fig. 55. In many of the Danaides, which
-we have already considered in relation to
-mimicry, the scent apparatus is even more
-perfect, for it is sunk in a fairly deep pocket
-on the posterior wings, and in this the scent-producing,
-hair-like scales lie concealed until
-the butterfly wishes to allow the fragrance to
-stream forth. In many South American and
-Indian species of <i>Papilio</i> the fragrant hairs are disposed in a
-sort of mane on a fold of the edge of the posterior wing, and
-so on. The diversity of these arrangements is extreme, and they
-are widely distributed among both diurnal and nocturnal Lepidoptera,
-in the latter sometimes in the form of a thick, glistening,<span class="pagenum"><a id="Page_219"></a>[Pg 219]</span>
-white felt which fills a folded-over portion of the edge of the
-posterior wing. In many cases the perfume can be retained, and
-then, by a sudden turning out of the wing-fold, be allowed to stream
-forth. But there are a great many species of butterfly which do
-not possess odoriferous scales, and they are often wanting in near
-relatives of fragrant species; they are obviously of very late origin,
-and arose only after the majority of our modern species were already
-differentiated. It often seems as if they bore a compensatory relation
-to beauty of colour, somewhat in the same way as many modestly
-coloured flowers develop a strong perfume, while, conversely, many
-magnificently coloured flowers have no scent at all. Although among
-butterflies, as among flowers, there are species which possess both
-beauty and fragrance, yet our most beautiful diurnal butterflies,
-the Vanessas, the Apaturas, and Limenitis, possess no scent-scales;
-and many inconspicuous, that is, protectively coloured nocturnal
-Lepidoptera, are strongly fragrant, like most night-flowers: I need
-only mention the convolvulus hawk-moth (<i>Sphinx convolvuli</i>), whose
-musk-like odour was known to entomologists long before the
-discovery of scent-scales.</p>
-
-<p>It is, however, always only in the males that this odoriferous
-apparatus is present. It must not be believed on this account that
-this fragrance has the significance of a means of attraction comparable
-to the perfume of the flowers which induces butterflies to visit them;
-indeed, we cannot assume that the odour carries to a distance,
-for, as far as we can make out, it is perceptible only within
-a very short radius, and this is indicated also by the manifold
-arrangements of the odoriferous organs, which are all calculated to
-retain the fragrance, and then&mdash;in the immediate neighbourhood of the
-female&mdash;to let it suddenly stream forth. Obviously, this arrangement
-can have no other significance than that of a sexual excitant; its use
-is to incline the female to the male, to fascinate her, just as do the
-beautiful colours, in regard to which we must draw the same inference.
-It is in this direction that the already mentioned relation of compensation
-between beautiful colours and pleasant odours is particularly interesting,
-for it confirms our interpretation of the decorative colours as a means
-of sexual excitement. The most delicately fragrant or the most
-beautifully coloured males were those which most excited the females,
-and thus most easily attained to reproduction. The expression used
-by Darwin, that the females 'choose,' must be taken metaphorically;
-they do not exercise a conscious choice, but they follow the male
-which excites them most strongly. Thus there arises a process of
-selection among these distinctively male characteristics.</p>
-
-<p><span class="pagenum"><a id="Page_220"></a>[Pg 220]</span></p>
-
-<p>If the odoriferous organs we have been discussing had merely
-been a means of attraction, serving to announce the proximity of
-a member of the species, then they should have occurred, not in the
-males but in the females, for these are sought out by the males,
-not conversely. The males are able to track their desired mates
-from great distances, and many remarkable examples of this are
-known, some of them indeed sounding almost fabulous. The females
-must therefore also exhale a fragrance, and perhaps continually, but
-it is much more delicate, carries extraordinarily far, and is quite
-imperceptible to our weak sense of smell. It is possible that it streams
-out from all the scales covering the wings and body, for, as I long
-ago pointed out, all the scales retain a connexion with the living cells
-of the skin, however minute these may be, and it is therefore quite
-possible that the cells produce scent imperceptible by us, and let it
-exhale through the ordinary scales, since the male scent-scales owe
-their ethereal oil to the large gland-like cells of the hypodermis on
-which they are placed.</p>
-
-<p>Here we see very clearly the difference between ordinary natural
-selection and sexual selection. The male odoriferous organs depend
-on the latter, for they do not serve for the maintenance of the species,
-but are of advantage in the courting competitions among the males
-for the possession of the females, while the assumed fragrant cells of
-the females must depend on natural selection, since they are of general
-importance for the mutual discovery of the sexes, which would otherwise
-be in most cases impossible. This hypothetical 'species scent,' as
-we may call it, is first of all useful in securing the existence of the
-species, and must therefore be referred to natural selection. The other,
-the 'male scent,' might be, and actually is, wanting in many species,
-although it may be necessary to reproduction in cases where it has
-become a male specific character, and could not be absent from any
-male without dooming him to sterility.</p>
-
-<p>That the 'species scent' really exists admits of no doubt, although
-we may be unable to perceive it. Entomologists have long been in
-the habit of catching the males of the rarer Lepidoptera, especially of
-the nocturnal forms, by freely exposing a captive female. Some
-years ago I kept for some time in my study, with a view to certain
-experiments, females of the eyed hawk-moth (<i>Smerinthus ocellatus</i>),
-and placed them at first, without any special intention, in a gauze-covered
-vessel near the open window. The very next morning several
-males had gathered and were sitting on the window-sill, or on the
-wall of the room close to the vessel, and by continuing the experiment
-I caught, in the course of nine nights, no fewer than forty-two males<span class="pagenum"><a id="Page_221"></a>[Pg 221]</span>
-of this species, which I had never believed to be so numerous in the
-gardens of the town. The males of the nocturnal Lepidoptera
-obviously possess an incredibly delicate organ of smell, and its bearers,
-the antennæ, are usually larger and more complex in structure in the
-male sex than in the female.</p>
-
-<p>Butterflies are by no means the only creatures that produce a peculiar
-odour at the breeding season; many other animals do the same,
-though in their case it does not seem so pleasant to our sense of smell.
-It is true that the scent of the musk-deer and that of the beaver
-(<i>Castoreum</i>), when much diluted, are agreeable to man, but others, like
-the odours exhaled by stags or by beasts of prey, are very disagreeable
-to us, though they have for the species that produce them the same
-significance as the others, and are therefore to be referred to sexual
-selection.</p>
-
-<p>Darwin referred all the different <i>mechanisms for the production
-of sounds</i>, up to the song of birds, to sexual selection, but it is probable
-that natural selection has also to do with this in many ways. It is
-certainly only the males which produce the well-known song of the
-Cicadas, crickets, grasshoppers and birds, and I do not see any reason
-to doubt that this 'music' affects the females by arousing sexual
-excitement. To some extent, then, the rivalry among the males for
-the possession of the females&mdash;that is to say, sexual selection&mdash;must
-have produced these mechanisms of song; and how long-continued
-and gradual the accumulations must have been which produced the song
-of the thrush or of the nightingale from the chirping of the sparrow we
-may learn from the innumerable species which, as regards beauty of
-song, may be ranged between these two extremes.</p>
-
-<p>My assumption that natural selection has also been operative in
-the case of the song of insects and birds is based on the fact that many
-of our songsters live widely scattered, and that the characteristic note
-must be a means by which the two sexes find each other. That they
-should find each other is an indispensable condition for the maintenance
-of the species. Thus it is well known that each species has
-a characteristic 'note' or love-call, which the male utters during the
-breeding season, and which is answered by the female. From this
-simple love-call the modern song of many species must have developed
-by means of sexual selection.</p>
-
-<p>It is remarkable that here again the various distinguishing
-characters of the male seem to be often mutually restrictive or
-mutually exclusive. The best singers among our birds are inconspicuously
-coloured, grey or brown-grey, and this can hardly be
-regarded as due to chance, but as the outcome of a greater sensitiveness<span class="pagenum"><a id="Page_222"></a>[Pg 222]</span>
-on the part of the females either to the song or to the beauty of their
-mates. And since, according to the theory, only those characters of
-the males could be increased which decided the choice, it therefore
-seems to me that this mutual exclusiveness of the two kinds of distinguishing
-characters is another indication of the reality of sexual
-selection. It proves&mdash;so at least I am inclined to believe&mdash;that the
-excitement of the female has been essentially affected by <i>only one</i> of
-the characters of the male, that in the bird of Paradise it was mainly
-the brilliance of the plumage which roused excitement, while in the
-nightingale it was mainly the song.</p>
-
-<p>It might be objected to this that there are brilliant butterflies
-which also possess scent-scales. This is really the case; thus a magnificent
-blue iridescent <i>Apatura</i> from Brazil has on the posterior
-wings a large yellow brush of scent-hairs, and even the beautiful
-blue males of our Lycænids have scent-scales in addition to their
-beautiful colour. But this can hardly be considered as a contradiction,
-but is rather an exception, which is the easier to explain since the
-odoriferous apparatus is a relatively simple arrangement, which did
-not require such a long series of generations for its evolution
-as the complicated song-box and brain-mechanism of the singing-birds.</p>
-
-<p>Moreover, it may also be that the scent-scales have arisen later
-than the decorative colouring, and they would do so the more easily
-since the brilliant blue, when once it was perfectly developed, and was
-common to all the males of the species in an equal degree, was no longer
-distinctive, and would have no specially exciting effect, while a novel
-preferential character in the male would have a much stronger effect.
-In the same way, the different parts of the body would be furnished
-in succession with decorative and, therefore, exciting distinctive
-characters. To understand this effect on the opposite sex we need
-only think of analogous phenomena in human kind, and of the
-strongly exciting effect that the sight of the secondary sexual
-characters of the woman has upon the man.</p>
-
-<p>By the successive additions of new decorative characters after
-the older ones became general and reached a climax, the origin of
-the extraordinary diversity of the decorative plumage in one and the
-same species of bird, can be readily understood, and the same is true
-of the complicated decorative coloration of the butterflies in so far as it
-depends on sexual selection, and not on other factors. The details did
-not arise all at once, but one after the other, and every character
-went on increasing till it had reached its limit of increase, but whenever
-it was common in its highest development to all the males it<span class="pagenum"><a id="Page_223"></a>[Pg 223]</span>
-was no longer an object of preference or the cause of specially violent
-excitement, so that a new process of selection would begin in reference
-to some other part of the body. We thus understand how, among
-male birds of Paradise and humming-birds, such a marvellous
-diversity of colours and of decorative feathers is found combined
-in one and the same species.</p>
-
-<p>Whoever has seen the Gould Collection of humming-birds in
-London must have observed with amazement that among the 130 or
-so species of these beautiful little birds nearly every group of feathers
-in the body has been affected by the decorative colouring. In one
-species the little feathers on the region of the throat are emerald
-green, metallic blue, or rose; in another the feathers of the neck have
-been transformed into an erectile collar of rose-coloured feathers with
-a metallic sheen; or, again, it is the little feathers round the ear that
-stand erect and are brilliantly coloured. Sometimes we find that the
-feathers of the tail are lengthened, it may be only two of them, or the
-various lengths may be graduated like steps; sometimes the tail has
-assumed the form of a wedge, or is fan-like, or is shaped like the tail
-of a swallow, and all this in combination with the most diverse colours
-and patterns, black and white, ultramarine blue, and so forth. Or it
-may be the outermost tail-feathers which are the longest, the inner
-ones the shortest, or the four outer feathers are broad, pointed, directed
-outwards, and only half as long as the other two, which are very long
-and straight. Some species exhibit a sort of fine swan's down on the
-legs, others have a gorgeous metallic red cap on the head&mdash;in short,
-the variety is beyond description, just as we should expect it to be if
-now this and now that chance variation attracted the favourable
-regard of the selecting sex, and thus attained to its highest pitch
-of development.</p>
-
-<p>The decorative colouring of male birds may be replaced, not only
-by the power of song, but in other ways also. Not all the male birds
-of Paradise possess the familiar feather ornaments. The Italian
-traveller Beccari has called attention to a species, the males of which
-are simply coloured brown, like the females of other species. This
-<i>Amblyornis inornata</i> entices its mate to itself in the pairing time in
-a very peculiar manner, for it arranges in the midst of the primitive
-forests of New Guinea a little 'love garden' or bower, a spot several
-feet in extent, strewn with white sand, on which it places shining
-stones and shells, and brightly coloured berries. In this case a special
-instinct has developed, which has replaced the personal charm of the
-bird in the eyes of the female. For this very reason the case seems
-to me to have some theoretical importance, for it serves indirectly to<span class="pagenum"><a id="Page_224"></a>[Pg 224]</span>
-show that the personal excellences do actually function as a means
-of exciting and attracting, if any one should still doubt it.</p>
-
-<p>All the distinguishing characters of the male which we have
-hitherto considered have had reference to gaining the favour of the
-female, but there are many other secondary sexual characters which
-are employed in quite a different manner to secure possession of the
-female. I have already mentioned that in many butterflies the males
-possess a much larger organ of smell. The antennæ of the males of
-numerous beetles, such as the cockchafer and its relatives, are also
-much larger, and furnished with much broader accessory branches,
-than those of the female, and the same is the case in many of the
-lower crustaceans, like the large transparent Daphnid of our lakes,
-<i>Leptodora hyalina</i>. Here the anterior antenna bears (Fig. 56,
-<i>A</i> and <i>B</i>, <i>at´</i>) olfactory filaments; in the female this appendage is small
-and stump-like, while in the male (<i>A</i>) it grows to a long, somewhat
-curved rod, which is extended obliquely into the water, and in addition
-to the nine olfactory filaments of the female (<i>ri</i>) bears from sixty to
-ninety more (<i>ri´</i>).</p>
-
-<div class="figcenter" id="f60">
-<img src="images/fig60.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 56.</span> <i>Leptodora hyalina.</i> <i>A</i>, head of the male. <i>B</i>, head of the female.
-<i>Au</i>, eye. <i>g. opt</i>, optic ganglion. <i>gh</i>, brain. <i>at´</i>, first antenna with olfactory
-filaments <i>ri</i> and <i>ri´</i>. <i>sr</i>, œsophageal nerve-ring. <i>n</i>, nerve. <i>m</i>, muscles.</p>
-</div>
-
-<p>In this and many other such cases it is not the struggle of
-the species for existence which has so markedly augmented this distinctive
-characteristic of the male; it is undoubtedly the struggle of
-the males among themselves, their competition for the possession of
-the females. In regard to decorative distinctions, the reality of a
-rivalry in wooing and the ultimate victory of the most decorative
-may perhaps be still doubted; but it is quite certain that, on an
-average, the male which can smell and track best will also gain
-possession of the females more easily than one less well equipped.
-Exactly the same is also true of those cases in which the male dis<span class="pagenum"><a id="Page_225"></a>[Pg 225]</span>tinguishing
-character does not refer merely to finding the female, but
-to holding her fast, or, as we may say, to capturing her.</p>
-
-<p>Thus the males of the Copepods possess on their anterior
-antennæ an arrangement which enables them to throw a long whiplike
-structure like a lasso round the head of the female as she rapidly
-swims away. The antennæ of the male Daphnids, too, are in one
-genus (<i>Moina</i>) developed into a grasping apparatus, instead of into
-smelling organs as in <i>Leptodora</i>. Fig. 57 shows the male, Fig. 58 the
-female of <i>Moina paradoxa</i>; the first antennæ of the male are not
-only much longer and stronger than those of the female (<i>at</i><sup>1</sup>), but
-they are also armed with claws at the end, so that the males can catch
-their mates as with a fork, and hold them fast. And even that was
-not enough, for, in addition, the males of most Daphnids possess
-a large sickle-shaped but blunt claw on the first pair of legs (Fig.
-57, <i>fkr</i>), which enables them to cling to the smooth shell of the
-female, and to clamber up on it to get into the proper position for
-copulation.</p>
-
-<div class="figcenter" id="f61">
-<img src="images/fig61.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 57.</span> <i>Moina paradoxa</i>, male. <i>at<sup>1</sup></i>, first antennæ, with claws at the tip for
-capturing the female. <i>at<sup>2</sup></i>, second antennæ. <i>fkr</i>, claws on the first pair of legs
-for clambering. <i>gh</i>, brain. <i>lbr</i>, upper lip. <i>md</i>, mandible. <i>md</i>, mid-gut, with the
-liver lobes (<i>lh</i>). <i>h</i>, heart. <i>sp</i>, testis. <i>aft</i>, anus. <i>sb</i>, caudal setæ. <i>skr</i>, caudal claws.
-<i>sch</i>, shell. <i>schr</i>, cavity of the shell. <i>kie</i>, gill-plates. Magnified 100 times.</p>
-</div>
-
-<p>If we inquire into the manner of the origin of secondary sexual
-characters of this kind, we shall find that both may have been increased
-by sexual selection, for a male with a better sickle will succeed
-more quickly in getting into the proper position for copulation than
-one with a less perfect mechanism. This assumption does not rest<span class="pagenum"><a id="Page_226"></a>[Pg 226]</span>
-on mere theory, for I was once able, by a happy chance, to observe for
-a considerable time, under the microscope, a female to whose shell
-two males were clinging, each trying to push the other off. Nevertheless
-it seems to me very questionable whether the origin of this
-sickle-claw can be referred to sexual selection, for without this
-clamping-organ copulation in most Daphnids would not be possible.
-It was thus not as an advantage which one male had over another
-that the clamping-sickle evolved, but rather as a necessary acquisition
-of the whole family, which must have developed in all the species at
-the same time as the other peculiarities, and notably those of the shell.
-The competition of the males among themselves is thus in this case
-simply an expression of the struggle for existence on the part of the
-species as such, and it is not a question merely of a character which
-makes it easier for the males to gain possession of the females, but of one
-which had necessarily to arise lest the species should become extinct.
-In other words, in this case natural selection and sexual selection
-coincide.</p>
-
-<div class="figcenter" id="f62">
-<img src="images/fig62.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 58.</span> <i>Moina paradoxa</i>, female. The letters of Fig. 57 apply
-<i>mutatis mutandis</i>. <i>brr</i>, brood-pouch. <i>ov</i>, ovary. <i>sr</i>, margin of shell.</p>
-</div>
-
-<p>The case of the antennæ of <i>Moina</i>, which have been modified
-into grasping organs, is quite different; these owe their origin not to
-natural selection, but to sexual selection, for antennæ of that kind
-are not indispensable to the existence of the species, as we can see
-from the closely related genera, <i>Daphnia</i> and <i>Simocephalus</i>, where
-the males have quite short stump-like antennæ, furnished with
-olfactory filaments not much more numerous than the females possess.
-Just as these supernumerary olfactory filaments were produced by<span class="pagenum"><a id="Page_227"></a>[Pg 227]</span>
-sexual selection, and not by the ordinary natural selection, because
-those males with the more acute sense of smell had an advantage over
-those in which it was blunter, so the males of the genus <i>Moina</i> which
-could grasp most securely had an advantage over those that gripped
-less firmly, and thus arose these two different kinds of male characteristics.
-Neither of them is of advantage to the species as such, but
-only to the males in their competition for the possession of the
-females.</p>
-
-<p>But, where the production of a novel character in the male is
-concerned, natural selection cannot proceed in a different manner
-from sexual selection; the process of selection is exactly the same:
-the better equipped males survive, the less well-equipped die without
-begetting offspring; the difference lies only in the fact that in the one
-case the improvement is in the species as such, in the other case only
-in one sex without the existence of the species being thereby made
-more secure. Such cases are instructive, because they make a denial
-of the process of sexual selection quite impossible if that of species-selection
-is admitted. If processes of selection are operative at all as
-factors in transformation, they must act even where the advantage is
-not to the species but only 'intra-sexual,' and the one process must
-often run into the other, so that it is often quite impossible to draw
-an exact line of demarcation between them.</p>
-
-<p>Numerous secondary sexual differences probably depend purely
-on species selection, that is to say, they include an improvement of
-the species in relation to the struggle for existence. We may find a
-case in point in the dwarf-like smallness of the males in many
-parasitic crustaceans, in some worms, in many Rotifers, and in the
-Cirripedes. It can hardly have been of advantage for the individual
-male to be smaller than his fellows, but it was of advantage for the
-species to produce as many males as possible in order to ensure a
-meeting with the females, and, as the enormous production of males
-made it advantageous for the species that as little material as possible
-should be used in their individual production, we can readily understand
-the minuteness of the males, and in some cases, as in the Rotifers
-and <i>Bonellia</i>, their poor equipment, lack of nutritive organs, and
-ephemeral existence. The marine worm, <i>Bonellia viridis</i>, whose
-female may be a foot in length, is not the only case in which a
-microscopically small male lives like a parasite inside the female.
-Among the round-worms, too, there is a species called <i>Trichosomum
-crassicauda</i>, discovered by Leuckart in the rat, the dwarf males of
-which live in the reproductive organs of the female. All these are
-arrangements for securing the propagation of the species, which<span class="pagenum"><a id="Page_228"></a>[Pg 228]</span>
-might have been endangered if the males had had to seek out the
-females, which, in the case of <i>Bonellia</i>, live in holes in the rocks on
-the sea-floor, and, in the case of <i>Trichosomum</i>, are concealed in the
-urinary bladder of the rat. Obviously, this is the reason which, in
-addition to the one already mentioned, has conditioned and produced,
-or helped to produce, the remarkable minuteness of certain males.</p>
-
-<p>From another category of sexual differences we see in how many
-ways species-selection and sexual selection play into each other's
-hands. In many species of animals the males are eager for combat,
-and they are equipped with special weapons, or excel the females in
-general strength of body. As these males struggle, in the literal
-sense of the word, for the possession of the females, Darwin referred
-to sexual selection those distinguishing characters which gave the
-stronger male the victory over the weaker, and thus raised the
-victorious characters to the rank of general characters of the species.
-And it certainly cannot be doubted that, for instance, the strength
-and the antlers of the stag must have been increased through the
-combats which recurred every year at the breeding season, for the
-stronger always win in these battles. The case is the same with the
-strength and the weapons of many other male animals. The lion is
-effectively protected by his mane from the bite of a rival, and the
-same protective arrangement occurs in quite a different family of
-mammals&mdash;in an eared seal, which is called the 'sea-lion' for this
-very reason. Among the seals the secondary sexual characters are often
-very strongly developed, at least in all the polygamous species, for in
-these the struggle for the females is very keen. In the 'sea-lions' and
-'sea-elephants' there are often fifty females to one male, and the
-latter are 'enormously larger' than the females, while in monogamous
-species of seal the two sexes are alike in size.</p>
-
-<p>Darwin has shown that actual combat for the females takes
-place among most mammals, not only among stags, lions, and seals,
-but even among the moles and the timid hares. Even among birds
-such combats occur, and this is sometimes particularly noteworthy in
-those species in which the males possess the most decorative colouring,
-like the humming-birds. In some cases among birds there has also
-been a development of weapons. Witness the spur of the cock, whose
-merciless combats with his rivals Man has, as is well known, made
-positively atrocious for his own amusement, by preventing the
-flight of the vanquished.</p>
-
-<p>In Darwin's great work on sexual selection a considerable
-number of cases are cited from among lower vertebrates, such as
-crocodiles and fishes, and even from insects, in which the males fight<span class="pagenum"><a id="Page_229"></a>[Pg 229]</span>
-for the possession of the females, and exhibit distinctive masculine
-characters adapted to such combats. But I do not propose to enter
-upon a discussion of such cases, since my aim is rather to elucidate
-the relation between sexual selection and species-selection than to
-discuss all the phenomena of the former in detail. But the combats
-of males illustrate with particular clearness the relation of sexual
-selection and species-selection, since many of the weapons or protective
-arrangements which may have arisen through sexual selection imply
-at the same time an improvement to the species in relation to the
-struggle for existence. Thus greater strength or sharper and larger
-teeth in the males mean a gain to the species, and it is indifferent to
-the species whether the weaker males succumb to a strange enemy
-(species-selection) or to their stronger rivals (sexual selection), provided
-only that the better equipped survive and leave descendants
-similarly endowed.</p>
-
-<p>I have intentionally begun the consideration of sexual selection
-with the cases most difficult to interpret on this theory, with those
-which have called forth the greatest divergence of opinion&mdash;the
-decorative colours and forms, the song of birds and of insects, the
-alluring odours&mdash;in short, all the courtship-adaptations of the males;
-these are the most difficult to deal with, because it is not easy to
-demonstrate directly that the females <i>do</i> choose. But if we revise
-them briefly in reverse order, I believe that all doubt as to the reality
-of choice on the part of the females will disappear. Thus the last-mentioned
-sexual characters of greater strength and greater perfection of
-weapons and defence in the males have been evolved by sexual selection
-in close co-operation with species-selection. We should have to deny
-species-selection altogether if we were to dispute this form of sexual
-selection, which is closely connected with pure species-selection, such,
-for instance, as is revealed in the production of dwarf males, where
-there does not seem to be any aid from sexual selection at all.</p>
-
-<p>Then came the cases in which the tracking and grasping organs
-of the males were strengthened or were increased in number, and here
-too species-selection may have had its share, for instance, in evolving
-the sickle-claws of the Daphnids, which were inevitably advanced
-and perfected through sexual selection, which must in this case have
-operated independently of any choice on the part of the female. In
-other cases the result may be referred to pure sexual selection, as in
-the grasping antennæ of the male <i>Moina</i>, or in the highly developed
-olfactory antennæ of the male <i>Leptodora</i>. That new organs, too, can
-arise in this way is shown by the 'turban eyes'&mdash;to which little
-attention has hitherto been paid&mdash;of some Ephemerids of the genera<span class="pagenum"><a id="Page_230"></a>[Pg 230]</span>
-<i>Cloë</i> and <i>Potamanthus</i>, which were long ago described by Pictet, the
-monographer of this family. These are large turban-shaped compound
-eyes, occurring beside the ordinary eyes in the males alone,
-which in these genera are in a majority of sixty to one. Whole
-swarms of these males fly about over the water on the search for
-females, and their highly developed organ of vision seems to decide
-matters for them just as the organ of smell does for <i>Leptodora</i>.
-Neither of these sense-organs can have any other advantage than
-that of making their possessors aware of the female, for the whole
-activity of the short-lived adult Ephemerides is limited to reproduction;
-they take no food, and have nothing whatever to do except
-to reproduce.</p>
-
-<p>Finally, when in an enormous number of cases we find in
-addition to one or the other of the already mentioned male distinguishing
-characters some which do not directly lead to gaining
-possession of the female, but do so only by sexually exciting her, can
-we doubt that the same principle has been operative, that here too
-processes of selection are fundamental, depending on the fact that
-in the wooing of the female the successful male is the one who most
-strongly excites her? There is no question of æsthetic pleasure in
-this, as the opponents of the theory of sexual selection have often
-urged, but only of sexual excitement, which may be aroused by very
-different means, by colours and shapes, but also by love-calls, songs,
-or odours. There are a few tropical birds (<i>Chasmorhynchus</i>) which
-have as the only distinguishing character of the male sex a hollow
-and soft appendage several inches long borne on the head. Usually
-it hangs down limply at the side of the head, but during the breeding
-season it is inflated from the mouth-cavity, and then stands erect like
-a spur. One species of this genus has as many as three of these
-horns, one of which is upright, while the other two stand out laterally
-from the head. Can it be supposed that these remarkable horns
-satisfy the female's 'sense of beauty'? To human beings they
-appear rather ugly than beautiful, both when limp and when inflated,
-but at any rate they are striking, and will be regarded by the female
-bird as something out of the common, and, since they are only
-fully displayed during the breeding season, that is, when the male
-is sexually excited, they will have an exciting effect on the female
-too. These inflated horns are symptoms of excitement, and they arouse
-it in the female. In exactly the same way the decorative feathers, the
-ruby-red and emerald-green feather collars of the humming-birds and
-birds of Paradise, are only erected and displayed when the males are
-wooing, and they, too, act as signs of excitement. This is not to say<span class="pagenum"><a id="Page_231"></a>[Pg 231]</span>
-that the gorgeousness of colour, the eye-spots on the train of the
-peacock and the Argus pheasant, and the hundreds of different kinds
-of beautiful feathers, do not also exercise a fascinating influence; on
-the contrary, we cannot avoid assuming this, since otherwise we could
-find no sufficient reason for their origin. But the primary effect in
-wooing is not due to the mere pleasure in the sight, or in the odour,
-or in the song, but to the contagious excitement which these express.
-The females do not behave as dispassionate judges, but as excitable
-persons which fall to the lot of the male who is able to excite them
-most strongly. It may be, however, that a sense of æsthetic satisfaction
-in perceiving such symptoms of excitement may also have
-been evolved as an accessory effect, at least in the higher and more
-intelligent animals.</p>
-
-<p>In the lower animals, which are lacking not only in intelligence
-but also in the higher and more complex differentiation of the sensory
-system, the development of such secondary sex characters is rare or
-altogether absent. Animals which have no sense of hearing can
-develop no song, and animals which do not see cannot acquire
-gorgeous colours as a means of exciting one sex through the other.
-But distinctive sex coloration may arise even in lowly animals,
-though there can be no question of æsthetic pleasure associated therewith;
-if the animals are able to see the colours at all, sexual
-excitement may be associated with these.</p>
-
-<p>We need not wonder, therefore, that in the somewhat stupid
-fishes, in the butterflies, and in the lower crustaceans, like the
-Daphnids, we still find brilliant colours, which we can hardly
-interpret otherwise than as the results of sexual selection. On the
-other hand, the absence of such characters in animals of a still lower
-order, with still simpler sense-organs, like the Polyps, Medusæ,
-Echinoderms, most Worms, and the Sponges, affords an indirect confirmation
-of the correctness of our view as to the reality of a sexual
-selection in the more highly organized animals.</p>
-
-<p>We see, then, that numerous peculiarities which distinguish the
-males of a species from the females depend on the process of sexual
-selection. This may be said of ornamental outgrowths, colours,
-remarkable feathers and feather-groups, peculiar odoriferous organs,
-vocal organs, artistic instincts, and also weapons, like antlers, tusks,
-and spurs, notable size and strength of body, and protective devices
-like manes; and again, the various organs for catching and holding
-the females, or for finding them out by sight or smell, must also be
-referred, at least in part, to sexual selection. The diversity of the
-male sexual characters is so great that I cannot give more than<span class="pagenum"><a id="Page_232"></a>[Pg 232]</span>
-a faint idea of them without entering on a long catalogue; whoever
-wishes a complete survey has only to consult Darwin's <i>Descent of
-Man</i>.</p>
-
-<p>But the significance of sexual selection is by no means exhausted
-with the production of the male sexual characters, for these characters
-are often more or less completely transferred to the females, and thus
-give rise to a transformation of the whole species, and not only of the
-male section of it. This is obviously a very important consequence of
-sexual selection, one which, as we shall see, materially deepens our
-insight into the mode of origin of new species.</p>
-
-<p>First let us try to determine the facts. Many male characters are
-not represented in the female in any degree, and therefore have never
-been transmitted to them at all. Such are the mane of the lion, the
-grasping antennæ of <i>Moina</i>, the turban eyes of the Ephemerides, the
-intensification of the sense of smell in <i>Leptodora</i>, the lasso-like antennæ
-of the Copepods, the scent-scales of the butterflies, and the musk glands
-of the alligators and stags. But in other cases there has been
-transmission, though only to a slight extent. Thus many female
-humming-birds have a faint indication of the magnificent metallic
-colouring of the males; many female blue butterflies have a tinge
-of the beautiful blue of their mates; the females of the stag-beetle
-(<i>Lucanus cervus</i>) possess a diminutive suggestion of the antler-like
-jaws of the male, and the female crickets, although they do not chirp,
-have a slight indication of the 'musical' mechanism of the male on
-the wing-coverts, and some of them even produce feeble notes at
-certain times.</p>
-
-<p>It can be proved, however, that such transmissions may, in the
-course of many successive generations, become intensified until the
-characters are exhibited by the females in the same degree as in the
-males. I know no better example of this than that afforded by the
-beautiful butterflies of the genus <i>Lycæna</i>. In this genus, which is
-rich in species and widely distributed over the whole earth, and
-must therefore be an old one, the upper surface of the wing is blue in
-by far the greater number of species, at least in the male sex. But
-there are three or four species which are dark-brown, and quite
-or nearly alike in the two sexes; such are the species <i>Lycæna agestis</i>,
-<i>L. eumedon</i>, <i>L. admetus</i>, and others. Everything indicates that this
-is the primitive colour of the genus. Moreover, there are some species
-with brown females, in which the males are not completely blue,
-but which have a slight bluish tinge, like <i>L. alsus</i>, the smallest of our
-indigenous Blues. Then follows a host of beautiful species, like
-<i>L. alexis</i>, <i>L. adonis</i>, <i>L. damon</i>, <i>L. corydon</i>, and many others, with<span class="pagenum"><a id="Page_233"></a>[Pg 233]</span>
-brown females, and among these there occasionally occur females more
-or less tinged with blue. These lead on to <i>L. meleager</i>, which has two
-forms of female, a common brown and a rarer blue; and thus we
-reach <i>L. tiresias</i>, <i>L. optilete</i>, and <i>L. argiolus</i>, in which all the females
-are blue, although less intensely and completely so than their mates.
-The climax of this evolutionary series is reached by some species like
-<i>L. beatica</i>, belonging to tropical or at least warm countries, in which
-both sexes are of an equally intense blue. As we know that, in species
-with an excess of males, sexual characters always begin in the males,
-there can be no doubt as to the direction of evolution&mdash;from brown to
-blue&mdash;in this series. Furthermore, the entire absence of scent-scales
-in most of the species with brown males indicates the great age of
-these species, for, as far as I have been able to investigate, all the
-males of the blue species possess them.</p>
-
-<p>Darwin regarded this transferring of the male characters to the
-females as due to inheritance, and it really seems as if it were simply
-a case of transmission by inheritance to one sex of what has been
-acquired by the other. Yet we have to ask whether we can continue
-to regard the facts in this light. In any case this 'transmission'
-is not an inevitable physiological process, necessarily resulting from
-the intrinsic conditions of inheritance, for we see that it often does
-not occur, even in many cases in which we can see no external reasons
-why it should not do so, though in other cases the failure may be
-presumably correlated with the external conditions of life. Thus,
-for instance, the persistent retention of the brown colour in the
-majority of our female Lycænidæ has probably its reason in the
-greater need of protection on the part of the much rarer females,
-and this must be so also in the case of many birds in which the
-brilliant colours of the males have not been transferred to the females.
-Wallace first pointed out that all birds whose females brood in exposed
-nests are inconspicuously coloured in the female sex, even if the
-males are brightly coloured, while those whose nests are concealed
-in holes of trees or the like, or which build domes over them, not
-rarely exhibit brilliant colouring in both sexes. This is the case
-in woodpeckers and parrots, while the gallinaceous birds, which
-brood in the open, have usually inconspicuously coloured females,
-for the most part very well adapted to their surroundings.</p>
-
-<p>If we grasp the fact that a transference of the characters
-which have arisen through sexual selection can take place, we have
-a valuable aid in the interpretation of many phenomena which would
-otherwise remain quite inexplicable. What is the meaning of the gay
-colours of the parrots, which occur in such incredibly diverse com<span class="pagenum"><a id="Page_234"></a>[Pg 234]</span>binations
-in this large and widely distributed family? Or of the
-marvellously complex markings and colour-patterns of the butterflies?
-In some cases they may be protective, as is the green of
-many parrots; in others, warning signs of unpalatability, like the
-bright colours and contrasted markings of many Heliconiidæ and
-Eusemiidæ and other butterflies with a nauseous taste; but there
-remain a great many cases to which neither of these explanations
-applies, which could only be regarded as pure freaks of nature if
-we did not know that male sexual characters can be transferred
-to the females, and that thus all the individuals of a species can
-be totally altered in their colouring.</p>
-
-<p>Thus the occurrence not only of conspicuous, but of complicated,
-coloration is explained.</p>
-
-<p>Darwin has shown that, in the equipment developed by the
-males in their competition for the possession of the females, it is by
-no means only those characters which may be considered 'beautiful'
-in themselves that have to be considered; it is rather the
-striking characteristics which mark their possessor and distinguish it
-from others that are primarily important. In fact, it is the principle
-of 'mode' or 'fashion' which is operative; something new is demanded,
-and as far as possible something quite different from that
-which was previously considered beautiful. Thus the starting-point
-for such processes of selection may have been afforded by white
-spots on a black ground, or, indeed, by any light spots on a dark
-ground, which may have been the primitive colour in most cases.
-If in the course of a long series of generations these spots became
-the common property of all the males, a possibility of further
-change was opened up as soon as a new contrast cropped up as a
-chance variation, which would then, under favourable conditions, be
-the starting-point of a new process of selection. Darwin has cited
-some cases in which, from a comparison of the dress of the young bird
-with that of the adult, we may conclude that a transformation of the
-colouring of the whole plumage must have taken place in the course
-of the phylogenetic history.</p>
-
-<p>In other cases the course of the process of selection has been such
-that, though the general colouring has not been changed, variations
-have appeared in particular regions of the body&mdash;spots or stripes
-which accumulated through the ages and co-operated to form an
-increasingly diverse and complex colour-scheme, such a 'marking' of
-the animal as we may observe to-day, especially in butterflies, but
-also in birds.</p>
-
-<p>It is a fine corroboration of the origin of bright colours through<span class="pagenum"><a id="Page_235"></a>[Pg 235]</span>
-sexual selection that, even in those groups of the animal kingdom
-which are in general sexually monomorphic, there always occur some
-species in which male and female are quite different, and a host of
-species in which both sexes are alike in the main, yet with differences
-in certain minor points. Among the parrots similarity of colouring
-prevails as a general rule, but in New Guinea there lives a parrot the
-female of which is a gorgeous blood-red and the male a beautiful
-light-green; minor differences occur in many species, for instance, the
-female of the horned parrot (<i>Cyanorhamphus cornutus</i> Gm.) lacks
-the two long black and red feathers on the head, that of the grass-parakeet
-(<i>Melopsittacus undulatus</i>) is a slightly paler green and has not
-the beautiful blue spots on the cheeks which the male possesses.
-Innumerable similar instances might be cited, serving to show that
-all these distinguishing characters of the males have been acquired
-step by step and piece by piece, and are slowly and independently
-transferred to the females&mdash;if, indeed, at all.</p>
-
-<p>In yet another way the correctness of the Darwinian theory of
-sexual selection may be deduced from the markings and coloration
-of birds and butterflies.</p>
-
-<p>It has frequently struck me, during the long period in which
-I have been studying brightly coloured birds and butterflies, that
-those colour-patterns which are referable to sexual selection are much
-simpler than those which must be referred to species-selection, especially
-in the case of what we call 'sympathetic coloration.' How
-crude is the decorative pattern of most parrots, notwithstanding all
-the brilliance of their colour. Large tracts of the body are red,
-others green, yellow, blue, and occasionally one finds a red and blue
-striped feather collar, a head which is red above and yellow underneath,
-but it is seldom that the colours vary enough in a small space
-to give rise to a delicate decorative pattern. The gayest of parrots
-are the Brush Tongues (<i>Trichoglossus</i>), and even among them subtlety
-of coloration does not go further than the combination of three
-colours on one of the long tail-feathers, or the production of a double
-band round the neck, and so forth. If we compare with this the
-complex markings of the inconspicuously coloured females of the
-pheasants, of the partridges, or that of the upper surface of the
-many birds in mingled grey, blackish-brown and white, which
-resemble the ground or the dried leaves when they crouch, we find
-that the colour-pattern in these cases is infinitely finer and more
-complex.</p>
-
-<p>This seems to me quite intelligible when we remember, on the one
-hand, that species-selection must operate far more intensively than<span class="pagenum"><a id="Page_236"></a>[Pg 236]</span>
-sexual selection, and that in the production of a protective colouring
-it is a question of deceiving the eye of a sharp-sighted enemy, while
-the aim of sexual selection is to secure the approval of others of the
-same species. As long as the enemy on the search for prey perceives
-the difference between the markings of its victim and those of the
-surroundings, so long will the gradual and steady improvement of the
-protective coloration continue, so long will new shades and new lines
-be added. We can thus understand how there would be gradually
-reached a complexity of marking to which sexual selection can never
-attain, or at least only in regard to a few specially favourable points.
-The eye-spots on the train feathers of the Argus pheasant and the
-peacock are such points, and these occur among polygamous birds
-in which sexual selection must be very intense; they are placed, too,
-on a part of the body, the wheel-shaped train, which is peculiarly
-suited for communicating the excitement of the male to the female,
-and must therefore be especially influenced by the latter. In general,
-however, we may say on <i>a priori</i> grounds that the intensity of
-species-selection is greater than that of sexual selection, because the
-former ceaselessly and pitilessly eliminates the less perfect, while the
-claims of the latter are in any case less imperative, and are also
-often mollified by a variety of chance circumstances.</p>
-
-<p>But in the case of insects, in particular, we have to add that the
-protective colours and the decorative colours have been, so to speak,
-painted by different artists&mdash;the former by birds, lizards, and other
-persecutors endowed with well-developed eyes, the latter by the
-insects themselves, whose eyes can hardly possess, for objects not
-quite near, that acuteness of vision which the bird's eye has. Thus
-we find that the protective coloration of butterflies has often a very
-complex marking, while the same butterfly may exhibit only a rather
-crude though brilliant pattern on its upper surface, where the
-coloration is due to sexual selection. Thus the famous <i>Kallima</i> has
-on its under surface the likeness of a dry or decayed leaf composed of
-a number of colour-tones&mdash;quite a complex painting. But if we look
-at the upper surface we see a deep brown with a shimmer of steel
-blue as the ground-colour of the wings, and on it a broad yellow band
-and a white spot: that is the whole pattern. We find a similar state
-of things among many of the forest butterflies of Brazil, and also
-among our indigenous butterflies. The pattern of our gayest diurnal
-butterflies, the red Admiral and the tortoiseshell butterfly (<i>Vanessa
-atalanta</i> and <i>Vanessa cardui</i>), is somewhat crude on the upper surface,
-and very simple compared with the protective colouring of the under
-surface, which is made up of hundreds of points, spots, strokes, and<span class="pagenum"><a id="Page_237"></a>[Pg 237]</span>
-lines of every shape and colour. On the other hand, the upper surface
-of the anterior wings in the hawk-moths and the Noctuidæ exhibits
-protective coloration, and is made up of curious zigzag complex lines,
-strokes, and spots, so that it resembles the bark of a tree or a bit of an
-old wooden fence&mdash;a painting, like the modern impressionist work,
-which, with an apparently meaningless confusion of colour splashes,
-conveys a perfect impression even of the details of a landscape. In the
-owl-moths (Noctuidæ) the wing surfaces, which are brightly coloured,
-are simple, almost crude, in pattern, as in the moths of the genus
-<i>Catocala</i>, with their red, blue, or yellow posterior wings, traversed
-by a large black band; while in the Geometer-moths, whose wings
-are spread out flat when at rest, the protective upper surface of
-all four wings is covered with a complex pattern of lines, spots, and
-streaks in different shades of grey, yellow, white, and black, so that it
-bears a deceptive resemblance to the bark of a tree or the side of a
-wall. For a long time I could not understand how such a definite
-and constant pattern could arise through natural selection if it was
-a case of mimicking the impression of bark or of any other irregularly
-covered surface, the colours of which are not mingled in exactly the
-same way everywhere. But now I think I understand it; for in the
-apparently meaningless colour-splashes of an 'impressionist' landscape
-the different splashes must be exactly where they are, otherwise
-on stepping back from the picture one would see, not a Haarlem
-hyacinth-field, or an avenue of poplars with their golden autumn
-leaves, but a mere unintelligible daub. It is the <i>type</i> of the colour-pattern
-that must be attained, and in nature this is attained very
-slowly, step by step, spot after spot, and therefore, obviously, no
-correct stroke once attained will be given up again, since, in combination
-with the rest, it secures the proper type of colour-pattern.
-Only thus, it seems to me, can we understand how apparently
-meaningless lines, like the figures 1840 on the under surface of
-<i>Vanessa atalanta</i>, could have become a constant characteristic of
-the species.</p>
-
-<p>To sum up briefly, we may say that sexual selection is a much
-more powerful factor in transformation than we should at first be
-inclined to believe. It cannot, of course, have been operative in the
-case of plants, nor can it be taken into consideration in regard to the
-lower animals, for these, like the plants, do not pair, or, at any rate,
-do so without any possibility of choice. Animals which live on the
-sea-floor, or which are attached there, must simply liberate their
-reproductive cells into the water, and cannot secure that they unite
-with those of this or that individual. This is the case among sponges,<span class="pagenum"><a id="Page_238"></a>[Pg 238]</span>
-corals, and Hydroid polyps. In some other classes the sense organs
-are too poorly developed, and the eyes in particular too imperfect to
-be excited in different degrees by any peculiarities in the appearance
-or behaviour of the males. This is what Darwin meant when he
-ascribed to these animals 'too imperfect senses and much too low
-intelligence' 'to estimate the beauty or other attractive points of the
-opposite sex, or to feel anything like rivalry.' Accordingly, in the
-Protozoa, Echinoderms, Medusæ, and Ctenophores, secondary sexual
-characters are entirely absent, as pairing also is.</p>
-
-<p>In those worms that pair we first meet with secondary sexual
-characters, and from this level upwards they are never quite absent
-from any large group, and gradually play an increasingly important
-rôle.</p>
-
-<p>But the significance of sexual selection lies, as we have seen, not
-only in the fact that one sex of a species, usually the male, is modified,
-but in the possibility of the transference of this modification to the
-females, and further, in the fact that the process of variation may
-start afresh at any time, and thus one variation may be developed
-upon or alongside of another. In this way we can explain certain
-complex and often fantastic forms and colourings which we could not
-otherwise understand; thus the extraordinary number of nearly related
-species in some animal groups, such as butterflies and birds, in which
-the differences mainly concern the colour-patterns.</p>
-
-<p>Darwin has shown convincingly that a surprising number of
-characters in animals, from worms upwards, have their roots in sexual
-selection, and has pointed out the probability that this process has
-played an important part in the evolution of the human race also,
-though, in this case, all is not yet so clearly and certainly known as
-among animals.</p>
-
-<p>To conclude this section, I should like once again to call attention
-to the deficiency which is necessarily involved in the assumption of
-any selection, sexual selection included, namely, that the first
-beginning of the character which has been intensified by selection
-remains obscure. Darwin attached importance to the occurrence
-of ordinary individual variation, but it is open to question
-whether the insignificant variations thus produced could give an
-adequate advantage in the competition for the possession of the
-females; and, further, whether we have not grounds for the assumption
-that larger variations also occur. This question may also be asked in
-regard to ordinary natural selection, although in that case we can
-imagine the beginnings to be smaller, since here the advantage of
-a variation lies only in the fact that it is useful, not in its being<span class="pagenum"><a id="Page_239"></a>[Pg 239]</span>
-appreciated by others. As a matter of fact, this very difficulty as to
-the first beginnings of variations has been frequently urged against
-both hypotheses of selection, and rightly so, inasmuch as this must be
-above all else the point of attack for further investigations. But it is
-a mistake to deny the whole processes of selection simply because this
-point is not yet clear. Later on we shall attempt to gain some insight
-into the causes of variation, and then we shall return to this question
-of the beginnings of the selective processes. In the meantime let
-it suffice to say that Darwin was very well aware that, in addition to
-the ordinary individual variations, there were also larger deviations
-which occurred discontinuously in single forms. He believed, however,
-that such occurrences were very rare, and, on the whole, he was
-not inclined to ascribe to them any particular importance in the
-transformation of species. He rather referred the organic transformations
-which have taken place in the course of the earth's
-history, in the main, to the intensification of the ordinary individual
-variations, and I believe that he was right in so doing, since adaptations
-from their very nature cannot have been brought about by
-sudden chance leaps in organization, but can only have become exactly
-suited to chance conditions of life through a gradual accumulation of
-minute variations in the direction of utility. Whether, however,
-purely sexual distinctions may not have had their primary roots in
-discontinuous variations must be inquired into later. Theoretically,
-there is nothing against this assumption, when such characters are
-not adaptations like the lasso antennæ of the Copepods, or the turban
-eyes of the Ephemerids; mere distinctive markings, decorative coloration,
-peculiar outgrowths, and the like, may, if they arose discontinuously,
-very well have formed the basis for further sexual
-selection, as long as they were not prejudicial to the existence of the
-species.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_240"></a>[Pg 240]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XII">LECTURE XII</h2>
-</div>
-
-<p class="c">INTRA-SELECTION OR SELECTION AMONG TISSUES</p>
-
-<div class="blockquot">
-
-<p>Does the Lamarckian principle really play a part in the transformations of
-species?&mdash;Darwin's position in regard to this question&mdash;Doubts expressed by Galton
-and others&mdash;Neo-Lamarckians and Neo-Darwinians&mdash;Results of exercise and practice:
-functional adaptation&mdash;Wilhelm Roux, <i>Kampf der Theile</i>.</p></div>
-
-
-<p><span class="smcap">We</span> have devoted a whole series of lectures to studying the
-Darwin-Wallace principle of Natural Selection and the range of its
-operation. It seemed to us to make innumerable adaptations intelligible
-up to a certain point. We now understand how the purposefulness,
-which we meet with everywhere among organisms, can have
-arisen without the direct interference of a Power working intentionally
-towards an end&mdash;simply as the outcome and result of the survival
-of the fittest. The two forms of the processes of selection, 'natural
-selection' in the narrower sense, and 'sexual selection,' dominate, so
-to speak, all parts and all functions of the organism, and are striving
-to adapt these as well as possible to the conditions of their life. And
-although the range of operation of Natural Selection is incomparably
-greater, because it actually affects every part, yet we must attribute
-to sexual selection also, at least among animals, a range of influence
-by no means unimportant, since through it, as far as we can see at
-present, not only do the secondary sexual characters in all their
-diversity arise, but by the transference of these to the other sex that
-too is modified, and thus the whole species may be influenced, and
-may indeed be started afresh on an unlimited series of further transformations.</p>
-
-<p>But although the processes of selection play such an important
-part in the transformations of the forms of life, we have to inquire
-whether they are the <i>sole</i> factors in these transformations, whether
-the accumulation of chance variations in the direction of utility has
-been the sole factor in bringing about the evolution of the animate
-world, or whether other factors have not also co-operated with it.</p>
-
-<p>We are all aware that Lamarck regarded the direct influence of
-use and disuse as the most essential factor in transformation, and that
-Darwin, though hesitatingly and cautiously, recognized and accepted
-this factor, which he believed to be indispensable. Indeed, it seems<span class="pagenum"><a id="Page_241"></a>[Pg 241]</span>
-at first sight to be so. There is a whole range of facts which seem to
-be intelligible only in terms of the Lamarckian theory; in particular,
-the existence of numberless vestigial or rudimentary organs which
-have degenerated through disuse, the remains of eyes in animals which
-live in darkness, of wings in running birds, of hind legs in swimming
-mammals (whales), and of ear muscles in Man, who no longer points
-his ears, and so forth through a long list.</p>
-
-<p>According to Wiedersheim, there are in Man alone about two
-hundred of these vestigial or rudimentary organs, and there is no
-higher animal which does not possess some. In all, therefore, a piece
-of the past history of the species is embodied in the actually existing
-organism, and bears witness to the fact that much of what the
-ancestors possessed is now superfluous, and is either transformed,
-or is gradually set aside, or is still in process of being set aside. It
-seems obvious that this gradual dwindling and degeneration of an
-organ no longer needed cannot be explained through natural selection
-in the Darwin-Wallace sense, for the process goes on so exceedingly
-slowly that the minute differences in the size of an organ, which may
-occur among individuals of the species at any given time during the
-retrogressive process, cannot possibly have a selection value. Whether
-the degenerate and now functionless hind leg of the whale is a little
-larger or a little smaller can have no importance in the struggle for
-existence; the smaller organ cannot be considered either as a lesser
-hindrance in swimming or as a greater economy of material, and the
-case is the same in regard to most other instances of degeneration
-through disuse. We therefore require another interpretation, and at
-first sight this seems to be supplied by the Lamarckian principle.</p>
-
-<p>But the reverse process, the strengthening, the enlarging, and the
-more perfect development of a part, very often goes on proportionately
-to its more frequent use, and here again the Lamarckian principle
-seems to afford a simple explanation. For we know that exercise
-strengthens a part, as disuse weakens it, and if we could assume that
-these results of use and disuse were transmitted from the individual
-who brought them about or 'acquired' them in the course of his life
-to his offspring, then there would be nothing to object to in the
-Lamarckian principle. But it is precisely here that the difficulty lies.
-Can we assume such a transmission of 'acquired' characters? Does
-it exist? Can it be demonstrated?</p>
-
-<p>That Lamarck did not even put this question to himself, but
-assumed such transmission as a matter of course, is readily intelligible
-when we consider the time at which he lived. He was himself one of
-the first to grasp the idea of the transmutation-hypothesis, and he<span class="pagenum"><a id="Page_242"></a>[Pg 242]</span>
-was only too glad to have any sort of principle of interpretation
-ready to work with. But Charles Darwin, too, attributed a not
-inconsiderable influence to this principle, although the transmission of
-'acquired' characters which it took for granted was not accepted
-without reflective hesitation. He even directed his own particular
-theory of heredity, as we shall see, especially to the explanation
-of this supposed form of inheritance, and we can very well understand
-this, after what I have said as to the impossibility of explaining
-the disappearance of organs which have become superfluous by
-the Darwin-Wallace theory of Natural Selection. Darwin needed
-the Lamarckian principle for the explanation of these phenomena, and
-it was this that decided him to assume the transmission of 'acquired'
-characters, although the proofs of it can hardly have satisfied him.
-For when we are confronted with facts which we see no possibility of
-understanding save on a single hypothesis, even though it be an
-undemonstrable one, we are naturally led to accept the hypothesis, at
-least until a better one can be found. It is in this way, obviously,
-that we are to understand Darwin's attitude to the Lamarckian
-principle; he did not reject it, because it seemed to him to offer the
-only possible explanation of the disappearance of characters which
-have become useless; he adhered to it, although the transmission of
-acquired characters which it assumed must have seemed, and, in point
-of fact, did seem to him doubtful, or at least not definitely proved.
-Doubts, some faint, some stronger, as to this assumed form of
-inheritance were hardly expressed till somewhat late in the day&mdash;almost
-twenty years after the appearance of the <i>Origin of Species</i>&mdash;first
-by Francis Galton (1875), then by His, who definitely declared
-himself at least against any inheritance of mutilations, and by
-Du Bois-Reymond, who, in his address <i>Ueber die Uebung</i> in 1881,
-said: 'If we are to be honest, we must admit that the inheritance of
-acquired characters is a hypothesis inferred solely from the facts
-which have to be explained, and that it is in itself quite obscure.'</p>
-
-<p>This is how it must appear to every one who examines it simply
-in respect of its theoretical possibility, its conceivability. This is how
-it appeared to me when I attempted, in 1883, to arrive at clearness
-on the subject, and I then expressed my conviction that such a
-form of inheritance was not only unproved, but that it was even
-theoretically unthinkable, and that we ought to try to explain the
-fact of the disappearance of disused parts in some other way, and
-I attempted to give an explanation, as will be seen later.</p>
-
-<p>Thus war was declared against the Lamarckian principle of
-the direct effect of use and disuse, and there arose a strife which<span class="pagenum"><a id="Page_243"></a>[Pg 243]</span>
-has continued down to the present day, the strife between the Neo-Lamarckians
-and the Neo-Darwinians, as the two disputing parties
-have been called.</p>
-
-<p>In order to form an independent opinion in regard to this famous
-dispute, it is, first of all, necessary to examine what actually takes
-place when an organ is exercised or is left inactive, and further,
-whether we can assume that the results of this exercise or inaction
-can be transmitted to descendants.</p>
-
-<p>That exercise in general has a strengthening, and neglect of it
-a weakening influence on the relevant organ has long been known
-and is familiar to all; gymnastics make the muscles stronger, the
-thickness of the exercised muscle and the number of its fibres
-increases; the right arm, which is much more used than the left,
-is capable of performing twenty per cent. more work. Similarly,
-the activity of glands is increased by exercise, and the glands
-themselves are increased in size, as are the milk-glands of the cow
-through frequent milking; and that even the nerve-elements
-can be favourably influenced by exercise is proved by actors and
-professors of mnemonics, who have by practice increased their powers
-of memory to an almost incredible degree. I have heard of a singer
-who had learned by heart 160 operas; and which of us has not
-experienced how quickly the capacity for learning by rote can be
-again increased by practice, even after it has been neglected or left
-unexercised for a long time?</p>
-
-<p>I have always been particularly struck with the practising of
-a piece of music, with its long succession of periods of different
-phrase, with its changes in melody, rhythm, and harmony, which
-nevertheless becomes so firmly stamped on the memory that it can be
-played, not only consciously, but quite unconsciously, when the player
-is thinking intensely of other things. It is in this case not the
-memory alone, but the whole complicated mechanism of successive
-muscle-impulses, with all the details of fast and slow, loud and soft,
-that is engraved on the brain elements, just like a long series of
-reflex movements which set one another a-going. Though in this case
-we cannot demonstrate the material changes which have taken place
-in the nervous elements, there can be no doubt that changes have
-taken place, and that these consist in a strengthening of definite
-elements and parts of elements. The strengthening causes certain
-ganglion-cells to give a stronger impulse in a particular direction, and
-this impulse acquires increasing transmissive power, and so on.</p>
-
-<p>Our first theoretical insight into these relations came through
-Wilhelm Roux, who, in 1881, gave expression to what had previously<span class="pagenum"><a id="Page_244"></a>[Pg 244]</span>
-been an open, if not quite conscious, secret, that 'functional stimulus
-strengthens the organ,' that is to say, that an organ increases through
-its own specific activity. Up till that time it had been believed that
-it was merely the increased flow of blood that caused the increase in
-the size of a much-used part. Roux showed that there is a 'quantitative
-self-regulation of the organ according to the strength of the stimulus
-supplied to it'; that the stimulated organ, that is, the organ which is
-performing its normal function, may, in spite of the increased breaking
-down or combustion (dissimilation), assimilate all the more rapidly;
-that its used-up material is 'over-compensated,' and that therefore it
-grows. He called this the 'trophic' or nutritive effect of the stimulus,
-and in terms of this he explained the increase and the heightened
-functional capacity of the much-used organ. Conversely, he referred
-the decrease of a disused organ to 'functional atrophy,' which sets in
-when there is a deficient compensation for the substance used up in
-the metabolism.</p>
-
-<p>But if we press for deeper analysis, we must ask: 'On what does
-this trophic effect of functional stimulus depend?' Roux could not
-answer this question when he wrote, nor can we do so now, as Plate
-has justly emphasized. We are here face to face with the fundamental
-phenomenon of life, metabolism; and, since we do not understand the
-causes of this, we are not in a position to say why it varies in this
-way or in that according to the 'stimulus.' But the fact itself is
-certain that the organs respond up to a certain point to the claims
-made upon them; they increase in proportion as they function more
-frequently or more vigorously, they are able to respond to increased
-functional demands, and this Roux has called 'functional adaptation.'
-As an animal adapts itself to the claims of the conditions of its life,
-for instance, by taking on a green or a brown protective colour
-according as it lives on green or brown parts of plants, so the
-individual organ adapts itself to the strength of the stimulus which
-impels it to function, and increases or decreases in proportion to
-it. If <i>one</i> kidney in Man degenerate, or be surgically removed, the
-other begins to grow, and goes on increasing until it has reached
-nearly twice its former size. The specific stimulus which is brought
-to bear upon it by the urea contained in the blood, and which forces
-it to grow, is twice as great in the absence of the other kidney, and
-therefore the remaining kidney grows in response to the increased
-stimulus and its 'trophic effect' until its increase in size has reduced
-the functional intensity to the normal proportion.</p>
-
-<p>Adaptation of an organ in the opposite direction takes place
-when the function diminishes or ceases. If a nerve supplying a<span class="pagenum"><a id="Page_245"></a>[Pg 245]</span>
-muscle or a gland be cut through, the organ concerned begins to
-degenerate and to lose its normal structure to a greater or less degree.
-Sensory nerves also degenerate in their peripheral part when they are
-cut through. In such cases there may be no alteration either in the
-nutritive mechanism or in the blood-vessels, &amp;c., but the functional
-stimulus&mdash;in the case of the muscle, the stimulus from the will&mdash;no
-longer affects the organ, and its metabolism is so much lowered in
-consequence that it begins to degenerate.</p>
-
-<p>When we admit that the fit adaptation of the organism, as far
-as we understand it, must depend upon processes of selection, we
-may refer this 'functional adaptation' also to primitive processes of
-selection, which prevailed at the very beginning of life upon our
-earth, and represented, so to speak, the first adaptation that was
-established, but we can say nothing with certainty in regard to this
-matter as long as we do not understand the essence of assimilation.
-It is conceivable, however, that a <i>primary</i> adaptiveness may have
-arisen, so to speak, abruptly, through a concurrence of favourable circumstances,
-as we shall endeavour to show later on when we discuss the
-beginnings of life.</p>
-
-<p>Even although we cannot lay bare the primary roots of 'functional
-adaptation' we can gain from the fact itself very valuable
-insight into phenomena which would otherwise be unintelligible and
-mysterious: <i>the perfectly adapted structure of many tissues and their
-power of adaptation to changed conditions</i>. In this lies, in the main,
-the advance in our knowledge which is due to Roux's <i>Kampf der
-Theile</i>.</p>
-
-<p>If a number of embryonic cells of different capacity, say <i>A</i>, <i>B</i>,
-and <i>C</i>, be affected by different kinds of functional stimuli, <i>a</i>, <i>b</i>, and <i>c</i>,
-those cells will grow most rapidly which are most frequently affected
-by the stimulus appropriate to them. The proportion in which the cells
-<i>A</i>, <i>B</i>, and <i>C</i> will ultimately be present in the tissues will depend upon the
-frequency with which the stimuli <i>a</i>, <i>b</i>, and <i>c</i> act upon the tissue. But
-the tissue will be still more precisely determined as to its structure if the
-three kinds of stimuli affect the cell-mass, not uniformly all over, but
-only at certain spots, or along particular paths, one in this, the other
-in that. Thus the cells <i>A</i> will predominate over the cells <i>B</i> and <i>C</i> at
-all the places which are most frequently affected by the stimulus <i>a</i>,
-the cells <i>B</i> in the sphere of the stimulus <i>b</i>, and the cells <i>C</i> in that of
-the stimulus <i>c</i>; there they will increase most rapidly and so crowd
-out the other kinds of cells, and thus a spatial arrangement will be
-established within the tissue, a 'structure' which corresponds and
-is well adapted to its end. This is what Roux deduced from his<span class="pagenum"><a id="Page_246"></a>[Pg 246]</span>
-<i>Struggle of the Parts</i>, and I subsequently defined the process as
-histonal or tissue selection.</p>
-
-<p>Let us first take an example. The anatomist Hermann Meyer
-showed in 1869 that the so-called 'spongiosa,' that is, the bony tissue
-of spongy structure within the terminal portions of the long bones
-in Man and Mammals, has a minute structure conspicuously well
-adapted to its office. The thin bone lamellæ of this 'spongiosa' lie
-precisely in the direction of the strongest strain or pressure which is
-exerted upon the bone at the particular area. Arch-like in form, they
-are kept apart by means of buttresses, and no architect could have
-done better if he had been entrusted with the task of making
-a complicated system of arches with the greatest possible carrying
-and resisting power combined with the greatest possible economy of
-material.</p>
-
-<p>This well-adapted structure is now interpreted through the
-<i>Struggle of the Parts</i> as a self-differentiation, for if there be in the
-rudiments or primordia of the bone differently endowed elements<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a>, that
-is, cells which respond in diverse ways to different stimuli, these must
-arrange themselves locally, owing to the struggle for space and food,
-in a manner corresponding to the distribution of the different stimuli
-in the bone. The largest amount of bone substance will be formed in
-the directions of the strongest strain and the greatest pressure, because
-the bone-forming cells are excited by this, their functional stimulus,
-to growth and multiplication. Thus the buttress and arch structure
-comes about, and between the delicate bone lamellæ spaces will remain
-free, and these, being relieved from the burden of strain and pressure
-by the aforesaid bony lamellæ, will offer suitable conditions of life to
-cells with other functional properties, such as connective tissue cells
-or vascular cells.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> I do not here enter into the question whether we have not in this case to do with
-similar elements, which have the power of differentiating into one or another kind
-of cell according to the nature of the external stimuli by which they are influenced.</p>
-
-</div>
-
-<p>The structure of the bone spongiosa is not everywhere the same,
-and it is demonstrably related with precision to the conditions of
-strain and pressure at each particular region. Thus, just below the
-soft cartilaginous covering of the joints there are no long pillars with
-short arches, but only rounded meshes, because the pressure is here
-almost equally strong from all sides. The long parallel pillars only
-occur further down in the bone, and they lie in two directions which
-intersect each other obliquely, corresponding to the two main
-directions of pressure. But it is only under the functional stimulus
-of pressure that the bone-forming cells have an advantage over the<span class="pagenum"><a id="Page_247"></a>[Pg 247]</span>
-others, and multiply more quickly, thus crowding out those that are
-not attuned to the appropriate functional stimulus.</p>
-
-<p>In a similar manner Roux interprets, in the light of the struggle
-of the parts, the striking adaptations in the course, the branching,
-and the lumen-formation of the blood-vessels, in the direction of the
-intersecting connective tissue strands in the tail-fin of the dolphin,
-in the direction of the fibres in the tympanum, and in many other
-adaptations in the histological structure of complex tissues.</p>
-
-<p>In this there is manifestly an important step of progress, for
-it is obvious that the direction of the bone-lamellæ and such like
-could not have been determined by individual selection, and the same
-is true in regard to many other histological details. It cannot be disputed,
-however, that there is a kind of selection-process here also,
-similar to that which we think of, with Darwin and Wallace, as
-occurring between individual organisms. Just as in the latter, which
-we shall henceforward call <i>personal selection</i>, variability and inheritance
-lead, in the struggle for existence, to the survival of the fittest,
-so, in histonal differentiation, the same three factors lead to the
-victory of what is best suited to the parts of the body in question.
-The tissues and the parts of the tissues have to distribute and arrange
-themselves so that each comes to fill the place in which it is most
-effectively and frequently affected by its specific stimulus, that is, the
-stimulus in regard to which it is superior to other parts; but these
-places are also those the occupation of which by the best re-acting parts
-makes the whole tissue capable of more effective function, and therefore
-makes its structure the fittest. Variability&mdash;in this case that
-of embryonic cells, with different primary constituents&mdash;must be
-assumed; inheritance is implied in the multiplication of the cells by
-division; and the 'struggle for existence' here assumes its frequent
-form of a competition for food and space; the cells which assimilate
-more rapidly because of the more frequent functional stimulus
-increase more rapidly, draw away nourishment from the more slowly-multiplying
-cells around them, and thus crowd these out to a greater
-or less extent.</p>
-
-<p>We might even speak of histonal selection among unicellulars,
-for it is conceivable that in primitive living substance, such as that of
-a moneron, there may be minute differences among the vital particles,
-involving also functional distinctions, which, under the influence of
-diverse stimuli, may gradually give rise to an increasingly complex
-differentiation. For the variations in the primary living substance
-most strongly affected by a particular stimulus would tend to accumulate
-at the places most frequently reached by that stimulus, and<span class="pagenum"><a id="Page_248"></a>[Pg 248]</span>
-would crowd out other variations at that spot, just as the body
-and its individual parts may be said to have taken their architectural
-form in exact response to the demands made upon them by function.
-In this case, of course, personal selection and histonal selection co-operate,
-for every improvement in the organization of the fundamental
-living substance means at the same time a lasting improvement in the
-whole individual.</p>
-
-<p>In many-celled organisms, however, we must admit that there is
-an essential difference between personal and histonal selection, inasmuch
-as the latter can give rise to adaptive structural modifications
-corresponding to the needs of the tissue at the moment, but not to
-permanent and cumulative changes in the individual elements of the
-tissue. If a broken bone heals crookedly, the spongy substance within
-the healed portion does not remain as it was before, for the pillars
-and arches, which now no longer run in the direction best suited to
-their function, break up, and a new system of arches is formed,
-not in line with the earlier one, but adapted to the new conditions
-of pressure. This is certainly an adaptation through selection, but
-the elements, that is the cells which form the bone substance in
-response to strain and pressure, or those which in response to the
-stimulus of the blood flowing into the spaces form the blood-vessels,
-or those which being quite freed from one-sided pressure develop into
-connective tissue, must be presupposed. These kinds of cells must be
-virtually implied in the germ-rudiment; they are themselves adaptations
-of the organism, and can therefore only be referred to <i>personal
-selection</i>. And this is true of all adaptations of the <i>elements</i> of
-multicellular organisms, and thus of the <i>cells</i>. Their adaptation
-according to the principle of division of labour, their differentiation
-into muscle, nerve, and gland cells can only be referred to natural
-selection in the Darwin-Wallace sense, and cannot depend upon
-histonal selection. In the spongy substance of the bone a better
-bone-cell does not struggle with an inferior one and leave behind
-it by its survival a host of descendants which are, if possible, better
-than itself; the struggle for existence and for descendants, in this
-case, is between two kinds of cell which were different from the
-beginning, and of which one has the advantage at one spot, another
-at another. The case may be compared to that of a flock of nearly
-allied species of bird, of which one species thrives best in the plains,
-another among the hills, and a third among the mountain forests, all
-mingled together in a vast new territory to which they had migrated,
-and in which all three kinds of conditions were represented. A
-struggle would arise among the different species, in which in every<span class="pagenum"><a id="Page_249"></a>[Pg 249]</span>
-case the particular species would be victorious which was best adapted
-to the local conditions. But each would thrive best in the region in
-which it was superior to the others, and very soon the three species
-would be distributed as they were in the land from which they
-came&mdash;in the plains, the high lands, and the mountain forests. This
-would be the result of a struggle between the three species, <i>not
-between individuals within each species</i>, and it could not therefore
-bring about an improvement of a single species, but only the local
-prevalence of one or another. The characters which made one
-species adapted for the plain, another for the mountain forest were
-<i>already there</i>; they can only be referred to personal selection, which
-brought about the adaptation of their ancestors in the course of ages
-to the conditions of their life. Something similar is true of the
-adaptations of the tissues; the differentiation of the individual kinds
-of cells is an ancient inheritance, and depends upon personal selection,
-but their distribution and arrangement into specially adapted tissues,
-so far as there is any plasticity at all, depends upon histonal selection.
-Obviously, however, only as far as the tissue is plastic, that is, with
-the power of adjusting itself to particular local conditions. Only
-adaptations of this kind can be referred to histonal selection; the
-ground-plan, even of the most complicated tissue, such as the large
-glands of mammals, the kidneys, the liver, and so on, must have been
-implicit in the germ, and must therefore be referred to personal
-selection. A precise limitation of the respective spheres of action
-of personal selection and histonal selection is not possible as yet, since
-hardly any investigations on the subject are available.</p>
-
-<p>Roux undoubtedly over-estimated the influence of his 'struggle
-of parts' when he believed that the most delicate adaptations of the
-different kinds of cells depended on it. I admit that, for a considerable
-time, I made the same mistake, until it became clear to me,
-as it did first in regard to the sex-cells, that this is not, and cannot be
-the case. How, for instance, could the diverse and minutely detailed
-adaptations of the sex-cells&mdash;which we are to discuss in a subsequent
-lecture&mdash;have arisen in this way? As far as the individual sperm-cell
-is concerned, it is a matter of indifference whether its head is a little
-thinner or thicker, its point a little sharper or blunter, its tail a little
-stronger or weaker. This does not decide whether the cell is to
-thrive better, or to occur in greater numbers than some other variety.
-But it does decide whether it is to be able to penetrate through the
-minute micropyle, or through the firm egg-envelope, into the egg,
-there to effect fertilization. An individual with less well formed
-sperm-cells will be able to fertilize fewer eggs, and therefore to leave<span class="pagenum"><a id="Page_250"></a>[Pg 250]</span>
-fewer descendants which might inherit its tendency to produce inferior
-sperm-cells, and conversely. Thus it is not the sperm-cells of any one
-individual which are selected according to their fitness, it is the
-individuals themselves which compete with one another in the production
-of germ-cells which shall fertilize best, that is, most certainly.
-The struggle is thus not intercellular, but a struggle between persons.</p>
-
-<p>The same is true of all cells differentiated for particular functions;
-every new kind of glandular, muscular, or nerve cell, such as have
-arisen a thousandfold in the course of phylogeny, can only have
-resulted from a struggle between individuals which turned on the
-possession of the best cells of a particular kind, <i>not from a struggle
-between the cells themselves</i>, since these would gain no advantage
-from serving the organism, as a whole, better than others of their
-kind. In regard to the sex-cells we might admit, in addition to
-personal selection, the possibility of an internal struggle between the
-sperm-cells or egg-cells of the same individual, inasmuch as each of
-these cells is the primordium of a new individual, and as those better
-adapted for reproduction might transmit their better quality to these
-new individuals. I will not here enter into my reasons for regarding
-this idea as erroneous, for in any case this interpretation would not
-apply to any other kind of cells. If, for instance, it were a question
-of the transformation of an ordinary mucus or salivary gland into
-a poison gland, it would not matter in the least to the individual cell
-whether it yielded a harmless or a poisonous secretion; but individuals
-with many poisonous cells would have an advantage in the struggle
-for existence.</p>
-
-<p>I agree so far with Plate when he refers the differentiation of the
-tissues entirely to personal selection, but not in his further conclusion
-that histonal selection does not exist. The ground-plan of the
-architectural structure of the organ depends upon personal selection,
-but the realization of the plan in particular cases is not predetermined
-down to the minutest details, but is regulated by histonal selection,
-and is thus to a certain extent an adaptation to local conditions of
-stimulus. The direction, strength, and size of every single bone lamella
-is not predetermined from the germ, but only the occurrence and
-nature of bone-cells and bone lamellæ in general. The direction
-and the strength which these bone lamellæ may assume depends on
-the local conditions of strain and pressure which affect the cell-mass,
-as is shown very clearly by the spongiosa of an obliquely healed bone,
-which we have already described.</p>
-
-<p>But let us now turn to the question which is here most important
-for us: <i>whether functional adaptations can be transmitted</i>. We must<span class="pagenum"><a id="Page_251"></a>[Pg 251]</span>
-admit that the insight we have so far gained into the causes of these
-adaptations does not make it much easier to answer the question.
-Histonal selection is a purely <i>local</i> process of adaptation to the conditions
-of stimuli prevailing at the moment, and no one will be likely
-to suppose that the distorted position of the spongiosa of a badly
-healed fracture could reappear in the straight bone of a descendant;
-this would be quite contrary to the principle, for the crooked lamellæ
-would in that case no longer be the best adapted. Even the question
-<i>whether the strengthening of a muscle through use can be transmitted</i>
-cannot be answered in the light of the knowledge we have hitherto
-gained. The 'trophic effect of the functional stimulus' is brought
-into activity through entirely local influences, through which only
-the parts most strongly affected by the stimulus can be caused to vary.
-Thus the problem remains unaltered, How can purely local changes,
-not based in the germ, but called forth by the chances of life, be
-transmitted to descendants?</p>
-
-<p>If all species, even in the highest groups, reproduced by dividing
-into two, we might imagine that a direct transmission of the changes
-acquired in the course of the individual life through use or disuse
-took place, though this would presuppose a much more complicated
-mechanism than is apparent at first sight. But it is well known that
-multiplication by fission is for the most part restricted to simple
-organisms, and that the great majority of modern plants and animals
-reproduce by means of germ-cells, which develop within the organism
-in regions often very remote from the parts, the results of the exercise
-of which are said to be transmitted. Moreover, the germ-cells are of
-very simple structure, at least as far as our eyes can discern; for we
-see in a germ-cell neither muscles nor bones nor ligaments, glands nor
-nerves, but only a cell-body consisting of that semifluid living matter
-to which the general name of protoplasm has been given, and of
-a nucleus, in regard to which we cannot say that it differs in any
-essential or definite way from the nucleus of any other cell. How
-then could the changes which take place in a muscle through exercise,
-or in the degeneration of a joint in consequence of disuse, communicate
-themselves to a germ-cell lying inside the body, and do so in such
-a fashion that this germ-cell is able, when it grows into a new
-organism, to produce of itself, in the relevant muscle and joint,
-a change the same as that which had arisen in the parent through
-use and disuse? That is the question which forced itself upon me
-very early, and in following it up I have been led to an absolute
-denial of the transmission of this kind of 'acquired characters.'</p>
-
-<p>In order to explain how I reached this result, and what it is<span class="pagenum"><a id="Page_252"></a>[Pg 252]</span>
-based upon, it is indispensable that we should first make ourselves
-acquainted with the phenomena of heredity in general, and the
-inseparably associated phenomena of reproduction, so that we may
-form some sort of theoretic conception of the process of inheritance&mdash;a
-picture, necessarily provisional and imperfect, of the mechanism
-which enables the germ-cell to reproduce the whole organism, and
-not merely, like other cells, others like itself. We are thus led to
-an investigation of reproduction and heredity, at the conclusion of
-which we shall feel justified in returning to the question of the
-inheritance of acquired characters, in order to give a verdict as to
-the retention or dismissal of the Lamarckian principle.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_253"></a>[Pg 253]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XIII">LECTURE XIII</h2>
-</div>
-
-<p class="c">REPRODUCTION IN UNICELLULAR ORGANISMS</p>
-
-<div class="blockquot">
-
-<p>Reproduction by division&mdash;In Amœbæ&mdash;In Infusorians&mdash;Divisions following one
-another in immediate succession&mdash;Formation of germ-cells in the Metazoa&mdash;Contrast
-between germ-cells and body-cells&mdash;Potential immortality of unicellular organisms&mdash;Beginning
-of natural death&mdash;Budding and division in the Metazoa.</p></div>
-
-
-<p><span class="smcap">We</span> wish to consider the reproduction of organisms with special
-reference to the problem of heredity, and it is most instructive
-to begin with the lowest forms of life&mdash;the unicellulars&mdash;because
-their structure, as far as we can see with the instruments at our
-command, is very simple, and, what is
-even more important, is relatively homogeneous.</p>
-
-<div class="figright" id="f63">
-<img src="images/fig63.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 59.</span> An Amœba: the process<br />
-of division. <i>A</i>, before the<br />
-beginning of the division. <i>B</i>,<br />
-the nucleus divided into two.<br />
-<i>C</i>, the two daughter-Amœbæ.<br />
-Magnified about 400 times.</p>
-</div>
-
-<p>Suppose that there are bacteria-like
-organisms of quite homogeneous structure,
-and that these multiply by simply dividing
-into two, each rod-like creature dividing
-transversely in the middle of its length,
-the two halves would represent independent
-daughter-organisms, whose structure
-would correspond exactly with that of
-the mother-organism, could not indeed in
-any way deviate from it, and consequently
-would take over all its characters,
-that is, would inherit them. The size of
-body is the only feature which is not
-obviously inherited, but in reality it is
-potentially heritable, since the structure
-of the divided portions involves the
-capacity and the limits of their possible
-growth. Moreover, the size of body is not invariable in any species;
-a particular size is only reproduced under similar conditions of
-development. Inheritance here consists simply in a continuation
-of the mother-organism into its two daughter-cells.</p>
-
-<p>Even in an Amœba (Fig. 59) we might picture the process of
-inheritance as equally simple, though in so doing we should probably
-be making a fallacious inference, for the structure of these lowest
-<span class="pagenum"><a id="Page_254"></a>[Pg 254]</span>unicellular animals probably seems to us simpler and more homogeneous
-than it really is. Among Infusorians it is quite obvious that inheritance
-implies more than the mere halving of the mother-animal into
-the two daughter-cells; something more must be involved. For among
-these unicellular animals the differentiation of the body is not only
-great, but it is unsymmetrical. The posterior and the anterior ends
-are different, and the transverse division of the animal, in which the
-process of reproduction here consists, does not produce two halves,
-but two very unequal portions. In the division of <i>Stentor</i>, the
-so-called trumpet-animalcule (<a href="#f64">Fig. 60</a>), the anterior portion contains
-the funnel-shaped mouth and gullet with its complicated nutritive
-apparatus, the circular peristome with its spirally curved rows of composite
-ciliated plates, the so-called membranellæ, and so forth; the
-posterior half contains nothing of all this, but possesses the foot of the
-mother-Stentor with its attaching organ, which the anterior half lacks.
-But each of the two portions possesses the power of 'regeneration,'
-that is, it is able to develop anew the missing parts, mouth or foot,
-and so on. So that here there is no longer merely a simple continuance
-of the maternal organization in the daughter-animals, there is<span class="pagenum"><a id="Page_255"></a>[Pg 255]</span>
-something new added, something which requires explanation; we are
-confronted with the first riddle of heredity. Simple growth does not
-explain the phenomenon, for what has to be added to complete the
-halved portions has a different structure, a different form, different
-accessory apparatus from any that the halves themselves possess. It
-in no way affects this state of matters that in the normal process of
-division in Infusorians the formation of the new mouth and peristome-region
-begins before the halves have actually separated, for even if a
-Stentor be cut in two artificially the cut halves form complete animals.
-And, indeed, a Stentor may be cut into three or four pieces, and in
-certain conditions each piece will develop into a complete animal.
-These pieces must therefore possess something more than the mere
-power of growth. We shall try later on to discover whether this
-marvellous invisible transmission of characters, this implication of the
-whole in each of the parts, can be in any way theoretically expressed
-and included in our scheme of conceptual formulation.</p>
-
-<div class="figcenter" id="f64">
-<img src="images/fig64.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 60.</span> <i>Stentor rœselii</i>, trumpet-animalcule. Process
-of division. <i>wsp</i>, ciliated spiral leading to the mouth
-(<i>m</i>); <i>cv</i>, contractile vacuole. <i>A</i>, in preparation for
-division, the nucleus (<i>k</i>) has coalesced into a long twisted
-band. <i>B</i>, a second ciliated spiral (<i>wsp´</i>) has begun to
-be formed; the nucleus (<i>k</i>) is contracted. <i>C</i>, just before
-the constricting off of the two daughter-Stentors.
-Magnified about 400 times. After Stein.</p>
-</div>
-
-<p>Now that we have become familiar with these facts it will no
-longer surprise us to learn that the reproduction of unicellular animals
-does not always depend on <i>equal</i> division, but that unequal spontaneous
-divisions are also possible, so that one or several smaller portions of
-the cell-body, containing a portion of the cell-nucleus, can separate off
-from the mother-animal. This occurs especially among the suctorial
-Infusorians or Acinetæ. In relation to the phenomena of inheritance
-the problem raised by the equal division of the Infusorians repeats
-itself, and it is in no way affected by the fact that equal division can
-take place several times, or many times in succession, so that from one
-animal a large number of small pieces of the same size may be
-produced in rapid succession. The characteristic marks of the mother-animal
-are not infrequently lost sight of, wholly or partially, when
-this occurs, and the divided portions seem to consist of a homogeneous
-cell-body and nucleus; but they possess the power of regenerating
-themselves, or of developing, if the expression be preferred, into
-animals similar to the maternal-organism. Such divided portions
-might very well be called germs, only it must not be forgotten that
-the relation of the mother-animal to these germs is a different one
-from that of a higher animal or plant to its germ-cells; the unicellular
-animal breaks up by continued division into these 'germs,' while the
-Metazoon continues its individual existence unimpaired by the production
-of its germ-cells.</p>
-
-<p>The beginning of a so-called 'spore-formation' is to be found in
-many Infusorians. Thus the holotrichous species, <i>Holophrya multifiliis</i>
-(Fig. 61), reproduces by first becoming enclosed in a cyst or capsule,<span class="pagenum"><a id="Page_256"></a>[Pg 256]</span>
-and then dividing many times in rapid succession, so that 2, 4, 8,
-16, &amp;c. individuals arise consecutively, and subsequently burst forth
-from the cyst (Fig. 61, <i>B</i>). In the Gregarines and other Sporozoa the
-period of division lasts much longer, and the encysted animal divides
-into 128, 256, or even more portions; but in this case also each part
-or 'spore' receives a piece of the maternal cell-body and cell-nucleus,
-so that there is no difference in principle between this and the simple
-division into two exhibited by <i>Stentor</i>; as in that case, so here, it is
-not the fully differentiated structure of the animal which is handed on
-to the divided parts; it is only the power to redevelop this anew on
-their own account. Thus here again we are face to face with the
-fundamental problem of heredity: How is it possible that the power
-of reproducing the complex whole can be inherent in the simple parts?</p>
-
-<div class="figcenter" id="f65">
-<img src="images/fig65.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 61.</span> <i>Holophrya multifiliis</i>, an Infusorian parasitic on the
-skin of fishes. <i>A</i>, in its usual condition; <i>ma</i>, macronucleus; <i>mi</i>,
-micronucleus; <i>cv</i>, contractile vacuole; <i>m</i>, mouth. <i>B</i>, after binary
-fission has been several times repeated within the cyst (<i>cy</i>); <i>tt</i>,
-results of the division. <i>C</i>, one of these units much enlarged.</p>
-</div>
-
-<p>In contrast to the unicellular organisms, the great majority of the
-multicellulars, the Metazoa and Metaphyta, many-celled animals and
-plants, differ not only in the multitude of their cells, but even more
-in the manifold differentiation of these cells according to the principle
-of division of labour, so that the various functions of the animal are
-not performed by all the cells uniformly, but each function is relegated
-to a particular set of cells specially organized with reference to it.
-Thus there is differentiation between motile, nutritive, and reproductive
-cells, and there may also be glandular, nerve, muscle, and skin cells,
-and we know how this differentiation into a great number of different
-kinds of cells with highly specialized functions has arisen, especially
-among the higher animals, in a multiplicity which cannot easily <span class="pagenum"><a id="Page_257"></a>[Pg 257]</span>be
-overlooked. Thus we find a large number of the most diverse kinds
-of cells, all of which serve for the maintenance of the body, in contrast
-to the simply reproductive cells or germ-cells. These alone possess
-the power of reproducing, under certain conditions, a new individual
-of the same species. We can contrast with these germ-cells, which
-serve, not for the maintenance of the individual, but only for that of
-the species, all the other kinds of cells under the name of somatic or
-body-cells. The problem which we have to solve now lies before us
-in the question, How comes it that the germ-cell is able to bring forth
-from itself all the other cells in definite sequence and arrangement,
-and is thus able to build up the body of a new individual?</p>
-
-<p>The similarity of this problem to that formulated in regard to
-unicellular organisms is at once obvious, but it becomes still more
-emphatic when we remember that the gulf between unicellular
-organisms and the higher animals and plants is bridged over by
-certain transition forms which are of the greatest interest, especially
-in relation to the problems of inheritance.</p>
-
-<div class="figcenter" id="f66">
-<img src="images/fig66.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 62.</span> <i>Pandorina morum</i>; after Pringsheim. I, A young colony, consisting of 16 cells. II, Another
-colony, whose cells have reproduced daughter-colonies; all the cells uniformly alike. III, A young
-Volvox-colony; <i>sz</i>, somatic cells; <i>kz</i>, germ-cells.</p>
-</div>
-
-<p>Among the lower Algæ there is a family, the Volvocineæ, in
-which the differentiation of the many-celled body on the principle of
-division of labour has just set in; in some genera it has been actually
-effected, though in the simplest way imaginable, and in others it has
-not yet begun. Thus in the genus <i>Pandorina</i> the individual consists
-of sixteen green cells, united into a ball (Fig. 62, I), each one exactly
-like the other, and all functioning alike. They are all united into a<span class="pagenum"><a id="Page_258"></a>[Pg 258]</span>
-spherical body, a whole, by a gelatinous matrix which they all secrete,
-and thus they form a cell-colony, a cell-stock, a many-celled individual;
-but each of these cells has not only all the typical parts&mdash;cell-body,
-nucleus, and contractile vacuole&mdash;but each possesses a pair of flagella or
-motor organs, an eye-spot, and a chlorophyll body which enables them
-to assimilate nourishment from the water and the air. Each one of
-these cells thus performs all the somatic functions, that is, all that are
-necessary to the maintenance of the individual life. But each also
-possesses the power of reproducing the whole colony from itself, that
-is, it also performs the function of reproduction necessary to the
-maintenance of the species. When such a colony, whose sixteen cells
-are continually growing, has led for some time a free-swimming life
-in the water, the cells retract their flagella, and each begins to
-multiply by dividing into 2, 4, 8, finally into 16 cells of the same
-kind, which remain together, forming a spherical mass enclosed in a
-gelatinous secretion (Fig. 62, II). Thus there are now, instead of
-sixteen cells in the mother-colony, sixteen daughter-colonies, each
-with sixteen cells which soon acquire flagella and eye-spots, and are
-then ready to burst forth from the dissolving jelly of the maternal
-stock as independent individuals. This <i>Pandorina</i> shows no trace of
-a differentiation of its component cells to particular and different
-functions, but a nearly allied genus of the same family, the genus
-<i>Volvox</i> (Fig. 62, III), consists of two kinds of cells&mdash;on the one hand
-of small cells (<i>sz</i>) which occur in large numbers and compose the wall
-of the hollow gelatinous mass, forming, so to speak, the skeleton of
-the <i>Volvox</i>; and, on the other hand, of a much smaller number of
-cells which are very much larger (<i>kz</i>). The former, the 'body' or
-'somatic' cells, are green, and have a red 'eye-spot' and two flagella;
-they are connected with each other by processes from their cell-bodies,
-and are able, by means of the co-ordinated action of their flagella, to
-propel the whole colony with a slow rotatory movement through
-the water. Many of my readers are doubtless familiar with these
-light green spheres, which are quite recognizable with the naked eye,
-and people our marsh pools and ponds in Spring in such abundance
-that it is only necessary to draw a glass of water to procure a large
-number of them.</p>
-
-<p>The little flagellated cells just described serve not only for the
-locomotion of the colony, but also for nutrition, for the secretion of
-the jelly, and for the excretion of waste products; in short, they
-perform all the functions necessary to the maintenance of life, but not
-that of reproduction. They can, indeed, multiply by dividing when
-the colony is young, like the cells of <i>Pandorina</i>, but they cannot<span class="pagenum"><a id="Page_259"></a>[Pg 259]</span>
-reproduce the whole colony but only cells like themselves, that is,
-other somatic cells. In <i>Volvox</i> the maintenance of the species, the
-production of a daughter-colony, is the function of the second and
-larger kind of cells, the reproductive cells, which are contained in the
-interior (filled with a watery fluid) of the gelatinous sphere. They
-possess no flagella (<i>kz</i>), and so take no share in the swimming
-movements of the somatic cells. For the present we need not allude
-to the fact that there are several kinds of these cells, and need only
-state that the simplest among them, the so-called 'Parthenogonidia,'
-after they have reached a considerable size, begin a process of division
-which results in the formation of a daughter-colony. Usually there
-are several of these large reproductive cells in a <i>Volvox</i> colony, and
-as soon as these have developed into a similar number of daughter-colonies
-they burst out through a rupture in the now flaccid jelly of
-the maternal sphere and begin to lead an independent life. The
-mother-sphere, which now consists only of somatic cells, is unable to
-produce new reproductive cells; it gradually loses its spherical form,
-sinks to the ground, and dies.</p>
-
-<p>In <i>Volvox</i> we have, for the first time, a cell-colony in which a
-distinction has been established between body or somatic cells and
-reproductive or germ-cells. In contrast to <i>Pandorina</i>, a large
-number, indeed the majority of the cells of the colony, have lost the
-power of reproducing the whole by division, and only the few
-reproductive cells possess this, while they, in turn, have lost other
-functions, notably that of locomotion. Their power of reproducing
-the whole, that is to say, their hereditary capacity, gives them a
-greater theoretical interest than the cells of <i>Pandorina</i>, for the latter
-require only to produce others like themselves, because there is only
-one kind of cell in the colony, while in <i>Volvox</i> the reproductive cell
-can not only produce others like itself, by division, but can
-produce the body-cells as well. The problem is quite analogous to
-the one which we have had to face in regard to the unicellular
-animals of complex structure, the Infusorians. The question, How
-can the part of the trumpet-animalcule which is mouthless develop
-from itself a new mouth and ciliated apparatus? here transforms itself
-into the question, How can a cell by division give rise not only to
-others like itself, but also to the body-cells, which are of quite
-different structure? This is, in its simplest form, the fundamental
-problem of all reproduction through germ-cells, to which we must
-now pass on. But first a short digression.</p>
-
-<p>We have already noted that unicellular organisms multiply by
-division, and originally, as well as in the great majority of cases<span class="pagenum"><a id="Page_260"></a>[Pg 260]</span>
-to-day, by division into two. It follows, therefore, that there is no
-<i>natural</i> death among them, for, if there were, the species would die
-out as the individuals grew old; but this does not happen. The two
-daughter organisms which arise from the binary fission of an Infusorian
-are in no way different in regard to their power of life; each
-of them possesses an equal power of doubling itself again by division,
-and so it goes on, as far as we can see, for an unlimited time. Thus
-the unicellular organisms are not subject to natural death; their body
-is indeed used up in the course of ordinary life so that the formation
-of new cilia and so on is necessary, but it is not worn away in the
-same sense in which our body is and that of all Metazoa and Metaphytes,
-where, through functioning, the organs are gradually worn away
-until they become incapable of function. Our body grows old, and
-can at last no longer continue to live; but among unicellular organisms
-there is no growing old, and no death in the normal course of the
-development of the individual. The unicellulars are, as we may say,
-immortal; that is, while individuals may be annihilated, by external
-agencies, boiling heat, poisons, being crushed, or eaten, and so on, at
-every period some individuals escape such a fate, and perpetuate
-themselves through succeeding ages. For, strictly speaking, the
-daughter-individual is only a continuation of the mother-individual;
-it contains not only half of the substance, but also the organization,
-and life is continued directly from mother to daughter. The daughter
-is simply half of the mother, which is subsequently regenerated; and
-the other half of the mother lives on in the other daughter, so that
-nothing dies in this multiplication. It may be said that the daughter
-has to develop the other half of its body anew, and that therefore it is a
-new individuality, and not merely a continuation of the old, and that
-therefore the unicellular animals are not immortal. The 'immortality'
-of the Protozoa may be scoffed at; the idea may seem absurd
-that the 'immortal' Protozoa are still the same individuals which
-lived upon the earth millions of years ago, but all such objections
-mean no more than doctrinaire quibbling with the concepts of
-'individual' and 'immortality,' which do not exist in nature at all,
-but are mere human abstractions, and therefore only of relative value.
-My thesis as to the potential immortality of the Unicellulars aims at
-nothing more than impressing on Science the fact that the occurrence
-of physiological, that is, natural, death is causally associated with the
-transition from single-celled to many-celled organisms; and this is
-a truth which will not be overthrown by any sophisms. It is the
-Volvocineæ which show us, so to speak, the exact point at which
-natural death set in, at which it was introduced into the world of life.<span class="pagenum"><a id="Page_261"></a>[Pg 261]</span>
-In <i>Pandorina</i> the state of things is still the same as in single-celled
-organisms, for each cell is still all in all, each can bring forth the
-whole, none dies from physiological causes involved in the course of
-development, and they are therefore 'immortal' in the sense stated.
-But in <i>Volvox</i> the 'individual' dies when it has given off its reproductive
-cells, because here the contrast between germ-cells and body
-has developed. Only the body is mortal in the sense of being subject
-to natural death; the germ-cells possess the potential immortality of
-the single-celled animals, and it is necessary that they should possess
-it if the species is to continue to exist.</p>
-
-<p>From this alone it does not seem quite clear why the body or
-soma should be subject to death, and when I first endeavoured to
-arrive at clearness in regard to these matters I tried to find out why
-a natural death of the body was necessitated by the course of
-evolution. I did not at once discover the true explanation, but
-without delaying to discuss my mistakes I shall proceed to expound
-what I believe to be the true reason. It lies simply in the fact, which
-we shall inquire into later on in more detail, that every function and
-every organ disappears as soon as it becomes superfluous for the
-maintenance of the particular form of life in question. The power of
-being able to live on without limit is useless for the somatic cells, and
-thus also for the body, since these cannot produce new reproductive
-cells after those that had been present are liberated; and with this
-the individual ceases to be of any value for the preservation of the
-species. What advantage would it be to the species if the <i>Volvox</i>
-balls were to continue living for an unlimited time after the reproductive
-cells were developed and had been liberated? Obviously
-their further fate can have no influence whatever in determining or
-preserving the characters of the species, and it is quite indifferent to
-the continuance of the species whether and how long they go on
-living. Therefore the soma has lost the capacity which conditions
-endless continuance of life and continued renewal of body-cells.</p>
-
-<p>In regard to these views it has been asked jeeringly, how
-'immortality,' if it were really a property of the Unicellulars and of
-undifferentiated cell-colonies, could be lost, as if the world, which we
-believe to be everlasting, should give up its everlastingness. But the
-jeer recoils on the superficial outlook which is unable to distinguish
-between the immortality dreamed of by the poets, religious and
-secular, and the real power that certain forms of life have to resist
-being permanently exhausted by their own metabolism. That we
-should call this 'immortality' does not seem to me to require any
-apology, for the right has always been conceded to science to transfer<span class="pagenum"><a id="Page_262"></a>[Pg 262]</span>
-popular words and ideas in a restricted and somewhat altered sense
-to scientific conceptions when it seems necessary. That the word
-'immortality' in this case expresses the state of matters more precisely
-and better than any other cannot be doubted, any more than we can
-doubt that there exists in regard to natural death a real difference,
-which we must take account of, between the Unicellulars and the
-higher organisms. What enables the species in the case of the higher
-organisms, like ourselves for instance, to last through ages is not the
-immortality of the individual, of the person, but only that of the
-germ-cells; these alone, among the cells of the whole body, have
-retained the primæval power. A small piece of the individual is still
-immortal, but only a minute part, which cannot be considered as
-equivalent to the whole, either morphologically or from the point of
-view of the conception of individuality. Can anyone consider himself
-identical with his children? If any one should imagine this, it would
-still not be the case, for he himself would in the course of time suffer
-natural death, and his children would continue to live on until they
-too had brought forth children, and in their turn also came to die.
-It is quite different with an Infusorian, which never lies down to die,
-but simply splits itself afresh into two halves which continue to live.</p>
-
-<p>It is hardly credible that such a simple and clear truth should
-have remained so long undiscovered, and it is even more incredible
-that since it was enunciated it should have been until quite recently
-laughed at as false, as a piece of pseudo-science, and as valueless.
-But it is the fate of all knowledge which rests on an intelligent and
-comprehensive working up of facts to be attacked, until it gradually
-bears down antagonism by the weight of its truth, and compels at
-least a silent recognition.</p>
-
-<p>The fact that natural death made its appearance with the appearance
-of a 'body,' a soma, as distinguished from the germ-cells, will
-sooner or later compel recognition. When I pointed out above that
-the explanation of natural death lay in the fact that it would be
-superfluous for the soma to continue to live on unlimitedly, after it
-had discharged its germ-cells, and so fulfilled its duty to the species, I
-only intended to say that this was the general reason for the introduction
-of natural death. I have no doubt that the actual beginning
-of this phenomenon could have, and probably did come about in other
-ways. Many kinds of cells in higher animals perish as a result
-of their function; it is, so to speak, their business to perish, to
-break up; this is the case with many glandular and epithelial
-cells. It may very well be that, in many of the highly differentiated
-tissue-cells, such as nerve, muscle, and glandular cells, the high<span class="pagenum"><a id="Page_263"></a>[Pg 263]</span>
-differentiation in itself excludes the possibility of unlimited length of
-life and multiplication. Through this alone, therefore, the exhaustion
-of the body and an ultimate death may be explicable from internal
-causes. But the deeper cause remains what I have already indicated,
-for it is obvious that if the continued life, that is, the immortality of
-the soma, were necessary to the preservation of the species it would
-have survived through natural selection; that is to say, had it been
-so, then histological differentiations incompatible with immortality
-would not have made their appearance; they would always have been
-eliminated on their way to development, since only that which is
-adapted to its end survives. Only if the immortality of the soma
-were indifferent for the species could the soma have become so highly
-organized that it became subject to death.</p>
-
-<p>Thus the old song of the transitoriness of life does not apply to
-all the forms of life: natural death is a phenomenon which made its
-appearance comparatively late in the development of the organic
-world, a phenomenon which, up to a certain point, we can quite well
-understand from the standpoint of purposefulness.</p>
-
-<p>It would take me too far from the goal towards which we are at
-present making if I were now to attempt to show, in connexion with
-natural death, that the durability of the soma, or what we usually
-call the normal duration of life, is also exactly regulated by natural
-selection, so that each species possesses exactly that duration of life
-which is most favourable to it, according to its physical constitution,
-its physiological capacity, and the conditions of life to which it has
-to adapt itself<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a>. But, interesting as this subject is, I must not digress
-further, but return to our proper subject of study, namely, reproduction
-in its relation to inheritance.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_11" href="#FNanchor_11" class="label">[11]</a> See Weismann, <i>Ueber die Dauer des Lebens</i>, Jena, 1882. Translated in <i>Essays on
-Heredity</i>.</p>
-
-</div>
-
-<p>We digressed from this study after having seen that all, even
-the most complex, multicellular plants and animals, in which the
-differentiation of the cells into a number of cell-groups with the
-most diverse functions has attained the highest degree of complexity,
-are able to produce special cells, the germ-cells, which have the
-power of reproducing from themselves another organism of the
-same species, and with the same complex structure. It might be
-thought that such cells must necessarily be very complex in their
-own structure, but in most cases nothing of the kind is to be seen,
-and the germ-cells often appear simpler in organization than many
-of the tissue-cells, such as the glandular-cells; and where there is an
-unusual size or complexity of structure in the germ-cell it usually<span class="pagenum"><a id="Page_264"></a>[Pg 264]</span>
-bears no relation to the grade of organization of the young creature
-that is to arise from it, but is due solely to the special conditions
-imposed on the particular germ-cell, if a young organism is to be
-evolved from it. We shall soon see what is meant by this.</p>
-
-<p>I must note here that plants and animals do not multiply by means
-of germ-cells alone, but that many species&mdash;the majority of plants
-and the simpler forms of animals&mdash;also exhibit multiplication by
-budding or division. All animals and plants which do not stop short
-at the stage of the individual, the 'person,' but rise to the higher
-stage of the 'stock' (or corm), illustrate this. The first person from
-which the formation of the stock proceeds gives rise by budding or
-division to new persons which remain attached to it, and in turn by
-repeated production of buds give rise to a third, fourth, or <i>n</i><sup>th</sup>
-generation of persons, all remaining in connexion with the first, and
-together forming the composite individuality of the animal-colony or
-plant-stock. Such colonies or stocks are seen in polyps and corals,
-Siphonophoræ and Bryozoa, and among plants, according to Alexander
-Braun, in all phanerogams which do not consist only of a single
-shoot. In these cases we find that definite, or perhaps indefinite
-groups of cells in the stock may give rise to a new person, and we
-have to inquire how this power may be theoretically interpreted.</p>
-
-<p>New stocks may also have their origin from such buds, or from
-single persons of the stock. The fresh-water polyp (<i>Hydra</i>) gives
-rise by budding to a small stock of at most three or four persons;
-but the young animals budded off only remain attached to the
-parent hydra until they have attained their full development; then
-they detach themselves and settle down independently, and begin
-to bud off in turn a similar and transitory stock. Among plants
-there are many which, like <i>Dentaria bulbifera</i> and <i>Marchantia
-polymorpha</i>, multiply by so-called 'brood-buds,' that is, buds which
-fall from the stock and grow into new plants. The whole horticultural
-propagation of plants by cuttings also depends on the process
-of budding, for what is cut off from the parent plant and stuck into
-the earth is a single shoot, that is, a 'person' which possesses the
-power of sending down roots into the earth, and by continual budding
-giving rise to new shoots or persons which together make up a new
-plant-stock.</p>
-
-<p>I must not, however, spend much time over this so-called
-'asexual' reproduction by budding and division, because it does not
-suggest any way by which we may penetrate more deeply into the
-processes of inheritance, and we may be content if we can bring them
-into harmony with other theoretical views which we deduce from<span class="pagenum"><a id="Page_265"></a>[Pg 265]</span>
-other phenomena. These forms of reproduction were long regarded
-as the oldest and the simplest, and it is only since the time of Francis
-Balfour that the conviction has gradually gained ground that this
-cannot be so, but that they are rather secondary methods of multiplication
-in the Metazoa and Metaphyta, which therefore rest on a very
-complex basis. We have seen that the germ-cells made their appearance
-along with the multicellular body, and the step from <i>Pandorina</i>
-to <i>Volvox</i> is as small a step as can be well imagined. It is thus proved
-that the oldest mode of multiplication among multicellular organisms
-was that through germ-cells, at least along this line of evolution.
-<i>Volvox</i> does not reproduce by dividing, or by the development of
-buds from any part of the spherical colony of cells. What is known
-as budding among single-celled organisms is only an unequal cell-division,
-and has nothing but its external appearance in common with
-the budding of higher plants and animals. The latter, therefore, is
-something new, of later and independent origin; <i>the primitive mode
-is reproduction by unicellular germs</i>.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_266"></a>[Pg 266]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XIV">LECTURE XIV</h2>
-</div>
-
-<p class="c">REPRODUCTION BY GERM-CELLS.</p>
-
-<div class="blockquot">
-
-<p>Historical&mdash;Differentiation of germ-cells into male and female&mdash;Pandorina&mdash;Volvox&mdash;Sperm-cells
-and ova in Algæ&mdash;Zoosperm form of the male germ-cells&mdash;Zoosperms
-of the Barnacles&mdash;Adaptation of the sperm-cells to the conditions of fertilization&mdash;Daphnids&mdash;Spermatozoa
-in different animal groups&mdash;Their minute structure&mdash;Form
-and structure of the egg-cell&mdash;Adaptation of the ovum to given conditions&mdash;Dimorphic
-ova in the same species&mdash;Nutritive cells associated with egg-cells&mdash;Complex
-structure of the bird's egg.</p></div>
-
-
-<p><span class="smcap">If</span> we now turn to the reproduction of the Metazoa and Metaphyta
-by means of germ-cells we find that a great number of lowly
-plants produce germ-cells which require nothing more for the development
-of a new plant beyond certain favourable external conditions,
-above all, moisture and warmth. Such, for instance, are the 'spores'
-of the ferns, which are formed on the under surface of the fronds
-in little clusters of a brown or yellow colour, easily visible to the
-naked eye. These spores are individually very small, so that thousands
-go to form one spore-cluster or sporangium, and millions of spores are
-given off annually by a single fern. Each spore is a germ-cell
-enclosed in a protective capsule, and may, if carried by the wind
-to a spot favourable to germination, become a young plant, the
-so-called prothallium, from which the fern-plant proper subsequently
-develops.</p>
-
-<p>This reproduction by spores has been regarded as a form of
-'asexual reproduction' so-called, and has been classed along with
-budding and fission under this head. But it has nothing in common
-with these forms of multiplication except the negative character that
-the act of fertilization, which we shall inquire into later on, does not
-in this case occur. This mode of classification has no longer any
-more justification than the division of the animal kingdom into backboned
-and backboneless animals, in which the negative character of
-the absence of vertebræ has led to the slumping of quite heterogeneous
-forms in one group. I do not mean to dispute that both these
-classifications were fully justified in their own time; indeed they
-expressed a step of progress. Nowadays, however, the division
-'Invertebrata' or 'backboneless animals' as a scientific conception
-has been abandoned, and the same should be done with the category<span class="pagenum"><a id="Page_267"></a>[Pg 267]</span>
-'asexual reproduction,' since it groups together quite different things,
-such as multiplication by single-celled and many-celled 'germs,' and is
-moreover based on a quite erroneous idea of what 'fertilization' really
-is. Both terms may very well be retained as a mere matter of convenience,
-but it is much to be desired that the two apt designations
-proposed by Haeckel&mdash;Monogony for asexual, and Amphigony for
-sexual reproduction&mdash;should come into general use.</p>
-
-<p>Meanwhile it is enough to say that reproduction by 'spores'
-occurs normally in Algæ, fungi, mosses, and fern-like plants, and that
-there are also animals in which the germ-cells possess the power of
-giving rise of themselves to a new individual. But the cases which I
-am chiefly thinking of are those of so-called virgin birth or parthenogenesis,
-which are not to be compared with multiplication by spores
-in regard to their mode of origin; there is a peculiarity in the origin
-of this mode of multiplication which I can only make clear after we
-have studied the normal forms of what is called 'sexual reproduction.'</p>
-
-<p>We shall therefore pass on to this mode of reproduction. It is
-well known that, in all higher animals, just as in Man, an individual
-cannot reproduce by itself; the co-operation of two individuals is
-necessary, and these&mdash;the male and the female&mdash;differ essentially
-from each other in many particulars. Their union in the act of procreation
-induces the development of a new individual, whether this
-matures within the mother in a special receptacle, or whether it is
-deposited as a 'fertilized egg,' as in birds, the lower vertebrates, and
-most 'invertebrates.'</p>
-
-<p>As long as Man has lived he has regarded this process of
-procreation as the essential factor in the origin of new individuals,
-and as he had no insight into the essence of the process he had necessarily
-to regard reproduction as something entirely mysterious, and the
-co-operation of the two sexes as a <i>conditio sine qua non</i> of reproduction
-in general; thus copulation and reproduction seemed identical.</p>
-
-<p>This was in the main the state of opinion at the time of the
-discovery of innumerable minute filaments, the so-called 'spermatozoa'
-in the 'fertilizing' spermatic fluid of the male. The discovery was
-made in 1677 by Leeuwenhoek in the case of birds, mammals, and
-many other animals. Albrecht von Haller (1708-77) was at first
-inclined to regard these spermatozoa as the rudiments of the embryo,
-but later on, in the course of his long life, he withdrew this theory,
-and declared them to be a kind of parasite in the spermatic fluid
-without anything to do with fertilization. The same opinion was
-expressed in 1835 by K. E. von Baer, in opposition to the opinion
-of Prevost and Dumas, who had rightly interpreted the spermatozoa<span class="pagenum"><a id="Page_268"></a>[Pg 268]</span>
-as the essential elements of the spermatic fluid. When one follows the
-matter out in detail, one finds it almost incredible that such a number
-of mistakes should have been made, and so many circuitous paths
-traversed, before even the limited knowledge current in the middle
-of the nineteenth century was attained&mdash;that is to say, enough to give
-ground for the assertion that fertilization depends upon the contact of
-the spermatozoon with the body of the egg. In 1843 Martin Barry
-had found the spermatozoa within the egg-envelope of the rabbit ovum,
-but it was some time later (1852) that the investigations of Meissner,
-Bischoff, and Newport established the fact that the zoosperm penetrates
-through the egg-envelope. All else remained quite obscure, and could
-not be cleared up as long as it was believed, on the strength of
-observations which were in themselves correct enough, that <i>several</i>
-zoosperms were always necessary to fertilize one ovum.</p>
-
-<p>To an understanding of the process even in its most general outlines
-there was lacking, apart from technical methods, an appreciation of
-the morphological value of the ovum and the spermatozoon. It was
-necessary to recognize both ovum and spermatozoon as <i>cells</i> before
-their union in fertilization could be regarded as the fusion of two
-cells, as a copulation or conjugation of two minute elementary
-organisms. But this knowledge only gained ground very gradually,
-and even in the sixties opinions on the subject were very much
-divided. Moreover, there was an entire absence of knowledge in
-regard to 'sexual' reproduction among the lower plants, the Algæ,
-Fungi, Mosses, and Ferns, as well as of any detailed acquaintance with
-the processes of fertilization among flowering plants. All this had to
-be elucidated by the labours of many distinguished observers before
-it was possible to say so much even as this, that the process of
-fertilization depends in general on the union of two cells.</p>
-
-<p>I need not discuss the whole of this long process of scientific
-development, and have only touched upon it because I wished to
-emphasize that the conception of the process of fertilization was for
-a long time quite erroneous, and has only attained to clearness in
-recent times. Pairing as it is seen in the higher animals was for long
-regarded as the essential part of the process, and a mysterious life-awakening
-influence was assumed in regard to it; and even when it was
-understood that not the copulation, but the union of two living units
-which was always brought about thereby&mdash;the union of the male and
-the female germ-cells&mdash;was the essence of 'fertilization,' this was still
-regarded as a life-awakening process, and the way to a true understanding
-of the facts was thus once more blocked.</p>
-
-<p>The simplest form of sexual reproduction in many-celled animals<span class="pagenum"><a id="Page_269"></a>[Pg 269]</span>
-is found, among others, in the Volvocineæ, those green, spherical, freshwater
-cell-colonies which we have already studied in relation to reproduction
-by asexual germ-cells. Among them it is the rule that,
-after a long series of generations producing only 'asexual' germ-cells,
-colonies occur in which each germ-cell is no longer able to develop
-a new colony alone, but can do so only after it has united with
-another germ-cell.</p>
-
-<p>Now, as we have seen, there are Volvocineæ in which the
-differentiation of cells into those of the body (soma) and those
-concerned with reproduction has not been established, and all the
-cells are therefore alike. In these, as for instance in the genus
-<i>Pandorina</i> (<a href="#f66">Fig. 62, p. 257</a>), when sexual reproduction is to occur
-the whole colony breaks up into sixteen cells; these burst forth from
-the gelatinous matrix in which they have been hitherto enclosed,
-swim about in the water with the help of their two flagella, meet
-other similar free-swimming cells and conjugate with these. The two
-swimming cells come close to each other, draw in their flagella, sink
-to the ground in consequence, and fuse completely both as to the cell-body
-and the nucleus. They assume a spherical form, lose the eye-spot,
-become surrounded with a tough cell-skin or cyst, and so remain
-for a longer or shorter time as so-called 'zygotes' or lasting spores.
-Then they develop by repeated cell-division into one of the sixteen-celled
-<i>Pandorina</i> colonies with which we are already familiar; this
-bursts forth from the capsule and swims freely about in the water
-again.</p>
-
-<p>Here, therefore, the so-called sexual reproduction depends on the
-fusion of two cells similar in appearance, and when this phenomenon
-was first known it was regarded as something quite different from the
-corresponding reproduction in other multicellular organisms. But we
-now know that quite nearly related Volvocineæ belonging to the
-genus <i>Volvox</i> and to other genera, which exhibit a differentiation
-into body-cells and reproductive cells, may reproduce sexually by
-means of two <i>different</i> kinds of germ-cells; and we have also learned
-through Goebel and others that even genera like <i>Pandorina</i>, which
-consist of only one kind of cells, may yet produce male and female
-reproductive cells differing essentially in form from one another. In
-<i>Eudorina</i>, for instance, a gelatinous sphere containing sixteen or
-thirty-two individual cells, asexual reproduction occurs in exactly the
-same way as in <i>Pandorina</i>, that is, each of these cells divides four
-or five times in rapid succession, and thus forms a new colony, which
-then bursts forth; but when the time for sexual reproduction comes
-the colonies behave differently, for some become female and some
-<span class="pagenum"><a id="Page_270"></a>[Pg 270]</span>male. In the former the cells remain as they were before, but in the
-male colonies the sixteen or thirty-two cells undergo a peculiar
-process of division, which ends in each becoming a mass (16-32) of
-so-called 'zoosperms,' that is, minute, narrow, longitudinally elongated
-cells with two flagella (Fig. 63 at <i>D</i> shows those of <i>Volvox</i>). In
-<i>Eudorina</i> they differ from the female germ-cells or ova externally in
-form and size, as well as by being much more actively motile, and
-they contain green and subsequently yellow colouring matter, and
-a red eye-spot. We here find, for the first time among multicellular
-organisms, the differentiation of male and female germ-cells; and we
-learn from this that the essence of fertilization does not lie in this<span class="pagenum"><a id="Page_271"></a>[Pg 271]</span>
-differentiation, since it may be absent, but that this distinction of
-female and male cells is only of secondary moment. From the fact
-that the egg-cells are larger and less active, the 'sperm-cells' or
-zoosperms smaller and livelier, we can already anticipate what will be
-more definitely established as our knowledge of the facts increases&mdash;that
-a differentiation according to the principle of division of labour
-has taken place even in the germ-cells, and that the first effect of this
-is to render the meeting of the cells destined for conjugation easier
-and more certain. The much smaller and more slender zoosperms
-swim about in the water in clusters until they come in contact with
-a female colony; then they separate from each other, bore their way
-into the soft jelly of the female colony, and 'fertilize' the egg-cell,
-that is to say, each male cell fuses with a female cell and forms
-a 'lasting spore,' exactly as in <i>Pandorina</i>.</p>
-
-<div class="figcenter" id="f67">
-<img src="images/fig67.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 63.</span> <i>Volvox aureus</i>, after Klein and Schenck.
-<i>A</i>, besides the small flagellate somatic cells of the colony
-there are five large egg-cells (<i>t</i>) which are capable of
-parthenogenetic development, three recently fertilized egg-cells
-(<i>o</i>) and a number of male germ-cells (<i>a</i>) in process of
-multiplication. From each of these, by continued division,
-a bundle of spermatozoa arises. <i>B</i>, a bundle of thirty-two
-sperm-cells in process of development, seen from above.
-<i>C</i>, the same seen from the side. Magnified 687 times.
-<i>D</i> individual spermatozoa, magnified 824 times.</p>
-</div>
-
-<p>In <i>Volvox</i> the state of matters is similar to that in <i>Eudorina</i>;
-here again, in addition to the 'asexual' reproduction through the
-'Parthenogonidia' which are like egg-cells in appearance (Fig. 63, <i>A</i>, <i>t</i>),
-there are also male and female germ-cells which are usually produced
-alternately with the former, but sometimes at the same time, as in
-Fig. 63. The egg-cells are large and have no flagella, the sperm-cells
-lie together in clusters, and after they are mature (<i>D</i>) they swim freely
-in the water and then bore into another colony, where each unites
-with an egg-cell. The difference between the two kinds of cells
-consists therefore in the much greater number, the much smaller size,
-and the greater activity of the male cells, and in the smaller number
-but much larger size of the female cells&mdash;a differentiation in accordance
-with the principle of division of labour, depending on the fact
-that the two kinds of cells must reach each other, and yet must
-contain a certain mass of living protoplasm. While the small size but
-large number of male cells, combined with their motility, gives them
-an advantage in seeking out and boring into the female cells, the large
-size of the latter, on the other hand, makes up for the loss in mass
-which the fertilized egg would otherwise suffer from the diminution
-in size of the male cell. This difference in size may be greatly
-accentuated; thus in one of the brown sea-wracks, for instance, the
-spermatozoa are only 5 micro-millimetres in length, while the ova are
-spherical and have a diameter of 80-100 micro-millimetres, thus
-containing a mass 30-60,000 times greater (Möbius). Fig. 64 shows
-an ovum of this species surrounded by spermatozoa</p>
-
-<p>In the course of the evolution of species this contrast between
-female and male germ-cells became more and more marked, not
-always in the same direction, however, but in one or another according<span class="pagenum"><a id="Page_272"></a>[Pg 272]</span>
-to the conditions of fertilization. It would be erroneous to suppose
-that, with the higher differentiation of the organism as a whole, the
-differentiation of the germ-cells became increasingly complex. On
-the contrary we find even among Algæ, as the case of <i>Fucus</i> shows,
-a marked difference between the sex-cells, which rather decreases than
-increases among many of the higher plants. It is not on the more or
-less complex structure of the organism itself that the nature and
-degree of the dimorphism of the germ-cells depends, but on the
-special conditions which are involved in each case, both in the union
-of the two kinds of sex-cells and in the subsequent development of
-the product of this union, the 'fertilized ovum.'</p>
-
-<div class="figleft" id="f68">
-<img src="images/fig68.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 64.</span> <i>Fucus platycarpus</i>, brown sea-wrack.<br />
-<i>Ei</i>, ovum, surrounded by swarming<br />
-sperm-cells (<i>sp</i>). After Schenck.</p>
-</div>
-
-<p>Thus it comes about that the male or 'sperm-cells' of the lower
-plants, of the lower animals, and, again, of the highest animals are
-similar in structure. In all these
-organisms the male germ-cells
-exhibit the minuteness, the form,
-and the activity of the so-called
-'zoosperms' or 'spermatozoa,'
-that is to say, they are thread-like,
-very minute corpuscles,
-which move rapidly forwards in
-water or other fluid with undulatory
-movements, and penetrate
-into the ovum with similar boring
-movements when they have been
-fortunate enough to reach their
-goal. At the anterior end they
-possess a more or less conspicuous
-thickening, the so-called 'head' in which the nucleus lies, and this
-is followed by the 'tail,' a thread-like structure consisting of cytoplasm
-which effects undulatory movements comparable to those of the
-flagella of Infusorians and Volvocineæ. The whole spermatozoon is
-thus a specialized 'flagellate cell.'</p>
-
-<p>When these 'zoosperms' were recognized as the 'fertilizing
-elements' in higher animals, and when 'sperm-threads' had been
-found, not only in all mammals and birds, reptiles, amphibians, and
-fishes, but even in many 'invertebrates,' the conclusion was suggested
-that the function of fertilization might be discharged by this lively
-motile substance; for until the eighth decade of the nineteenth century
-fertilization was still regarded by many as an 'awakening of life' in
-the egg. Since life depends on movement, in truth on infinitely fine
-molecular movements, of which the movement of the whole germ-cell<span class="pagenum"><a id="Page_273"></a>[Pg 273]</span>
-from place to place is only a visible outcome, fertilization was pictured,
-by a not very luminous process of reasoning, as the awakening of life
-in the ovum&mdash;in itself incapable of further life&mdash;through the transference
-to it of movement through the agency of the zoosperm. Some
-investigators even went the length of regarding the ovum as 'dead
-organic material.'</p>
-
-<p>I mention this at this point, though I do not propose in the
-meantime to inquire further into the significance of the conjugation
-of the sex-cells. But the view just referred to is so completely refuted
-even by the external form of the male germ-cells in many groups of
-plants and animals, that I cannot discuss these differences in form
-without at the same time indicating the conclusions which they
-directly suggest.</p>
-
-<p>The great majority of plants and animals exhibit the zoosperm
-form of male germ-cells, and this must obviously be interpreted in
-the light of the fact that the ova to be fertilized are not generally to
-be found in direct proximity to the sperms shed by the male organism,
-but are at some distance from them. Among Medusæ and Polyps both
-male and female germ-cells are liberated into the water, simultaneously
-it may be, but separated from each other by distances of some feet or
-yards. The spermatozoa then swim about seeking the ova, which
-are also floating freely in the sea, guided by a power of attraction on
-the part of the latter&mdash;an attraction of whose nature we know nothing,
-though in the case of certain fern-ova it has been traced to the
-secretion of malic acid (Pfeffer).</p>
-
-<p>The same conditions obtain among Sponges. Here, again, the
-persons or stocks are either male or female; the latter produce large
-delicate ova, which lie in the interior of the sponge and there await
-the fertilizing sperms; the former give off the ripe sperms into the
-water in such abundance that thousands and millions of zoosperms
-burst forth simultaneously in all directions; these seek about for
-a female sponge, penetrate into its canal system, and so ultimately
-reach the ova. Of course only a very few of them reach their goal;
-the greater number are lost in the water and become the prey of
-Infusorians, Radiolarians, or other lowly animals. The fact that
-enormous numbers thus miss their true aim shows us why these
-zoosperms must be produced in such quantities; it is simply an
-adaptation to the extraordinarily high ratio of elimination in these
-cells, just as the number of young annually produced by an animal,
-or of seeds by a plant, is regulated by natural selection according to
-the ratio of elimination of the particular species. The more numerous
-the descendants which succumb each time to unfavourable circum<span class="pagenum"><a id="Page_274"></a>[Pg 274]</span>stances,
-to enemies, or to lack of food, the more prolific must the
-species be. The same holds true of the regulation of the number of
-male germ-cells to be produced by an individual; there must be so
-many developed that, in spite of the unavoidable enormous loss, on an
-average the number of mature ova necessary to the maintenance of the
-species always receive spermatozoa.</p>
-
-<p>Also associated with the prodigal production of zoosperms is their
-minuteness, for the more zoosperms that can be developed out of
-a given mass of organic substance the smaller they are. Each species
-is restricted within definite limits of production by its size and the
-volume of its body, and there is thus an advantage in having the
-zoosperms of the smallest possible size whenever the chance of the
-individual sperm successfully reaching an ovum is very small. In
-all such cases nature has abstained from burdening the male germ-cell
-with an appreciable contribution of material to the result of
-conjugation, that is, to the foundation of the new organism; the
-passive ovum contains in itself alone almost all that is necessary to
-the building up of the embryo. Fertilization of the ovum by the
-liberation of the sperm-cells into the water occurs not only in animals
-of low degree, such as Sponges, Medusæ, Star-fishes, Sea-urchins and
-their relatives, but also in much higher animals, such as many Fishes
-and Amphibians, and in these the male cells have the form of motile
-threads. But the spermatozoon-form of male cell does not occur
-only in animals and plants which live in the water, or in those which,
-like mosses and many vascular plants, are at least occasionally covered
-by a thin layer of rain or dew, in which the zoosperms can swim to
-the ova, it occurs also in a very large number of animals in which
-the sperms pass directly into the body of the female, in those,
-therefore, in which copulation takes place.</p>
-
-<p>But even where copulation occurs we find that in most cases, as,
-for instance, in Vertebrates, Molluscs, and Insects, the zoosperm-form
-is retained. The reason for this is obviously twofold: in the first
-place, in many cases the sperms do not directly reach the ovum as
-a consequence of copulation, but may have to go a long way within
-the body of the female, as in mammals; or even when the way is short
-and certain, the ovum may be encased in a firm covering or shell
-difficult to penetrate, and the thread-like zoosperm has to face the
-task of boring its way through this shell, or slipping in through
-a minute opening, the so-called micropyle. In either case it would
-be difficult to imagine a form of sperm-cell better adapted to the
-fulfilment of this task than that of a thread with a thin, pointed head-portion
-and a long motile tail, which enables the zoosperm to twist<span class="pagenum"><a id="Page_275"></a>[Pg 275]</span>
-itself like a screw through a narrow opening in the egg-envelope,
-whether the opening was previously present or not.</p>
-
-<p>We can thus understand why, among insects for instance, the
-male cells should always occur in the form of zoosperms, although
-in this case they reach a special receptacle in the female reproductive
-organs, the 'receptaculum seminis,' and are stored up in this. When
-a mature ovum gliding downwards through the oviduct comes to
-the place where this receptacle opens into it, the liberation of a few
-sperm-cells suffices to fertilize it with certainty, provided that they
-possess the thread-like form, which allows them to slip in through
-the very minute opening in the egg-envelope. It might be inferred
-from the certainty with which the ovum must in this case be found
-by the spermatozoon that only a small number of the latter would
-require to be produced, and yet even here the number is very large,
-though not so enormous as in the sea-urchins and other marine
-animals, which simply allow the sperm-cells to escape into the water.
-The large number in insects is due to the fact that many of the
-sperms may miss the micropyle; and also that in many insects a
-very large number of eggs have to be fertilized in succession. In the
-course of a life lasting three or four years the queen bee lays many
-thousand of eggs, most of which are fertilized, and that from a
-seminal receptacle which has been filled only once.</p>
-
-<p>There are, however, other sperm-cells of thread-like form which
-are not produced in such enormous multitudes, but in a much more
-moderate number, perhaps a few hundreds in the testicle. This is so
-in the little Crustaceans, known as Ostracods, all the freshwater species
-of which possess zoosperms only moderately numerous and of quite
-unusual size.</p>
-
-<p>The comparatively small number is explained by the certainty
-with which each of them reaches the ovum, and the large size may be
-accounted for in part by the small number which suffices, and which,
-therefore, admits of the male cell also carrying a considerable portion
-of the material for the building up of the embryo. Probably, however,
-the thickness and firmness of the covering of the ovum has something
-to do with it, for it has no opening for the entrance of the
-male cell, and it is fully hardened by the time fertilization takes
-place. Perhaps nowhere can we see more clearly how every little
-detail of the structure of the organism is dominated by the principle
-of adaptation than in the arrangements for fertilization, and notably
-in those which obtain in the Ostracods. I pass by the complicated
-apparatus for copulation, since we do not yet understand it fully in
-all particulars. According to my own investigations and those of my<span class="pagenum"><a id="Page_276"></a>[Pg 276]</span>
-former students, Dr. Stuhlmann and Dr. Schwarz, the essential point
-seems to be that the colossally large zoosperms, which show no activity
-within the body of the male, leave it one at a time, so to speak, in
-single file. In copulation they are pressed out singly, one after the
-other, through a very fine tube, and then they enter, still singly,
-through the reproductive aperture of the female into an equally fine
-passage with spiral windings, through which they ultimately reach a
-roomy pear-shaped receptacle, the 'receptaculum seminis' of the female.
-There they lie in a long band composed of several hundreds, and
-only now attain their full maturity by throwing off an outer cuticle&mdash;moulting,
-so to speak. It is only when they get into a fluid medium
-that they show the power of undulatory movement, feeble at first,
-but gradually more energetic
-and more violent.
-And these movements
-enable them to penetrate
-like gimlets into the
-calcareous egg-shell. In
-the normal course it happens
-that when a mature
-ovum is deposited from the
-opening of the oviduct,
-one of the giant zoosperms
-at the same time,
-or shortly afterwards,
-leaves the 'receptaculum
-seminis' of the female by
-way of the spiral passage,
-and reaches the exterior
-just behind the ovum.
-The actual process of penetration has not been observed as yet, but
-the zoosperm has been seen at a slightly later stage spirally coiled
-inside the ovum.</p>
-
-<div class="figleft" id="f69">
-<img src="images/fig69.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 65.</span> Copulation in a Daphnid (Lyncæid).<br />
-Emptying of the sperm (<i>sp</i>) into the brood-chamber<br />
-of the female (♀). <i>abd</i> ♂, the abdomen of the male.<br />
-Magnified 100 times.</p>
-</div>
-
-<p>In these Ostracods the sperms are often visible with the naked
-eye, and in some species they are twice the length of the animal;
-they are thus emphatically giant cells, which can develop a very
-considerable boring power.</p>
-
-<p>In respect to the various adaptations of the sperm-cells to the
-conditions of fertilization there is hardly any group more interesting
-than the water-fleas or Daphnids.</p>
-
-<p>It is amazing how greatly the size of the sperms varies among
-the Daphnids, and how it stands in inverse proportion to their
-<span class="pagenum"><a id="Page_277"></a>[Pg 277]</span>number, and how both are obviously regulated in relation to the
-difficulties which stand in the way of each sperm-cell before it can
-reach the ovum. In some species the sperm-cells are very large, in
-others extremely small. In the genera <i>Daphnia</i>, <i>Lynceus</i>, and others,
-copulation occurs as shown in Fig. 65; the sperm-cells (<i>sp</i>) are
-liberated by the male into the capacious brood-cavity of the female,
-which at the moment is closed to some extent by the abdomen of the
-male, in reality closed only partially at the posterior end and at
-the sides. It seems inevitable that a large proportion of the male
-elements should stream out again and be lost because of the violent
-movements of both animals. Accordingly, we find that the sperm-cells
-are only about the
-hundredth part of a
-millimetre in length
-and of round or rod-like
-form, and are
-voided in multitudes
-into the brood-cavity.
-Fig. 66, <i>f</i>, <i>g</i>, and <i>h</i>,
-show such cells in different
-species, as they
-occur in the testes to
-the number of many
-thousands. But in all
-the species in which the
-brood-cavity is <i>closed</i>,
-and in which therefore
-there is not such a
-serious loss of sperm-cells,
-the elements are
-much larger, and at the same time less numerous. They are largest
-and least numerous in species of genera like <i>Daphnella</i>, <i>Polyphemus</i>,
-and <i>Bythotrephes</i>, in which the males have a copulatory organ, so that
-the possibility of loss of the male cells is excluded. Thus the round,
-delicate, and viscid sperm-cells of <i>Bythotrephes</i> (Fig. 66, <i>b</i>) are more
-than a tenth of a millimetre in length, but they are developed in proportionately
-smaller numbers, so that more than twenty are never
-found in the testis, and often only six or eight, while in copulation
-only from three to five are ejected. But as there are only two eggs
-to be fertilized at a time, and as the male cells are expressed into the
-brood-cavity directly upon the eggs, so that they immediately adhere
-to them, this small number is amply sufficient.</p>
-
-<p><span class="pagenum"><a id="Page_278"></a>[Pg 278]</span></p>
-
-<div class="figright" id="f70">
-<img src="images/fig70.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 66.</span> Spermatozoa of various Daphnids. <i>a, Sida.</i><br />
-<i>b, Bythotrephes.</i> <i>c, Daphnella.</i> <i>d, Moina paradoxa.</i> <i>e,<br />
-Moina rectirostris.</i> <i>f, Eurycercus lamellatus.</i> <i>g, Alonella<br />
-pygmæa.</i> <i>h, Peracantha truncata.</i> All magnified 300<br />
-times.</p>
-</div>
-
-<p>It is remarkable how different the sperm-cells sometimes are in
-quite nearly related species of Daphnids, as a glance at Fig. 66 will
-show; and, on the other hand, how similar they may be in two
-species which belong to different families, like <i>Bythotrephes longimanus</i>
-(<i>b</i>), and <i>Daphnella hyalina</i> (<i>c</i>). The last fact may be explained as an
-adaptation to similar conditions of fertilization. Both species have
-effective copulatory organs, and their large delicate sperm-cells must
-immediately adhere when they come into contact with the shell-less
-ovum, and penetrate into it by means of amœboid processes. Conversely,
-the difference between sperm-cells of allied species like <i>Sida
-crystallina</i> (<i>a</i>), <i>Moina rectirostris</i> (<i>e</i>) and <i>M. paradoxa</i> (<i>d</i>) is related to
-different adaptations to nearly the same conditions of fertilization.
-In <i>Sida</i> (Fig. 66 <i>a</i>) the large flat sperm-cells, with their fringed ends
-and their large soft surface, adhere easily to the ova, and the same
-end is attained in <i>Moina rectirostris</i> by means of stiff radiating
-processes, while in the nearly related species, <i>Moina paradoxa</i>, the
-male cell (<i>d</i>) resembles an Australian boomerang and presses in
-like a wedge between the ova and the wall of the brood-sac.</p>
-
-<div class="figcenter" id="f71">
-<img src="images/fig71.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 67.</span> Spermatozoa of various animals, after Ballowitz, Kölliker, and vom Rath.
-1, man. 2, bat (<i>Vesperugo</i>). 3, pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (<i>Raja</i>).
-8, beetle. 9, mole-cricket (<i>Gryllotalpa</i>). 10, freshwater snail (<i>Paludina</i>). 11, sea-urchin.
-Much magnified.</p>
-</div>
-
-<p>In Fig. 67 a small selection of animal male cells is figured, all of<span class="pagenum"><a id="Page_279"></a>[Pg 279]</span>
-which take the form of sperm-threads or spermatozoa, and yet they
-differ very much from one another in detail. It would undoubtedly
-be of great interest to follow out these minute adaptations of the
-sperm-cells to the conditions of fertilization, and to demonstrate that
-their size, and especially their form, in the different species of animals
-are adjusted to the special constitution of the ovum, its envelope,
-and its micropyles, and to the ease or difficulty with which it can be
-reached; but much information must be forthcoming
-before we can even suggest, for instance, why the sperm-cell
-of the salamander is so enormously long, large, and
-pointed at the head, while that of Man (Fig. 67, 1) is
-comparatively short, with broad, flat head and a recently
-discovered minute apex; or why those of Man and many
-fishes (such as <i>Cobitis</i>) should be so much alike, and so
-on. From many sides, however, we are led to conclude
-that even down to the minutest details nothing is in
-vain, and that everything depends on adaptation.</p>
-
-<p>In general, even the peculiarities of form already
-indicate this; thus the spirally coiled structure of the
-head, which is especially well developed in the spermatozoa
-of birds (Fig. 67, 5), in those of the skate (7),
-and of the freshwater snail (<i>Paludina</i>) (10), works like
-a corkscrew, and makes it possible for the spermatozoon
-to pierce through the resistant envelope of the
-ovum. Similarly, the sharply pointed head of the insect
-spermatozoon (Fig. 67, 8 &amp; 9) seems adapted for slipping
-through the minute pre-formed micropyle in the hard
-egg-shell.</p>
-
-<p>Of the detailed and complicated structure of
-spermatozoa we have only recently been made aware
-through the increasing perfection of the microscope and
-of technical methods of investigation. Fig. 68 shows
-one after a diagrammatic figure by Wilson. We see the
-apical point (<i>sp</i>) for boring into the ovum, the nucleus
-(<i>n</i>) surrounded by a thin layer of protoplasm, which
-together form the head, then the middle portion (<i>m</i>) which contains
-the 'centrosome' (<i>c</i>), and the 'tail' or 'flagellum' which effects the
-movement of the whole and which itself possesses a complex structure
-with an 'axial filament' (<i>ax</i>) and an enveloping layer, the latter often
-drawn out into a spirally twisted, undulating membrane of the most
-extreme delicacy, as is most clearly seen in the newt (Fig. 67, 6).</p>
-
-<div class="figright" id="f72">
-<img src="images/fig72.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 68.</span> Diagram of a<br /> spermatozoon, after Wilson.<br />
-<i>sp</i>, apical point.<br /> <i>n</i>, nucleus.<br /> <i>c</i>, centrospere.<br />
-<i>m</i>, middle piece.<br /> <i>ax</i>, axial filament.<br /> <i>e</i>, terminal filament.</p>
-</div>
-
-<p>Not in the Daphnids alone, but in other groups of Crustaceans as<span class="pagenum"><a id="Page_280"></a>[Pg 280]</span>
-well, sperm-cells of quite peculiar form occur, as, for instance, in the
-crayfish and its marine relatives, the crabs and the long-tailed
-Decapods. In these cases the spermatozoa bear long and stiff thorn-like
-processes, which, as in the sperm-cells of <i>Moina</i>, make them adhesive,
-and, according to Brandes, render it possible for them to cling
-among the bristles on the abdomen of the female until one of the
-many eggs leaving the oviduct comes within reach. For among these
-Crustacea there is no true copulation, but the masses of sperm-cells
-are packed together into sperm-packets or 'spermatophores,' and are
-affixed by the male near the opening of the oviduct, where they burst
-and pour forth their contents between the appendages of the female.</p>
-
-<p>All these remarkable and widely divergent structures and arrangements
-depend not upon chance or on the fantastic expression of a
-'formative power,' as an earlier generation was wont to phrase it; they
-are undoubtedly without exception adaptations to the most intimate
-conditions of fertilization in each individual case. I lay particular
-stress upon a recognition of this, because it permits us to infer with
-certainty that even the variations of the single cell, if they are
-sufficiently important for the species, may be controlled by natural
-selection. It is obvious that the adaptations of the sex-cells must
-depend not on histonal selection, but only upon personal selection, since
-it is indifferent for the individual sperm-cells whether fertilization is
-accomplished successfully or not, while it is by no means indifferent
-for the species. The organism dies without descendants if its sperm-cells
-do not fertilize, and the carrying on of the species must be left to
-those of its fellows which produced sperm-cells which fertilize with
-more certainty; thus it is not the sperm-cells themselves, but the
-individual organisms which are selected, and that in relation to the
-quality of the sex-cells they produce.</p>
-
-<p>In contrast with the great diversity of form exhibited by the
-spermatozoa, the differentiation of the ovum appears very uniform, at
-least in regard to form and activity. The main form is spherical, but
-it is subject to many variations in the way of elongation or flattening.
-In the lower forms of life, as, for instance, among the sponges, and also
-in the polyps and Medusæ the egg-cells possess, until they are mature,
-the locomotor capacity of unicellular organisms; they creep about after
-the manner of amœbæ, and indeed, as I showed years ago, this
-movement from place to place in many polyps is exactly regulated;
-thus at a definite time they may leave the place where they originated
-and may, for instance, creep from the outer layer of cells (ectoderm)
-of the animal into the inner layer (endoderm) by boring through the
-so-called 'supporting lamella,' then they may creep further in the<span class="pagenum"><a id="Page_281"></a>[Pg 281]</span>
-endoderm, and finally return to quite definite and often remote spots
-in the ectoderm (<i>Eudendrium</i>, <a href="#f99">Fig. 95</a>). In another hydroid polyp
-(<i>Corydendrium parasiticum</i>) the mature egg-cells leave their former
-position within the endoderm and creep entirely outside of the animal
-which produced them, establishing themselves in a definite spot on its
-external surface, where they await the fertilizing zoosperms. Many
-ova can accomplish slight amœboid movements, but in most animals
-these do not suffice for movement from place to place, and the ova
-remain quietly in the
-spot where they were
-developed, or are passively
-pushed to another.
-Cases such as that of the
-polyp I have cited, in
-which the ovum actually
-comes to meet the male
-element, are quite exceptional,
-for in general
-the ovum is the passive
-and the spermatozoon
-the active or exploring
-element in fertilization.
-The female cell is entrusted
-with procuring
-and storing the material
-necessary to the building
-up of the embryo; and
-its peculiarities depend
-chiefly on this.</p>
-
-<div class="figright" id="f73">
-<img src="images/fig73.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 69.</span> Ovum of the Sea-urchin, <i>Toxopneustes lividus</i>,<br />
-after Wilson. <i>zk</i>, cell-body. <i>k</i>, nucleus or so-called<br />
-'germinal vesicle,' <i>n</i>, nucleolus or so-called 'germinal<br />
-spot.' Below there is a spermatozoon of the<br />
-same animal, with the same magnification (750<br />
-times).</p>
-</div>
-
-<p>It is true that in
-plants this stored material
-is seldom considerable, and that is because the ovum so frequently remains
-even after fertilization within the living tissues of the plant, and
-is thence supplied, often very abundantly, with food-stuffs; and, moreover,
-because the young plant that springs from the fertilized ovum maybe
-very small and simple, and yet capable of immediately procuring
-its own nourishment. But there are exceptions to this; thus the ova
-of the brown sea-wracks, or Fucaceæ, for instance, are quite twenty
-times as large as the ordinary cells of the algæ (<a href="#f68">Fig. 64</a>), and contain
-a quantity of food-stuff within themselves. In this case the ova are
-liberated into the water even before fertilization, and the nutrition of
-the embryo from the mother-plant is excluded.</p>
-
-<p><span class="pagenum"><a id="Page_282"></a>[Pg 282]</span></p>
-
-<p>In these Algæ we meet, for the first time, with a special organ in
-which the ova arise. In animals this is much more generally the
-case, and from sponges upwards there are always quite definite parts
-and tissues of the body which are alone able to develop eggs, and
-these are usually well-defined organs of special structure, the ovaries.
-Similarly, in male animals the spermatozoa arise in special places, and
-usually in special organs, the spermaries or testes.</p>
-
-<p>Animal ova often consist of more than the simple cell-body, the
-protoplasm and its nucleus; they almost always contain in the cell-body
-a so-called 'Deutoplasm,' as Van Beneden has fittingly named the
-yolk-substance. This consists of fats, carbohydrates, or albuminoids,
-which often lie in the cell-body in the form of spherules, flakes, or
-grains&mdash;a nutritive material that is often surrounded and enclosed
-by a small quantity of living matter or formative protoplasm. Apart
-from these stores of yolk it would be impossible for a young animal
-to develop from the ovum of a snake or a bird, for such highly
-differentiated animals could not be formed from an egg of microscopic
-dimensions if this remained without some supply of food from outside
-of itself during the period of development. There is obviously need
-for a considerable amount of building material, so that all the organs
-and parts, which are composed of thousands and millions of cells, may
-be developed.</p>
-
-<p>Thus the size of the animal-ovum depends essentially on the
-quantity of yolk that has to be supplied to the egg, and this
-depends in the main on whether the egg is still drawing nourishment
-from the mother during the development of the young animal. Therefore,
-as a general rule, eggs which are laid, and are surrounded and
-protected by a shell, are usually much larger than the eggs of animals
-which go through their development within the body of the mother.
-The best known illustration of this proposition is offered by mammals
-and birds, animals of equally high organization and comparable in
-bodily size. While the eggs of birds may be as much as 15 centimetres
-in length, and may weigh 1½ kilogrammes, those of most
-mammals remain microscopically minute, and scarcely exceed a length
-of 0.3 millimetres. The same principle is often illustrated within one
-and the same small group of animals, and even in the same species.
-Here, again, the Daphnids or water-fleas may serve as an example.</p>
-
-<p>Among these there are two kinds of eggs, summer and winter
-eggs, of which the former go through their development into a young
-animal within a brood-cavity on the back of the female, while the others
-are liberated into the water, and are surrounded by a hard shell. The
-summer eggs receive more or less nourishment from the mother by<span class="pagenum"><a id="Page_283"></a>[Pg 283]</span>
-the extravasation of the nutritive constituents of the blood into the
-brood-cavity, and they thus require a smaller provision of yolk than
-the winter eggs, which are thrown entirely upon their own resources.
-Accordingly we find that in all Daphnids the summer eggs are at
-least a little smaller and have less yolk than the winter eggs, as in
-the genus <i>Daphnella</i> (Fig. 70, <i>A</i> and <i>B</i>), while in some species, e.g. of
-<i>Bythotrephes</i>, this difference increases so much that the summer eggs
-are almost without yolk, and therefore very minute (Fig. 71, <i>B</i>). The
-reason of this lies in the fact that in this case the brood-sac is filled
-with a nutritive fluid rich in albuminoid substances, so that the
-embryo during its development is continually supplied with concentrated
-nourishment. This is not the case with the winter eggs,
-because these are liberated into the water, and we therefore find that
-they are of enormous size and quite filled with yolk (Fig. 71, <i>A</i>).</p>
-
-<div class="figleft" id="f74">
-<img src="images/fig74.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 70.</span> <i>Daphnella.</i> <i>A</i>, summer<br />
-egg. <i>B</i>, winter egg. <i>Oe</i>,<br />
-'oil-globules' of the summer<br />
-egg.</p>
-</div>
-
-<div class="figright" id="f75">
-<img src="images/fig75.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 71.</span> <i>Bythotrephes longimanus.</i> <i>A</i>, the brood-sac<br />
-(<i>Br</i>) of the female containing two winter-ova<br />
-(<i>Wei</i>), on which five large sperm-cells (<i>sp</i>) are lying.<br />
-<i>R</i>, dorsal surface of the animal. <i>Dr</i>, glandular layer<br />
-which secretes the shell-substance. <i>BK</i>, copulatory<br />
-canal. <i>B</i>, the brood-sac (<i>Br</i>) containing two summer-ova<br />
-(<i>Sei</i>). Both figures under the same magnification<br />
-(100).</p>
-</div>
-
-<p>In this instance, as in all the simpler eggs, the yolk constituents
-are secretions of the cell-body of the ovum; but nature employs many
-devices, if I may so speak, to bring up the mass of the egg, and
-especially of the yolk, to the highest attainable point. Thus in many
-orders of Crustaceans, for instance in the water-fleas just mentioned,
-there are special egg-nourishing cells, that is, young ovum-cells which<span class="pagenum"><a id="Page_284"></a>[Pg 284]</span>
-do not differ from the rest either in origin or in appearance, only they
-do not become mature eggs, but at a definite time cease to make
-progress, and then slowly break up, so that their substance may be
-absorbed as food by the true ova. Thus there is a much greater and
-at the same time more rapid growth than could be attained through
-nourishment from the blood alone. In the Daphnids the ovaries
-consist of groups of four cells each, only one of which becomes an
-ovum (Fig. 72, <i>Ei</i>), while the other three (1, 2, and 4) form nutritive
-cells which break up. This is so in all summer eggs; but in the
-winter eggs a much larger number of nutritive cells may take part
-in equipping a single ovum, and in the genus <i>Moina</i> over forty
-do so. But here the difference in size between the two kinds of
-eggs is very marked, the winter eggs being twice the diameter of the
-summer eggs.</p>
-
-<div class="figcenter" id="f76">
-<img src="images/fig76.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 72.</span> <i>Sida crystallina</i>, a Daphnid: a fragment of the ovary showing one of the
-groups of four cells, of which 1, 2, and 4 are nutritive cells, and only 3 becomes an
-ovum. Magnified 300 times.</p>
-</div>
-
-<p>In many insects also, e.g. in beetles and bees, similar nutritive
-cells occur, but there is in these forms a different arrangement which
-serves at the same time for the formation of the shell, and the
-supplying to the ovum of the necessary yolk-stuffs&mdash;the ovum is
-surrounded with a dense layer of epithelial cells, a so-called 'follicle.'
-In mammals and birds also these 'follicle cells' certainly play an
-important part in the nutrition of the ovum, though it is not yet
-quite clear how they act&mdash;whether they produce within themselves
-grains of yolk and other nutritive substances and convey these to the
-ovum by means of fine radiating processes, or whether they themselves
-ultimately migrate into the ovum and there break up. In any case
-it is worthy of note that all these follicular cells in insects and
-vertebrates have the same origin as the egg-cells, that is, they are
-modified germ-cells. The case is therefore essentially the same as in
-the nutritive cells of the Daphnids; nature sacrifices the greater
-number of the germ-cells in order to be able to provide more
-abundantly for the minority. She thus succeeds in raising the egg<span class="pagenum"><a id="Page_285"></a>[Pg 285]</span>
-beyond itself, so to speak, and provides the means for a growth
-which could obviously not be attained by means of the ordinary
-nourishment supplied by the blood.</p>
-
-<div class="figright" id="f77">
-<img src="images/fig77.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 73.</span> Diagrammatic longitudinal section of a<br />
-hen's egg before incubation, after Allen Thomson.<br />
-<i>Bl</i>, germinal disk. <i>GD</i>, yellow yolk. <i>WD</i>, white yolk.<br />
-<i>DM</i>, vitelline membrane. <i>EW</i>, albumen. <i>Ch</i>, chalaza.<br />
-<i>S</i>, shell membrane. <i>KS</i>, shell. <i>LR</i>, air chamber.</p>
-</div>
-
-<p>We now understand why the eggs of many animals should be
-of such enormous size
-and often of such complex
-structure. The eggs
-of birds are especially
-remarkable in this respect,
-and it has till
-recently been disputed
-whether they are really
-morphologically equivalent
-to a single cell. But
-this is undoubtedly the
-case, and though only
-the small thin germinal
-disk (Fig. 73, <i>Bl</i>) with
-its nucleus is the active
-part of this cell&mdash;the
-cell-body proper&mdash;yet
-all the rest&mdash;the enormous sphere of yolk with its regular layers of
-yellow (<i>GD</i>) and white (<i>WD</i>) yolk, the concentric layers of fluid
-albumen (<i>EW</i>) round about this, the chalazæ (<i>Ch</i>), and finally, the
-delicate shell membrane (<i>S</i>) and the limy shell (<i>KS</i>)&mdash;belong to this
-cell, and have arisen in connexion with it (Fig. 73).</p>
-
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_286"></a>[Pg 286]</span></p>
-<h2 class="nobreak" id="LECTURE_XV">LECTURE XV</h2>
-</div>
-
-<p class="c">THE PROCESS OF FERTILIZATION</p>
-
-<div class="blockquot">
-
-<p>Cell-division and nuclear division&mdash;The chromatin as the material basis of
-inheritance&mdash;The rôle of the centrosphere in the mechanism of division&mdash;The
-Chromosomes&mdash;Fertilization of the egg of the sea-urchin according to Hertwig&mdash;Of
-the egg of Ascaris according to Van Beneden&mdash;The directive divisions, or the extrusion
-of the polar bodies&mdash;Halving of the number of chromosomes&mdash;The same in the
-sperm-cell&mdash;Reducing division in parthenogenetic eggs&mdash;In the bee&mdash;Exceptional
-and artificial parthenogenesis&mdash;Rôle of the centrosphere in fertilization and in
-parthenogenesis.</p></div>
-
-
-<p><span class="smcap">Now</span> that we have made ourselves acquainted with the two kinds
-of germ-cells on the union of which 'sexual reproduction' depends, we
-may proceed to a more detailed discussion of the process of fertilization
-itself. But it is indispensable that we should take account of the
-processes of nuclear and cell-division, as these have been gradually
-recognized and understood in the course of the last decade. It may
-appear strange that the processes of division should throw light on the
-apparently opposite processes of cell-union, but it is the case, and no
-understanding of the latter is possible without a knowledge of the
-former.</p>
-
-<p>From the time of the discovery of the cell until well on in the
-sixties the process of cell-division was looked on as a perfectly simple
-process, as a mere constriction in the middle of the cell. It was
-observed that a cell in the act of dividing (<a href="#f63">Fig. 59</a>, <i>A</i>) stretched itself
-out, that its nucleus also became longer, became thinner in the middle,
-assumed a dumb-bell form, and was then gradually constricted, giving
-rise to two nuclei (<a href="#f63"><i>B</i></a>), whereupon the body of the cell also constricted
-and the two daughter-cells were formed (<a href="#f63"><i>C</i></a>). In certain worn-out or
-highly differentiated cells a cell-division of this kind really seems to
-occur&mdash;the so-called 'direct' division&mdash;but in young cells, and indeed
-in all vigorous cells, the process, which looks simple, is, in reality,
-exceedingly complex. Not only is the structure of the nucleus incomparably
-more complex than was recognized a quarter of a century ago,
-but nature has placed within the cell a special and marvellously
-intricate apparatus, by means of which the component parts of the
-nucleus are divided between the two daughter-nuclei.</p>
-
-<p>For a long time all that was distinguished in the cell-nucleus was<span class="pagenum"><a id="Page_287"></a>[Pg 287]</span>
-the nuclear membrane and a fluid content in which one or more
-nuclear bodies or nucleoli float. But this does not by any means
-exhaust what can now be recognized in the structure of the nucleus,
-and the most important constituents are not even among these, for
-recent researches, especially those of Häcker, have shown that the
-nucleolus or the nucleoli, to which there was formerly an inclination
-to attach a very high importance, must be regarded as only transient
-formations and not living elements&mdash;in fact, as mere collections of
-organic substance&mdash;'bye-products of the metabolism,' which at a
-definite time, that is just before the division of the nucleus, disappear
-from the nuclear space and are used up. We now know that in the
-resting cell, that is, in the cell which is not in the act of dividing
-(<a href="#f78">Fig. 74</a>, <i>A</i>), a very fine network of pale threads, often very difficult
-to make visible, fills the whole nuclear cavity, like a spider's web or
-the finest soap bubbles, and that in this so-called nuclear framework
-there are embedded granules of rounded or angular form (<i>A</i>, <i>chr</i>)
-which consist of a substance which stains deeply with such pigments
-as carmine, hæmatoxylin, all aniline dyes, &amp;c., and which has therefore
-received the name of chromatin. Often, indeed generally,
-these granules are exceedingly small, but sometimes they are bigger,
-and in that case they are less numerous and more easily made visible;
-in all cases, however, they are in a certain sense the most important
-part of the nucleus, for we must assume that it is their influence
-which determines the nature of the cell, which, so to speak, impresses
-it with the specific stamp, and makes the young cell a muscle-cell or a
-nerve-cell, which even gives the germ-cell the power of producing,
-by continued multiplication through division, a whole multicellular
-organism of a particular structure and definite differentiation, in
-short, a new individual of the particular species to which the parents
-belong. We call the substance of which these chromatin granules
-consist by the name first introduced into science by Nägeli, though
-only to designate a postulated substance which had not at that time
-been observed, but which he imagined to be contained within the
-cell-body&mdash;by the name <i>Idioplasm</i>, that is to say, a living substance
-determining the individual nature (εἶδος = form). I am anticipating
-here, and I reserve a more detailed explanation until I can gradually
-bring together all the facts which justify the conception I have just
-indicated of the 'chromatin grains' as an 'idioplasm,' or, as we may
-also call it, a 'hereditary substance.'</p>
-
-<p>That this chromatin must be something quite special we see from
-the processes of cell and nuclear division, which I shall now briefly
-describe.</p>
-
-<p><span class="pagenum"><a id="Page_288"></a>[Pg 288]</span></p>
-
-<div class="figcenter" id="f78">
-<img src="images/fig78.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 74.</span> Diagram of nuclear division, adapted from E. B. Wilson. <i>A</i>, resting cell
-with cell-substance (<i>zk</i>), centrosphere (<i>csph</i>) which contains two centrosomes, nucleolus
-(<i>kk</i>); and chromosomes (<i>chr</i>), the last distributed in the nuclear reticulum. <i>B</i>, the
-chromatin united in a coiled thread; the centrosphere divided into two and giving off
-rays which unite the halves. <i>C</i>, the nuclear spindle (<i>ksp</i>) formed, the rays more
-strongly developed, the nuclear membrane (<i>km</i>) in process of dissolution, the chromatin
-thread divided into eight similar pieces (<i>chrs</i>), the rays are attaching themselves to the
-chromosomes. <i>D</i>, perfected nuclear spindle with the two centrospheres at the poles
-(<i>csph</i>) and the eight chromosomes (<i>chrs</i>) in the equator of the spindle, all now longitudinally
-split. <i>E</i>, daughter-chromosomes diverging from one another, but still
-united by filaments, the centrosomes (<i>cs</i>) are already doubled for the next division.
-<i>F</i>, daughter-chromosomes, quite separated from one another, are already beginning
-to give off processes; the cell-substance is beginning to be constricted. <i>G</i>, end of the
-process of division: two daughter-cells (<i>tz</i>) with similar nuclear reticulum (<i>tk</i>) and
-centrospheres (<i>csph</i>), as in <i>A</i>.</p>
-</div>
-
-<p>When a cell is on the eve of dividing we observe first that the
-chromatin grains, which have till then been scattered throughout the
-network of the nucleus, approach each other and arrange themselves<span class="pagenum"><a id="Page_289"></a>[Pg 289]</span>
-into a long thin thread which, irregularly intertwined, forms a loose
-skein, the so-called coil-stage (Fig. 74, <i>B</i>). The thread then begins to
-thicken, and somewhat later it can be seen to have broken up into
-a number of pieces of equal length, as if it had been cut into equal
-pieces with scissors (<i>C</i>).</p>
-
-<p>These pieces or chromosomes become shorter by slowly contracting,
-and thus each takes the form of an angular loop, a straight rod, or
-a roundish, oval, or spherical body (Fig. 74, <i>C</i>, <i>chrs</i>). While this is
-happening, we can see at the side of the nucleus, and closely apposed
-to it, a pale longitudinally striped figure with a swelling, similar to
-a handle, at both ends&mdash;the so-called nuclear spindle or central
-spindle (<i>ksp</i>). This is the apparatus for the division of the nucleus,
-and it was previously represented by a small body susceptible to
-certain stains&mdash;the centrosome, which was surrounded by a halo-like
-zone, the centrosphere or 'sphere.' This body was long overlooked,
-but now the majority of investigators assume that, though it is often
-inconspicuous and very difficult to make visible, it is nevertheless
-present in every cell which is capable of division, and that it is
-therefore a permanent and indispensable constituent of the cell
-(Fig. 74, <i>A</i> and <i>B</i>, <i>csph</i>).</p>
-
-<p>When a cell is on the point of dividing, this remarkable cell-organ,
-which has hitherto seemed no more than an insignificant, pale,
-little sphere, now becomes active. First of all, often before the
-formation of the chromatin coil, it doubles by division (<a href="#f78"><i>A</i> and <i>B</i> <i>csph</i></a>),
-at first only as regards the centrosome, and then as regards the
-sphere also (<a href="#f78"><i>B</i></a>); and while division is going on fine protoplasmic
-filaments issue from the dividing sphere and radiate like rays from
-a sun into the cell-substance. As they only retain their connexion
-with each other at the surfaces of the dividing halves of the sphere
-which are turned towards each other, we might almost say that fine
-threads are drawn out between the two halves as they separate, and
-these become longer the further apart the halves diverge. In this
-manner the much-talked-of 'spindle figure' arises, which was first
-described in the seventies through the researches of A. Schneider,
-Auerbach, and Bütschli, but the significance and origin of which have
-claimed the labours of many later investigators down to our own day.</p>
-
-<p>The processes now to be described do not always take place in
-exactly the same manner, but the gist of the business is everywhere
-the same, and it consists in this, that the two ends or 'poles' of the
-spindle diverge further and further apart, and between them lies
-the nucleus whose membrane now disappears (<a href="#f78"><i>C</i>, <i>km</i></a>) while the spindle
-threads traverse its interior. Sometimes the membrane is retained,<span class="pagenum"><a id="Page_290"></a>[Pg 290]</span>
-but nevertheless the spindle threads penetrate into the interior of
-the nucleus. But the chromosomes always range themselves quite
-regularly in the 'equatorial plane' of the spindle (<a href="#f78"><i>D</i>, <i>aeq</i></a>)&mdash;a process
-the precise mechanism of which is by no means clearly understood,
-and indeed the play of the forces in the whole process of nuclear
-division is still very imperfectly revealed to our intelligence.</p>
-
-<p>Thus we have now before us a pale, spindle-shaped figure, which
-takes only a faint stain, with the 'suns' (<a href="#f78"><i>cs</i></a>) at its 'poles,' and in its
-equatorial plane the loop- or rod-shaped, or spherical chromosomes
-(<a href="#f78"><i>chrs</i></a>). The whole is designated the 'karyokinetic,' the 'mitotic,' or
-the 'nuclear division figure.'</p>
-
-<p>The meaning and importance of this, at first sight, puzzling figure
-will at once become clear from what follows. It may be observed
-at this stage, if not even long before, that each of the chromatin rods
-or loops has split along its whole length like a log of wood, and that
-the split halves are beginning slowly and hardly noticeably to move
-away from each other, one half towards one, the other towards the
-other pole of the spindle (<a href="#f78">Fig. <i>D</i> and <i>F</i></a>). Directly in front of the
-centrosome they make a halt, and now the material for the two
-daughter-nuclei is in its proper place (<a href="#f78"><i>F</i>, <i>chrs</i></a>). These develop
-quickly, each chromosome group surrounding itself with a nuclear
-membrane (<a href="#f78">Fig. <i>G</i></a>) within which the chromosomes gradually become
-transformed again into a nuclear network. Within the chromatin
-substance proper this is scattered about in small roundish or angular
-granules, lying especially at the intersecting points of the network.
-It may be stated at once, though the full significance of the statement
-can only be appreciated later, that we may assume with probability
-that this breaking up of the chromosomes is only apparent, and that
-these rods or spheres really continue to exist in the nuclear network,
-only in a different form, greatly spread out, somewhat after the
-manner of a Rhizopod which stretches out fine processes in all
-directions. These processes branch and anastomose, so that the body,
-which previously seemed compact, now appears as a fine network.
-In point of fact, it can be directly observed that the chromosomes,
-after the nucleus is completely divided into two daughter-nuclei, send
-out pointed processes (<a href="#f78"><i>F</i> and <i>G</i></a>) which gradually increase in length and
-branch, while the body of the chromosome itself becomes gradually
-smaller. It is thus probable that, when such a daughter-nucleus
-is on the point of dividing anew, it may, by a drawing together of the
-processes or pseudopodia of the chromosomes, produce the same rods
-or spheres as those which previously gave rise to the network. More
-definite reasons for this interpretation will be adduced later on. In<span class="pagenum"><a id="Page_291"></a>[Pg 291]</span>
-any case, the chromosomes, even in their compact rod-like state,
-consist of two kinds of substance, the chromatin proper, which stains
-deeply, and the linin, which is difficult to stain; and it is the latter
-which, by breaking up, forms the pale part of the nuclear network.</p>
-
-<p>Thus we can understand that the number of chromosomes remains
-the same in every cell-generation throughout development, as it is
-the same in all the individuals of a species. The numbers are known
-for many species: in some worms there are only two or four
-chromosomes, while in other related worms there are eight; in the
-grasshopper there are twelve, and in a marine worm, <i>Sagitta</i>, eighteen;
-in the mouse, the trout, and the lily there are twenty-four; in some
-snails thirty-two; in the sharks thirty-six, and in <i>Artemia</i>, a little
-salt-water crustacean, 168 chromosomes. In Man the chromosomes are
-so small that their normal number is not certain&mdash;sixteen have been
-counted. This counting can only be done during the process of
-nuclear division, for afterwards the chromosomes flow indistinguishably
-together, or rather apart, only to reappear, however, in the old
-form and number whenever the nucleus again begins to divide.</p>
-
-<p>It remains to be told what becomes of the centrosphere in cell-division.
-As soon as the formation of the daughter-nuclei has been
-brought about by the divergence of the split halves of the loops, the
-spindle figure begins to retrograde, its threads become pale and
-gradually disappear, as does the whole radiate halo of the centrosphere
-(<a href="#f78">Fig. <i>F</i> and <i>G</i></a>). The cell-body has by this time also divided
-in the equatorial plane of the nuclear spindle, and the centrosome
-remains usually as a very inconspicuous pale body lying in the cytoplasm
-close to the nucleus, reawakening to renewed activity when
-cell-division is about to recommence (<a href="#f78"><i>G</i>, <i>csph</i></a>).</p>
-
-<p>These, briefly, are the remarkable processes of nuclear division.
-Their net result is obvious; the chromatin substance is divided between
-the daughter-nuclei with the greatest conceivable accuracy.</p>
-
-<p>It is not so easy to understand the mechanism of this partition,
-and there are various divergent theories on this point. According to
-the older idea of Van Beneden, the spindle fibres work like muscles, and
-by contracting draw the halves of the chromosomes which adhere to
-them towards the pole, while the rest of the fibres radiating out from
-the polar corpuscles act as resisting and supporting elements. This
-view, with many modifications however, has still its champions,
-and M. Heidenhain in particular has made a notable attempt to
-establish it and to work it out in detail. Opposed to it stand the
-views of those who, like O. Hertwig, Bütschli, Häcker, and others
-regard the rays not as specific elements which were pre-formed in the<span class="pagenum"><a id="Page_292"></a>[Pg 292]</span>
-cell, but as the expression of the orientation of certain protoplasmic
-particles&mdash;an orientation evoked by forces which have their seat
-within the central corpuscles, and act in the manner of magnetic or
-electric forces. That the central corpuscles are centres of attraction
-seems to me hardly open to doubt, and I cannot regard the regular
-arrangement of the chromosomes in the equatorial plane of the
-spindle as due to a mere adhesion to contractile threads. Some still
-unknown forces&mdash;chemotactic or otherwise&mdash;must be at work here.
-Later on we shall study the phenomenon of the migration of the
-sperm-nucleus into the ovum, when it is accompanied by its
-central body and its halo of rays. Häcker seems to me justified in
-inferring from this phenomenon alone that the sudden origin of the
-rays is due to forces resident in the central corpuscle. But undoubtedly
-even this 'dynamic' explanation of karyokinesis is still
-only at the stage of hypothesis and reasoning from analogy, and is
-far removed from a definite knowledge of the forces at work.</p>
-
-<p>For the problems with which we are here chiefly concerned, the
-problems of heredity, it is enough to know that the cells of multicellular
-organisms possess an extremely complex apparatus for
-division, whose chief importance lies in the fact that through it the
-chromatin units of the nucleus are divided into precisely equal parts,
-and so separated from each other that one half forms one daughter-nucleus,
-the other half the other. It is not merely that there is an
-exact division of the whole chromatin in the mass, which could have
-been effected much more simply, but that there is <i>a regulated distribution
-of the different qualities of the chromatin</i>, as we shall see
-later.</p>
-
-<p>It must here be emphasized that the splitting of the chromosomes
-does not depend on external forces, but on internal ones involved in
-their organization, and in the definite attractions and repulsions of
-their component particles which come about in the course of growth.
-The chromosomes do not split like a trunk that has been broken open
-with an axe, but rather like a tree burst apart by the frost, that is, by
-the freezing of the water within itself. I consider it very important
-that we should recognize this, even though we do not yet know what
-the forces are that have control in this case, because it leads us to
-conclude that the structure of the chromosomes is extremely complex,
-that they are, so to speak, a world in themselves, that they possess an
-infinitely complex and delicate though invisible organization, in
-which intrinsic chemico-physical forces produce the regulated succession
-of changes which we observe. We shall afterwards see that we
-are led to the same conclusion from another direction&mdash;that is, from<span class="pagenum"><a id="Page_293"></a>[Pg 293]</span>
-the phenomena of inheritance. We shall then recognize that the rod-
-or loop-shaped chromosomes cannot be simple elements, but are
-composed of linear series of 10, 20, or more globular single-chromosomes,
-each of which represents a particular kind of chromatin or
-hereditary substance. If we consider this carefully, we shall see that
-it would hardly be possible to think out a mode of nuclear division
-which would so exactly and securely fulfil the purpose of conveying
-these many kinds of chromatin to the two daughter-nuclei in like
-proportions as does the mechanism of distribution actually brought
-about by nature. The longitudinal splitting of the rods halves the
-chromosomes, and the spindle apparatus secures the proper distribution
-of the halves between the two daughter-nuclei.</p>
-
-<p>So much, at least, is certain, that no such complicated mechanism
-for 'mitotic' division would have arisen if the very precise division
-of a substance <i>of the highest importance</i> had not been concerned, and in
-this conclusion lies the first hint of the interpretation of the chromatin
-substance as the bearer of the hereditary qualities.</p>
-
-<p>We are now familiar with the cell-nucleus and the apparatus for
-its division, and we are thus fully prepared to begin the study of
-the phenomena of 'fertilization.' Here also the processes depend essentially
-on the behaviour of the cell-nuclei, for even the first observations
-made by O. Hertwig on the behaviour of the spermatozoon after it has
-penetrated into the ovum led to the suggestion that the essential
-fact is the union of two nuclei; and numerous later, more and more
-deeply penetrating researches have furnished abundant evidence that
-the so-called 'fertilization' <i>is essentially a nuclear fusion</i>.</p>
-
-<p>Let us begin with O. Hertwig's observations on the ovum of the
-sea-urchin. Eggs of this animal, which have been taken out of the ovary
-of the female, may easily be fertilized artificially by pouring over
-them spermatic fluid taken from a male, and diluted with sea-water.
-Before this is done only one nucleus can be observed in the ovum, but
-shortly afterwards two nucleus-like structures of unequal size can be
-seen within the ovum, and the smaller is surrounded by a circle of
-rays. Hertwig rightly interpreted this smaller nucleus as the modified
-remains of the penetrating spermatozoon, which then slowly
-approaches the nucleus of the egg, and ultimately fuses with it to
-form a 'segmentation nucleus.' From this starts the so-called
-'segmentation' of the ovum, that is, the series of repeated divisions
-resulting in the formation of an ordered mass of cells, which by
-continued division of cells builds up the embryo.</p>
-
-<p>Simple as this process of nuclear conjugation may seem, it was by
-no means so easy to recognize, and several investigators, especially<span class="pagenum"><a id="Page_294"></a>[Pg 294]</span>
-Auerbach, Schneider, and Bütschli, had seen stages of the process at
-an earlier date without arriving at the true interpretation of the
-phenomena. This was chiefly due to the fact that, in addition to the
-phenomena of fertilization proper, which we have briefly sketched,
-other nuclear changes take place in the maturing ovum, and these are
-not very easy to distinguish from the former; we refer to the phenomena
-of the so-called 'maturation of the ovum.' When the ovum-cell
-has attained its full size within the ovary it is not yet capable of being
-fertilized, but must first undergo two processes of division, to the
-right understanding of which Hertwig's investigations, and afterwards
-those of Fol, have contributed much.</p>
-
-<p>For a long time it had been a familiar observation that small
-refractive corpuscles were extruded from one pole of the ovum shortly
-before the beginning of embryonic development. These were called
-'polar bodies,' because it was believed that they marked the place
-which would afterwards be intersected by the first plane of division;
-it was only known at that time that they had to be extruded from the
-egg, but no one had the remotest idea of their real nature.</p>
-
-<p>We now know that they are cells, and that their origin depends
-on a twice repeated division of the egg-cell; but it is a very unequal
-division, for these 'directive cells' or 'polar bodies' are always much
-smaller than the ovum, and indeed are usually so small that it is easy
-to understand why their cellular nature was for so long overlooked.
-Yet they have always a cell-body, and in many ova, for instance those
-of certain marine Nudibranchs, this is quite considerable; and they
-have likewise always a nucleus, which, notwithstanding the smallness
-of the cell-body, is in all cases exactly of the same size as the sister
-nucleus which remains behind in the ovum after division&mdash;a fact
-which is in itself enough to indicate that we have here to do
-essentially with readjustments and changes in the nucleus of the
-ovum.</p>
-
-<p>Long before the polar or directive divisions were recognized as
-divisions of the egg-cell it was known that the nucleus of the ovum
-disappeared as soon as the latter attained to its full size within the
-ovary. It was also known that this nucleus&mdash;the large so-called
-'germinal vesicle' lying in the middle of the ovum&mdash;left its central
-position and moved to the upper surface of the ovum, there to become
-paler and paler, and ultimately to disappear altogether from the sight
-of the observer. By many it was believed that it broke up, and that
-the 'segmentation nucleus,' which is afterwards obvious, is a new
-formation. The truth is that the germinal vesicle, at the time of its
-disappearance, is transformed into a division figure which is invisible<span class="pagenum"><a id="Page_295"></a>[Pg 295]</span>
-without the aid of artificial staining. The nuclear membrane breaks
-up; the centrosome of the ovum, which, although hardly visible, had
-previously lain beside the germinal vesicle, divides into two centrosomes
-and their centrospheres, and these now form the 'mitotic figure' by
-moving away from each other and sending out their protoplasmic rays.
-This nuclear spindle soon ranges itself at right angles to the surface
-of the egg, which at the same time arches itself into a protuberance,
-and soon two daughter-nuclei are formed, one of them lying within
-the protuberance (<a href="#f79">Fig. 75</a>, <i>A</i>, <i>Rk1</i>). This soon separates itself off from
-the ovum, surrounded by a small quantity of cell-substance. The
-other daughter-nucleus remains within the ovum, but neither of them
-remains in a state of rest; both are again transformed into a spindle
-and divide once more; the minute first 'polar body' dividing into two
-'secondary polar bodies' of half the size (<i>B</i>, <i>Rk1</i>), while the nuclear
-spindle within the egg brings about a second division of the ovum
-(<i>B</i>, <i>Rk2</i>) whose unequal products are the second polar cell and the
-mature ovum&mdash;that is, the ovum ready for fertilization. The process
-is now complete; the egg-cell, which has lost very little plasmic
-material through the 'polar bodies' and has not become visibly
-smaller, has now a nucleus (<i>B</i>, <i>Eik</i>) which has become considerably
-smaller through the two rapidly successive divisions, and, as we shall
-see later, has also undergone internal changes. In this state it is
-'ripe,' that is, it is ready to enter into conjugation with the nucleus of
-a male cell, and this we have already recognized as the essential
-element in the process of fertilization.</p>
-
-<p>These processes of 'maturation of the ovum' are common to all
-animal ova which require fertilization, and they follow almost the
-same course, only that in many cases the second division of the first
-polar body does not take place, so that only two polar bodies in all
-are formed. All these processes have nothing directly to do with
-fertilization, but it is only through them that the ovum becomes
-capable of fertilization. This does not prevent the spermatozoon from
-previously making its way into the ovum, for this is usually the case
-(Fig. 75, <i>A</i>, <i>sp</i>); there it waits until the second 'directive division' of
-the ovum has been accomplished, utilizing the time to become transformed
-in the manner necessary for the conjugation of the two nuclei.
-Only in a few species, for example in the sea-urchin, does the egg
-complete its polar divisions within the ovary, therefore before it has
-come into contact with the sperm at all.</p>
-
-<div class="figcenter" id="f79">
-<img src="images/fig79.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 75.</span> Process of fertilization in <i>Ascaris megalocephala</i>, the thread-worm of the
-horse, adapted from Boveri and Van Beneden. <i>A</i>, ovum in process of the first directive
-division; <i>Rk</i> 1, first polar body; <i>sp</i>, spermatozoon with two chromosomes in its nucleus,
-attaching itself to the ovum, and about to penetrate into it; a protrusion of the egg-protoplasm
-is meeting it. <i>B</i>, the second directive division has been completed; <i>Rk2</i>,
-the second polar body; <i>Eik</i>, the reduced nucleus of the ovum. The first polar body
-(<i>Rk</i> 1) has divided into two daughter-cells, <i>spk</i>; the nucleus of the spermatozoon
-remains visible with its two centrospheres (<i>csph</i>). <i>C</i>, the sperm nucleus (♂<i>k</i>) and the
-ovum nucleus (♀<i>k</i>) have grown, each has two loop-like chromosomes; only the male
-nucleus has a centrosphere, which has already divided into two (<i>csph</i>). <i>D</i>, the two
-nuclei lie apposed between the poles of the nuclear spindle. <i>E</i>, the four chromosomes
-have split longitudinally; the spindle for the first division of the ovum (the segmentation
-spindle, <i>fsp</i>) has been formed. <i>F</i>, divergence of the daughter-chromosomes towards
-the two poles; division of the ovum into the first two cleavage cells or embryonic
-cells.</p>
-</div>
-
-<p>That we may be able to penetrate still more deeply into the
-processes of fertilization, the best illustration to take seems to me to
-be, as yet, the ovum of the thread-worm of the horse (<i>Ascaris</i><span class="pagenum"><a id="Page_296"></a>[Pg 296]</span>
-<i>megalocephala</i>), which has become famous through the classical
-observations of Ed. van Beneden. Many favourable circumstances
-unite in this case to make the essentials of the process clearly recognizable.
-Fertilization takes place within the body of the female, in an<span class="pagenum"><a id="Page_297"></a>[Pg 297]</span>
-enlarged portion of the oviduct, within which a number of the
-remarkable sperm-cells are always found in a mature female. They
-are remarkable in being not thread-like, but rather spheroidal cells,
-bearing, however, a small protuberance something like a pointed horn
-(Fig. 75, <i>A</i>, <i>sp</i>). When such a sperm-cell comes in contact with the
-upper surface of an ovum a swelling forms at the place touched, and
-the sperm-cell attaches itself firmly to this, and is drawn by it into the
-ovum. Without doubt, amœboid movements on the part of the sperm-cell
-itself play some part in this, as can be most plainly seen in the
-large sperm-cells of many Daphnids which we have already discussed.
-In the egg of the thread-worm the whole sperm-cell with its nucleus
-can soon be detected within the substance of the ovum, and it then
-changes rapidly. Its main body fades more and more completely,
-until at last it disappears altogether, while the nucleus becomes
-vesicle-like and soon attains a considerable size (Fig. 75, <i>B</i>, <i>spk</i>).
-Meanwhile the residue of the germinal vesicle which remained behind
-in the ovum after the second directive division (<i>B</i>, <i>Eik</i>) has changed
-into a large vesicle-like nucleus (<i>C</i>, ♀ <i>k</i>), which in the ovum of <i>Ascaris</i>,
-as well as in the spermatozoon, at first contains a nuclear reticulum
-with irregular fragments of chromatin. Later on, these form a spiral
-coil in the manner we have already described, and finally this breaks
-up into two large and relatively thick angular loops or chromosomes
-(Fig. 75, <i>C</i> and <i>D</i>, <i>chr</i>).</p>
-
-<p>At the same time a nuclear division apparatus has formed in the
-space between the two nuclei&mdash;the so-called male and female 'pronuclei'
-(♂ <i>k</i>, ♀ <i>k</i>)&mdash;two centrospheres (<i>csph</i>) become visible, at first
-lying close together, but afterwards moving apart (<i>D</i>) to form the
-poles of a nuclear spindle, in the equatorial plane of which the four
-chromosomes of the male and female pronuclei are now arranged.
-The nuclear membranes disappear, and the two nuclei now unite to
-form one, the segmentation nucleus (<i>D</i>). A dividing spindle then
-develops and brings about the first embryonic cell-division (<i>E</i>), and
-thus the beginning of the 'segmentation' of the ovum; each of the
-four chromatin loops splits longitudinally, and each of the split halves
-migrates, one to one, the other to the other daughter-nucleus (<i>F</i>). As
-this same method of distribution of the chromatin substance is repeated
-at every successive cell-division throughout embryogenesis, and indeed
-through the whole of development, it follows that the result of
-fertilization is, that all the cells of the body of the new animal which
-develops from the ovum contain an equal quantity of paternal and
-of maternal chromatin. If we are right in regarding the chromatin
-substance as the hereditary substance, it becomes immediately apparent<span class="pagenum"><a id="Page_298"></a>[Pg 298]</span>
-that this equal division is of the most far-reaching importance, for it
-shows us that the so-called process of fertilization is the union of
-equal quantities of hereditary substance of paternal and maternal
-origin.</p>
-
-<p>The process of fertilization is now known in all its details in
-a great number of animals in the most diverse groups; it is everywhere
-the same in its essential features; there is always only one
-sperm-cell which normally enters into conjugation with the ovum-nucleus,
-and in every case the sperm-cell, however minute it may be
-to begin with, forms a nucleus nearly or exactly as large as the
-nucleus of the ovum, and in all cases it contains the same number of
-chromosomes as the nucleus of the ovum. Of special interest, however,
-is the fact that this number is always half the number of the
-chromosomes exhibited by the somatic cells of the particular animal
-in question, and that the reduction of the number of chromosomes
-to half the normal is effected in both male and female germ-cells by
-the last divisions of these cells, which take place before they have
-attained to a state of maturity. In the ovum the reduction occurs in
-the directive divisions, to which we must therefore turn our attention
-once more, with special reference to the number of chromosomes.</p>
-
-<p>We saw that, in the full-grown ovarian egg, the germinal vesicle
-rises to the surface and there becomes transformed into the first polar
-spindle. Now this shows, in its equatorial plane, double the number
-of chromosomes normal to the species. This duplication comes about,
-not directly before the nuclear division, but much earlier in the young
-mother-egg-cell; it is only the change in the time of the splitting of
-the chromosomes that is unusual. The first maturation division takes
-place nevertheless in accordance with the usual plan of nuclear
-division; it is, as I have called it, an 'equation division,' that is, both
-daughter-nuclei again receive the same number of chromosomes as
-the young mother-egg-cell had to start with, namely, the normal
-number of the species. Thus, if the young mother-egg-cell had four
-chromosomes (Fig. 76, <i>A</i>), this number would double to eight at an
-early stage (<i>B</i>), but the first maturing division would give each
-daughter-nucleus four (<i>C</i> and <i>D</i>). In the second maturation division
-the case is different, for here no splitting and duplicating of the
-number of chromosomes takes place, but the existing number, by
-being distributed between the two daughter-nuclei, is reduced to half
-in each (<i>E</i> and <i>F</i>). For this reason I have called it a 'reducing
-division.' In our example, therefore, the ovum, as well as the second
-polar body, would contain only two chromosomes (Fig. 76, <i>F</i>).</p>
-
-<div class="figcenter" id="f80">
-<img src="images/fig80.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 76.</span> Diagram of the maturation divisions of the ovum. <i>A</i>, primitive germ-cell.
-<i>B</i>, mother-egg-cell, which has grown and has doubled the number of its
-chromosomes. <i>C</i>, first maturation division. <i>D</i>, immediately thereafter; <i>Rk1</i>, the first
-directive cell or polar body. <i>E</i>, the second maturation spindle has been formed; the
-first polar body has divided into two (2 and 3); the four chromosomes remaining in
-the ovum lie in the second directive spindle. <i>F</i>, immediately after the second
-maturation division; 1, the mature ovum; 2, 3, and 4, the three polar cells, each of
-these four cells containing two chromosomes.</p>
-</div>
-
-<p>I cannot enter into the details of the process here, for we are<span class="pagenum"><a id="Page_299"></a>[Pg 299]</span>
-dealing with essentials and not with isolated and, so to speak, chance
-details, but I must emphasize the fact that the same process of reduction
-of the number of chromosomes takes place in this or an analogous
-manner in all animal ova, and can be demonstrated also in most of
-the chief groups of plants. Whether it be, as many have maintained,
-that the reduction is not always first effected by the 'maturation
-divisions,' but in some cases takes place earlier in the primitive
-egg-cell<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a>, so much is certain, that the nuclei which come together
-for 'fertilization' only contain half the normal number of chromosomes,
-and this is true not only of the ovum but also of the sperm-nucleus.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> See the discussion of this point in chapter xxii.</p>
-
-</div>
-
-<p>Arguing from general considerations, but especially from the
-theory which regards the chromosomes as the bearers of the hereditary
-substance, I had come to the conclusion, before there was any full<span class="pagenum"><a id="Page_300"></a>[Pg 300]</span>
-knowledge of the phenomena of the maturation of the ovum, that
-a reduction of the chromosomes by half <i>must</i> take place, and had
-postulated a similar 'reducing division' for the sperm-cell, and
-further, for plants as well as animals&mdash;indeed, for all sexually reproducing
-forms of life. The two divisions in the sperm-cell corresponding
-to the polar divisions of the ovum with their reduction
-of chromosomes were demonstrated by Oscar Hertwig in the case
-of the thread-worm of the horse (<i>Ascaris megalocephala</i>)&mdash;a form
-which has proved so very important in relation to the whole theory
-of fertilization. It is true that in this case the course of the phenomena
-of reduction is less convincing than in some other forms which
-have been investigated more recently, as, for instance, the mole-cricket
-and the bugs. In these instances, at any rate, a 'reducing division'
-in spermatogenesis, quite corresponding to that of the egg-cell, has
-been demonstrated, and this demonstration is of particular value
-owing to the fact that the development of the sperm-cell, as we shall
-presently see, throws an entirely new light on that of the ovum, and
-especially on the phyletic significance of the polar bodies.</p>
-
-<p>We began our consideration of the processes of reduction with
-the full-grown egg-cell, but now let us go back to the earliest
-rudiments of the ovary of the embryo, and we find that it consists
-of a single primitive egg-cell, from which, by division, all the other
-egg-cells arise. In the same way the first rudiment in the testis or
-spermary is formed by a primitive sperm-cell, which does not differ
-visibly from the primitive egg-cell. Both now multiply by division
-for a considerable time, and in the ovary this is followed by a period
-of growth, during which multiplication ceases, and each cell increases
-considerably in size and lays in a store of yolk. Each cell thus
-ultimately reaches the condition with which we started previously,
-that of the full-grown <i>mother-egg-cell</i>.</p>
-
-<p>Although the primitive sperm-cells do not exhibit such pronounced
-growth as the ova, they have likewise their period of growth, during
-which multiplication by division ceases, and the cells increase only in
-size (Fig. 77, <i>A</i>). When they have attained their maximum of growth
-the number of chromosomes is seen to have been doubled by longitudinal
-splitting (as in the diagram, Fig. 77, <i>B</i>, from four to eight).
-From this <i>mother-sperm-cell</i> there now arise by two divisions in
-rapid succession (<i>C</i>-<i>F</i>) four sperm-cells, and the same reduction of the
-number of chromosomes to half is effected as in the polar divisions of
-the egg-cell. In the first division, four chromosomes go to each
-daughter-cell (<i>D</i>), in the second, two (<i>F</i>). The only essential difference
-between the corresponding processes in the egg-cell and the sperm-<span class="pagenum"><a id="Page_301"></a>[Pg 301]</span>cell
-lies in the fact that the divisions of the so-called 'spermatocyte'
-or mother-sperm-cell are equal, so that four granddaughter-cells of
-equal size arise, while in the mother-egg-cell or 'ovocyte' the
-divisions are very unequal. In the former the result of the divisions
-is <i>four</i> cells capable of fertilizing, in the latter <i>one</i> cell capable of
-being fertilized and three minute 'polar cells' which are incapable
-of conjugating with a sperm-cell and giving rise to a new individual.</p>
-
-<div class="figcenter" id="f81">
-<img src="images/fig81.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 77.</span> Diagram of the maturation-divisions of the sperm-cell, adapted from
-O. Hertwig. <i>A</i>, primitive sperm-cell. <i>B</i>, mother-sperm-cell. <i>C</i>, first maturation
-division. <i>D</i> 1 and 2, the two daughter-cells. <i>E</i>, the second maturation division, by
-which the four cells of <i>F</i> arise, each with half the number of chromosomes.</p>
-</div>
-
-<p>There can thus be no doubt that the polar cells, as Mark and
-Bütschli long ago suggested, are abortive ova, that is, that, at a
-remote period in the evolution of animal life, each of these four
-descendants of a mother-egg-cell became a germ-cell capable of
-development. It is not difficult to infer that the unequal division,
-which now leads to an insufficient size in three of these descendants,
-has gone on <i>pari passu</i> with the continually increasing size
-of the mature ovum, and had its reason in the fact that it was
-important above all things to store in the ovum as much protoplasm
-and yolk as possible. We have already seen that even the dissolution
-of a number of the sister-cells of the ovum is sometimes demanded, so
-that the ovum may be surrounded by nutritive follicular cells. In
-short, the greatest possible quantity of nourishment is conveyed to the<span class="pagenum"><a id="Page_302"></a>[Pg 302]</span>
-ovum in every conceivable way, and it is thus stimulated to a growth
-which no single cell could attain to if it were dependent on the
-ordinary nutrition supplied by the blood. And we can understand
-that nature&mdash;to speak metaphorically&mdash;did not wish to destroy her
-own work by finally distributing among four ova all the nourishment
-she had succeeded in heaping up in all sorts of ways within the
-mother-egg-cell.</p>
-
-<p>But it may be asked, Why have all these unnecessary divisions
-been maintained up till the present day? Why have they not long ago
-been given up, since they can and do only lead to the production of
-three abortive ova, which are foredoomed to perish? Are they mere
-vestiges, processes which are in themselves meaningless, but have, so
-to speak, been maintained by the principle of inertia? This principle
-is certainly operative in some sense and to some extent even in living
-nature; a process which has been regularly repeated through a long
-series of generations does not at once cease to be performed when it
-is no longer of use to the organism concerned. The eyes of animals
-which have migrated to lightless depths do not disappear all at once
-and leave no trace; they degenerate very gradually and only in the
-course of many generations; and it would thus be quite possible to
-defend the position that these polar or 'maturation divisions' of the
-ovum are purely <i>phyletic reminiscences</i> without actual significance.</p>
-
-<p>But I cannot agree with this opinion. If it were actually so we
-should expect that the formation of the polar bodies would not still take
-place in all cases in almost the same manner, for all rudimentary parts
-and processes vary greatly; we should expect that in many animal
-groups the polar divisions would not occur, or perhaps that only half
-the number would occur. But this is not so; in all multicellular
-organisms, from the lowest to the highest, two reducing divisions take
-place, and always in almost the same manner, with the exception of a
-single category of ova, of which I shall presently have to speak. We
-shall see later that even in unicellular organisms analogous processes
-may be observed.</p>
-
-<p>But it is also intelligible that this twice repeated division of the
-mother-egg-cell is necessary if the reduction in the number of
-chromosomes to half is only possible in this way, since <i>this reduction
-is indispensable</i>. If each of the two conjugating germ-cells contained
-the full normal number of chromosomes, the segmentation-nucleus
-would contain a double number, and if that went on, the number of
-chromosomes would increase in arithmetical proportion from generation
-to generation, and would soon become enormous. Even though we
-were not otherwise certain that these chromosomes are units of a<span class="pagenum"><a id="Page_303"></a>[Pg 303]</span>
-permanent nature, which only apparently break up in the nuclear
-reticulum, but in reality persist, the fact of reduction would point in
-this direction. For if they were not permanent structures and distinct
-from one another, and if their number depended solely on the quantity
-of chromatin which the nucleus contains, the reduction in number might
-be secured if the chromosomes in the growing egg and sperm-cells
-increased in size more slowly than the cell-body and the other parts of
-the cell. But from the fact that the reduction takes place not in this
-simple way, but, in sperm-cells and in ova which require to be
-fertilized, only through cell-division and a specific mode of nuclear
-division, we may conclude that it cannot happen otherwise, that
-chromosomes are not mere aggregates of organic substance, but organs
-whose number can only be reduced by the extrusion of some of them
-from the cell.</p>
-
-<p>It is true that there are ova in which the process of reduction
-does not follow the course we have described, but the exceptions only
-serve to confirm our view of the reducing significance of the polar
-divisions, and of their persistence because of the necessity for
-reduction.</p>
-
-<p>As far back as the middle of the nineteenth century it was
-known that in various animals the eggs develop without fertilization.
-This reproduction by 'parthenogenesis' was first established with
-certainty by the German bee-keeper Dzierzon in 1845, and then
-scientifically corroborated by Rudolph Leuckart and C. Th. von
-Siebold. Although parthenogenesis was at first observed only in a
-few groups of the animal kingdom, in bees and some nocturnal
-Lepidoptera (Psychidæ and Tineidæ), it has become more and more
-apparent in the course of years that this 'virgin reproduction' is by
-no means a rare form of reproduction, and that it occurs regularly and
-normally in many cases, especially in the very diverse groups of the
-great series of Arthropoda. Thus among insects it is found in certain
-saw-flies, gall-flies, ichneumon-flies, in the honey bee, and in common
-wasps, and it is particularly widespread among plant-lice (Aphides)
-such as the vine-aphis (<i>Phylloxera</i>), whose prodigious multiplication
-in a short time depends partly on the fact that all the generations,
-with the exception of one, consist only of females with a parthenogenetic
-mode of reproduction.</p>
-
-<p>Among the lower Crustaceans also parthenogenesis plays a large
-rôle, and in many species it even occurs as the sole mode of reproduction,
-but more often&mdash;as is also the case among insects&mdash;it occurs
-alternately with bi-sexual reproduction. For parthenogenesis must not
-be regarded as asexual reproduction, but rather as <i>unisexual</i>, that is,<span class="pagenum"><a id="Page_304"></a>[Pg 304]</span>
-as arising from sexually differentiated individuals (females), and from
-germ-cells (true ova), but brought about by the agency of individuals
-of only one sex, the female. These parthenogenetic eggs emancipate
-themselves, so to speak, from the law that was previously regarded as
-without exception, that all ova require fertilization to enable them to
-develop. That this law admits of many exceptions is now universally
-admitted; thus in the small family of water-fleas (Daphnids) there are
-even two kinds of eggs, the summer and winter eggs we have already
-mentioned, which are produced by the same female, and yet the
-former kind develop without fertilization, while the latter require to
-be fertilized before they can develop.</p>
-
-<p>It was obviously important to learn the state of affairs in regard
-to reducing divisions in parthenogenetic ova, to find out whether here
-also, three, or, in some circumstances, two polar bodies were formed,
-and whether the second polar division reduced the number of chromosomes
-to half. If the theory previously advanced as to the importance
-of the chromatin, and especially of the reducing effect of the
-second maturing division be correct, we should expect the second
-division to be wanting in parthenogenetic eggs, since otherwise the
-number of chromosomes would be reduced to half in each generation,
-and would thus gradually disappear or sink to one.</p>
-
-<p>Having directed my attention to this problem, I succeeded in
-establishing for a Daphnid, <i>Polyphemus</i>, that the second polar
-division does not occur, and that only one polar body is formed.
-Blochmann found the same in the parthenogenetic eggs of plant-lice
-or Aphides, in which, moreover, the eggs requiring fertilization
-exhibit, like the winter eggs of Daphnids, two polar divisions. It
-was thus established that at least those eggs of Aphides and Daphnids
-which are wholly parthenogenetic retain the full number of chromosomes
-of their species, as is represented in the diagram, <a href="#f82">Fig. 78</a>.
-When parthenogenesis set in the polar divisions were limited to one,
-and that this could happen justifies us in concluding <i>a posteriori</i>
-that it could have happened also in the case of ova which required
-fertilization if that had been necessary or even merely indifferent.
-The polar divisions are thus not mere 'vestigial' processes; they
-have an immediate significance, and it lies in the reduction of the
-number of chromosomes.</p>
-
-<p>But I must make a reservation here; it is not universally true
-of parthenogenetic eggs that maturation takes place without the
-second polar division. The first exception was observed in the salt-water
-crustacean, <i>Artemia salina</i>. In this case only one polar body
-is actually extruded and the number of chromosomes remains normal,<span class="pagenum"><a id="Page_305"></a>[Pg 305]</span>
-as I was able to demonstrate with the small number of ova at my
-disposal; but according to the investigations of Brauer on more
-abundant material it appears that, while the second polar division is
-suppressed in the majority of the ova, and the external extrusion of
-a second polar body never occurs, the second polar division does
-nevertheless sometimes take place. The two daughter-nuclei arising
-from this division unite again immediately afterwards to form a
-single nucleus, and this now functions as a segmentation nucleus. Of
-course it again contains the full number of chromosomes, namely,
-twice 84=168.</p>
-
-<p>In <i>Artemia</i>, therefore, the adaptation of the ova to parthenogenetic
-development is not yet fully established, and the complete
-abandonment of the second polar division seems to be phyletically
-striven for, since, although the division still takes place, its effect is
-neutralized immediately afterwards.</p>
-
-<div class="figcenter" id="f82">
-<img src="images/fig82.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 78.</span> Diagram of the maturation of a parthenogenetic ovum. The number of
-chromosomes normal to the species has been assumed to be four. <i>Uei</i>, a primitive
-germ-cell. <i>M Eiz</i>, a mother-egg-cell, with twice the normal number of chromosomes.
-<i>Eiz</i>, mature ovum after the separation of the first and only polar body. <i>Rk</i><sup>1</sup>.</p>
-</div>
-
-<p>Among bees the state of affairs is again exceptional. Here the
-female, the so-called queen bee, possesses a capacious sperm-sac, in
-which the spermatozoa received in copulation remain living for years,
-and the fertilization of an ovum is effected in the usual way from
-this sac while the egg from the ovary is passing down the oviduct.
-The queen bee has the power of releasing some spermatozoa from the
-receptacle, or of not doing so, and thus of fertilizing the egg, or of
-not fertilizing it. Since the notable observations of Dzierzon and the
-investigations of von Siebold and Leuckart which followed them, it
-has been assumed that only those eggs were fertilized which were
-laid in the cells destined for rearing females (workers or queens),
-while those which were to give rise to 'drones' or males remained<span class="pagenum"><a id="Page_306"></a>[Pg 306]</span>
-normally unfertilized. Only in the last decade of the past century
-did the bee-keepers begin to cast doubt on this so-called 'Dzierzon
-theory'; various violent and obstinate attacks were made upon
-it, and these were supported by new and apparently convincing experiments.
-Dickel, a teacher in Darmstadt, has been particularly
-strenuous in attempting to overthrow the old theory, by emphasizing
-the fact that von Siebold's old investigations on bee eggs afforded
-no convincing proof. Von Siebold made his investigations on eggs
-freshly taken from the hive, and was never able to find spermatozoa
-in 'drone eggs' (that is, eggs laid in drone cells and therefore
-destined to develop into drones), while he was often able to demonstrate
-the presence of from one to four spermatozoa in 'worker eggs.'
-But he only examined 'drone eggs' which were already twelve hours
-old, and in these, as we now know, he would not have found
-spermatozoa in any case, even if they had been fertilized, because
-in ova at that stage the development of the embryo has already
-fully begun, and nothing remains of the spermatozoa. In the bee,
-according to Buttel-Reepen, the fertilizing spermatozoon is transformed
-in twenty minutes after it has penetrated into the egg into
-a minute 'sperm-nucleus' which is almost invisible even in sections,
-and certainly nothing whatever could be seen of it by the old method
-of squeezing the fresh ovum.</p>
-
-<p>It had therefore to be admitted that Dzierzon's theory rested
-on an insecure foundation, and I accordingly set two of my students
-at that time, Dr. Paulcke and Dr. Petrunkewitsch, to examine the
-eggs of the bee anew with regard to the point in question, using the
-greatly improved methods at their disposal. These investigations
-have been carried out in the Freiburg Zoological Institute during the
-last three years, and have resulted in establishing the absolute correctness
-of Dzierzon's theory: the 'drone eggs' do remain unfertilized,
-while the eggs from which females are to develop are fertilized
-without exception.</p>
-
-<p>In this case, therefore, we have, in the same animal, eggs which
-can be fertilized and eggs which, without fertilization, develop
-parthenogenetically, and it is therefore of the greatest possible interest
-to know the state of matters in them in regard to the directive
-divisions and the reduction of the chromosomes.</p>
-
-<p>Dr. Petrunkewitsch's investigations have shown that in both
-cases, that is, whether a spermatozoon penetrates into the ovum or
-does not, a twice-repeated division of the nuclear material in the
-ovum takes place. Moreover, the two daughter-nuclei which result
-from the second division do not, as Brauer showed was sometimes the<span class="pagenum"><a id="Page_307"></a>[Pg 307]</span>
-case in <i>Artemia</i>, unite again afterwards; they remain separate, and
-the number of chromosomes&mdash;there are sixteen of them&mdash;is thereby
-reduced to half in the segmentation nucleus. But this is not all, for
-before embryonic development has begun the normal number can be
-again seen in the segmentation nucleus; the chromosomes must therefore
-have <i>doubled their number by division within the nucleus</i>.</p>
-
-<p>It is probable that something similar takes place in the cases of
-exceptional parthenogenesis which have long been known, but this
-point has not yet been sufficiently investigated. Nevertheless I cannot
-pass them over, as they are instructive from another point of view.</p>
-
-<div class="figright" id="f83">
-<img src="images/fig83.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 79.</span> The two maturation divisions of the<br />
-'drone eggs' (unfertilized eggs) of the Bee, after<br />
-Petrunkewitsch. <i>Rsp 1</i>, the first directive spindle.<br />
-<i>k 1</i> and <i>k 2</i>, the two daughter-nuclei of the same.<br />
-<i>Rsp 2</i>, the second directive spindle. <i>k 3</i> and <i>k 4</i>, the<br />
-two daughter-nuclei. In the next stage <i>k 2</i> and <i>k 3</i><br />
-unite to form the primitive sex-nucleus. Highly<br />
-magnified.</p>
-</div>
-
-<p>In some silk-moths (Bombycidæ) and hawk-moths (Sphingidæ),
-especially in the silk-moth
-proper (<i>Bombyx mori</i>), in
-<i>Liparis dispar</i>, and in
-quite a number of other
-Lepidoptera, it sometimes
-happens that, out of a
-large number of unfertilized
-eggs, a few will
-develop and produce
-caterpillars. This is interesting
-enough, but it
-gains increased importance
-through the investigations
-of the Russian naturalist,
-Tichomiroff, who succeeded
-in considerably increasing
-the number of
-unfertilized eggs that developed
-by gently rubbing
-them with a paint-brush, or by dipping them for a little in dilute
-sulphuric acid. It is thus possible to make eggs, which would not
-ordinarily develop without being fertilized, capable of parthenogenetic
-development by means of mechanical or chemical stimulus. This
-sounds almost incredible, but it is beyond a doubt, and it is still further
-corroborated by the fact that Prof. Jacques Loeb has succeeded in
-inciting the eggs of a sea-urchin to parthenogenetic development by
-means of a chemical stimulus. When he added to the sea-water in
-which the eggs were laid a certain quantity of chloride of magnesium
-the ova developed, and not only went through the process of segmentation,
-but even reached the stage of the quaint easel-like Pluteus larva.
-Quite recently Hans Winkler has made the interesting observation<span class="pagenum"><a id="Page_308"></a>[Pg 308]</span>
-that from sea-urchin sperms which have been killed by heat it is
-possible to extract in aqueous solution a substance capable of exciting
-unfertilized sea-urchin eggs to development, although they only go as
-far as to the sixteen-cell stage.</p>
-
-<p>From all these results we can at least infer so much, that
-chemical changes and influences may determine whether the ripe
-ovum shall go on to embryonic development or not, and that these
-influences, may be very diverse in nature in different cases. I shall
-return later to these important facts.</p>
-
-<p>When we sum up the facts we have cited with reference to the
-reduction of the number of chromosomes, it appears that nature is,
-as it were, striving to keep the number constant for each species;
-that in germ-cells which are destined for amphimixis they are reduced
-to half the normal number, but that this halving of the number is
-suppressed where fertilization is always absent, or that the reduction
-to half is compensated for again in various ways, whether by subsequent
-fusion of the two daughter-nuclei, which have arisen from
-the process of reduction, or by an independent duplicating of the
-chromosomes in the segmentation nucleus.</p>
-
-<p>We might perhaps be inclined to conclude from all this that the
-occurrence of development depended on the presence of the normal
-number of chromosomes; and I used to regard this as possible. But
-facts which have been more recently brought to light have excluded
-this view. Above all, we now know that every nuclear division
-depends on the presence of a dividing apparatus, a centrosphere, but
-that this organ degenerates in the ova of most animals and is completely
-lost after the second polar division has been effected. The
-mature ovum is therefore in itself incapable of entering on its
-embryonic development, no matter how many chromosomes its
-nucleus contains; it is only capable of further division when the
-fertilizing sperm-cell brings with it its dividing apparatus or centrosphere.
-In thread-like sperms this lies in the median portion (<a href="#f72">Fig.
-68</a><i>C</i>), and after the tail-piece has been dissolved, which happens soon
-after the sperm enters the egg, the central corpuscle, at first very
-small, can be recognized in front of the sperm-nucleus, where it is
-soon transformed into an 'aster' and divides into two. Then both
-spheres move apart (<a href="#f79">Fig. 75</a><i>D</i>, p. 296) and form the nuclear spindle
-between them by the confluence of their rays.</p>
-
-<p>From this the division of the ovum into the two first embryonic
-cells proceeds. The two pronuclei in the ovum, the male and the
-female, are thus exactly alike as to number of the chromosomes, and
-frequently at least as to size and appearance (<a href="#f79">Fig. 75</a><i>C</i>). But they<span class="pagenum"><a id="Page_309"></a>[Pg 309]</span>
-differ in the possession or absence of a dividing apparatus, and in
-the great majority of cases it is the male nucleus that brings with it
-the central corpuscle which seems to be indispensable to embryonic
-development (<a href="#f79"><i>B</i></a>, <i>cspt</i>). Hitherto, at least, only two exceptions to this
-are known. In the little segmented worm, <i>Myzostoma</i>, which is
-parasitic on sea-lilies or Crinoids, Wheeler observed that the ovum
-retained its central corpuscle even after the polar divisions, while the
-sperm-cell which penetrated into the egg had none. More recently
-Conklin made the interesting discovery that in the egg of a marine
-Gasteropod (<i>Crepidula</i>) both the egg-nucleus and the sperm-nucleus
-retain their centrosphere and together form the segmentation spindle,
-one lying at one pole and the other at the opposite.</p>
-
-<p>All these observations confirm the view that the sperm and the
-egg-cell are alike in this respect also. Each of them can, in certain
-circumstances, bring with it the dividing apparatus indispensable to
-development, though it is usually the sperm-cell that does so.</p>
-
-<p>I should indeed assume that the sperm-cell and the egg-cell were
-essentially alike, even although there were no exception to this rule,
-that is, although the centrosome of the ovum perished in all eggs
-which were fertilized. For this is obviously a secondary arrangement,
-an adaptation to fertilization, that the ovum should be incapable
-of development without fertilization, and it is made so by the disappearance
-of its centrosome. In all other cells, as far as is known,
-the central corpuscle persists after division, so that this remarkable
-cell-organ is transmitted from cell to cell just like the nucleus, and
-like it, never rises <i>de novo</i>. It is only in the egg-cell that it disappears,
-though even there often very late, for it may be present, as
-an aster, even after the sperm has penetrated into the ovum and
-disclosed its own central body, or even brought it the length of
-dividing into two (Fig. 80, <i>A</i> and <i>B</i>). But the ovum-centrosome
-disappears as soon as the second polar division is accomplished.</p>
-
-<p>That this disappearance is really a secondary arrangement, which
-may be again departed from, is proved by the case of those eggs
-which are able to develop parthenogenetically, for in them the central
-body does not disappear, but persists in the ovum after the first polar
-division, as Brauer showed in <i>Artemia</i>. It then behaves exactly like
-the sphere of the sperm-nucleus in the fertilized ovum, that is, it
-duplicates itself and forms the segmentation spindle.</p>
-
-<div class="figcenter" id="f84">
-<img src="images/fig84.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 80.</span> Fertilization of the ovum of a Gasteropod (<i>Physa</i>), after Kostanecki and
-Wierzejski. <i>A</i>, the whole spermatozoon lies in the ovum. <i>sp</i>, its already divided
-centrosphere. <i>Rk 1</i>, the first polar body. <i>Rsp 2</i>, the second directive spindle. <i>B</i>, <i>spk</i>,
-the sperm-nucleus, the second directive spindle still has its centrosphere, which
-afterwards disappears. The first polar body (<i>Rk 1</i>) has divided into two. Highly
-magnified.</p>
-</div>
-
-<p>Thus the beginning of embryonic development in the ovum
-depends not on a definite number of chromosomes, but on the presence
-of an apparatus for division. Upon what the awakening of this to
-activity just at that time depends cannot as yet be exactly stated; we
-<span class="pagenum"><a id="Page_310"></a>[Pg 310]</span>can only indicate that all parts of the cell have interrelations with
-each other, and that, therefore, the division mechanism is dependent on
-the condition of the rest of the cell-parts at the moment, and on the
-substances which they contain or produce. From what we know
-experimentally in regard to artificial parthenogenesis it is not difficult
-to imagine that some sort of chemical substances are necessary to
-stimulate the central corpuscle to activity. In any case, the whole
-nutrition of the central corpuscle depends on the cell in which it lies,
-as is shown by the fact that the sperm-nucleus, whose centrosome
-before the entrance of the sperm into the ovum was inactive and
-scarcely recognizable, grows rapidly after entrance and forms a large
-aster round itself&mdash;is, in short, in the highest degree active (Fig. 80).
-As the chromosomes certainly play an important part in the life of the
-cell, and materially help to determine its various phases, it cannot be
-disputed that they also may share in awakening the activity of the
-central corpuscle. But this influence is only indirect; it is not the
-mere number of chromosomes that decides whether the central
-corpuscle is to become active or remain inactive. This cannot be
-assumed, because we have in the maturation divisions a proof that<span class="pagenum"><a id="Page_311"></a>[Pg 311]</span>
-division may take place with a double number of chromosomes as
-well as with the undoubled number; while in the divisions of the
-mother-egg-cells and the mother-sperm-cells we have proof that
-a doubled number of chromosomes does not in itself compel to
-division.</p>
-
-<p>The exceptional and artificially produced cases of parthenogenesis
-which we have discussed above are probably to be interpreted thus:
-through slight differences in the constitution of the ovum, or through
-certain mechanical or chemical stimuli, the metabolic processes in the
-ovum are so altered that the centrosome of the ovum, instead of
-breaking up, is stimulated to growth, and thus produces the active
-dividing apparatus which is otherwise only brought into it by the
-sperm. This is a more exact definition of the interpretation I gave
-earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which
-was then the only case known, when I said 'the nucleoplasm of some
-ova must possess the power of growth in a greater degree than the
-majority.'</p>
-
-<p>But we are not yet in a position to go further, or to define more
-exactly the nature of the processes of metabolism which are involved.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_312"></a>[Pg 312]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XVI">LECTURE XVI</h2>
-</div>
-
-<p class="c">FERTILIZATION IN PLANTS AND UNICELLULAR<br />
-ORGANISMS, AND ITS IMMEDIATE<br />
-SIGNIFICANCE</p>
-
-<div class="blockquot">
-
-<p>Fertilization in a lichen, Basidiobolus&mdash;In Phanerogams&mdash;Here too there is reduction
-of the number of chromosomes by a half&mdash;'Polar cells' in lower and higher plants&mdash;Conjugation
-among unicellular organisms&mdash;Noctiluca&mdash;The maternal and paternal
-chromosomes remain apart&mdash;Actinophrys&mdash;Infusoria&mdash;Sexual differentiation of the
-two conjugates in Vorticella&mdash;Importance of the process of Amphimixis&mdash;Not a 'life-awakening'
-process&mdash;May occur independently of multiplication&mdash;The Rejuvenescence
-hypothesis&mdash;Pure parthenogenesis&mdash;The cycle idea&mdash;Does Amphimixis prevent natural
-death?&mdash;Maupas' experiments with Infusorians&mdash;Bütschli's view&mdash;Potential immortality
-of unicellular organisms&mdash;The immortality of unicellular organisms and of the
-germ-cells depends on the fact that there is no time-limit to the multiplication of the
-smallest living particles&mdash;Parthenogenesis is not self-fertilization&mdash;Petrunkewitsch's
-observations on the ova of bees&mdash;Is the chromatin really the 'hereditary substance'?&mdash;Nägeli's
-conclusion from the difference in size between ovum and spermatozoon&mdash;Artificial
-division of Infusorians&mdash;Boveri's experiments with the fertilization of pieces
-of ova not containing a nucleus&mdash;Fertilization gives an impulse to development even
-to non-nucleated pieces of ova&mdash;Merogony&mdash;The female and male nuclear substances
-are essentially alike&mdash;Summary.</p></div>
-
-
-<p><span class="smcap">I now</span> turn to the consideration of the process of fertilization in
-plants and unicellular organisms.</p>
-
-<p>With regard to plants, it can now be definitely asserted that in
-them, too, fertilization is essentially a conjugation of nuclei; it
-depends on the union of the nuclei of the two 'sex-cells.' These
-sex-cells are usually very small among lower plants, indeed up to the
-phanerogams; this is especially true of the zoosperm-like male germ-cells,
-but it usually holds also true of the ovum, which is but seldom
-burdened with an abundant supply of yolk. In spite of the many
-difficulties which this smallness of size puts in the way of observation,
-the untiring exertions of a host of excellent investigators have
-succeeded in following the process of fertilization in all the larger
-groups of plants&mdash;in algæ, fungi, mosses, ferns, and horse-tails
-among cryptogams, and in phanerogams.</p>
-
-<p>I shall first give an example from among the lower plants
-(Fig. 81). In one of the lichens, <i>Basidiobolus ranarum</i>, each of two
-adjacent cells in the fungus-thread gives off a bill-like process, and the<span class="pagenum"><a id="Page_313"></a>[Pg 313]</span>
-two processes become closely apposed (Fig. 81, <i>a</i>). The nucleus of
-each cell moves into the bill-shaped process, is there transformed into
-a nuclear spindle (<i>B</i>, <i>ksp</i>) and divides, so that one daughter-nucleus
-comes to lie in the apex point of the bill, the other at the base. The
-cell-body also divides, though very unequally, and the final outcome
-of the process is two cells in each, of which one is small and occupies
-the apex of the bill, while the other is large and fills all the rest of
-the cell-space. The former do not play any further part of importance,
-but break up, the latter are the sex-cells, the cytoplasm of which now
-coalesces through a gap in the cell-walls, while their nuclei become
-closely apposed and ultimately unite (<i>C</i>, ♂ and ♀ <i>k</i>). From this union
-arises the fertilized spore, the so-called 'zygote' (<i>D</i>). The two small
-abortive cells so greatly resemble in their origin the polar cells of the
-animal ovum that it is difficult to resist the supposition that they
-bring about a reduction in the number of chromosomes. But the
-number of the chromosomes has not yet been determined either in
-them or in the sex-nuclei.</p>
-
-<div class="figcenter" id="f85">
-<img src="images/fig85.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 81.</span> Formation of polar bodies in a lichen, <i>Basidiobolus ranarum</i>. <i>A</i>, the two
-conjugating cells with the bill-like processes in which the nuclei lie. <i>B</i>, the nuclei
-dividing. <i>ksp</i>, the nuclear spindle. <i>C</i>, after the division into a polar body (<i>rk</i>) and
-a sex-nucleus (♂ <i>k</i> and ♀ <i>k</i>). <i>D</i>, after the union of the nuclei to form a conjugation
-nucleus (<i>copk</i>); the fertilized ovum is surrounded by envelopes and modified into
-a lasting spore. After Fairchild.</p>
-</div>
-
-<p>We have come to know the processes of fertilization among
-phanerogams chiefly through Strasburger, Guignard, and more
-recently through the Japanese botanist Hirase. The agreement with
-the animal process is surprisingly great, notwithstanding the notable
-differences in the external conditions of fertilization.</p>
-
-<p><span class="pagenum"><a id="Page_314"></a>[Pg 314]</span></p>
-
-<p>As is well known, the male cells in the highest flowering plants
-are not zoosperms but roundish cells, each of which, enclosed,
-together with a sister-cell&mdash;the so-called 'vegetative' cell&mdash;in a thick
-cellulose capsule constitutes a pollen-grain. The pollen-grains reach
-the stigma, under which, buried deep within the 'ovule,' the female
-sex-cell rests, enclosed in a long, sac-like structure called the 'embryo-sac'
-(Fig. 82, <i>A</i>). Beside it (<i>eiz</i>) there lie several other cells, usually
-seven in number, two of which, the so-called 'synergidæ' (<i>sy</i>), have
-their place at one end of the embryo-sac, just in front of the
-ovum (<i>eiz</i>). Probably these give off a secretion which exercises an
-attractive (chemotactic) influence on the male fertilizing body ('the
-pollen-tube'), and thus, so to speak, show it the way to the ovum.</p>
-
-<div class="figcenter" id="f86">
-<img src="images/fig86.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 82.</span> Fertilization in the Lily, <i>Lilium martagon</i>, after Guignard. <i>A</i>, the embryo-sac
-before fertilization; <i>sy</i>, synergidæ; <i>eiz</i>, ovum; <i>op</i> and <i>up</i>, upper and lower 'polar
-nuclei'; <i>ap</i>, antipodal cells. <i>B</i>, the upper part of the embryo-sac, into which the pollen-tube
-(<i>pschl</i>) has penetrated with the male sex-nucleus (♂<i>k</i>) and its centrosphere; below
-that is the ovum with its (also doubled) centrosphere (<i>csph</i>). <i>C</i>, remains of the pollen-tube
-(<i>pschl</i>); the two sex-nuclei are closely apposed. Highly magnified.</p>
-</div>
-
-<p>When a pollen-grain has reached the stigma it sends out a tube,
-usually after a few hours, which penetrates into the soft tissue of the
-style, and grows deep down into the interior of the ovule, ultimately
-penetrating as far as the embryo-sac through a special little opening
-in the covering of the ovule, the so-called 'micropyle' (Fig. 82<i>B</i>, <i>pschl</i>).
-Its blunt end is now closely apposed to this, so that the true sperm-nucleus
-(<i>B</i>, ♂<i>k</i>), surrounded by some protoplasm, can leave the
-pollen-tube and wander in among the cells of the embryo-sac. Later
-on we shall see that two generative nuclei migrate from the pollen-tube,
-but in the meantime we shall devote our attention only to one<span class="pagenum"><a id="Page_315"></a>[Pg 315]</span>
-of them, the fertilizing nucleus, which immediately moves towards
-the ovum-nucleus and apposes itself closely to it. Then follows the
-fusion or conjugation of the two nuclei, which are alike in size and
-appearance, just as in the fertilization of the animal ovum (<i>C</i>, ♂ <i>k</i> and
-♀ <i>k</i>). Whether in this case, too, the sperm-nucleus brings with it
-a central corpuscle, or whether, as Guignard believed he observed, the
-ovum retains its central corpuscle (<i>C</i>, <i>csph</i>), or finally, whether both
-modes occur, is not yet known with certainty. The fact that, as
-a rule, seeds capable of reproduction only form in an ovule when the
-stigma has been previously dusted with pollen, leads us to suppose
-that, in this case, as among animals, the ovum lacks something that
-is necessary to induce embryonic development, only retaining this
-power in very exceptional cases, namely, when adapted for parthenogenesis.
-And this something may very well be the dividing apparatus
-of the cell, the centrosome with the centrosphere. But whether this
-supposition prove correct or not, a nuclear spindle always forms
-simultaneously with the fusion of the two sex-nuclei into a segmentation
-nucleus, and this spindle is the starting-point of the young plant,
-thus exactly corresponding to the first segmentation of the animal
-ovum. It agrees with it also in the important respect that it again
-contains the full number of chromosomes&mdash;twenty-four in the lily&mdash;while
-the two nuclei, male and female, only exhibit half the number
-each, that is, twelve.</p>
-
-<p>Thus a reduction in the number of chromosomes to half takes
-place in plants also, but it is not yet known with certainty whether
-this is brought about in the same way as among animals, namely, by
-reducing divisions. Without entering more fully into this still
-unsolved and very complex problem, I should like to state that
-I consider this very probable; indeed, I agree with the view of
-V. Häcker<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a>, that the reducing divisions of plants are only more
-difficult to recognize as such, and, furthermore, are often disguised by
-the fact that they often occur alongside of, or between divisions which
-are not reducing. If it were possible to reduce the number of chromosomes
-in a cell to half without the aid of cell-division, if, for instance,
-only half were to integrate again from the chromatin-network, this
-must have been quite as possible in the case of animal cells, and then,
-moreover, the single chromosome would not have had the significance
-of an individuality, and no special form of nuclear division would
-have been introduced to reduce their number. That it has been
-introduced seems to me to prove that it was necessary, and since it<span class="pagenum"><a id="Page_316"></a>[Pg 316]</span>
-was so among animals, it could not have been dispensed with among
-plants either.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> See V. Häcker, <i>Praxis und Theorie der Zellen- und Befruchtungslehre</i>, Jena, 1899, pp. 144-5.</p>
-
-</div>
-
-<p>Moreover, throughout the vegetable kingdom divisions often
-occur in connexion with the origin of the sex-cells which can be
-compared, in occurrence and result, with the maturation divisions of
-animal germ-cells. In the lichen, <i>Basidiobolus</i>, we have already seen
-that an abortive cell separates itself off from the sex-cell before the
-latter becomes capable of reproduction (<a href="#f85">Fig. 81</a>, <i>C</i>). Similar cell-divisions
-occur in many if not in all groups of plants. In the marine
-algæ of the genus <i>Fucus</i> it has even been proved that the division of
-the first primordial cell of the ovary into the so-called 'stalk-cell'
-and the primitive egg-cell is a reducing division, and brings down the
-number of chromosomes from thirty-two to sixteen. In vascular
-plants the reduction is not postponed until the formation of the sex-cells,
-but occurs earlier in the formation of the spores, as Calkins has
-demonstrated for ferns; in the Conifers and other Gymnosperms
-several so-called 'preparatory' divisions precede the formation of the
-germ-cells, and we know by comparison with the alternation of
-generations in vascular plants that these are related to the gradual
-waning of the strictly sexual generation. As the 'polar bodies' or
-'directive corpuscles' of the animal ovum are rudimentary egg-cells,
-so the cells which, in the pollen-grains, separate themselves from the
-sex-cells proper are rudimentary Prothallium-cells, and, like the
-animal cells, they perish without playing any further physiological
-rôle. I will not assert that it is precisely in these divisions that the
-reducing divisions are concealed, for the analogy with the spore-formation
-of ferns leads us rather to suppose that it may lie further
-back; but in any case there is no lack of opportunity in the ontogeny of
-phanerogamic plants for the interpolation of a reducing division, and
-as long as it remains unproved that a reduction of the chromosomes
-can take place directly, that is, without the help of nuclear division,
-we shall continue to expect with confidence that the reducing divisions
-of phanerogams will be discovered in the future. Processes of a
-similar kind are known among unicellular organisms, and there, too,
-they are associated with nuclear divisions.</p>
-
-<p>In passing to the so-called 'sexual reproduction' of unicellular
-organisms, I should like first to call attention to the fact that the
-expression 'reproduction' is not very suitable in this case, for the
-process in question does not always effect an increase in the number
-of individuals as reproduction ought to do, but leads, in fact, in many
-cases, even to a decrease, when two individuals unite to form one.
-Even if the phenomena of sexual 'reproduction' among higher<span class="pagenum"><a id="Page_317"></a>[Pg 317]</span>
-organisms, which we have already studied, had not made it clear to us
-that there are two associated processes, quite different in nature, the
-conjugation of unicellular organisms would have led us to that
-conclusion. It has long been known that two unicellular plants or
-animals occasionally become closely apposed and fuse; and this process
-of 'conjugation' was many years ago regarded as an analogue to
-'fertilization,' although it is only through the laborious investigations of
-the last two or three decades that this supposition has been proved to be
-correct. We now know that a process quite analogous to that which
-we have learnt to know
-as 'fertilization' takes
-place among unicellulars,
-only in this case it is not
-directly connected with
-reproduction and multiplication,
-but occurs independently
-of them,
-and, in its most primitive
-form, it results, not in
-an increase but&mdash;for a
-short time at least&mdash;in
-a diminution of the number
-of individuals. This
-occurrence of the process
-independently of reproduction
-appears to me
-of inestimable value theoretically,
-for it frees us
-completely from the old
-deep-rooted preconceptions
-in the interpretation of fertilization.</p>
-
-<div class="figright" id="f87">
-<img src="images/fig87.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 83.</span> Conjugation of Noctiluca, after Ischikawa.<br />
-<i>A</i>, two Noctilucas beginning to coalesce; <i>pr</i>, the protoplasm<br />
-drawn out into processes which traverse the<br />
-gelatinous substance of the cell; <i>k</i>, the nucleus.<br />
-<i>B</i>, the cells and their gelatinous substance have fused;<br />
-the nuclei, in which the chromosomes are visible, are<br />
-closely apposed; <i>CK</i>, centrospheres. <i>C</i>, the two nuclei<br />
-are united in one nuclear spindle; beginning of<br />
-division. <i>D</i>, completion of the division. Highly<br />
-magnified.</p>
-</div>
-
-<p>First let us briefly sketch the process itself in the main forms of
-its occurrence.</p>
-
-<p>The most primitive form of conjugation is undoubtedly the complete
-fusion of two unicellular organisms of the same species, as
-we see it to-day in unicellular plants, and also among the lowest
-unicellular animals, such as the flagellate Infusorians, Gregarines, and
-Rhizopods. It is well seen, for instance, in the Noctilucæ, those
-unicellular flagellate organisms which cause the familiar marine phosphorescence
-extending uniformly over wide surfaces of water (Fig. 83).
-In these forms Prof. Ischikawa of Tokio was able to trace the whole
-process of conjugation. To begin with, two Noctilucas range them<span class="pagenum"><a id="Page_318"></a>[Pg 318]</span>selves
-side by side (Fig. 83) and coalesce at the surfaces in contact,
-both as to the spherical gelatinous envelope (<i>A</i>, <i>G</i>) and the protoplasm
-(<i>pr</i>) itself, which branches in amœboid fashion into the jelly.
-The union becomes gradually complete, and the two animals form
-a single sphere (<i>B</i>) with one cell-body. But the two nuclei (<i>K</i>) also
-place themselves side by side (<i>B</i>), and though they do not actually
-fuse, they form together, under the guidance of two centrospheres (<i>C</i>),
-a single nuclear division-figure, which is obviously analogous to the
-segmentation spindle of the fertilized egg. Then follows a division,
-by means of which the chromatin substance of the nuclei of both
-animals is divided between the two daughter-nuclei, and after this
-has been accomplished the united individual again separates into two
-independent Noctilucas (<i>D</i>). Although I have spoken here&mdash;that is,
-in referring to the Protozoa&mdash;of chromosomes, I must immediately
-add that these have not yet been seen with full clearness in Noctiluca
-itself; nothing more has been recognized than deeply staining thickenings
-of the spindle fibrils, which move from the equator of the
-nuclear spindle towards the pole. Since, however, in other Protozoa,
-as, for instance, in the beautiful freshwater Rhizopod (<i>Euglypha
-alveolata</i>), these thickenings of the nuclear spindle fibrils have been
-clearly recognized as chromosomes, doubt on this point is hardly
-justifiable. Apart from this, the assumption that each of the two
-daughter-nuclei receives half the chromosomes of each of the conjugated
-nuclei rests on a secure basis, not only because otherwise the
-whole process would have no meaning, but because the position of the
-mitotic figure conditions this. Even the fact that the two conjugation-nuclei
-lying side by side remain apart during nuclear division is not
-without parallel; Häcker and Rückert observed it also in the segmentation-nucleus
-of much higher animals, the Copepods, and it has
-no effect in altering the process of division, but only proves that the
-chromosomes of maternal and those of paternal origin in the combination-nucleus
-remain independent&mdash;a fact the significance of which
-I shall discuss later on.</p>
-
-<p>The process of conjugation occurs, in the same manner as in
-<i>Noctiluca</i>, in a freshwater Rhizopod, the well-known Sun-animalcule,
-<i>Actinophrys sol</i> (Fig. 84), but in this case complete fusion of the two
-nuclei takes place (Fig. 84, <i>V</i>) before the formation of the division-spindle
-(<i>VI</i>, <i>sp</i>), which, with the simultaneous division of the cell-body,
-gives rise to two new individuals. The process in this case is
-especially interesting, because Schaudinn has succeeded in observing
-a maturation division (<i>III</i>, <i>Rsp</i>, directive spindle) as well as in
-demonstrating polar bodies (<i>IV</i>, <i>Rk</i>). Thus the analogy with the<span class="pagenum"><a id="Page_319"></a>[Pg 319]</span>
-process of fertilization in the Metazoa and the Metaphyta is almost
-complete.</p>
-
-<p>But that the conjugation of unicellular organisms, like the
-fertilization of multicellular organisms, is essentially a matter of
-nuclear conjugation is shown more distinctly still by the ciliated
-Infusorians, the most highly organized of the Protozoa.</p>
-
-<div class="figcenter" id="f88">
-<img src="images/fig88.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 84.</span> Conjugation and polar body formation in the Sun-animalcule, <i>Actinophrys
-sol</i>, after Schaudinn. <i>I</i>, two free-swimming conjugated individuals, which in <i>II</i> have
-become surrounded by a transparent gelatinous cyst. <i>III</i>, formation of the directive
-spindles (<i>RSp</i>). <i>IV</i>, the polar bodies are formed (<i>RK</i>); <i>K</i>, the two sex-nuclei. <i>V</i>, these
-are fused to form the conjugation-nucleus (<i>K</i>). <i>VI</i>, the conjugation-nucleus is transformed
-into the division-spindle; the polar bodies (<i>RK</i>) have penetrated the internal
-cyst-wall, and are in process of degeneration.</p>
-</div>
-
-<p>Here there is usually no complete union of the cell-bodies of the
-two animals, but only an adhering of the apposed surfaces. In the
-relatively large <i>Paramœcium caudatum</i> the process of conjugation
-is very exactly known through the beautiful investigations of Maupas
-and R. Hertwig. In this case the mouth-surfaces of the two animals
-come together and unite over a short area, and then the two animals
-swim about together in this conjugated state. During this time very
-remarkable changes take place in their nuclei.</p>
-
-<p>It is well known that these Infusorians have a double nucleus,
-a large one, the macronucleus (<a href="#f89">Fig. 85</a>, <i>ma</i>), and one which is usually
-very small, the micronucleus (<i>mi</i>). We may ascribe to the former
-of these the guidance and regulation of the everyday processes of life,
-that is, briefly, of metabolism, and the preservation of the integrity
-of the whole animal. The small nucleus has often been designated
-the 'reproductive nucleus,' but as it plays no other part in repro<span class="pagenum"><a id="Page_320"></a>[Pg 320]</span>duction,
-as far as can be recognized, than that of dividing into two
-daughter-nuclei, I cannot regard this designation as suitable; it
-obviously originated in the mistaken interpretation, prevalent till
-very lately, of conjugation as a 'kind of reproduction,' and this in
-its turn depends on the conception, transferred from multicellular
-organisms, of fertilization as a 'sexual reproduction.' We shall
-immediately see that the micronucleus plays the main part in conjugation,
-and from this we may suppose that it otherwise fills no rôle
-in the life of the animal, and therefore it may best be designated the
-'supplementary' or reserve nucleus. In every conjugation the macronucleus,
-which has hitherto been active, breaks up and becomes
-completely absorbed, very much like a ball of food. This of course
-takes place slowly; the large nucleus elongates, becomes indented,
-falls into several pieces, and these are so gradually absorbed that,
-even after the act of conjugation has been accomplished, irregular
-fragments of the macronucleus often lie about in the animal
-(<a href="#f89">Fig. 85</a>, 9).</p>
-
-<p>But while the macronucleus falls to pieces the previously minute
-micronucleus grows enormously and forms a distinct longitudinally
-striated spindle (1, <i>mi</i>). About the same time these divide in both
-animals, and each of the daughter-nuclei immediately divides again,
-so that after these two divisions four spindle-shaped descendants of
-the micronucleus are to be seen in each animal (<a href="#f89">Fig. 85</a>, 4). We
-have previously noted that the apparatus for nuclear division in
-unicellular organisms was similar to that in multicellular organisms,
-and yet was different from it. In these ciliated infusorians we see
-an essential difference, for the striated spindle, after the division into
-daughter-chromosomes has taken place, lengthens out enormously,
-and becomes so thin in the middle of its length (2) that the two
-daughter-nuclei at the ends of this long stalk suggest the appearance
-of a very long and thin dumb-bell, or of a long silk purse. Of asters
-(centrospheres) there is nothing to be seen, and the mechanism of
-division is still very obscure; it almost seems as if a rapidly growing
-substance forced the two groups of chromosomes apart.</p>
-
-<p>Hardly have these four descendants of the micronucleus arisen
-when three of them begin to break up and very shortly disappear;
-only the fourth is of any further importance, and it divides once
-more (5), and so gives rise to the two nuclei which play the chief part
-in the process of conjugation&mdash;the copulation-nuclei, exactly analogous
-to the male and female pronuclei in the fertilized ovum (5, <i>mi</i><sup>4</sup>).
-But in this case each of the two animals functions doubly, that is,
-both as male and female, for each sends one of the two copulation-nuclei
-<span class="pagenum"><a id="Page_321"></a>[Pg 321]<br /><a id="Page_322"></a>[Pg 322]</span>across the bridge formed by the union of the apposed surfaces
-into the other animal (6, <i>mi</i> ♂), so that it may form, by union with
-the nucleus which has remained there, a double nucleus (7), a structure
-which corresponds to the segmentation nucleus of the ovum (<i>copk</i>).
-From it there then arises by division a new macronucleus and a new
-micronucleus, not usually directly, however, that is, not by a single
-division, but through several successive nuclear divisions, into the
-meaning of which I cannot here enter. Immediately after the union
-of the two sex-nuclei the two animals sever their connexion with each
-other; each begins again to feed, and is subject to multiplication by
-division just as it was before conjugation took place (8 and 9).</p>
-
-<div class="figcenter" id="f89">
-<a id="fig89" href="images/fig89big.jpg">
-<img src="images/fig89.jpg" alt=""/></a>
-
-<p class="caption"><span class="smcap">Fig. 85.</span> Diagram of the conjugation of an Infusorian, <i>Paramœcium</i>, after R. Hertwig and Maupas. 1, two animals with the mouth-openings
-apposed; <i>ma</i>, the macronucleus beginning to degenerate; <i>mi</i><sup>1</sup>, the micronucleus has already increased considerably in size and
-is beginning to divide. 2. each micronucleus has divided into two daughter-nuclei (<i>mi</i><sup>2</sup>), which are connected only by the division-strand
-(<i>ts</i>). 3, to the left each of the daughter-micronuclei (<i>mi</i><sup>2</sup>) is beginning to divide; to the right this division is already completed
-and the grand-daughter-nuclei of the original micronucleus hang together by their division-strands (<i>ts</i>). 4, in each of the animals there
-are now four grand-daughter-micronuclei (<i>mi</i><sup>3</sup>). 5, three of these are in process of dissolution, the fourth is dividing into two great-grand-daughter-nuclei
-(<i>mi</i><sup>4</sup>), which are the two sex-nuclei. 6, one (the male) sex-nucleus (<i>mi</i> ♂) migrates into the other animal, and
-there unites with the remaining (female) sex-nucleus. 7, the conjugation-nucleus (<i>copk</i>) being formed. 8, the animals have separated;
-the conjugation-nucleus divides into (9) the new macronucleus (<i>n ma</i>) and the new micronucleus (<i>n mi</i>).</p>
-</div>
-
-<p>Although the course of this remarkable process exhibits all
-manner of differences in detail in different species, it is everywhere
-the same in its essential feature, and this essential feature is undoubtedly
-the union of an equal quantity of the nuclear substance
-of two animals to form a new nucleus. It is thus essentially the
-same process which we have already recognized among higher animals
-as 'fertilization.' The differences are of minor importance, and they
-arise partly from the fact that the sex-cells of multicellular animals
-are not independent self-supporting units, and partly from their
-differentiation into 'male' and 'female' cells. The minuteness of
-the sperm-cell, for instance, conditions its penetration of the ovum,
-which is always much larger and passive, and also the thorough
-fusion of its cell-body with the cell-body of the ovum. That this
-difference has very little deep significance is best seen from the fact
-that, even among Infusorians, there are forms in which the two
-conjugating individuals are quite different, especially in size, and in
-which the much smaller 'male' animal fuses completely with the
-much larger 'female,' and indeed bores its way into it after the
-manner of a sperm-cell. This is the case among the bell-animalcules
-(Vorticellinæ) (<a href="#f90">Fig. 86</a>), the conjugating pairs of which had been
-observed long before our present insight into these processes had
-been attained. Indeed, the facts had been interpreted as a kind of
-'budding process,' the minute and differently shaped 'male' animal
-(<i>mi</i>), which at the time of conjugation is attached to the larger
-'female' (<i>ma</i>), was regarded as its bud. This supposed bud, however,
-does not grow out from the animal, but into it!</p>
-
-<p>Thus we see here again that a differentiation of individuals as
-males and females may occur among unicellular organisms, just as in
-the sex-cells of higher animals and plants, and this proves to us once
-more that all these differences of sex, whether in reproductive cells of
-multicellular organisms, or in the entire multicellular animal or plant,<span class="pagenum"><a id="Page_323"></a>[Pg 323]</span>
-or finally, in unicellular organisms, are not of essential, but only of
-secondary significance, however important they may be for securing
-fertilization or conjugation in each special case. They are always
-only adaptations to the special conditions, and only occur where they
-are necessary to ensure the union, and always in such a manner that
-the union of the two cells is facilitated. In most Infusorians such
-a differentiation into male and female animals was not necessary,
-because these organisms are very motile, and are thus readily able
-to meet and unite; it was therefore sufficient for them to remain
-hermaphrodite. The bell-animalcules, however, are sedentary, and
-for them it was obviously an advantage that, at the time of conjugation,
-smaller, free-swimming, and also more simply organized individuals
-should arise, which were able to seek out the larger sedentary
-forms. Here, then, as in many other unicellular animals, these little
-male individuals only occur when they are necessary, that is, at the
-time of conjugation. Similarly, in the green alga, <i>Volvox</i>, male and
-female cells arise only at the time of conjugation, reproduction being
-at other times effected by means of parthenogonidia, that is, by
-elements which require no fertilization.</p>
-
-<div class="figcenter" id="f90">
-<img src="images/fig90.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 86.</span> Conjugation of an Infusorian. <i>Vorticella nebulifera</i>, showing sexual differentiation
-of the whole organism. After Greef. <i>I</i>, the 'microgonidium' or male
-individual (<i>mi</i>) attaches itself to the 'macrogonidium' or female individual (<i>ma</i>);
-<i>cv</i>, contractile vacuole; <i>st</i>, contractile stalk. <i>II</i>, the ciliated circle on the male individual
-has disappeared. The male has become firmly embedded in the female by means of
-a sucker-like retraction of its lower end. <i>III</i>, the fusion of the two individuals has
-been completed; the bristly residue of the male (<i>ct</i>) is about to be thrown off; the
-stalk (<i>st</i>) is contracted into a spiral. Magnified about 300 times.</p>
-</div>
-
-<p>As these differences are only adaptations to the necessity that
-the animals or cells shall find each other and unite, so also are all the
-other differences of a sexual kind, the thousand-fold differences
-between the sperm-cell and the egg-cell, and the not less numerous
-differences between male and female animals, both in 'primary' and
-especially in the diverse 'secondary' sexual characters which we have<span class="pagenum"><a id="Page_324"></a>[Pg 324]</span>
-previously discussed; all these are only means for bringing about the
-process of the union of two germ-cells to form a fertilized 'ovum'
-which is capable of development. The essential part of this so-called
-'sexual reproduction' does not, however, depend on these differences,
-neither on the sexual differences of the germ-cells nor on those of the
-whole organism; it lies solely in the actual union of the two germ-cells.
-Let us remember the idea we have already emphasized, that
-the <i>essential part</i> of the so-called 'sexual reproduction' does not
-depend on these differences, and let us hold fast to the idea already
-indicated, that the chromosomes of the nucleus are the real bearers
-of the hereditary tendencies; then we see that the mingling, or, better,
-the union of the hereditary substances of two different individuals,
-whether single-celled or many-celled, is the result of the process
-which we have hitherto called fertilization or conjugation, but which
-we shall henceforward designate by the more general term 'Amphimixis'
-which means the mingling of substances contributed from two
-distinct sources.</p>
-
-<p>Having made ourselves acquainted with the phenomena of
-amphimixis in animals, plants, and unicellular organisms, we have to
-face the problem of the significance of this remarkable and complicated
-process. What is it that happens, and what meaning can
-we attach to it?</p>
-
-<p>The first thing to be done is to show that the old and long-prevailing
-conception of fertilization as <i>a life-awakening process</i>
-must be entirely abandoned. That a new individual can arise even
-among highly organized animals, quite independently of fertilization,
-is proved by the parthenogenetic eggs of insects and crustaceans;
-fertilization is not the spark 'which falls into the powder-cask' and
-causes the explosion; it is only an indispensable condition of development.
-As we have seen, there are germ-cells which are not sexually
-differentiated, such as the spores of the lower plants, which are all
-capable of development without amphimixis; and parthenogenetic ova
-prove that even differentiated female germ-cells, that is, germ-cells
-originally adapted for amphimixis, may in certain circumstances
-develop without it; amphimixis is thus not the fundamental cause
-of development, but is only, for many germ-cells, one of the conditions
-which must be fulfilled before development can set in. It
-is a condition which, under certain circumstances, may be dispensed
-with.</p>
-
-<p>If, then, the multiplication of individuals by single-celled germs
-can take place independently of amphimixis, we may conclude that
-the establishment of amphimixis has nothing to do with the capacity<span class="pagenum"><a id="Page_325"></a>[Pg 325]</span>
-for multiplication, that it is not a life-awakening process, but is
-a process of a unique kind, which means something quite different.
-The whole conception of the awakening of life in the germ is antiquated
-and out of harmony with the present state of our knowledge.
-<i>Life never begins anew</i>, as far as we can see, and apart from the
-possibility that, unknown to us, a spontaneous generation (<i>Urzeugung</i>)
-of the lowest forms of life is still taking place, life is continuous and
-consists of an infinite series of living forms between which there is
-no real interruption. Life, in fact, is like a continuous stream, the
-larger and smaller waves of which are particular species and individuals.
-Only a few decennia ago a morphologist, who was rightly
-held in high esteem, could champion the idea that the mature ovum
-of animals was lifeless material, which had to be quickened in order
-to develop, but now such a theory is untenable, since we have
-become aware of the phenomena of maturation in the ovum, and
-know that most important vital processes, the reducing divisions,
-take place at the time of maturation, quite independently of fertilization.</p>
-
-<p>Thus we do not even require to take into account the conjugation
-of unicellular organisms to make it clear that amphimixis is not
-the cause of the origin of new individuals, but a process, <i>sui generis</i>,
-which may indeed be associated with the beginning of embryonic
-development, but which may also occur independently of it, as we
-see in the case of unicellular organisms. If, on the one hand, we see
-development taking place in spores and parthenogenetic ova independently
-of amphimixis, and on the other hand amphimixis
-occurring without reproduction in unicellular organisms, we must
-regard the two phenomena, amphimixis and reproduction, as processes
-of a distinct kind, which may, however, occur in association with and
-interdependence upon each other.</p>
-
-<p>It was by chance that human observation brought the latter fact
-to light first, and therefore led us for so long to accept the idea that
-<i>fertilization</i>, that is, amphimixis, and <i>development</i>, that is, reproduction,
-are one and the same; and thus it happens that even now there
-are many naturalists who cannot rid themselves of the idea that
-amphimixis, if not a life-awakening, is at least a <i>life-renewing</i> process,
-a so-called 'process of rejuvenescence.'</p>
-
-<p>More than ten years ago<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a> I disputed this view, and since then
-the facts which make it untenable have become more and more clear.
-Notwithstanding this I see that it is still adhered to, at least in a<span class="pagenum"><a id="Page_326"></a>[Pg 326]</span>
-modified form, by many esteemed naturalists, and so it does not seem
-superfluous to discuss it in more detail.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> <i>Die Bedeutung der sexuellen Fortpflanzung für die Selektionstheorie</i>, Jena, 1886.</p>
-
-</div>
-
-<p>I have already noted that we see in conjugation an amphimixis
-without reproduction, and in spores and parthenogenetic ova reproduction
-without amphimixis, and I do not doubt that every
-unprejudiced critic will admit this; many among us, however, are
-not unprejudiced, but are under the spell of earlier ideas, so that they
-cannot forget that it was long believed that fertilization was an
-indispensable condition of development; they therefore regard the
-divisions which recommence sooner or later after conjugation, and
-which may be repeated hundreds of times, <i>as conditioned by the
-conjugation which preceded them</i>, and compare them to the series of
-cells which, in the Metazoa, lead from the fertilized ovum to the fully-formed
-animal. They regard both series of cell-generations as a
-developmental cycle, which leads from fertilization to fertilization
-again, from conjugation to conjugation, and which would be impossible
-without either fertilization or conjugation.</p>
-
-<p>This play with the idea of a 'cycle' reminds me vividly of
-similar fantastic play from the time of the much-despised 'Naturphilosophie'
-of a hundred years ago. As men sought to find the
-analogues of 'solar' and 'planetary' systems in animal and plant, and
-believed they had stated something when they compared the motile
-animals to planets and the sedentary plants to the sun (!), so it is now
-imagined that a deeper insight has been gained by the recognition of
-cycles of development. By all means let us regard the development
-of a multicellular organism as cyclic; it returns again to its starting-point,
-but this no more explains the forces which produce the cycle,
-and thus the meaning of fertilization, than a comparison with the
-circling planets explains the causes of locomotion in animals. With
-quite as much reason the cycle of development might be made to start
-from the parthenogenetic ovum, and then the whole conclusion of the
-fanciful cycle idea in regard to the meaning of fertilization falls to
-the ground, for in this case the cycle begins without fertilization.
-Attempts are made to get over this difficulty by showing that in many
-cases parthenogenesis alternates regularly or irregularly with sexual
-reproduction, as in the water-fleas (Daphnids), the Aphides, and so on.
-The mysterious rejuvenating power of amphimixis is supposed to
-suffice for several generations, a purely gratuitous assumption, which
-is also in open contradiction to the facts. For there are species which
-now reproduce exclusively by parthenogenesis, among plants for
-instance, a number of fungi, among animals a few species of
-Crustaceans. Of the latter it can be demonstrated that ages ago they<span class="pagenum"><a id="Page_327"></a>[Pg 327]</span>
-reproduced sexually, for they still possess the sac which serves for
-receiving spermatozoa, but this sac remains empty, for there are now
-no males, at least in any habitat of the species known to us. To this
-set belongs an inhabitant of stagnant water, <i>Limnadia hermanni</i>, a
-species of Crustacean which was found thirty years ago in hundreds,
-all of the female sex, near Strassburg, and also many of the little
-Ostracods (<i>Cypris</i>) which inhabit especially the muddy bottom of our
-pools and marshes. I bred one of these (<i>Cypris reptans</i>) in numerous
-aquaria for sixteen years, during which there were about eighty
-generations, and throughout this time no male ever appeared, nor did
-the sperm-sac of the female ever contain spermatozoa. The after-effects
-of the 'rejuvenating' power of an amphimixis supposed to have
-taken place earlier must in this case have been enduring indeed!</p>
-
-<p>For these reasons it seems to me useless to make comparisons
-between the developmental cycle of unicellular organisms and the
-ontogeny of multicellular organisms. Both processes have indeed
-many points of resemblance&mdash;long series of cells, then interruption of
-the divisions and the occurrence of amphimixis&mdash;so that we may quite
-well speak of cyclic development in the physiological sense, in as
-far as certain internal conditions periodically recur and compel the
-organism to conjugation, but we must not suppose that there is more
-in this than, for instance, in the 'cyclic development' of Man, which
-consists in the fact that he finds himself periodically impelled to take
-food. The feeling of hunger which forces him to do so is the signal
-which warns the organism that it is time to supply fresh combustible
-material to the metabolism. In the same way, after a long series
-of generations of Infusorians the necessity for conjugation arises;
-the whole colony suffers an 'epidemic of conjugation,' and the animals
-unite in pairs; in the meantime we know not why, and must content
-ourselves with formulating what is observable, that <i>the nuclear
-substances of two individuals are thereby mingled in each conjugate</i>.</p>
-
-<p>Obviously the impulse to conjugation is a signal in the same
-sense as the feeling of hunger is, and we know well from the higher
-animals what a mighty influence it exerts, an influence hardly less
-potent than that of hunger. In Schiller's words, 'Durch Hunger
-und durch Liebe, erhält sich dies Weltgetriebe.'</p>
-
-<p>We can see clearly enough why Nature should have given
-animals the feeling of hunger, but the reason for the need of conjugation
-is not so plain; we can only say in the meantime that it
-must be of some value in maintaining the forms of life, for only that
-which fulfils a purpose can be permanently established.</p>
-
-<p>I shall return later to the problem of the meaning of 'sexual<span class="pagenum"><a id="Page_328"></a>[Pg 328]</span>
-reproduction,' and try to probe more deeply into the meaning of its
-establishment; in the meantime I must restrict myself to having
-shown its significance in the union of the hereditary substances of
-two individuals, and at the same time to controverting the theory
-of the 'rejuvenating power' of amphimixis. I use this expression in
-its original sense, which indicates that every life is gradually wearing
-itself away and would become extinct were it not fanned to flame
-again by amphimixis&mdash;by an artifice of Nature, we may say. This
-conception rests on the fact that the cells of the multicellular body
-possess for the most part only a limited length of life, for they are
-used up by the processes of life, and they break up and die, some
-sooner, some later. As it is observed that all true somatic cells,
-among higher animals at least, are subject to this law of mortality,
-but that the germ-cells are not, and that, furthermore, the germ-cells
-only develop when they are fertilized, the cause of the potential
-immortality of the germ-cells is believed to lie in amphimixis, and
-a 'rejuvenating' power in fertilization, or, more generally, in amphimixis,
-is inferred. Mystical as this sounds, and little as it agrees
-with our otherwise mechanical conceptions of the economy of life,
-it was until very recently a widespread view, although perhaps it is
-now abandoned by many who formerly held it, and has been imperceptibly
-modified into a quite different conception, for which the
-word 'rejuvenescence' is retained, but with the altered meaning of a
-mere 'strengthening of the metabolism' or 'of the constitution.' By
-many authors, indeed, the two meanings of the word are not clearly
-kept apart. I shall return later to the modified meaning of the word
-'rejuvenescence,' and shall keep in the meantime to the original
-meaning of the word, which implies a renewal of life which would
-otherwise die out.</p>
-
-<p>This meaning seemed to gain a firm hold, when, about fifteen years
-ago, the French investigator Maupas published his remarkable observations
-on the conjugation of Infusorians. These seemed to show
-that colonies of Infusorians which were artificially prevented from
-conjugating gradually died out; not of course at once, but after
-many, often several hundred, generations; ultimately a degeneration
-of all the animals in such colonies set in, and ended only with their
-utter extinction. Maupas himself interpreted this as <i>a senile degeneration</i>
-which took place because conjugation had been prevented,
-and he therefore regarded conjugation as a '<i>rajeunissement karyogamique</i>,'
-a rejuvenescence, and therefore a means of preventing the
-ageing and final dying off of the individuals&mdash;of obviating, in short,
-the natural death to which in his opinion they would otherwise be<span class="pagenum"><a id="Page_329"></a>[Pg 329]</span>
-subject. This conception was greeted with general approval, and
-there are many people who still regard conjugation as a process by
-which the capacity for life is renewed&mdash;a view which I must still
-dispute as emphatically as I did some years ago.</p>
-
-<p>In the first place, the observations on which this theory is based
-admit of another interpretation, quite different from that which has
-been assumed to be the only possible one. Maupas prevented conjugation,
-not perhaps because he had isolated individuals and their
-progeny, but by exposing the whole colony of near relatives to
-unusual conditions when conjugation was just about to set in, namely,
-by supplying them with particularly abundant food. The need for
-conjugation then disappeared, as, conversely, it could be called forth
-at any time in a colony by hunger. But these are artificial conditions,
-and indeed the breeding of Infusorians for months in a small quantity
-of water on the object-glass certainly does not correspond to natural
-conditions. We must admire the skill of the investigator who was
-able to keep his colonies alive for months and years under such
-artificial conditions, but we may venture to doubt whether the fate
-of extinction which did ultimately overtake them was really due to
-the absence of conjugation, and not to the unnaturalness of the
-conditions.</p>
-
-<p>In any case a repetition and modification of Maupas' experiments
-is very desirable, and would be of lasting value<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a>.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> Since the above was written Calkins has made a series of new experiments, the
-results of which differed in several respects from those yielded by Maupas' experiments.
-When his infusorian-cultures began to grow weaker, as happened frequently and at
-irregular intervals, he was always able to restore them to more vigorous life by a change
-of diet, and especially by substituting grated meat, liver, and the like for infusions of
-hay. Certain salts, too, had the same effect: the animals became perfectly vigorous
-again. Calkins believes that chemical agents, and especially salts, must be supplied to
-the protoplasm from time to time. He reared 620 generations of Paramœcium
-without conjugation. But the 620th was weakly and without energy. The addition
-of an extract of sheep's brains made them perfectly fresh and vigorous again. Further
-experiments in this direction are to be desired, but, according to those of Calkins, it is
-probable that Infusorians can continue to live for an unlimited time even without
-conjugation.</p>
-
-</div>
-
-<p>Let us, however, assume for the moment not only that Maupas'
-observations were correct, which I do not doubt, but also that they
-were rightly interpreted. Would they in that case afford a proof
-that amphimixis means a rejuvenescence of the power of life? To my
-thinking, not in the remotest degree.</p>
-
-<p>It certainly seems as if this were true at the first glance; the
-colony which is prevented from conjugating goes on multiplying for
-a considerable time, often indeed for hundreds of generations, but this
-may be compared with sufferers from hunger, whose life does not
-cease at once if the feeling of hunger is not appeased.</p>
-
-<p><span class="pagenum"><a id="Page_330"></a>[Pg 330]</span></p>
-
-<p>It was certainly made evident by these experiments that Infusorians
-which were prevented from conjugating were incapable
-of unlimited persistence. But even this in no way proves that
-amphimixis has a power of rejuvenating life, but simply that these
-animals are adapted for conjugation, and that they degenerate without
-it, just as the sperm-cell or the ovum dies if it does not attain to
-amphimixis.</p>
-
-<p>My opponents take it as axiomatic that the life-movement must
-come to a standstill of itself, and that it therefore requires help.
-Even so distinguished a specialist on the Protozoa as Bütschli argues
-that organisms are not <i>perpetua mobilia</i>, and when one remembers
-the physicist's theory of the impossibility of a <i>perpetuum mobile</i> this
-looks at first sight like a formidable objection. But does the organism
-always remain the same as long as it lives, like a pendulum which
-friction causes to swing more and more slowly till ultimately it comes
-to a standstill? We know surely that the phenomena of life arise
-from a continual process of combustion, which is followed by a constant
-replacement of the used-up particles by new particles; we know that
-life depends on an unceasing metabolism, which brings about changes
-in the material basis of the organism every moment, so that it is
-constantly becoming new again.</p>
-
-<p>I shall attempt to show later on that the cells cannot be the
-ultimate elements of the organism, but that the life-units visible with
-the microscope must be made up of smaller invisible units. These,
-therefore, undergo 'metabolism,' which conditions their multiplication
-and their destruction, and this 'metabolism' is not to be seen only in
-the building up and breaking down of 'albuminoid substances,' as the
-physiologists say, but in the alternation between the multiplication
-and the dissolution of these smallest vital particles. Therefore, it
-seems to me that the movement of life, whether in a single-celled or
-in a many-celled organism, is not to be compared to one pendulum,
-but to an endless number of pendulums which succeed one another
-imperceptibly in the course of the metabolism, always producing anew
-the same result, which therefore may continue <i>ad infinitum</i>. Suppose,
-then, that we possessed our present conception of life as a process of
-combustion, and of metabolism as the agency which continually
-provides new combustible material in the shape of new vital particles,
-but that we knew nothing about multicellular organisms and their
-transitory existence, but were acquainted only with unicellular
-organisms and their unlimited multiplication by division. If we were
-then to make the observation that all multicellular organisms are
-mortal, subject to natural and inevitable death, it would at first<span class="pagenum"><a id="Page_331"></a>[Pg 331]</span>
-appear to us quite unintelligible, since we should be aware that in
-these also the fire of life is continually being fed by the supply of
-new combustible material. Not the potential immortality of unicellular
-organisms would then appear to us remarkable and surprising,
-but the limitation of the life of multicellular organisms&mdash;the occurrence
-of natural death. Who knows whether, in that case, many of those
-investigators trained in regard to unicellular organisms alone would
-not say just the opposite of what Bütschli has said, that there could
-be no natural death in many-celled organisms, since single-celled
-organisms prove to us that life is an endless chain of transitory
-minute vital units?</p>
-
-<p>Furthermore, our physiologists are still far from being able to
-explain the natural death of many-celled organisms from below&mdash;I
-mean from a knowledge of its necessary causes; on the contrary,
-they argue from the known occurrence of natural death to the causes
-which underlie it; and thus they have arrived at the idea, undoubtedly
-correct, that the somatic cells of the body are gradually so altered by
-their own activity that they are ultimately unable to function any
-longer and must die off. Therefore, if we were unacquainted with
-death, we should not have been able to infer it from our physiological
-knowledge, and still less from our knowledge of the unicellulars.</p>
-
-<p>As our insight has in point of fact grown by starting from the
-mortal many-celled organisms, and has only later penetrated down to
-the unicellular organisms, so we can understand the genesis of the
-conclusion, deduced from the mortality of the many-celled organisms,
-that unicellular organisms also are unable to continue without limit
-the renewal of material and of vital particles, and that consequently
-they would be subject to natural death if nature had not found in
-conjugation a 'remedy' for 'the physiological difficulties which ensue
-automatically and necessarily from the constitution and from the
-continual functioning' even of unicellular organisms.</p>
-
-<p>But we ask in vain for a shadow of proof of this remarkable
-conception; it is an axiom deduced from our knowledge of natural
-death among multicellular organisms, and bolstered up by a mistaken
-application of the idea of 'perpetual motion.' Or may we regard it
-as a proof of this if it should be found that all unicellular organisms
-are adapted for conjugation?</p>
-
-<p>We shall see later on that amphimixis has certainly quite a
-different and, undoubtedly, a very important effect, namely, that it
-increases the capacity of the species for adaptation; and a life-renewing
-effect in Bütschli's sense could only be ascribed to it in
-addition if the assumption of the necessity of a natural death in<span class="pagenum"><a id="Page_332"></a>[Pg 332]</span>
-unicellular organisms were not directly contrary to the clear facts of
-the case; but this is just what it is.</p>
-
-<p>We are acquainted with such contradictory facts, not perhaps
-among the unicellulars themselves, where it is difficult to procure
-direct proof, but in regard to the germ-cells of many-celled organisms
-which correspond to unicellular organisms. We know that under
-certain circumstances the ovum is capable of persisting by itself&mdash;in
-cases of parthenogenesis&mdash;how then can we conclude that amphimixis
-is in the case of Metazoan germ-cells the cause of their capacity for
-development? We can only conclude, it seems to me, that their
-power of developing is usually bound up with the occurrence of
-amphimixis. So we may conclude in regard to the unicellulars that
-their unlimited power of multiplication is bound up with the occurrence
-of amphimixis, but not that amphimixis is the cause of this
-power, or that it implies a rejuvenescence of life. If unicellular
-organisms could have been made immortal through amphimixis, then
-what I maintain would be proved&mdash;that they possess potential immortality;
-but if they did not possess it, no artifice in the world could
-give it to them; amphimixis could be at most only the condition with
-the fulfilment of which the realization of their immortality was
-bound up.</p>
-
-<p>One may ask, How then can amphimixis be a condition of their
-survival? why should Infusorians which have not conjugated at the
-proper time be doomed to extinction? And from the standpoint of our
-present knowledge I am as little able to give a precise answer as my
-opponents. But I can give one in relation to the amphimixis of
-multicellular organisms, for in regard to these we know that each
-of the germ-cells&mdash;male and female&mdash;uniting in fertilization, is of
-itself incapable of development and doomed to perish, the sperm-cell
-because it is too small in mass to be able to develop the whole
-organism, and the ovum because, in order to become capable of being
-fertilized, it must undergo certain changes which make it incapable of
-independent development. We have seen that after the two maturing
-divisions in the egg-cell have been accomplished the ovum no longer
-contains a mechanism of division, as the centrosphere breaks up after
-the second division; embryonic development can therefore only begin
-when a new centrosphere has been introduced into the ovum, and
-this is normally brought about by fertilization, that is, by the
-entrance of the sperm-cell, whose nucleus is accompanied by a centrosphere.</p>
-
-<p>Thus amphimixis is seen to be really a condition of development.
-But we now know that the ovum can emancipate itself from this condition,
-<span class="pagenum"><a id="Page_333"></a>[Pg 333]</span>by only going through a part of the processes of maturation
-which are related to the subsequent amphimixis, and by thus retaining
-its own centrosome. Nothing is more instructive in this connexion
-than the cases we have already briefly discussed of facultative or
-occasional parthenogenesis. We have seen that in some insects, for
-instance in the silk-moths, there are sometimes, among thousands
-of unfertilized eggs, a few that develop little caterpillars. If we
-examine a large number of such unfertilized eggs we not infrequently
-find among them several which, although they have not gone through
-the whole course of development, have at least gone through the
-earlier stages, and others which may have advanced somewhat further
-and then come to a standstill; in short, we can see that several of
-these eggs were capable of parthenogenetic development, although in
-varying degrees.</p>
-
-<p>The cause of this parthenogenetic capacity has not as yet been
-definitely determined by observation, but we shall hardly go wrong
-if we seek it in the fact that the centrosphere of the ovum does not
-always perish immediately and completely during maturation, and
-may persist, rarely in its integrity, but sometimes in a weakened state.
-Future observations will probably reveal some differences in the size
-or aster-forming power of the centrospheres of such eggs; in any case
-it is of the greatest interest that stimuli of various kinds&mdash;mechanical
-or chemical&mdash;can strengthen the disappearing centrosphere of the
-ovum, although as yet we are far from being able to say how this
-comes about.</p>
-
-<p>The experiments already mentioned of Tichomiroff, Loeb, and
-Winkler give us at least an indication how we must picture to ourselves
-the origin of parthenogenesis, namely, through the fact that
-the breaking up of the apparatus for division, introduced for the sake
-of compelling amphimixis, is prevented. Minute changes in the
-chemistry of the ovum, similar to those caused artificially in the ova
-of the sea-urchin by the introduction of an infinitesimal quantity of
-chloride of magnesium (Loeb), in the ovum of the silk-moth by
-friction or by sulphuric acid (Tichomiroff), or in the sea-urchin ovum
-by an extract of the sperm of the same animal (H. Winkler), will
-effect this modification, and normal parthenogenesis is induced.</p>
-
-<p>For the ovum, therefore, amphimixis is certainly not a life-renewing
-or rejuvenating factor; it only appears as such because
-the process has in the course of nature been made compulsory by
-making the two uniting cells each incapable of developing by itself.
-As we have seen, this is true also of the sperm-cell, for although it
-contains a centrosphere, and would be capable of division as far as<span class="pagenum"><a id="Page_334"></a>[Pg 334]</span>
-that is concerned, yet in almost all animals and plants it consists of
-such a minimal quantity of living matter that it is unable to build up
-a new multicellular organism by itself. Only in one alga (<i>Ectocarpus
-siliculosus</i>) has it been observed that not only the female germ-cell
-can develop parthenogenetically under certain circumstances, but that
-the male-cell may also do so. In this case, however, the difference
-in size between the two is not great, and it is noteworthy that the
-male plant, in correspondence with the smaller size of the zoosperm,
-tends to be a somewhat poorly developed organism.</p>
-
-<p>If we are forced to the conclusion in regard to multicellular
-organisms that amphimixis does not supply the power of development
-to the ovum, but that, on the contrary, the power of development
-is withdrawn from the ovum, so that amphimixis can, so to
-speak, be forced, must we not assume something similar for unicellular
-organisms also? May not amphimixis be made compulsory in their
-case also, in that the Infusorians in preparation for conjugation go
-through changes which make their unlimited persistence possible
-only on condition that they conjugate? In my opinion the division
-of labour in the nucleus, which is differentiated into a macronucleus
-and a micronucleus, and the transitory nature of the former, may
-be regarded as an adaptation in this direction. In any case, it is
-striking that an organ which otherwise persists without limit among
-unicellular organisms, the nucleus, is here subject to natural death
-after the manner of the body of multicellular organisms, that it
-breaks up and must be reformed from the micronucleus which in
-this case is alone endowed with potential immortality. I am inclined
-to regard this as an arrangement for compelling conjugation, since
-it is only after conjugation that the micronucleus forms a new macronucleus,
-although the latter is indispensable to life, as we see from
-experiments in dividing Infusorians artificially.</p>
-
-<p>Suppose we had to create the world of life, and it was said to us
-that amphimixis must&mdash;wherever possible&mdash;be secured periodically to
-all unicellular and multicellular organisms, what better could we do
-than arrange devices which should exclude individuals which, by
-chance or constitution, could not attain to amphimixis from the
-possibility of further life? But would amphimixis then be the cause
-of persistence or a principle of rejuvenescence?</p>
-
-<p>I do not see that there can be any ground for such an assumption
-other than the tenacious and probably usually unconscious adherence
-to the inherited and deep-rooted idea of the dynamic significance of
-'fertilization,' no longer, perhaps in its original form, which regarded
-the sperm as the vital spark which awakened new life in the dead<span class="pagenum"><a id="Page_335"></a>[Pg 335]</span>
-ovum, but in the modified form of the 'rejuvenating' power of
-amphimixis.</p>
-
-<p>Quite recently an attempt has been made to modify the idea of
-the 'rejuvenating' effect of amphimixis so that it should mean only
-an advantage, not an actual condition of persistence. Hartog, in
-particular, admits so much, that the occurrence of purely asexual and
-purely parthenogenetic reproduction excludes the possibility of our
-regarding the process of amphimixis as a condition of the maintenance
-of life. But then we must also cease to regard the 'ageing' and
-dying off of Infusorians which have been prevented from conjugating
-as an outcome of the primary constitution of the living substance, and
-should entirely abandon the misleading expression 'rejuvenescence.'</p>
-
-<p>If we fix our attention on the numberless kinds of cells in higher
-organisms and on multicellular organisms as intact unities, we see
-that they all die off, that they are subject to a natural death, that is,
-a cessation of vital movement from internal causes, yet no one is likely
-to refer their transitoriness to the fact that they do not enter into
-amphimixis. We find it quite 'intelligible' that the cells of our body
-should be used up sooner or later as a result of their own function,
-though we are very far from being able to demonstrate the necessity
-for this, and so really to 'understand' it.</p>
-
-<p>It is only from the standpoint of utility that we can understand
-the occurrence of natural death; we see that the germ-cells <i>must</i> be
-potentially immortal like the unicellular organisms, but that the cells
-which make up the tissues of the body <i>may</i> be transient, and indeed
-<i>must</i> be so in the interests of their differentiation&mdash;often great and in
-one direction&mdash;which determines the services they render to the body.
-They required to become so differentiated that they could not continue
-to live on without limit, and they did become so differentiated
-because only thus could an ever-increasing functional capacity of the
-whole organism be rendered possible; but they die not because 'rejuvenescence
-through amphimixis is denied them, but because their
-physical constitution is what it is.' And we must explain the death
-of the whole many-celled individual in a similar way. When we
-were trying in a previous study to establish the unlimited continuance,
-the potential immortality, of unicellular organisms, we noted that an
-eternal continuance of the life of the body of multicellular organisms
-could certainly not be a necessity, since the continuance of these
-forms of life is secured by their germ-cells. A continuance of the
-body cannot even be regarded as useful from any point of view.
-And what is not useful for a form of life <i>does not arise as a lasting
-adaptation</i>, which is of course not to say that an immortality of<span class="pagenum"><a id="Page_336"></a>[Pg 336]</span>
-multicellular organisms, such as they are now, would even have been
-possible. If these organisms were to attain to such a high degree of
-functional capacity and of structural complexity as they now exhibit,
-they obviously could not also have been adapted at the same time to
-an eternal persistence of life.</p>
-
-<p>This is in perfect harmony with our whole conception of the
-impelling forces in the development of the organic world; the ever-increasing
-functional capacity of the structure arose from the advantage
-which this afforded in the struggle for existence, in comparison
-with which the apparent advantage of the endless life of the individual
-was of no account whatever.</p>
-
-<p>I will not here follow out this idea. I have merely touched on it
-in order to make clear that the death of individuals in all multicellular
-organisms gives us no ground for thinking of the unlimited
-life of the germ-cells as dependent on a special artifice of nature, such
-as amphimixis is often supposed to be. Let us always remember that
-there is parthenogenesis, and that there are unicellular germs (spores)
-which are never fertilized, and that the reproduction of many species
-of animals and plants occurs in this way without the intervention of
-amphimixis at all.</p>
-
-<hr class="tb" />
-
-<p>Attempts have recently been made to prove that parthenogenesis
-is a kind of self-fertilization, and these have been based on the
-observations of Blochmann and Brauer, which showed that in the bee
-and in the salt-water Crustacean, <i>Artemia salina</i>, the reducing second
-maturation division of the ovum-nucleus is not suppressed, but is
-regularly accomplished, and that the two daughter-nuclei which result
-from this division unite with each other subsequently. I have already
-noted that these statements do not hold true, at least with regard
-to the bee. In this case the second maturing division takes place
-without any subsequent fusion of the two daughter-nuclei. According
-to the observations of Dr. Petrunkewitsch, which I have already
-mentioned, and for the exactness of which I can vouch, the second
-maturation-spindle is unusually long, so that the two daughter-nuclei
-are pushed very far apart (<a href="#f83a">Fig. 79</a>, <i>Rsp 2</i>), and only the inner of the
-two nuclei (<i>K 4</i>) becomes a segmentation nucleus, while the outer
-undergoes a remarkable fate; it unites with the inner nucleus which
-results from the division of the <i>first maturation cell</i> (<i>K 2</i>), and from
-this union the primitive <i>genital cells of the animal appear to arise</i>&mdash;an
-observation the eventual theoretical importance of which can only
-be estimated later.</p>
-
-<p><span class="pagenum"><a id="Page_337"></a>[Pg 337]</span></p>
-
-<p>Meantime all we can gain from it is a certain mistrust of the
-interpretation of the processes of maturation in <i>Artemia</i> which have
-hitherto been given; at least we are tempted to suppose that the
-copulation of two nuclei which Brauer observed in <i>Artemia</i> may not
-have led to the formation of the segmentation nucleus there either,
-but may have had some other significance.</p>
-
-<p>But, even if we leave this point entirely out of account, there
-remain all the cases of regular parthenogenesis in which this mode of
-reproduction occurs alone and not in alternation with the sexual mode.
-In these only one maturing division is undergone, and only one polar
-body is formed, and thus
-there can lie no possibility
-of supposing a self-fertilization
-of the ovum.</p>
-
-<div class="figright" id="f83a">
-<img src="images/fig83.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 79.</span> The two maturation divisions in the<br />
-unfertilized (drone-forming) egg of the bee, after<br />
-Petrunkewitsch. <i>Rsp 1</i>, first polar-body in division.<br />
-<i>K 1</i> and <i>K 2</i>, the two daughter-nuclei thereof.<br />
-<i>Rsp 2</i>, second directive spindle. <i>K 3</i> and <i>K 4</i>, the<br />
-two daughter-nuclei thereof. In the subsequent<br />
-stage <i>K 2</i> and <i>K 3</i> unite to form the primordial<br />
-sex-cell nucleus. Highly magnified.</p>
-</div>
-
-<p>It is possible that we
-may yet discover species
-among unicellular organisms
-which multiply without
-limit in the absence of
-any amphimixis. R. Hertwig
-has recently observed
-phenomena in Infusorians
-which he is inclined to refer
-to the suppression of an
-earlier habit of conjugation,
-and so to a kind of parthenogenesis.
-But even if it
-should be shown that amphimixis
-plays a part regularly
-and without exception in the life of <i>all</i> unicellular organisms,
-the facts in regard to multicellular organisms are not affected; and,
-finally, the process of amphimixis is one which we have not the
-slightest ground for assuming to be either an awakener or a maintainer
-of life, and so I return to the most essential part of the whole problem,
-the meaning of the chromatin structures, the combination of which
-is the undoubted result of amphimixis. Do they really represent,
-as we assumed earlier, <i>the hereditary substance</i>, and what do we
-mean by this term?</p>
-
-<p>As far as I know the literature and the development of biological
-theories, the botanist Nägeli was the first to deduce, from the considerable
-difference in size between the egg-cell and the sperm-cell, the
-conclusion that the material basis on which the hereditary tendencies<span class="pagenum"><a id="Page_338"></a>[Pg 338]</span>
-depend must be a <i>minimal</i> quantity of substance. The difference is
-especially great in animals, even in those species whose eggs may be
-called small, for instance, those of sea-urchins or of mammals; even in
-these the mass of spermatozoon is scarcely a thousandth part, often
-scarcely a hundred-thousandth part of the mass of the ovum. And
-yet the inheritance from the father and from the mother is equally
-great. Now as we know that vital powers have always a material
-basis, a minute quantity, such as is contained, for instance, in the
-spermatozoon of Man, must have implicitly in it all the hereditary
-tendencies of the father; and the conclusion is inevitable that in
-the ovum there can only be an equally minimal quantity of substance
-which is the bearer of the hereditary powers, for if there were a larger
-quantity of hereditary substance in the ovum its power of transmission
-would also be greater<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a>.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> The improbable assumption that the hereditary substance of the father may be in
-quality altogether different from that of the mother, and so may have the same power
-of transmission, and yet take up much less room, I leave out of the question altogether.</p>
-
-</div>
-
-<div class="figleft" id="f73a">
-<img src="images/fig73.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 69.</span> Ovum of Sea-urchin (<i>Toxopneustes lividus</i>),<br />
-after E. B. Wilson, <i>zk</i>, cell-substance. <i>k</i>, nucleus<br />
-(so-called germinal vesicle). <i>n</i>, nucleolus (so-called<br />
-germinal spot). Below there is a spermatozoon of<br />
-the same animal (<i>sp</i>), magnified in the same proportion,<br />
-about 750 times.</p>
-</div>
-
-<div class="figright" id="f72a">
-<img src="images/fig72.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 68.</span> Diagram of a<br />
-spermatozoon. After E. B.<br />
-Wilson. <i>sp</i>, apex. <i>n</i>, nucleus.<br />
-<i>c</i>, centrosphere. <i>m</i>, middle<br />
-portion. <i>ax</i>, axial filament.<br />
-<i>e</i>, terminal filament.</p>
-</div>
-
-<p>If we inquire as to the part of the spermatozoon which bears
-this hereditary substance, we may exclude both the tail-thread and
-the middle piece (Fig. 68), the former because it obviously fulfils<span class="pagenum"><a id="Page_339"></a>[Pg 339]</span>
-quite a specialized physiological function and is histologically adapted
-to this function, the latter because, from observation on the spermatozoon
-which has made its way into the ovum, we know that it
-contains the centrosome, the dividing apparatus of the nucleus. Thus
-there only remains the 'head' of the spermatozoon, which includes the
-nucleus, as the possible vehicle of the heritable substance. Therefore
-we are led to seek for the hereditary substance in the nucleus. But
-the hereditary substance cannot be a perishable substance which may
-at need be dissolved, in the literal sense of the word, and be formed
-anew; therefore we cannot look for it in the nuclear membrane,
-and just as little in the 'nuclear sap' which fills the meshes of the
-nuclear network, since the material on which heredity depends must
-necessarily be solid. Nägeli has clearly shown that we must assume
-a stable, that is, a solid molecular architecture. There thus remains
-only the nuclear reticulum with its chromatin granules, and when we
-remember what we have learnt of the behaviour of this chromatin
-substance during division and amphimixis we can entertain no doubt
-that the sought-for bearer of the inheritance is contained in the substance
-of the chromosomes.</p>
-
-<p>The great care with which the chromosomes are halved by means
-of the complicated division apparatus led us earlier to regard them as
-a substance of complex and manifold qualities and of great physiological
-importance; their constant number in any one species, and
-the reduction of that number to half by means of the reducing
-divisions, justify us in concluding that they are permanent structures,
-physiological and morphological units, which undergo no more than an
-apparent irregular dispersion during the resting state of the nucleus.
-Finally, the fact that these supposed vehicles of inheritance occur in
-equal numbers in each of the conjugating germ-cells, and that this
-number is <i>always</i>, both in animals and in plants, half of the normal
-number occurring in somatic cells, is decisive. The logical necessity
-that the hereditary substance of both parents should be transmitted
-to the offspring in equal quantity could not be more precisely met
-than it is by the fact that half the normal number of chromosomes
-occurs in each of the sex-nuclei in the ovum. Personally, I have long
-been certain, on these grounds, that the chromosomes of the nucleus
-are the hereditary substance, and I expressed my conviction on this
-point almost simultaneously with Strasburger and O. Hertwig<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a>.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> More precisely, my conclusions were published several months later than those of
-the investigators named (1885). I think, however, that no one who is familiar with
-my writings for the years immediately preceding, which are collected in <i>Aufsätzen über
-Vererbung und verwandte biologische Fragen</i> (Jena, 1892), will dispute that the idea was
-reached by me independently. I attach importance to this because all my later work
-is based upon this idea.</p>
-
-<p><span class="pagenum"><a id="Page_340"></a>[Pg 340]</span></p>
-
-</div>
-
-<p>But there is also a physiological proof of the meaning of the
-nuclear substance; and this we owe, again, to the simultaneous and
-independent researches of two investigators, M. Nussbaum and A.
-Gruber, the latter working in the Zoological Institute here (in
-Freiburg), and at my request. They made experiments on regeneration
-in unicellular organisms, and found that Infusorians which were
-artificially divided into two, three, or four pieces were able to build
-up a whole animal out of each piece, provided that it contained a
-portion of the nucleus (macronucleus). The large blue trumpet-animalcule,
-<i>Stentor cœruleus</i>, is well suited for such experiments,
-not only on account of its size, but because it possesses a very long
-rosary-like nucleus, which can be easily cut two or three times.
-When a piece is cut off which does not contain a portion of the
-nucleus, it may indeed live for some days and swim about and
-contract, but it is incapable of reconstructing the lost parts, and thus
-of forming a whole animal, and it perishes. It is in the nucleus,
-therefore, that we have to look for the substance which stamps the
-material of the cell-body with a particular form and organization,
-namely, the form and organization of its ancestors. But that is
-exactly the conception of a hereditary substance or idioplasm (Nägeli).
-Some modern biologists deny that there is any hereditary substance
-<i>per se</i>, and believe that the whole of the germ-cell, cell-body and
-nucleus together effects transmission. But though it must be admitted
-that the nucleus without the cell-body cannot express inheritance
-any more than the cell-body without the nucleus, this is dependent
-on the fact that the nucleus cannot live without the cell-body; if it
-be removed from the cell and put, say, into water, it bursts and is
-dissolved. But the cell-body without the nucleus lives on, though
-of course only for a few hours or days, and its metabolism ceases
-only when it is brought to a standstill by the failure to replace by
-nutrition the used-up material. Thus the argument used by those
-who deny the existence of a hereditary substance would be paralleled
-if we denied that Man possesses a thinking substance, and maintained
-that he thinks with his whole body, and even that the brain cannot
-think by itself without the body.</p>
-
-<p>I am convinced that it is just as mistaken to maintain that every
-part of an organism must contain the hereditary tendencies in the
-same degree, or that in unicellular organisms the cell-body is as
-important in inheritance as the nucleus (Conklin). If one feels
-any doubt on this point, one has only to call to mind Nägeli's inference,
-from the minuteness of the spermatozoon, that the hereditary
-substance must be minimal in quantity. But even theoretically there<span class="pagenum"><a id="Page_341"></a>[Pg 341]</span>
-is not the smallest ground for the assumption that the cell-body as
-well as the nucleus contains the hereditary qualities, since we find in
-general that functions are distributed among definite substances and
-parts of the whole organism, and it is just on this division of labour
-that the whole differentiation of the body depends. And why should
-this principle not have been employed just here where the most
-important of all functions is concerned? Why should all living
-substance be hereditary substance? Although Nägeli thought of his
-'idioplasm' otherwise than we now think of hereditary substance,
-although he wrongly imagined it in the form of strands running
-a parallel course through the cell-substance and forming a connected
-reticulum throughout the whole body, he recognized at least so much
-quite correctly, that there are two great categories of living substance&mdash;hereditary
-substance or idioplasm, and 'nutritive substance'
-or trophoplasm, and that the former is much smaller in mass than the
-latter. We now add to this, that the idioplasm must be sought for
-in the cell-nucleus, and indeed in the chromatin granules of the
-nuclear network and of the chromosomes.</p>
-
-<p>But incontrovertible proof of the fact that the nuclear substance
-<i>alone</i> is the hereditary substance was furnished when it was found
-possible to introduce into a non-nucleated piece of a mature ovum of
-one species the nucleus of another related species, and when it was
-seen that the larva that developed from the ovum so treated belonged
-to the <i>second</i> species. Boveri made this experiment with the ovum
-and spermatozoon of two species of sea-urchin, and believed that he
-had succeeded in getting from non-nucleated pieces of the ovum of
-the first species, fertilized with the sperm of the second, larvæ of this
-second species; but, unfortunately, later control-experiments made
-by several investigators, especially by Seeliger, have shown that this
-result cannot be regarded as quite certain and indubitable.</p>
-
-<p>I must emphasize again that I am far from regarding the cell-protoplasm
-of the ovum as an indifferent substance. It is certainly
-not only important but indispensable for the development of the
-embryo, and it has assuredly its own specific character, as in every
-other kind of cell. It represents, so to speak, the matrix and nutritive
-environment in which alone the hereditary substance can unfold its
-wonderful powers; it has developed historically, like every other kind
-of cell, but it contains nothing more than the inherited qualities of
-this one kind of cell-protoplasm, not those of the other cells of the
-body.</p>
-
-<p>But although the essence of fertilization lies, as we have seen, in
-the union of the hereditary substance of two individuals, and not<span class="pagenum"><a id="Page_342"></a>[Pg 342]</span>
-in a 'quickening' of the ovum, we may quite well speak of a quickening
-by fertilization in another sense, if we mean the impulse to
-embryonic development, for this is really supplied by the entrance
-of the sperm-nucleus with its centrosphere into the ovum. But even
-this impulse can, under certain circumstances, be given in another
-way, and certainly the awakening of it is not the <i>end</i> of fertilization,
-but only the condition without which the end, the union of two kinds
-of nuclear substance, could not be attained. There is no indication
-whatever that this 'quickening' of the ovum would be necessary for
-any other reason except that <i>the ovum was previously made incapable
-of development</i>. There would be no 'fertilization' were not the
-mingling of hereditary substances of fundamental importance for
-the organic world.</p>
-
-<p>Moreover, an ovum, or a fragment of an ovum, may also develop
-of itself, having only <i>one</i> of the sex-nuclei, and the union of the
-hereditary substance of two cells is therefore not indispensable for
-the mere production of a new individual.</p>
-
-<p>What has been observed in regard to fragments of ova is
-particularly interesting in this connexion. Ernst Ziegler first succeeded
-in halving a newly fertilized sea-urchin ovum, so that one half
-contained the female and the other the male pronucleus. The latter
-alone contained a centrosphere, and developed a blastula larva.
-Delage carried these experiments further, and cut an unfertilized but
-mature sea-urchin ovum into pieces, and then 'fertilized' the non-nucleated
-pieces with spermatozoa. These pieces developed and
-yielded young larvæ of the relevant species; so it is clearly seen that
-even a piece of mature ovum-protoplasm may undergo embryonic
-development, provided that a nucleus furnished with a dividing
-apparatus penetrates into it. Unfortunately it is technically impossible
-to cut such a non-nucleated and then fertilized fragment
-of ovum so that one half shall contain the male nucleus the other its
-centrosphere. Even without this <i>experimentum crucis</i> we may say
-that the half with the male nucleus would not multiply by division,
-and that the other probably would, though it would not go through
-the regular course of segmentation processes, because the hereditary
-substance absolutely necessary for these was wanting.</p>
-
-<p>But these and similar experiments prove something more, namely,
-that the nuclei of the sperm-cell and egg-cell do not, as was formerly
-believed, stand in a primary and essential contrast to each other,
-which may be described as male and female, but that both are alike
-in their deeper essence, and may replace each other. They only
-differ from each other as far as the cells to which they belong differ,<span class="pagenum"><a id="Page_343"></a>[Pg 343]</span>
-in this, namely, that they are mutually attractive; they find each
-other and unite, and then go on to develop, which each was previously
-unable to do by itself. Widely as the sperm-cell and egg-cell differ
-in size, constitution, and behaviour, in regard to essential character
-they are alike; they bear the relation&mdash;as I expressed it twenty years
-ago&mdash;of 1:1; that is, <i>they both contain an equal quantity of essentially
-similar hereditary substance</i>, and the quality of this substance is
-only individually variable. We should, therefore, speak not of a
-'male' and 'female,' but of a 'paternal' and a 'maternal' nucleus.</p>
-
-<p>All the more recent experiments on 'merogony,' that is, on the
-development of fragments of the ovum, confirm this view. Thus
-Boveri had already observed that even small pieces of sea-urchin ova
-which did not contain the nucleus of the ovum developed, after the
-spermatozoon had entered them, into small but otherwise normal
-larvæ of the species. More recently Hans Winkler proved the same
-thing for the ova of plants, by dividing the ovum of a marine alga
-(<i>Cystosira</i>) into two pieces, then fertilizing these with water containing
-sperms, with the result that he got from both pieces, the
-nucleated and the non-nucleated, an embryo of normal appearance.
-In the latter it could only have been a 'paternal' nucleus which
-directed the development.</p>
-
-<p>To sum up. Our investigation into the meaning of amphimixis
-has led us to the conclusion that it consists in the union of two equal
-complements of hereditary substance, contributed by two different
-individuals, into one unified nucleus, and that the sole immediate
-result of this is <i>the combination of the hereditary tendencies of two
-individuals in one</i>. Among multicellular organisms this one individual
-of dual origin always implies the beginning of a new life, since
-amphimixis is indissolubly associated with reproduction, and even
-among unicellular organisms it can hardly be disputed that the two
-Infusorians which separate after conjugation are no longer the same
-as they were before. After amphimixis they must contain a different
-combination of hereditary substance from what they had
-before, and this must reproduce the parts of the animal in a somewhat
-modified form. This is theoretically beyond doubt, although it
-can scarcely be established by observation.</p>
-
-<p>We thus know now what 'fertilization' is. Through the labours
-of the last decade the veil has been torn from a mystery of nature
-which for thousands of years confronted humanity as unapproachable;
-a riddle has been solved for the solution of which a few centuries
-ago men did not even dare to hope. Not a few have taken part in
-these labours; some I have already named, but it is impossible that<span class="pagenum"><a id="Page_344"></a>[Pg 344]</span>
-I should here mention all who have shared in the achievement by
-observation or reflection. Whoever has helped it on even a single
-step may say to himself that he has taken an active part in bringing
-about what must be called essential progress in human knowledge.</p>
-
-<p>But in the science of nature every new solution implies the
-cropping up of a new riddle, and we are immediately confronted with
-the problem, Why should nature, in the course of evolution, have
-interpolated this process of the mingling of different hereditary substances
-almost everywhere in the organic world? This, however, is
-a problem which we cannot attack until we have first made ourselves
-more fully acquainted with the phenomena of inheritance, and
-have attempted to reason back from these to the nature of the
-hereditary substance. We must, in short, think out a theory of
-heredity.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_345"></a>[Pg 345]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XVII">LECTURE XVII</h2>
-</div>
-
-<p class="c">THE GERM-PLASM THEORY</p>
-
-<div class="blockquot">
-
-<p>Conception of the 'id' deduced from the process of fertilization&mdash;Hereditary
-substance, 'idioplasm' and 'germ-plasm'&mdash;'Idants'&mdash;Evolution or Epigenesis&mdash;Herbert
-Spencer's uniform germinal substance&mdash;Determinants&mdash;Illustrations: <i>Lycæna
-agestis</i>&mdash;The leaf-butterflies&mdash;Insect metamorphosis, limbs of segmented animals&mdash;Heterotopia&mdash;The
-ultimate living units or biophors&mdash;Number of determinants&mdash;Stridulating
-organ of the grasshopper.</p></div>
-
-
-<p><span class="smcap">In</span> proceeding to expound the theory of heredity which has
-shaped itself in my mind in the course of my own scientific development,
-I should like to begin by pointing out that the hereditary
-substance of the germ-cell of an animal or of a plant contains not
-only the primary constituents (<i>Anlagen</i>) of a single individual of
-the species, but rather those of several, often even of many individuals.
-That this is so can be proved in several ways.</p>
-
-<p>I start from what I hold to be the proved proposition, that the
-chromatin substance of the nucleus is the hereditary substance. We
-have seen that this is present in the germ-cells of every species in
-the form of a definite number of chromosomes, and that in germ-cells
-destined for fertilization, that is, in sex-cells, this number is first
-reduced to half, the reduction being effected, as is now proved in
-regard to a whole series of animals, by the two last cell-divisions,
-the so-called maturation divisions.</p>
-
-<p>We know that the full number is only reached again through
-amphimixis, by which process the half number of chromosomes in
-the male and female germ-cells are united in a single cell, the
-'fertilized ovum,' and in a single nucleus, the so-called segmentation
-nucleus. Thus the hereditary substance of the child is formed half
-from the paternal, half from the maternal hereditary substance, and
-we have seen that this remains so during the whole development
-of the child, since, at every succeeding cell-division each of the
-paternal and each of the maternal chromosomes doubles by dividing,
-and the resulting halves are distributed between the two daughter-nuclei.</p>
-
-<p>Now if the complete hereditary substance of a germ-cell before
-the reducing divisions contains potentially all the primary constituents<span class="pagenum"><a id="Page_346"></a>[Pg 346]</span>
-of the body, which it does as a matter of course, then it follows that
-after the reduction each germ-cell must either contain only half the
-primary constituents of the parents or all the primary constituents
-must be contained in the half number of chromosomes. The latter
-seems to me the only possible assumption, as I shall immediately
-proceed to show, and this is as much as to say that the primary
-constituents of at least two complete individuals must be contained
-in the chromosomes of the segmentation nucleus.</p>
-
-<p>That this conclusion is correct is obvious from the fact that
-a whole, that is, a perfect individual with all its parts, develops
-from the ovum, and not a defective one. For suppose that each
-mature germ-cell contained only half the primary constituents of
-the body, it would be impossible that these halves should always
-exactly complete themselves to form a whole embryo when they
-are brought together in fertilization, after having been halved
-by mere chance during the preceding reducing division; it would
-be much more likely to happen that they did not complete themselves,
-and that their union would therefore result in an individual with
-certain parts wanting. If, for instance, in the sperm-cell only the
-anterior half of the body was potentially present, and this united
-with an ovum which likewise contained only the primary constituents
-of the anterior half, the embryo resulting from their union would
-lack the posterior half of the body, and so on. Of course so rough
-a division of the primary constituents is not to be thought of, but
-however fine we can imagine the halving of the mass of primary
-constituents to be, there would never be any guarantee that the
-two cells uniting in amphimixis would complete the mass of primary
-constituents again; indeed, the chance that the two exactly complementary
-halves of the mass would meet would rather become
-less the finer and more complex one imagines the halving by reducing
-divisions to be. A perfect embryo with all its parts would rarely
-arise, but now one group of parts, now another would be wanting,
-while another group might be developed double, or at least would be
-doubly present in the primary constituents.</p>
-
-<p>But in addition to this the facts of inheritance show us that
-the resemblance to mother and father may express itself simultaneously
-in all the parts, or at least in the same parts of the child,
-as may be seen with especial clearness among plant-hybrids, and
-thus the conclusion is inevitable that even in the half number
-of chromosomes all the primary constituents of the whole body are
-present.</p>
-
-<p>Let us go a generation further. If the species possess four<span class="pagenum"><a id="Page_347"></a>[Pg 347]</span>
-chromosomes the child will have in its cells two maternal chromosomes
-(<i>A</i>) and two paternal chromosomes (<i>B</i>); what form will this
-proportion take in the germ-cells produced by the child? The
-maturation division can effect the reduction to two chromosomes
-in different ways; there may, for instance, be two paternal chromosomes
-(<i>B</i>) left in the one, and two maternal chromosomes (<i>A</i>) in the
-other daughter-cell, or one paternal (<i>B</i>) and one maternal (<i>A</i>) in the one,
-and a similar combination in the other cell. Let us follow the latter
-case further. A sperm-cell which contained the combination <i>A</i> and <i>B</i>
-might meet in amphimixis with an egg-cell of different origin also
-containing a similar combination of chromosomes, let us say a chromosome
-<i>C</i> from the mother, and a chromosome <i>D</i> from the father. We
-should then have in the segmentation nucleus of the fertilized ovum
-four different chromosomes, each of which contained the hereditary
-substance of one grandparent; we should have the four chromosomes,
-<i>A</i>, <i>B</i>, <i>C</i>, <i>D</i>, as the hereditary substance of the grandchild.</p>
-
-<p><i>But since, as we have seen, the halved hereditary substance still
-contains the whole mass of primary constituents, each one of these
-chromosomes must contain the collective primary constituents of the
-whole body of the relevant grandparent</i><a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a>. <i>The hereditary substance
-in the fertilized ovum thus consists of several complexes of primary
-constituents (chromosomes) each of which (an 'id') comprises within
-itself all the primary constituents of a complete individual.</i></p>
-
-<div class="footnote">
-
-<p><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> When I say the 'collective' primary constituents of the whole body of the
-grandparent this is not expressing it quite precisely, for, as we shall see later, each
-individual must arise from the co-operation of different chromosomes of different
-origin, not merely from one of the chromosomes contained in its germ-plasm. In the
-example given above, the body of each grandparent cannot have arisen only from
-a single chromosome, which was transmitted to his grandchild, but from the co-operation
-of this chromosome with three others, which have distributed themselves along
-other genealogical paths. But this does not affect the above chain of reasoning, for
-here it is not a question of whether all the primary constituents of the grandparent
-are present in the child&mdash;that can never be the case&mdash;but whether the primary
-constituents transmitted by him represent the whole body of an individual.</p>
-
-</div>
-
-<p>It can be made clear in yet another way that, as a consequence
-of sexual reproduction, the germ-plasm of each species must be
-composed of several 'ids,' <i>individually different</i>. Let us assume
-that there was as yet no amphimixis, and that we could look on
-at its introduction into the organic world; the hereditary substance
-of the beings which had previously lived and multiplied by division
-would consist of more or less numerous chromosomes similar to each
-other, so that, for instance, each individual would contain sixteen
-identical 'ids.' But if amphimixis were now to take place for the
-first time, in the same manner as it does to-day&mdash;that is, after
-the reduction of the number of the ids to half&mdash;in the first amphimixis
-<span class="pagenum"><a id="Page_348"></a>[Pg 348]</span>eight paternal ids would unite with eight maternal ids to form
-the germ-plasm of the new individual, as is indicated in <a href="#f91">Fig. 87</a> by
-a circle of spheres, of which ten are white and ten black as a sign
-of their difference. We may think of the figure as representing
-the 'equatorial plate' of a nuclear spindle with its ids arranged
-in a circle. Now, if two organisms of this generation, with two
-kinds of ids, unite in amphimixis after previous reduction of the
-ids, we have figure <i>B</i>, in which the paternal ids (<i>pJ</i>) are seen to
-the left of the line and the maternal ids (<i>mJ</i>) to the right, while
-each semicircle is in its turn made up of two kinds of ids, those
-of the grandparents (<i>p</i><sup>2</sup><i>J</i> and <i>m</i><sup>2</sup><i>J</i>, <i>p</i><sup>2</sup><i>J</i><sup>1</sup> and <i>m</i><sup>2</sup><i>J</i><sup>1</sup>). The figures
-<i>C</i> and <i>D</i> show the two following generations, in which the number
-of identical ids is each time reduced to half, because eight strange
-ids are again mingled with them; in <i>C</i> only two ids are still identical,
-and in <i>D</i> all the ids are individually different, because they have come
-from different ancestors of the same species. Of course this would
-only be the case if inbreeding were excluded, because through it
-the ids of the same forefathers from two or more sides would meet;
-but prolonged inbreeding is a rare exception in free nature.</p>
-
-<div class="figcenter" id="f91">
-<img src="images/fig91.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 87.</span> Diagram to illustrate the operation of amphimixis on the
-composition of the germ-plasm out of diverse ancestral plasms or 'ids.'
-<i>A</i>-<i>D</i>, the ids of the germ-plasm of four successive generations: <i>A</i>, consisting
-of only two kinds of ids; <i>B</i>, of four; <i>C</i>, of eight; <i>D</i>, of sixteen kinds. <i>pJ</i> and <i>mJ</i>,
-paternal and maternal ids. <i>p</i><sup>2</sup><i>J</i>, grandpaternal; <i>p</i><sup>3</sup><i>J</i>, great-grandpaternal;
-<i>p</i><sup>4</sup><i>J</i>, great-great-grandpaternal ids. The marks in the ids themselves indicate
-their individually distinct characters.</p>
-</div>
-
-<p><span class="pagenum"><a id="Page_349"></a>[Pg 349]</span></p>
-
-<p>I shall now call the hereditary substance of a cell its 'idioplasm,'
-after Nägeli's example, although he sought it in the cell-substance,
-not in the nucleus, and had a different theoretical conception of its
-mode of action. It was he, however, who conceived and established
-the idea of the idioplasm as the bearer of the primary constituents,
-an <i>Anlagensubstanz</i>, determining the whole structure of the organism
-in contrast to the general nutritive protoplasm. Every cell contains
-idioplasm, since every cell-nucleus contains chromatin, but I call
-the idioplasm of the germ-cells <i>germ-plasm</i>, or the primary-constituent-substance
-of the whole organism, and the complexes of
-primary constituents necessary to the production of a complete individual&mdash;whose
-presence we have just shown to be theoretically
-necessary&mdash;I call <i>ids</i>. In many cases these 'ids' might be synonymous
-with chromosomes, at least in all the cases in which the chromosomes
-are simple, that is, are not composed of several similarly formed
-structures. Thus in the salt-water Crustacean, <i>Artemia salina</i>,
-which possesses 168 minute granular chromosomes, each of these
-chromosomes must be regarded as an id, for each can in certain
-circumstances be thrown out from the ovum by the reducing division,
-or it can be brought into the most various combinations with
-other chromosomes by fertilization. Each of them must therefore
-consist of perfect germ-plasm in the sense that all the parts of an
-individual are virtually contained in it; <i>each is a biological unity,
-an id</i>. But when we see in many animals larger band-shaped or
-rod-shaped 'chromosomes,' and when these are composed of a series
-of granules, as they are, for instance, in the often mentioned <i>Ascaris
-megalocephala</i>, each of these granules is to be regarded as an id.
-In point of fact, we find, instead of the two or four large rod-shaped
-chromosomes of <i>Ascaris megalocephala</i>, a larger number of smaller
-spherical chromosomes in other species of <i>Ascaris</i>.</p>
-
-<p>Compound chromosomes consisting of several ids, such as all
-rod or band-like elements of the nuclear substance probably are,
-I designate 'idants.' That they are composed of several individual
-ids is not always clearly apparent because of the smallness of the
-object, and even in larger ones this may only be seen in certain
-stages. Thus we have in <a href="#f92">Fig. 88</a>, <i>A</i> and <i>B</i>, two 'mother-sperm-cells'
-of the salamander; <i>A</i> at an earlier stage, in which the individual ids
-are not visible; <i>B</i> at a later stage, in which the band has split, and
-the rosary-like structure has become at once apparent. It is not
-possible, then, to see at once whether each chromosome corresponds
-to one or to several ids. A more exact investigation of the processes
-of reducing division has shown that there are chromosomes of simple<span class="pagenum"><a id="Page_350"></a>[Pg 350]</span>
-spherical form, that is, composed of several ids whose 'plurivalence'
-cannot be directly recognized, but can only be inferred from their
-further development; there are bivalent chromosomes of double
-value and quadrivalent chromosomes of fourfold value, which we
-have to think of as made up of two or four ids. It would lead
-us too far to go into this more precisely, nor does it fall within
-the scope and intention of these lectures to inquire into these intimate
-and still disputed details.</p>
-
-<p>The germ-plasm of every species of plant or animal is thus
-composed of a larger or smaller number of ids or primary constituents
-of an individual, and it is through the co-operation of these that the
-individual which develops from the ovum is determined.</p>
-
-<div class="figcenter" id="f92">
-<img src="images/fig92.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 88.</span> Sperm-mother-cells (spermatocytes) of the salamander. <i>A</i>, cross-section
-of the cell in the aster-stage; the chromosomes (<i>chr</i>) or idants do not
-reveal that they are compounded out of many ids, which are, however, quite
-distinctly seen in <i>B</i> (<i>Jd</i>), where the chromosomes or idants (<i>chr</i>) are already
-longitudinally split. <i>zk</i>, cell-substance. <i>csp</i>, centrosome. <i>c</i>, centrosome in
-division. After Hermann and Drüner.</p>
-</div>
-
-<p>We have further to inquire what conception we can form of the
-constitution of an id and of its mode of operation. I have already
-spoken of 'primary constituents' (<i>Anlagen</i>) of which the germ-plasm
-consists, but what right have we to think of the parts of an animal
-as already contained in the germ in any form whatever? Is it
-not equally possible that the germ consists of parts, none of which
-bear any definite relation in advance to the parts of the finished
-animal? Might not the germ-cell, along with its nucleus, undergo
-transformations and regular changes which would successively give
-rise to new conditions, namely, the different stages of development,
-until finally the complete animal was attained?</p>
-
-<p>We stand here before an old problem, before the two opposed
-interpretations&mdash;the theory of 'Evolution' and the theory of 'Epigenesis,'
-which were first ranged against each other long ago, and which
-are a cause of strife even now, although in somewhat different guise.</p>
-
-<p>The theory of 'Evolution' is especially associated with the name
-of Bonnet, who elaborated it in detail in the eighteenth century.<span class="pagenum"><a id="Page_351"></a>[Pg 351]</span>
-It maintains that the development of the ovum to the perfect animal
-is not really a new creation, but only an unfolding of invisible small
-parts, which were already present in the ovum. It assumes that
-the parts of the perfect organism are already preformed in the ovum,
-and on this account it is called the 'Preformation Theory.' Bonnet
-often speaks of the preformation of the perfect animal in the germ
-as a 'miniature model,' although his conception of 'evolution' was
-not really so crude as has been often alleged. He expressly
-emphasized that this miniature model was not exactly like the
-perfect animal, but consisted of 'elementary parts' only, which he
-thought of as a net whose meshes were filled up during development
-and by means of nutrition with an infinite number of other parts.
-But after all, his conceptions, and those of his time generally, were
-very far removed from the biological thinking of our own day, as
-may perhaps be most readily understood when I mention that he
-regarded death and decay as an 'involution,' as a folding back,
-so to speak, by means of which all the parts gained though nutrition
-were removed again, so that the net of the miniature model shrank
-together to the invisible minuteness that it had in the ovum. So
-it remained, he fancied, till it was reawakened at the resurrection,
-using the term in the religious sense! He afterwards dropped
-this fancy, because the objection was made to it that human beings
-who had lost a leg or an arm in this life would necessarily be maimed
-at the resurrection!</p>
-
-<p>In Bonnet's time the facts of development were quite unknown,
-and not even the stages of the development of the chick from the
-egg had been observed. When this was afterwards done the
-prevalent theory of 'evolution' necessarily collapsed, for men saw
-with their own eyes that a miniature model of the chick did not
-gradually grow into visibility and ultimately into the young chick,
-but that first of all parts showed themselves in the egg which bore
-no resemblance at all to the chick, that these first rudiments were
-then altered, and that through continual new formations and transformations
-the chick finally appeared. Upon this K. von Wolff based
-his theory of 'Epigenesis,' or development through new formations
-and transformations. He maintained that the doctrine of 'Evolutio'
-was false; that there is no miniature model invisibly contained within
-the egg; but that from the simple egg-substance there arises,
-through the agency of the formative powers inherent in it, a long
-series of stages of development, of which each succeeding one is more
-complex than the one before, until ultimately the perfect animal is
-reached.</p>
-
-<p><span class="pagenum"><a id="Page_352"></a>[Pg 352]</span></p>
-
-<p>This certainly marked considerable progress, for it meant the
-beginning of a science of embryology, that is, the science of the
-form-development of the animal or plant from the ovum. The
-result was not so important in its theoretical aspect, for though the
-knowledge had been gained that the young animal goes through
-a long series of different stages, it had not been discovered how
-nature works this wonder and causes an animal of complex
-structure to arise from the apparently simple substance of the ovum.
-A solution of the difficulty was found by attributing to the ovum
-a formative power, afterwards called by Blumenbach the <i>nisus
-formativus</i>, which possessed the capacity of developing a complex
-animal from the simple 'slime,' or, as we should say, the simple
-protoplasm.</p>
-
-<p>If we contrast the strictly theoretical part of the two theories,
-we find that Bonnet regarded the ovum as something only apparently
-simple, but in reality almost as complex as the animal which
-developed from it, and that he thought of the latter, not as being
-formed anew, but as being unfolded or evolved. That is to say, he
-thought that rudiments present from the outset in the ovum gradually
-revealed themselves and became visible. Wolff, on the other hand,
-regarded the ovum as being what it seemed, something quite simple,
-out of which only the <i>nisus formativus</i> could, by a series of
-transformations and new formations, build up a new organism of the
-relevant species.</p>
-
-<p>Wolff's Epigenesis routed Bonnet's theory so completely from the
-field that, until quite recently, epigenesis was regarded as the only
-scientifically justifiable theory, and a return to the 'evolutionist'
-position would have been looked upon as a retrograde step, as
-a reversion to a period of fancy which had been happily passed.
-I myself have been repeatedly told, with regard to my own
-'evolutionistic' theory, that the correctness of epigenesis was indisputably
-established, that is, was a fact, verifiable at any time
-by actual observation!</p>
-
-<p>But what are the facts? Surely only that there is a succession
-of numerous developmental stages, which we know very precisely
-in the case of a great many animals, and that the miniature model
-which Bonnet assumed to be in the egg does not exist. Both these
-facts are now no longer called in question. But that does not furnish
-us with a theory of development, for theory is not the observation
-of phenomena or of a series of phenomena, <i>it is the interpretation
-of them</i>. Epigenesis, as formulated first by Aristotle and again by
-Harvey, Wolff, and Blumenbach, certainly offered an interpretation<span class="pagenum"><a id="Page_353"></a>[Pg 353]</span>
-of development, not, however, by referring only to what was
-observable, but by going far beyond it; on the one hand taking the
-<i>appearance</i> of a homogeneous germ-substance for reality, and, on the
-other, assuming a special power, which caused a heterogeneous
-organism to arise from a homogeneous germ.</p>
-
-<p>We cannot now accept either of these assumptions, for we know
-that the germ-substance is not homogeneous, and indeed is not merely
-a substance but a living cell of complex structure; and we no longer
-believe in a special vital force, and therefore not in a special 'power
-of development,' which could only be a modification of the former.
-We are thus as little able to accept the old epigenesis as the old
-evolution, and we must establish a theory of Development and
-Heredity on a new basis.</p>
-
-<p>What this basis must be is in a general way beyond doubt.
-Since it is the endeavour of the whole of modern biology to
-interpret life more and more through the interactions of the physical
-and chemical forces bound up with matter, development, too, comes
-within this aim, for development is an expression of life. We seek to
-understand the mechanism of life, and, as a part of that, the mechanism
-of development and of heredity which is closely associated with it.</p>
-
-<p>If we wished to attack the problem of heredity at its roots we
-should first of all have to try to understand the process of life itself
-as a series of physico-chemical sequences. Perhaps this will be
-achieved up to a certain point in the future, but if we were to wait
-for this we should in the meantime have to abandon all attempts at
-a theoretical interpretation of the phenomena of development and
-heredity, and might indeed have to postpone them to the Greek
-Kalends. That would be as though, in the practice and theory
-of medicine, all investigation into and speculation regarding disease
-had to wait until the normal, healthy processes of life were thoroughly
-understood. In that case we should now know nothing of bacteria
-diseases and the hundred other acquisitions of pathological science:
-physiology too would have remained far behind its present level if
-it had lacked the fruitful influence of experience in cases of disease,
-and the ideas and theories, true and false, which have been based
-thereon. In the same way we require a theory of development and
-heredity if we are to penetrate deeper into these phenomena, and
-must have it in spite of the fact that we are still very far from
-having a complete causal knowledge of the processes of life. For the
-raw material of observation, which is to some extent fortuitous, will
-never bring us any further on; observation must be guided by an
-idea, and thus directed towards a particular goal.</p>
-
-<p><span class="pagenum"><a id="Page_354"></a>[Pg 354]</span></p>
-
-<p>It is, however, quite possible to leave aside for the present all
-attempts at an explanation of life, and simply to take the elements
-of life for granted, and on this basis to build up a theory of heredity.
-We have already taken a step in that direction by establishing that
-the whole substance of the fertilized ovum does not take part in
-heredity in the same degree, but that only a small part, the chromatin
-of the nucleus, is to be looked upon as the bearer of the hereditary
-qualities, and by deducing, further, that this chromatin is made up of
-a varying number of small but still visible units, the ids, each of
-which virtually represents the whole organism, or, as I have already
-expressed it, each of which contains within itself, as primary
-constituents, all the parts of a perfect animal.</p>
-
-<p>It was these 'primary constituents' which led us to the
-digression in regard to Bonnet's theory of 'Evolutio' and Wolff's
-'Epigenesis.'</p>
-
-<p>Let us now inquire what must be the constitution of such
-a chromatin globule, an id, so that, shut up within the nucleus of
-a living reproductive cell, it can direct the development of a new
-organism which resembles its parent. Two fundamental assumptions
-present themselves, and these can be related to every conception of
-a 'germ-plasm,' even independently of the assumption of ids. Either
-we may think of the id as made up of similar or of different kinds of
-parts, none of which has any constant relation to the parts of the
-perfect animal, or we think of it as composed of a mass <i>of different
-kinds of parts, each of which bears a relation to a particular part of
-the perfect animal</i>, and so to some extent represents its 'primary
-constituents' (<i>Anlagen</i>), although there may be no resemblance
-between these 'primary constituents' and the finished parts. The
-assumption of a germ-plasm composed of similar parts, which has
-been made, for instance, by Herbert Spencer, may be called the
-modern form of epigenesis, while the other assumption is the modern
-form of the 'evolution' theory. As the former theory can no longer
-call to its aid a 'formative power' as a <i>Deus ex machina</i>, it can only
-explain development as induced by the influence of external
-conditions&mdash;temperature, air, water, gravity, position of parts&mdash;upon
-the chemical components of the germ-plasm, which are everywhere
-uniformly mingled; and it makes no difference whether this uniform
-germ-plasm is thought of as composed of many different kinds of
-parts, as long as those parts are mingled uniformly to make the germ-plasm
-and bear no relation to definite parts of the developing animal.
-Oscar Hertwig has recently outlined such a theory. Although I cannot
-expound it here I must say at least so much with regard to it, and to<span class="pagenum"><a id="Page_355"></a>[Pg 355]</span>
-all other theories of development founded on a similar basis, that
-they could not be accepted even if they were able to offer a workable
-explanation of the development of the individual, and for this
-reason, that ontogeny is not an isolated phenomenon which can be
-interpreted without reference to the whole evolution of the living
-world, for it is most intimately associated with this, being indeed
-a piece of it, having, as we shall see, arisen from it, and, furthermore,
-preparing for its continued progress. <i>Ontogeny must be explained in
-harmony with phylogeny and on the same principles.</i> The assumption
-of a germ-plasm without primary constituents, or of a completely
-homogeneous germ-plasm, as Herbert Spencer maintained, is irreconcilable
-with this, for, as will be seen, it contradicts certain facts
-of inheritance and variation. Therefore all theories founded on
-this assumption must be rejected.</p>
-
-<p>There is another and, I believe, weighty consideration which
-forbids us to assume a germ-substance without primary constituents.
-I shall return to this later, but in the meantime I wish to build up
-more completely my own 'germ-plasm' theory.</p>
-
-<p>I assume that the germ-plasm consists of a large number of
-different living parts, each of which stands in a definite relation to
-particular cells or kinds of cells in the organism to be developed, that
-is, they are 'primary constituents' in the sense that their co-operation
-in the production of a particular part of the organism is
-indispensable, the part being <i>determined</i> both as to its existence and
-its nature by the predestined particles of the germ-plasm. I therefore
-call these last <i>Determinants</i> (<i>Bestimmungsstücke</i>), and the parts of the
-complete organism which they determine <i>Determinates</i>, or hereditary
-parts.</p>
-
-<p>It is easy to show on what basis this assumption rests; the
-phenomena of inheritance taken in conjunction with those of variation
-seem to me to compel us to it. We know that all the parts of an organism
-are variable, and that in one individual the same part may be larger,
-in another smaller. Not all variations are transmissible, but many of
-them, and some very minute ones, are. Thus, for instance, in many
-human families there occurs a small pit, hardly as large as the head
-of a pin, in the skin of the ear, whose transmission I have observed
-from the grandmother to the son and to several grandchildren. In
-such a case there must be a minute something in the germ-plasm,
-not present in that of other human beings, which causes the
-origin, in the course of development, of this little abnormality in
-the skin.</p>
-
-<p>There are human families in which individuals occur repeatedly,<span class="pagenum"><a id="Page_356"></a>[Pg 356]</span>
-and through several generations, who have a white lock of hair,
-in a particular spot, on an otherwise dark-haired head. This cannot
-be referred to external influences, it must depend on a difference in
-the germ, on one, too, which does not affect the whole body, not even
-all the hairs of the body, but only those of a particular spot on the
-surface of the head. It is a matter of indifference whether the white
-colouring of the hair-tuft is produced by an abnormal constitution of
-the matrix of the hair, or by other histological elements of the skin,
-as of the blood-vessels or nerves. It can only depend ultimately on
-a divergently constituted part of the germ-plasm, which can only affect
-this one spot on the head, and alter it, if it is itself different from
-what is usual. On this account I call <i>it</i> the <i>determinant</i> of the
-relevant skin-spot and hair-group. In Man such minute local
-variations are usually lost after a number of generations, but in
-animals there are innumerable phenomena which prove to us that
-single minute deviations can become permanent. Thus there lives in
-Central Europe a brown 'blue butterfly,' <i>Lycæna agestis</i>, which has
-a little black spot in the middle of its wing. The same species also
-occurs in Scotland, but there, instead of the black spot, it has a
-milk-white one, and so-called 'eye-spots' on the under surface of the
-wing have also lost their black centres. The species has thus varied
-transmissibly, but only in regard to these particular spots on the wing.
-A slight variation must therefore have taken place in the germ-plasm
-which only affects these few parts of the body, or, to express it
-otherwise, the germ-plasms of the ancestral species and of the variety
-can only be distinguished by a difference which determines exclusively
-the scale colour of these spots. The two germ-plasms differ, I should
-say, only as regards the <i>determinants</i> of these wing-scales.</p>
-
-<p>We know from the artificial selection to which Man has subjected
-and still subjects his domesticated animals and useful plants, that
-any spots and parts of the body which he chooses can be hereditarily
-altered, if the desired variations which present themselves are always
-selected for breeding, and that this does not necessarily cause variation
-in other parts of the body. When, for instance, in the case cited
-by Darwin, the comb of a Spanish cock which had previously hung
-downwards was made to stand upright because a prize had been
-offered for this character, or when a certain breed of hens was
-'furnished with beards,' the results were permanent variations affecting
-only the parts on which the fancier's attention had been fixed. In
-the same way, when the tail feathers of the Japanese cock are
-lengthened to three feet the rest of the plumage does not alter, still
-less any other part of the body. Of course there are numerous<span class="pagenum"><a id="Page_357"></a>[Pg 357]</span>
-'correlated' variations, and in very many cases the breeder causes
-a second or third character, on which he had not fixed his attention,
-to vary in addition to the one he was aiming at. But such concomitant
-variations are not necessary or inevitable in all cases; and
-indeed we need not refer them all to a true correlation of the parts,
-but may suppose that they depend not infrequently on the faultiness
-of our power of observation, which is not sufficiently keen to control
-several parts of the body at
-one time, and to notice minimal
-variations in parts on which
-we have not specially fixed our
-attention.</p>
-
-<div class="figright" id="f13a">
-<img src="images/fig13.jpg" alt=""/>
-<p class="caption1"><span class="smcap">Fig. 13.</span> <i>Kallima paralecta</i>, from India;<br />
-showing the right under surface in the<br />
-resting pose. <i>K</i>, head. <i>Lt</i>, palps. <i>B</i>, limbs.<br />
-<i>V</i>, fore wing. <i>H</i>, hind wing. <i>St</i>, 'tail' of the<br />
-latter, representing the stalk of the leaf.<br />
-<i>gl</i><sup>1</sup> and <i>gl</i><sup>2</sup>, transparent spots, <i>Aufl</i>, remains<br />
-of 'eye-spots.' <i>Sch</i>, a 'mould'-spot.</p>
-</div>
-
-<p>So much, at least, is certain,
-that in all these cases of the
-artificial alteration of individual
-characters the germ-plasm is in
-some way changed, but always
-in such a way that it differs
-from that of the ancestral form
-through such variations alone,
-and the effect of these is that
-only the altered parts are influenced
-thereby, and not the
-whole organism. This again is
-but another way of saying that
-only the <i>determinants</i> of these
-parts have altered.</p>
-
-<p>We can see from a thousand
-cases that exactly the same
-happens in a state of nature,
-that there, too, one part changes
-after another, until the highest
-possible degree of adaptation to
-the conditions has been attained.
-In the mimetic resemblance to
-leaves exhibited among butterflies this is most clearly seen, for here
-we are familiar with the model&mdash;the leaf&mdash;and we see how one species
-approximates to it in a general way only in the total colour, how others
-develop a brown stripe crossing the posterior wing obliquely, so that, to
-a certain extent, it resembles the midrib of a leaf, how in a third species
-this stripe is continued for some distance forward across the anterior
-wing, in a fourth it goes a little further, until, finally, in a fifth, it<span class="pagenum"><a id="Page_358"></a>[Pg 358]</span>
-is continued on to the tip of the anterior wing. This may be seen,
-for instance, in the genus <i>Anæa</i>, which is rich in species. But even
-then a still further increase of the resemblance is possible, for, as is
-well known, there are not infrequently imitations of the lateral veins
-of the leaf as well, or dark spots which faithfully reproduce the
-mould-spot on a damp, decaying leaf, or colourless transparent spots
-which probably simulate dewdrops, and so on. All these are variations
-relating to individual and distinct groups of wing-scales, which
-have varied transmissibly and independently, that is, each of them
-has been produced by a variation in the germ-plasm, which brought
-about a change in this particular area of the body and in no other.</p>
-
-<p>Let us for a moment assume the impossible, and suppose that we
-could look on at the evolution of such a leaf-butterfly; the beginning
-of the leaf-imitation might have its cause in the fact that an ancestral
-form of <i>Kallima</i>, which had previously lived in the meadows, exhibited
-on the part of some of its descendants a migration to the woods, and
-thus divided into two groups, with a different manner of life&mdash;a
-meadow form and a wood form. The latter adapted itself to sitting
-among leaves, and the midrib of a leaf developed on its wings. In
-a germ-plasm without 'primary constituents' this variation could
-only depend on a uniform variation of all the parts, for these parts
-are either alike among themselves, or at any rate have the same value
-for every part of the finished organism. But the germ-plasm of the
-new breed must somehow differ from that of the ancestral form,
-otherwise it could produce no new variety, but only the ancestral
-form over again. But how could an animal differing only in one
-minute part arise from a germ-plasm which has varied in all its parts,
-and how could such little steps of variation be repeated many times
-in the course of the phylogeny without the corresponding variations
-of the germ-plasm becoming so intense that not only the wing-markings
-but everything about the animal would be altered likewise?
-And yet these 'leaf-pictures' have not originated suddenly, but by
-many small steps, so that the germ-plasm must have varied <i>in toto</i>
-a hundred times in succession if there are no primary constituents.</p>
-
-<p>In the Indian species, <i>Kallima paralecta</i>, there are no fewer
-than five well-marked varieties, the differences between which depend
-solely on the manner in which the leaf-picture on the wing is
-elaborated, <i>for the upper surface of the wing is alike in all</i>. Even
-a cursory observation of a collection of these butterflies shows
-that the lateral veins of the leaf-picture are quite different in number,
-distinctness, and length in the different individuals. On the right
-half of the wing there may be as many as six of them indicated<span class="pagenum"><a id="Page_359"></a>[Pg 359]</span>
-(<a href="#f13a">Fig. 13</a>); and it can be observed that the three middle ones are the
-longest, most sharply defined, and darkest, while those lying near
-the tip and the base of the mimic leaf are shorter and often even
-shadowy. On the left side the second lateral vein in particular
-distinctly shows indentations indicative of the rings, inherited from
-the ancestral forms, which surrounded the still visible eye-spots
-(<i>Aufl</i>); the third lateral vein is quite indefinite and shadowy, but
-nevertheless it runs exactly parallel to the first two, and thus heightens
-the deceptive effect. We can thus distinguish older and more recent
-elements in the marking&mdash;a proof of the slow and successive origin of
-the picture.</p>
-
-<p>This is not reconcilable with the conception of a germ-plasm
-without primary constituents, however complex a mixture it may
-otherwise be. A substance which had to undergo thousands upon
-thousands of variations, arising from each other according to law
-and in the strictest succession, in order that it might become a definite
-organism, predetermined as to all its thousands of parts down to the
-most minute, cannot vary over and over again in its whole constitution
-without the consequences showing themselves in numerous, or indeed
-in <i>all</i>, the parts of the body. Such variations in the germ-plasm
-would be comparable to many successive deviations of a ship from
-her course, which, although the single ones would only cause a minimal
-deviation from the true course, would, when summed up in a voyage
-of some length, land the vessel at quite another coast than the one
-intended. If each individual adaptation of the species depended on
-a variation of the whole germ-plasm the wood <i>Kallima</i> would soon
-retain no resemblance to its ancestral form, the meadow species;
-yet we are acquainted with species of <i>Kallima</i> which do not show
-the special resemblance to a leaf, but, for instance, still exhibit the
-perfectly developed eye-spot of the ancestral form, and so forth. It
-follows, therefore, that the origin of the leaf-picture has not greatly
-influenced the general character of the species; and the fact that the
-upper surface of the wings has remained the same in all the varieties
-is in itself enough to prove this.</p>
-
-<p>Since, then, the resemblance to a leaf cannot have arisen without
-something in the germ-plasm varying, since the germ-plasm of a forest
-<i>Kallima</i> and a meadow <i>Kallima</i> must be different in something, and
-cannot be any more alike than the germ-plasm of a fantail-pigeon
-and a carrier, there <i>must be 'primary constituents' in the germ-plasm</i>,
-that is, vital units whose variation occasions the variation of definite
-parts of the organism, and of these alone.</p>
-
-<div class="figcenter" id="f17a">
-<img src="images/fig17.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 17.</span> Caterpillar of <i>Selenia tetralunaria</i> on a twig of birch. <i>K</i>, head.
-<i>F</i>, feet. <i>m</i>, protuberances resembling dormant buds. Natural size.</p>
-</div>
-
-<p>It is on such considerations as these that my assumption, that
-<span class="pagenum"><a id="Page_360"></a>[Pg 360]</span><i>the germ-plasm is composed of determinants</i>, depends. There must be
-as many of these as there are regions in the fully-formed organism
-capable of independent and transmissible variation, including all the
-stages of development. Every part, for instance, of the butterfly's
-wing, which is capable of independent and transmissible variation,
-must, so I conclude, be represented in the germ-plasm by an element
-which is likewise variable, the determinant; but the same must be
-true of every independently and transmissibly variable spot of the
-caterpillar from which the butterfly developed. We know how
-markedly caterpillars are adapted in form and colour to their environment.
-Let us assume that the caterpillar of the butterfly which we
-chose as an example of wing-marking had the habit of feeding only
-by night and during the daytime of resting on the trunk of a tree, or,
-more precisely, in the crevices of the bark. It would then resemble
-the caterpillar of the moths of the genus <i>Catocala</i> or the Geometers
-(Geometridæ), and possess the colour of the bark of the tree in
-question; the determinants of the skin would thus have varied to
-correspond with this mode of life on the part of the caterpillar, so that
-the skin would appear grey or brown. But there cannot be only <i>one</i><span class="pagenum"><a id="Page_361"></a>[Pg 361]</span>
-determinant of the caterpillar skin in the germ-plasm, for the bark-like
-colour of, for instance, a Geometer caterpillar is not a uniform
-grey, but has darker spots at certain places and lighter whitish spots
-at others, such as are to be seen on the bark of the twig on which
-the caterpillar is wont to rest, or brown-red spots, like those on the
-cover-scales of the buds, or little warts and protuberances which exactly
-correspond to similar roughnesses on the twigs, to cracks in the bark,
-and so on. All these markings are constant, and are to be found
-in the same spot in every caterpillar of the species. A large number
-of regions of the caterpillar skin must therefore be independently
-determined by the germ-plasm; the germ-plasm must contain parts
-the variations of which bring about variations only of an independently
-variable region of the caterpillar skin. In other words, in the germ-plasm
-of the butterfly ovum there must not only be determinants for
-many regions of the butterfly's wing, but also for many regions of the
-caterpillar's skin.</p>
-
-<p>This line of argument, of course, applies to all the bodily parts
-and organs of the butterfly and of the caterpillar, as well as to
-all the stages of development of the species as far as these parts
-are able to vary in such a way that the variation reappears in the
-following generation, that is to say, as far as it is transmissibly
-variable.</p>
-
-<p>But all parts must be transmissibly variable which have exhibited
-independent variation in relation to their ancestors. When,
-for instance, the eggs of a butterfly (<i>Vanessa levana</i>) bear a deceptive
-resemblance to the flower-buds of the stinging-nettle on which the
-caterpillar lives, not only in form and colour, but in their pillar-like
-arrangement, we may conclude that these eggs have varied transmissibly
-from those of their ancestors, which had not acquired the
-habit of living on the stinging-nettle, in these three respects independently,
-that is, uninfluenced by any other variations the species
-may have undergone; and that, consequently, the germ-plasm must
-contain determinants for the egg-shell, egg-colouring, and so on.
-The manner of laying the eggs in the form of pillars depends on a
-modification of the egg-laying instinct, which must in its turn depend
-on the variations of certain nerve-centres, and we learn from this
-that there must be in the germ-plasm determinants for the individual
-centres of the nervous system.</p>
-
-<p>It may, perhaps, be suggested that matters could be explained
-in a simpler way&mdash;that it is enough to assume the presence in
-the egg of determinants for all the parts of the caterpillar, and that
-those of the butterfly are only formed within the caterpillar.</p>
-
-<p><span class="pagenum"><a id="Page_362"></a>[Pg 362]</span></p>
-
-<p>This suggestion seems justifiable if we confine ourselves to
-superficial considerations. We read in every handbook of entomology
-that the wings only arise during the life of the caterpillar, and in
-a certain sense this is true, for the primary constituents or primordia
-of wings only develop into the fully formed wing during the larval
-period. But even if these primordia were only formed during the
-caterpillar-stage, what could they develop from? Only out of the
-material parts of the caterpillar, that is, from some of its living cells
-or cell-groups. The constitution of the wings would therefore be
-dependent on that of the cells of the caterpillar from which they
-arose, so that if these varied transmissibly through the variation
-of their determinants contained in the germ, the determinants of
-the butterfly which were just developing would vary with them;
-every transmissible variation of the caterpillar would necessarily
-cause a similar variation in the butterfly, and this does not happen.
-If any one hazarded the assumption that the determinants of the
-butterfly develop only in the caterpillar, but quite independently
-of its constitution, he would either be making an absurd statement,
-namely, that the characters of the butterfly were not transmissible
-at all, or he would be unconsciously admitting that the determinants
-of the butterfly were already contained in the parts of the caterpillar,
-and come direct from the germ-plasm.</p>
-
-<p>That the characters of the butterfly do vary independently of
-those of the caterpillar I demonstrated many years ago, when
-we were still very far away from the idea of the germ-plasm or
-of determinants. I demonstrated then that the constancy of the
-markings of a species can be quite different in the two chief stages;
-that the caterpillar may be very variable, while the butterfly or
-the moth may be very constant in all its markings, or conversely.
-I called attention to the dimorphic caterpillars which are green or
-brown, and yet become the same moth (for instance, <i>Deilephila
-elpenor</i> and <i>Sphinx convolvuli</i>); I cited the case of the spurge
-hawk-moth (<i>Deilephila euphorbiæ</i>), whose dark but at the same time
-motley caterpillars occur in the Riviera at Nice as a local variety
-(<i>Nicæa</i>), and there wear quite a different dress&mdash;pale clay-yellow, with
-a double row of large conspicuous dark yellow eye-spots&mdash;while the
-moth does not differ from our variety in a single definite character,
-except in its larger size. At that time, too, I instituted experiments
-with the caterpillars of the smallest of our indigenous Vanessa species
-(<i>Vanessa levana</i>), of which the majority are black with black thorns,
-while a minority are yellowish-brown with yellow thorns; reared
-separately, both yielded the same butterfly, though in this case one<span class="pagenum"><a id="Page_363"></a>[Pg 363]</span>
-would be inclined to suppose that there was some internal connexion
-between the colour of the caterpillar and that of the butterfly, since
-the butterfly also occurs in two colours. It was shown, however,
-that the colour of the butterfly had nothing to do with that of
-the caterpillar, for it is known to be dependent on the season, and
-is a seasonal dimorphism, 'while the two forms of caterpillar may
-occur side by side at all times of the year.'</p>
-
-<p>Subsequently I made a similar experiment with the dimorphic
-caterpillars of the 'fire'-butterfly (<i>Polyommatus phlæas</i>), and it yielded
-the same result. The pure green caterpillars became the same butterflies
-as those marked with broad red longitudinal stripes, and in this
-case we can definitely describe both colours as protective, for the
-green form is adapted to the green under surface of the leaf, the
-red-striped to the green red-edged stalk of the lesser sorrel (<i>Rumex
-acetosella</i>).</p>
-
-<p>There was really no necessity for special proofs that the
-caterpillar and butterfly vary transmissibly in complete independence
-of each other, for the facts of metamorphosis alone are enough to
-prove it. How would it have been possible otherwise that the jaws
-adapted for biting should, in the primitive insects, and in the locusts
-which are nearest to them, remain as a biting apparatus throughout
-life, while in the caterpillar they are modified during its pupal
-stage into the suctorial proboscis of the butterfly? The parts of
-insects, therefore, must be capable of transmissible variation in the
-stages of life independently of each other. Not only have the jaws
-of the leaf-eating caterpillars remained unaltered, while in the
-sexually mature animal they have been gradually modified into a
-very long and extremely complex suctorial apparatus, but when at
-a much later time this proboscis became superfluous in a species,
-because the butterfly or moth, from some cause or another, lost
-the habit of taking any nourishment at all, its degeneration exercised
-no effect on the jaws of the caterpillar, as we can observe in many
-hawk-moths, silk-moths and Geometridæ. How could such a
-degeneration become transmissible if the caterpillar's jaws, from
-which those of the adult are developed, remain the same? We are
-thus forced to assume that there is something in the latter which
-can vary from the germ, without the jaws themselves being altered
-thereby. This 'something' it is which I call 'determinants,' vital
-particles, which&mdash;however we may try to picture them&mdash;are indeed
-contained in the cells of the caterpillar's jaws, but are there inactive
-and do not influence the structure of these, while, on the other
-hand, it is their constitution which determines the form and structure<span class="pagenum"><a id="Page_364"></a>[Pg 364]</span>
-of the suctorial proboscis of the butterfly down to the minutest
-details. It must be these alone which cause the suctorial proboscis
-to develop, and in some cases to degenerate again, without bringing
-about any change in the corresponding parts in the caterpillar.</p>
-
-<div class="figleft" id="f93">
-<img src="images/fig93.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 89.</span> Anterior region of the larva<br />
-of a Midge (<i>Corethra plumicornis</i>). <i>K</i>, head.<br />
-<i>Th</i>, thorax. <i>ui</i>, inferior imaginal disks.<br />
-<i>oi</i>, superior imaginal disks. <i>ui</i><sup>1</sup>, <i>ui</i><sup>2</sup>, and<br />
-<i>ui</i><sup>3</sup>, the primordia of the limbs. <i>oi</i><sup>2</sup><br />
-and <i>oi</i><sup>3</sup>, the primordia of the wings and<br />
-'balancers.' <i>g</i>, brain. <i>bg</i>, chain of ventral<br />
-ganglia with nerves which enter the<br />
-imaginal disks. <i>trb</i>, tracheal vesicle.<br />
-Enlarged about 15 times.</p>
-</div>
-
-<p>This example seems to me to be preferable to that of the wings
-of insects in this respect, that there is no organ in the caterpillar
-with a specific function corresponding to the wing of the butterfly.
-Yet the two cases are exactly alike, and it would be a mistake to
-say that the first primordium of the wing within the caterpillar
-is not a part of the caterpillar at all. At first, certainly, it is only
-a group of cells on the skin, occurring at a particular spot on the
-dorsal surface of the second and third segments of the caterpillar,
-and doubtless arising from a single cell of the embryo, the 'primitive
-wing-cell,' which, however, has not
-as yet been demonstrated. But it
-is nevertheless an integral part of
-the caterpillar, which could neither
-be wanting, nor be larger or smaller,
-and so on; which, in short, does
-mean something for the caterpillar,
-although perhaps not more than
-any other of the skin-cells. For
-the butterfly, however, this area on
-the skin means the rudiment of the
-wing; for from it alone can there
-arise by multiplication the aggregate
-of cells which grows out into a hollow
-protuberance, enlarges by degrees
-into a disk, the imaginal disk, and
-eventually develops into the form of wing peculiar to the species. This
-imaginal disk is connected very early with nerves and with tracheæ, as
-may be beautifully seen especially in dipterous larvæ (Fig. 89, <i>oi</i>),
-and these become later the nerves and tracheæ of the wing, while
-thousands of peculiar scale-like hairs develop on the upper surface;
-in short, the rudiment becomes a perfect wing with its specific
-venation, and with the marking and colouring which is often so
-complicated in Lepidoptera. Almost every little spot and stripe
-of the latter is handed down with the most tenacious power of
-transmission from generation to generation, and each can at the
-same time be transmissibly varied; the same is true of the venation,
-which is so important systematically just because it is so strictly
-hereditary, yet it too can vary transmissibly, as can also the hooked<span class="pagenum"><a id="Page_365"></a>[Pg 365]</span>
-bristles, the odoriferous apparatus, and, in short, the whole complex
-structure of the wing, with all its specific adaptations to the mode
-of flight, to the manner of life, and to the colour of the environment.
-How is it possible that all this can develop from a skin-cell? Is
-it the influence of position that effects it, and could any other cell
-of the caterpillar's skin do the same if it were placed in the same
-position? Could any neighbour-cell of the primitive wing-cell replace
-it if it were destroyed? It is hardly probable, and I think I can
-even prove that this is not so. The experiment of killing such
-a cell in the living animal has not yet been made; if it should
-succeed, we may venture to say in advance that none of the neighbouring
-skin-cells will be able to do its work and take its place
-in developing a wing; the wing in question will simply remain
-undeveloped. In the summer of 1897 I hatched a specimen of
-<i>Vanessa antiope</i> from the pupa, which, though otherwise normal
-and well-developed, lacked the left posterior wing altogether; no
-trace of it could be recognized. In this case, from some cause
-which could no longer be discovered, the first formative cell of
-the wing in the hypodermis, or its descendants, must have been
-destroyed, and no substitution of another took place, as the defect
-showed.</p>
-
-<p>The young science of developmental mechanics attributes to
-the position of a cell in the midst of a group of cells a determining
-value as regards its further fate, and as far as the cells of the
-segmenting ovum are concerned this seems to be true in certain
-cases, but the assumption cannot be generally true except in a
-very subordinate sense. The formative cell of the wing does not
-become what it is because of its relative position in the organism.
-If this were so it could not happen that a wing should develop
-instead of a leg, as was observed in a <i>Zygæna</i>, nor could there be
-any of those deformities already referred to, to which the name
-'Heterotopia' is applied, and which consist in the development of
-organs of definite normal structure, or at any rate of apparently
-normal structure in quite unusual places, e. g. an antenna on the coxa
-of a leg, or of a leg instead of an antenna (in <i>Sirex</i>), or instead of
-a wing. It is therefore not some influence from without that makes
-that particular skin-cell of the caterpillar the rudiment of the wing,
-but the <i>reason lies within itself</i>, in its own constitution. As the
-whole mass of determinants for the whole body and for all the stages
-of its development must be contained within the ovum and the
-sperm-cell, so the primitive cell of the butterfly's wing must contain
-all the determinants for the building up of this complicated part;<span class="pagenum"><a id="Page_366"></a>[Pg 366]</span>
-and if the cell gets into a wrong position in the course of development
-because of some disturbance or other, a wing may develop from
-it in that position if the conditions are not too utterly divergent.
-These heterotopic phenomena afford a further proof of the existence
-of determinants, because they are quite unintelligible without the
-assumption of 'primary constituents' or <i>Anlagen</i>.</p>
-
-<p>The hypothesis of determinants in the germ-plasm is so fundamental
-to my theory of development that I should like to adduce
-another case in its support and justification. The limbs of the
-jointed-footed animals, or Arthropods, originally arose as a pair on
-each segment of the body, and they were at first alike or very similar
-both in their function and in their form. We find illustration of
-this in the millipedes, and still more in the species of the interesting
-genus <i>Peripatus</i>, which resembles them externally, as well as in the
-swimming and creeping bristle-footed marine worms (Chætopods)
-belonging to the Annelid phylum. We can quite well picture to
-ourselves that the whole series of these appendages was represented
-in the germ-plasm by a single determinant or group of determinants,
-which only required to be multiplied in development. Without
-disputing whether this has really been the case in the primitive
-Arthropods or not, it is certain that it can no longer be the case
-in the germ-plasm of the Arthropods of to-day. In these each pair
-of appendages must be represented by a particular determinant.
-We must infer this from the fact that the several pairs of these
-appendages have varied transmissibly, independently of each other,
-for some are jaws, others swimming legs, or merely bearers of the
-gills or of the eggs; others are walking legs, digging legs, or jumping
-legs. In Crustaceans a forceps-like claw is often borne by the first
-of the otherwise similarly constructed appendages, or also by the
-second or the third, or there may be no forceps, and so on; in short,
-we see that each individual pair has adapted itself independently
-to the mode of life of its species. This could only have been possible
-if each was represented in the germ-plasm by an element, whose
-variations caused <i>a variation only in that one pair of legs, and in
-no other</i>.</p>
-
-<p>It may perhaps be objected that the differences in the appendages
-may quite well have had their origin simply during the development
-of the animal, while the primary constituents were the same for all, so
-that a single determinant in the germ-plasm would suffice. But this
-could only be the case if the differences depended not on internal but
-on external causes, that is, if the same primary constituents gave rise
-to a set of appendages which became different because they were<span class="pagenum"><a id="Page_367"></a>[Pg 367]</span>
-subject in the course of their development to different modifying
-influences. But this is not the case, at least not to the extent that
-this supposition would necessitate. Can it be supposed that, for
-instance, the jumping legs of the water-flea (<i>Gammarus</i>) are a necessary
-consequence of the somewhat divergent form of the segments
-from which they grow? A direct proof to the contrary may be found
-in 'Heterotopia,' for in the place where a posterior limb, modified
-for holding the eggs, normally occurs in the crab an ordinary walking
-leg may exceptionally develop (Fig. 90, Bethe), or an appendage
-resembling an antenna may take the place of an extirpated eye
-(Herbst). But if there were really only one determinant in the
-germ-plasm for all the appendages these would of necessity be
-all alike, apart from the larger or smaller differences which might
-be stamped upon them by growing from segments different in
-size and in nutrition. Such differences, however, are far from being
-sufficient to explain the great deviations seen among the appendages
-of most kinds of Crustaceans, and still less to explain their adaptation
-to quite different functions.</p>
-
-<div class="figcenter" id="f94">
-<img src="images/fig94.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 90.</span> The Common Shore-Crab (<i>Carcinus mænas</i>), seen from below, with
-the abdomen forced back. In place of the swimmeret, which ought to be borne
-by the fifth abdominal swimmeret, a walking leg has grown on the left side,
-and one which properly should belong to the right side (6). 1-5, thoracic
-limbs, <i>ps</i>1-4, swimmerets of the right side. <i>s</i>6, <i>s</i>7, posterior segments of
-the abdomen. After Bethe.</p>
-</div>
-
-<p>It need not be imagined that my argument can be controverted
-by saying that <i>one</i> appendage-determinant in the germ may split
-itself in the course of development into a series of different appendage-determinants.
-The question would then arise, How is it able to do so?
-And the answer can be no other than that the single first determinant
-had within it several different kinds of elements, which subsequently<span class="pagenum"><a id="Page_368"></a>[Pg 368]</span>
-separated to determine in different ways the various appendages.
-But that is just another way of saying that this single determinant
-actually includes within itself several different determinants. For
-a determinant means nothing more than an element of the germ-substance
-by whose presence in the germ the specific development of
-a particular part of the body is conditioned. If we could remove the
-determinants of a particular appendage from the germ-plasm this
-appendage would not develop; if we could cause it to vary the
-appendage also would turn out differently.</p>
-
-<p>In this general sense the determinants of the germ-plasm are not
-hypothetical, but actual; just as surely as if we had seen them with
-our eyes, and followed their development. Hypothesis begins when
-we attempt to make creatures of flesh and blood out of these mere
-symbols, and to say how they are constituted. But even here there
-are some things which may be maintained with certainty; for
-instance, that they are <i>not</i> miniature models, in Bonnet's sense, of the
-parts which they determine; and, further, that they are not lifeless
-material, mere substances, but living parts, vital units. If this were not
-so they would not remain as they are throughout the course of
-development, but would be displaced and destroyed by the metabolism,
-instead of dominating it as living matter alone can do&mdash;doubtless
-undergoing oxidation, but at the same time assimilating material
-from without, and thereby growing. There cannot be lifeless determinants;
-they must be living units capable of nutrition, growth, and
-multiplication by division.</p>
-
-<p>And now we have arrived at the point at which a discussion of
-the organization of the living substance in general can best be interpolated.</p>
-
-<p>The Viennese physiologist, Ernst Brücke, forty years ago promulgated
-the theory that living matter could not be a mere mixture of
-chemical molecules of any kind whatever; it must be 'organized,' that
-is, it must be composed of small, invisible, vital units. If, as we must
-certainly assume, the mechanical theory of life is correct, if there is
-no vital force in the sense of the 'Natur-Philosophie,' Brücke's
-pronouncement is undoubtedly true; for a fortuitous mixture of
-molecules could no more produce the phenomena of life than a <i>single</i>
-molecule could, because, as far as our experience goes, molecules do
-not live; they neither assimilate, nor grow, nor multiply. Life can
-therefore arise only through a particular combination of diverse
-molecules, and all living substance must consist of such definite groups
-of molecules. Shortly after Brücke, Herbert Spencer likewise
-assumed the reality of such vital 'units,' and the same assumption<span class="pagenum"><a id="Page_369"></a>[Pg 369]</span>
-has been made in more recent times by Wiesner, De Vries, and
-myself. In the meantime we can say nothing more definite about
-the composition of these bearers of life, or 'biophors,' as I call them,
-than that albumen-molecules, water, salts, and some other substances
-play the chief part in their composition. This has been found out by
-analysis of dead protoplasm; but in what form these substances are
-contained in the biophors, and how they affect each other in order to
-produce the phenomena of life by going through a ceaseless cycle of
-disruptions and reconstructions, is still entirely hidden from us.</p>
-
-<p>We have, however, nothing to do with that here; we content
-ourselves with recognizing in the biophors the characteristics of life,
-and picturing to ourselves that all living substance, cell-substance,
-and nuclear substance, muscle-, nerve-, and gland-substance, in all
-their diverse forms, consist of biophors, though, of course, of the most
-varied composition. There must be innumerable kinds of biophors in
-all the diverse parts of the millions of forms of life which now live
-upon the earth; but all must be constructed on a certain fundamental
-plan, which conditions their marvellous capacity for life; all possess
-the fundamental characters of life&mdash;dissimilation, assimilation, growth,
-and multiplication by division. We must also ascribe to them in
-some degree the power of movement and sensibility.</p>
-
-<p>As to their size, we can only say that they are far below the
-limits of visibility, and that even the minutest granules which we
-can barely perceive by means of our most powerful microscopes
-cannot be small individual biophors, but must be aggregates of these.
-On the other hand, the biophors must be larger than any chemical
-molecule, because they themselves consist of a group of molecules,
-among which are some of complex composition, and therefore of
-relatively considerable size.</p>
-
-<p>It may now be asked whether the determinants, whose existence
-we have already inferred, are not identical with these 'biophors' or
-smallest living particles; but that is not the case, at least not
-generally. We called determinants those parts of the germ-substance
-which determine a 'hereditary character' of the body; that is, whose
-presence in the germ determines that a particular part of the body,
-whether it consists of a group of cells, a single cell, or a part of a cell,
-shall develop in a specific manner, and whose variations cause the
-variations of these particular parts alone.</p>
-
-<p>Again, it may be asked how large and how numerous such
-'hereditary parts' may be, whether they correspond to every distinct
-part of a cell, or to every cell of the body, or only to the larger cell
-groups. Obviously the areas which are individually determined from<span class="pagenum"><a id="Page_370"></a>[Pg 370]</span>
-the germ must differ in size, according as we have to do with an
-organism which is small or large, simple or more complex. Unicellular
-organisms, such as Infusorians, probably possess special
-determinants for a number of cell-organs and cell-parts, although we
-cannot directly observe the independent and transmissible variation
-of these organs; lowly multicellular animals, such as the calcareous
-sponges, will require a relatively small number of determinants, but
-in the higher multicellular organisms, as, for instance, in most
-Arthropods, the number must be very high, reaching many thousands
-if not hundreds of thousands, for in them almost everything in the
-body is specialized, and must have varied through independent
-variation from the germ. Thus in many Crustaceans the smelling-hairs
-occur singly on special joints of the antennæ, and the number
-of joints furnished with a smelling-hair is different in different
-species; the size, too, of the smelling-hairs themselves varies greatly,
-being, for instance, much smaller in our common Asellus than in the
-blind form from the depths of our lakes, in which the absence of sight
-is compensated for by an increased acuteness of the sense of smell.
-Thus the smelling-hairs may vary transmissibly in themselves, while
-any joint of the antennæ may also produce one independently
-through variation. In this case accordingly we must assume that
-there are special determinants for the smelling-hairs, and for the
-joints of the antennæ. But we cannot always and everywhere refer
-identical or approximately similar organs, when there are many of
-them, to a corresponding number of determinants. Certainly the
-hairs of mammals or the scales of butterflies' wings do not all vary
-individually and independently, but those of a certain region vary
-together, and are therefore probably represented in the germ-plasm
-by a single determinant. These regions often appear to be very small,
-as is best seen by the fine lines, spots, and bands which compose the
-marking of a butterfly's wing, and still more in the odoriferous scales
-occurring in some butterflies, as, for instance, in the blue butterflies
-(<i>Lycæna</i>). These little lute-shaped scales do not occur in all species,
-and they occur in very unequal numbers even in those which possess
-them; there are certain species which exhibit only about a dozen, and
-these are all on one little spot of the wing. Since these odoriferous
-scales must have arisen as modifications of the ordinary hair-like
-scales, as one of my pupils, Dr. Köhler, has demonstrated by comparative
-studies, these ordinary hair-like scales must have varied transmissibly
-at certain spots, that is, their determinants have varied while
-those of the surrounding scales have not.</p>
-
-<p>The case is the same in respect to the sound-producing apparatus<span class="pagenum"><a id="Page_371"></a>[Pg 371]</span>
-of many insects. Many grasshoppers produce sounds by fiddling
-with the thigh of the hind leg on the wing, others by rubbing one
-anterior wing upon the other, and, indeed, always with one particular
-vein in one upon a particular vein in the other. One of these serves
-as the bow, the other as the string, of the violin, and the bow is
-furnished with teeth (<a href="#f95">Fig. 91</a>), ranged beside each other in a long
-row, which have the same function as the colophonium of the violin,
-that is, to grasp and release the strings alternately, and thus to
-produce resounding vibrations. My pupils, Dr. Petrunkewitsch and
-Dr. Georg von Guaita, have recently proved that these teeth have
-arisen as modifications of the hairs which are scattered everywhere
-over the wing and leg. But only in this one place, on the so-called
-'stridulating-vein,' have they been
-modified to form stridulating teeth
-(<i>schr</i>). Thus this vein must be
-capable of transmissible variation by
-itself alone, that is, there must be
-parts contained in the germ-plasm,
-the variation of which causes a
-variation solely of this individual
-vein and its hairs, possibly even
-a variation only on certain hairs on
-this vein.</p>
-
-<div class="figright" id="f95">
-<img src="images/fig95.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 91.</span> Hind leg of a Locustid<br />
-(<i>Stenobothrus protorma</i>), after Graber. <i>fe</i>,<br />
-femur. <i>ti</i>, tibia. <i>ta</i>, tarsal joints. <i>schr</i>,<br />
-the stridulating ridge.</p>
-</div>
-
-<p>On the other hand, there are
-also large regions, whole cell-masses
-of the body, which in all probability
-vary only <i>en bloc</i>, as, for
-instance, the milliards of blood-cells
-in Man, the hundreds of thousands or
-millions of cells in the liver and other glandular organs, the thousands
-of fibres of a muscle, or of the sinews or fascia, the cells of a cartilage
-or a bone, and so on. In all these cases a single determinant, or at
-least a few in the germ-plasm, may be enough. But in numerous cases
-it is impossible to say how large the region is which is controlled by
-a single determinant, and it is, of course, of no importance to the
-theory. In unicellular organisms the determinants will control parts
-of cells, in multicellular organisms often whole cells and groups of
-cells.</p>
-
-<p>Perhaps an inference as to the nature of the determinants may
-be drawn from this with some probability, in as far as mere parts of
-cells may be supposed to have simpler determinants than whole cells
-and groups of cells. The determinants in the chromosomes of uni<span class="pagenum"><a id="Page_372"></a>[Pg 372]</span>cellular
-organisms may therefore often consist of single biophors, so
-that in this case the conception of biophors would coincide with that
-of determinants. In multicellular organisms, on the other hand,
-I should be inclined on the whole to picture the determinant as
-a group of biophors, which are bound together by internal forces to
-form a higher vital unity. This determinant must live as a whole,
-that is, assimilate, grow, and multiply by division, like every vital unit,
-and its biophors must be individually variable, so that the separate
-parts of a cell controlled by them may also be capable of transmissible
-variation. That they are so, every highly differentiated cell of
-a higher animal teaches us; even the smelling-hairs of a crab exhibit
-a stalk, a terminal knob, and an internal filament, and many muscle-,
-nerve-, and gland-cells are much more complex in structure.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_373"></a>[Pg 373]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XVIII">LECTURE XVIII</h2>
-</div>
-
-<p class="c">THE GERM-PLASM THEORY (<i>continued</i>)</p>
-
-<div class="blockquot">
-
-<p>Structure of the germ-plasm&mdash;Vital affinities&mdash;Division&mdash;O. Hertwig's chief objections
-to this theory&mdash;Male and female eggs in the Phylloxera show differential division&mdash;Dispersal
-of the germ-plasm in the course of Ontogeny&mdash;Active and passive state
-of the determinants&mdash;Predetermination of cells&mdash;There are no determinants of
-characters&mdash;Liberation of the determinants&mdash;Accessory idioplasm&mdash;Herbst's lithium
-larvæ&mdash;Plant galls&mdash;Cells with several facultative determinants&mdash;Connective tissue in
-vertebrates&mdash;Mesoderm cells of Echinoderms&mdash;Sexual dimorphism&mdash;Female and male
-ids&mdash;Polymorphism (<i>Papilio merope</i>)&mdash;Ants.</p></div>
-
-
-<p><span class="smcap">I have</span> endeavoured to prove that the germ-substance proper
-must be looked for in the chromatin of the nucleus of the germ-cell,
-and more precisely still in those ids or chromosomes which we conceive
-of as containing the primary constituents (<i>Anlagen</i>) of a
-complete organism. Such ids in larger or smaller numbers make up
-the whole germ-plasm of a germ-cell, and each id in its turn consists
-of primary constituents or determinants, i.e. of vital units, each of which
-determines the origin and development of a particular part of the
-organism. We have now to make an attempt to picture to ourselves
-how these determinants predetermine those cells or cell-groups to
-which they correspond. In doing so we have to fall back upon mere
-hypotheses, and in stating any such hypothesis I wish expressly to
-emphasize that I am only following up one of the possibilities which
-our imaginative faculty suggests. Nevertheless, to endeavour to
-form such a conception is certainly not without use, for it is only by
-elaborating a theory to the utmost that we are able to use it in
-application to concrete cases, thus stimulating the search for corroboratory
-or contradictory facts, and leading gradually to a recognition
-of the gaps or mistakes in the theory.</p>
-
-<p>The first condition that must be fulfilled in order that a determinant
-may be able to control a cell or cell-group is that it should
-succeed in getting into it. It must be guided through the numerous
-cell-divisions of ontogeny so that it shall ultimately come to lie in the
-cells which it is to control. This presupposes that each determinant
-has from the very beginning its definite position in relation to the
-rest, and that the germ-plasm, therefore, is not a mere loose aggregate
-of determinants, but that it possesses a structure, an architecture, in<span class="pagenum"><a id="Page_374"></a>[Pg 374]</span>
-which the individual determinants have each their definite place. The
-position of the determinants in relation to one another cannot be due to
-chance, but depends partly on their historical development from earlier
-ancestral determinants, partly on internal forces, such as we have
-already assumed for keeping the determinants together. We may
-best designate these hypothetical forces 'affinities,' and in order to
-distinguish them from mere chemical affinities we may call them
-'vital.' There must be forces interacting among the different determinants
-which bind them together into a living whole, the id, which
-can assimilate, grow, and multiply by division, in the same manner as
-we were forced to assume for the smaller units, the biophors and
-single determinants. In the ids, however, we can observe the working
-of these forces quite directly, since each chromosome splits into two
-halves of equal size at every nuclear division, and not through the
-agency of external forces, e.g. the attraction which we may assume to
-be exerted by the fibrils of the nuclear spindle, but through purely
-internal forces, often long before the nuclear spindle has been formed
-at all.</p>
-
-<p>But if the determinants must separate from each other in the
-course of development so as to penetrate singly into the cells they are
-to control, the id must not only have the power of dividing into
-daughter-ids of identical composition, it must also possess the power
-of dividing under certain influences into dissimilar halves, so that the
-two daughter-ids contain different complexes of determinants. The
-first mode of division of the id, and with it of the nucleus and of the
-cell, I call <i>erbgleich</i>, or integral, the second <i>erbungleich</i>, or differential.
-The first form of multiplication is the usual one, which we
-observe everywhere when unicellular organisms divide themselves
-into two equal daughter-units, or when the cells of multicellular
-bodies produce their like by division into two. The second is not
-directly observable, because a dissimilarity of the daughter-cells, as
-long as it lies only in the idioplasm, cannot be actually seen; it can
-only be inferred from the different rôle which the two daughter-cells
-play in the building up of the individual. When, for instance, one of
-two sister-cells of the embryo gives rise to the cells of the alimentary
-canal and the other to those of the skin and the nervous system, I
-infer that the mother-cell divided its nuclear substance in a differential
-way between the two daughter-cells, so that one contained the
-determinants of the endoderm, the other those of the ectoderm; or
-when a red and a black spot lie side by side and under exactly the
-same conditions on the wing of a butterfly, I conclude that the
-ancestral cells of these two spots have divided differentially, so that<span class="pagenum"><a id="Page_375"></a>[Pg 375]</span>
-one received the 'red,' the other the 'black' determinants. Our eyes
-can perceive no difference between the nuclear substance of the two
-cells, but the same is true of the chromosomes of the paternal and
-maternal nuclei in the fertilized ovum, although we know in this
-case that they contain different tendencies. In any case we are not
-justified in concluding from the apparent similarity of the chromosome-halves
-in nuclear division that there cannot be differential
-division. The theoretical possibility that there is such differential
-division cannot be disputed; indeed, I am inclined to say that it
-is more easily imagined than the division of the ids into absolutely
-similar halves. Both are only conceivable on the assumption that
-there are forces which control the mutual position of the determinants
-in the ids, that is, on the assumption of 'affinities.' I shall not follow
-this further, but that there are forces operative within the ids which
-are still entirely unknown to us is proved at every nuclear division
-by the <i>spontaneous</i> splitting of the chromosomes.</p>
-
-<p>It has been objected to my theory that such a complex whole as
-the id could not in any case multiply by division, since there is
-no apparatus present which can, in the division into two daughter-units,
-re-establish the architecture disturbed by the growth. But
-this objection is only valid if we refuse to admit the combining
-forces, the 'vital affinities' within the ids, and the same is true for
-the smaller vital units. An ordinary chemical molecule cannot
-increase by division; if it be forcibly divided it falls into different
-molecules altogether; it is only the living molecule, that is, the
-biophor, which possesses this marvellous property of growth and
-division into two halves similar to itself and to the ancestral molecule,
-and we may argue from this that in the division of the ids
-forces of attraction and repulsion must likewise be operative<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a>.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> In my book <i>The Germ-Plasm</i> I have already assumed the existence of 'forces of
-attraction' in the determinants and biophors, as in the cells. I did not, indeed, enter
-into details, but I argued on the same basis as now (<i>Germ-Plasm</i>, p. 64, English edition).
-My critics have overlooked this.</p>
-
-</div>
-
-<p>I see no reason why we should not assume the existence of such
-forces, when we make the assumption that the hundreds of atoms
-which, according to our modern conceptions, compose the molecule of
-albumen and determine its nature, are kept by affinities in this
-definite and exceedingly complex arrangement. Or must we suppose
-that between the atom-complex of the molecule and the next higher
-atom-complex of the biophor, determinant, and id there is an absolute
-line of demarcation, so that we must assume quite different forces in
-the latter from those we conceive of as operative in the former? The<span class="pagenum"><a id="Page_376"></a>[Pg 376]</span>
-biophor is ultimately only a group of molecules, the determinants
-a group of biophors, the id a group of determinants, and all the three
-inferred stages of vital organization only become real units through
-the forces operating within them and combining them into a whole.
-What compels the chromatin granules of the resting nucleus to
-approach each other at the time of cell-division, to unite into a long,
-band-like thread, and what is it that subsequently causes this thread
-to break up again into a definite number of pieces? Obviously only
-internal forces of which we know nothing further than that they are
-operative.</p>
-
-<p>We shall see later that this assumption of vital affinities must be
-made not only in regard to the cells, but also in regard to entire
-organisms whose parts are united by an internal bond, and whose
-co-ordination is regulated by forces of which we have as yet no
-secure knowledge. In the meantime we may designate these forces
-by the name of 'vital affinities.'</p>
-
-<p>It must be admitted, however, that some objections of a fundamental
-nature have been urged against the assumption of a differential
-nuclear division of the hereditary substance. O. Hertwig holds that
-the assumption of differential division is essentially untenable, because
-it is contradictory to 'one of the first principles of reproduction,' for
-'a physiologically fundamental character of every living being is the
-power of maintaining its species.'</p>
-
-<p>This certainly seems so, but a closer examination shows that this
-'principle,' although correct enough when taken in a very general
-sense, does not really cover the facts, and is therefore incapable of
-supporting the inferences drawn from it. If the proposition expressed
-the whole truth there could have been no evolution from the primitive
-organisms to higher ones, every living being must have simply
-reproduced exact copies of itself. Whether the transformations of
-species have been sudden or gradual, whether they have been brought
-about by large steps or by very small ones, they could only have
-come about by breaking through this so-called 'principle' of like
-begetting like. In fact, we may with more justice maintain the
-exact converse of the principle, and say that 'no living being is able
-to produce an exact copy of itself,' and this is true not only of sexual,
-but of asexual reproduction.</p>
-
-<p>In ontogeny we see exactly the same thing. There are no two
-daughter-cells of a mother-cell which are exactly alike, and the
-differences between them, if they increase in the same direction, may
-lead in later descendants to entire differences of structure. Indeed
-the whole process of development depends on such an augmentation<span class="pagenum"><a id="Page_377"></a>[Pg 377]</span>
-of the differences between two daughter-cells&mdash;on differences which
-proceed from within and are definitely pre-established. Here, again,
-the facts do not justify us in making a dogma of the proposition that
-it is a 'fundamental power' of every living being to maintain its
-species by producing replicas of itself. If we look at two directly
-successive cell-generations, we can hardly, it is true, in most cases,
-perceive any difference between them, just as in the generations
-of species; but if we compare the end of a long cell-lineage with the
-beginning, then the difference is marked, and we recognize that the
-difference is due to a gradual summing up of minute, invisible
-deviations. In my opinion these steps of difference cannot possibly
-depend merely on direct external influences; they proceed rather from
-the hereditary substance the cell receives from the ovum, which,
-therefore, in order to attain to such many-sided and far-reaching
-differentiation, must have undergone a frequently repeated splitting
-up of its qualities. That this splitting is not merely a variation
-to which the whole of the hereditary substance of the daughter-cells
-is uniformly subject, according to the influences dependent on their
-position in relation to other cells of the embryo, will be made clear
-from the case of the Ctenophora referred to in the next lecture.
-A scarcely less striking example is that of those animals in which
-the ova contain the primary constituents for only one sex, in which,
-in other words, there are 'male' ova and 'female' ova. This is the
-case, for instance, among Rotifers, and in plant-lice such as the vine-pest,
-<i>Phylloxera</i>. Here the eggs from which males develop are
-smaller than those which produce females. The primary constituents
-for both male and female are not, as in most animals, contained in the
-same ovum, to be liberated on one side or the other by influences
-unknown to us, but in each ovum there is only one of the two
-sets of primary constituents present, and in this case, therefore, the
-development of hermaphrodites, which not infrequently occur in
-other animals, would be impossible. But all these ova have been
-produced by one primitive reproductive cell, and consequently, at one
-of the divisions implied in the multiplication of this first cell, a separation
-of the male from the female primary constituents must have
-taken place, that is, a differential division of hereditary substance, for
-which no external and no intercellular influences can possibly account.</p>
-
-<p>If there is, then, a differential division of the ids and with them
-of the whole idioplasm, the germ-plasm of the fertilized ovum must
-be broken up in the course of ontogeny into ever smaller groups
-of determinants. I conceive of this as happening in the following
-manner.</p>
-
-<p><span class="pagenum"><a id="Page_378"></a>[Pg 378]</span></p>
-
-<p>In many animals the fertilized ovum divides at the first segmentation
-into two cells, one of which gives rise predominantly
-to the outer, the other to the inner germinal layer, as in molluscs, for
-instance. Let us now assume that this is the case altogether, so that
-one of the first two blastomeres gives rise to the whole of the ectoderm,
-the other to the whole of the endoderm; we should here have a differential
-division, for the developmental import (the 'prospective' of
-Driesch) of the primitive ectoderm-cell is quite different from that
-of the primitive endoderm-cell, the former giving origin to the skin
-and the nervous system, with the sense organs, while the second gives
-rise to the alimentary canal, with the liver, &amp;c. Through this step
-in segmentation, I conclude, the determinants of all the ectoderm-cells
-become separated from those of the endoderm-cells; the
-determinant architecture of the ids must be so constructed in such
-species that it can be segregated at the first egg-cleavage into ectodermal
-and endodermal groups of determinants. Such differential
-divisions will always occur in embryogenesis when it is necessary
-to divide a cell into two daughter-cells having dissimilar developmental
-import, and consequently they will continue to occur until the
-determinant architecture of the ids is completely analysed or segregated
-out into its different kinds of determinants, so that each cell
-ultimately contains only one kind of determinant, the one by which
-its own particular character is determined. This character of course
-consists not merely in its morphological structure and chemical content,
-but also in its collective physiological capacity, including its
-power of division and duration of life<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a>.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> Emery has lately called attention to another direct proof of the existence of
-differential cell- and nucleus-division. According to observations made by Giardina,
-in the water-beetle (<i>Dytiscus</i>), one primitive ovum-cell gives rise, through four successive
-divisions, to fifteen nutritive cells and one well-defined ovum-cell. But only half of
-the nuclear substance takes part in these divisions, the rest remains inactive in
-a condensed, cloudy condition. 'The meaning of the whole process is obviously that
-the germ-plasm mass as a whole is handed over to the ovum-cell, while the nutritive
-cells receive only the nuclear constituents which belong to them' (<i>Biol. Centralbl.</i>, May
-15, 1903).</p>
-
-</div>
-
-<p>But embryogenesis does not proceed by differential divisions
-alone, for integral divisions are often interpolated between them,
-always, for instance, when in a bilateral animal an embryonic cell
-has to produce by division into two a corresponding organ for the right
-and left sides of the body; for instance, in the division of the primitive
-genital cell into the rudiments of the right and left reproductive
-organs, or in the division of the primitive mesoderm-cell into the right
-and left initial mesoderm-cell, but also later on in the course of embryogenesis,
-when, for instance, the right or the left primitive reproductive<span class="pagenum"><a id="Page_379"></a>[Pg 379]</span>
-cell multiplies into a large number of primitive germ-cells, or in the
-multiplication of the blood-cells, or of the epithelial cells of a particular
-region; in short, whenever mother and daughter-cells have the same
-developmental import, that is, when they are to become nothing more
-than they already are. In all such cases a similar group of determinants,
-or a similar single determinant, must in the nuclear division
-penetrate into each of the two daughter-cells.</p>
-
-<p>It is in this way, it seems to me, that the determinants gain
-entrance into the cells they are to control, by a regulated splitting
-up of the ids into ever smaller groups of determinants, by a gradual
-analysis or segregation of the germ-plasm into the idioplasms of the
-different ontogenetic stages. When I first developed this idea
-I assumed that the splitting process would in all cases set in at the
-same time, namely, at the first division of the ovum. But since then,
-in the controversies excited by the theory, many facts have been
-brought to light which prove that the ova of the different animal
-groups behave differently, and that the splitting up of the aggregate
-of primary constituents may sometimes begin later&mdash;but I shall return
-to this later on.</p>
-
-<p>If we accept the segregation hypothesis, which is similar in
-purport to that advanced by Roux as the' mosaic theory,' it must strike
-us as remarkable that the chromatin mass of the nucleus does not
-become notably smaller in the course of ontogeny, and even ultimately
-sink to invisibility. Determinants lie far below the limits of visibility,
-and if there were really only a single determinant to control each cell
-there would be no chromatin visible in such a case. This objection has
-in point of fact been urged against me, although I expressly emphasized
-in advance the assumption that the determinants are continually
-multiplying throughout the whole ontogeny, so that in proportion
-as the number of the <i>kinds</i> of determinants lying within a cell
-diminishes the number of resting determinants of each kind increases.
-When, finally, only one kind of determinant is present there is a whole
-army of determinants of that kind.</p>
-
-<p>It follows from this conception of the gradual segregation of the
-components of the id in the course of development that we must
-attribute to the determinants two different states, at least in regard
-to their effect upon the cell in which they lie: an active state,
-in which they control the cell, and a passive state, in which they
-exert no influence upon the cell, although they multiply. From the
-egg onwards, therefore, a mass of determinants is handed on by the
-cell-divisions of embryogenesis, which will only later become active.</p>
-
-<p>My conception of the manner in which the determinants become<span class="pagenum"><a id="Page_380"></a>[Pg 380]</span>
-active is similar to that suggested by De Vries in regard to his
-'Pangens,' very minute vital particles which play a determining part
-in his 'pangen theory,' similar to that filled by the determinants
-in my germ-plasm theory. It seems to me that the determinants
-must ultimately break up into the smallest vital elements of which
-they are composed, the biophors, and that these migrate through the
-nuclear membrane into the cell-substance. But there a struggle for
-food and space must take place between the protoplasmic elements
-already present and the newcomers, and this gives rise to a more
-or less marked modification of the cell-structure.</p>
-
-<p>It might be supposed that the structure of these biophors corresponded
-in advance to certain constituent parts of the cell, that there
-were, for instance, muscle biophors, which make the muscle what
-it is, or that the plant-cells acquired their chlorophyll-making
-organs through chlorophyll biophors. De Vries gave expression
-to this view in his 'pangen theory,' and I confess that at the time
-there seemed to me much to be said for it, but I am now doubtful
-whether its general applicability can be admitted. In the first place,
-it does not seem to me theoretically necessary to assume that the
-particles which migrate into the cell-bodies should themselves be
-chlorophyll or muscle particles; they may quite well be only the
-architects of these, that is to say, particles which by their co-operation
-with the elements already present in the cell-body give rise to chlorophyll
-or muscle substance. As we are as yet unacquainted with the
-forces which dominate these smallest vital particles, as well as the
-processes which lead to the histological differentiation of the cells,
-it is useless in the meantime to make any further hypotheses in regard
-to them. But in any case the biophors which transform the general
-character of the embryonic cells into the specific character of a particular
-tissue-cell must themselves possess a specific structure different
-from that of other biophors, for they must keep up the continuity
-of the structures handed on from ancestors, chlorophyll and muscle-substance
-and the like, since we cannot assume that these structures,
-so peculiar and so complex in their chemical and physical constitution,
-are formed afresh, so to speak, by spontaneous generation in each new
-being, as De Vries has very rightly emphasized. A specific biophor,
-for instance, of muscle substance will produce this substance as soon
-as it has found its way into the appropriate cell-body, even though
-it may not be a contractile element itself.</p>
-
-<p>To this must be added that the structure of the body and the
-distinctive features of an organism do not depend merely on the
-histological differentiation of the cells, but quite as much on their<span class="pagenum"><a id="Page_381"></a>[Pg 381]</span>
-number and arrangement, and on the size and on the frequency of
-repetition of certain parts. These distinctive characters are just
-as constant and as strictly transmissible, and may be as heritably
-variable as those which depend on specific cell-differentiation, and
-they must therefore likewise be determinable by definite elements
-of the germ-plasm. Obviously enough, however, these elements are
-not of the same nature as the known specific histological elementary
-particles; they can be neither nerve-, muscle-, nor gland-biophors.
-They must rather be vital units of such a kind that they communicate
-to the cells and lineage of cells, into whose bodies they migrate from
-within the nucleus, a definite vital power, that is, an organization
-which regulates the size, form, number of divisions, and so on, of
-these cells&mdash;in short their whole prospective significance. Always,
-however, they act in co-operation with the cell-body into which they
-have penetrated.</p>
-
-<p>Throughout we must hold ourselves aloof from the idea that
-'characters' are transmissible. It is customary, indeed, to speak
-as if this were so, and it is also necessary, because we can only
-recognize the 'characters' of a body, and not the essential 'nature'
-on which these characters depend; but the determinants are not seed-grains
-of individual characters, but co-determinants of the nature
-of the parts which they influence. There are not special determinants
-of the size of a cell, others of its specific histological differentiation,
-and still others of its duration of life, power of multiplication, and
-so on; there are only determinants of the whole physiological nature
-of a cell, on which all these and many other 'characters' depend.
-For this reason alone I should object to the assumption that the
-determinants of the germ are ready-made histological substances.
-That is as unlikely as that their groups in the germ-plasm are
-'miniature models' of the finished parts of the body.</p>
-
-<p>I conceive of the process of cell-differentiation as follows: at
-every cell-stage in the ontogeny determinants attain to maturity, and
-break up so that their biophors can migrate into the cell-bodies,
-so that the quality of each cell is thus kept continually under control,
-and may be more or less modified, or may remain the same. By the
-'maturity' of a determinant I mean its condition when by continual
-division it has increased in number to such a point that its disintegration
-into biophors and their migration into the cell-substance can take place.</p>
-
-<p>One more point I must touch upon here, the question of the
-'liberation' or 'stimulation' of the determinants. The activity of
-an organ never depends on itself alone; the contraction of a muscle is
-induced by a nerve stimulus or by an electric current; the activity<span class="pagenum"><a id="Page_382"></a>[Pg 382]</span>
-of the nerve-cells of the brain requires the continual stimulus of the
-blood-stream, and cannot continue to exist without it; the specific
-sensory-nerves and sense-cells of the eye, ear, olfactory organ, and so
-on, are all prompted to activity by adequate stimuli. The same is
-true in regard to the determinants, they must be 'liberated' if they
-are to distribute themselves and migrate into the cell-body; and we
-have to ask how that happens, whether it is possibly due only to
-their own internal condition, which again would, of course, depend
-on the nutritive conditions of the cell in which they lie, or whether it
-is perhaps due to some specific stimulus which is necessary in addition
-to the fact of 'maturity,' just as a muscle is always ready to contract,
-yet only does so when it is affected by a specific stimulus.</p>
-
-<p>From the very first, therefore, I have considered whether it
-would not be better to elaborate the determinant theory in such a way
-that it would not be necessary to assume a disintegration of the id in
-the course of ontogeny, but simply to conceive of every expression
-of activity on the part of a determinant as dependent on a specific
-stimulus, which in many cases can only be supplied by a definite cell,
-that is, by internal influences, and in other cases may be due to
-external influences.</p>
-
-<p>Darwin assumed the first of these alternatives in his theory of
-Pangenesis, which we have still to outline. In it he attributes to his
-'gemmules' the power of giving rise to particular cells, which,
-however, they can only accomplish when they reach the cells which
-are the genetic antecedents of those which the gemmules are to
-control. Translated into the language of our theory this view would
-read as follows: the whole complex of determinants is contained
-within every cell, as it is contained in the germ-cell, but at every
-stage of ontogeny, that is, in each of the developing cells, only the
-determinant which is to control the immediately successive cells is
-'liberated,' and that through the stimulus which the specific nature of
-the cell supplies to the determinant. In that case there would
-necessarily be in every species of animal as many specific stimuli
-for determinants as there are determinants. This appeared to me
-improbable, and I rejected the hypothesis because of the enormous
-number of specific stimuli which it demands, but also on other grounds
-which will be touched upon in the course of these lectures.</p>
-
-<p>Although the assumption of an autonomic dissolution of the
-determinant complexes of the id in the course of ontogeny seems
-to me imperative, I do not by any means reject the interposition
-of liberating stimuli, indeed I regard their co-operation as indispensable.
-Later on we shall discuss cases in which it is definitely<span class="pagenum"><a id="Page_383"></a>[Pg 383]</span>
-demonstrable that there may be two alternative sets of homologous
-determinants present in a cell, but that on any occasion only one
-of these becomes active, a fact which we can only explain on the
-assumption that only one of these is affected by the specific liberating
-stimulus. The phenomena of regeneration, of polymorphism, of
-germ-cell formation, &amp;c., compel us to the assumption that numerous
-cells, even after the completion of the building up of the body, contain
-two or more kinds of determinants, as in a sense inactive 'accessory
-idioplasm,' each of which could control the cell alone, though in reality
-it only does control it when it is affected by the appropriate liberating
-stimulus. I stated this view some years ago when I attempted to
-define more precisely the rôle played by 'external influences as
-developmental stimuli<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a>'. It is not, then, that I underrate the
-importance of external influences on the organism, but I believe that
-a still larger part of the determination of what shall happen at a
-particular point depends on the primary constituents, and that these
-are not alike at all parts of the body.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> <i>Äussere Einflüsse als Entwicklungsreize</i> [External Influences as Stimuli to Development],
-Jena, 1894.</p>
-
-</div>
-
-<p>All living processes, therefore, both those of growing and of
-differentiation, depend always upon the interaction of external and
-internal factors, of the environment and the living substance, and the
-resultants of the interaction, namely, the structure of the body and its
-parts must necessarily turn out differently, not only when the germ-substance
-is different, but when the essential conditions of development
-are changed. But that the constitution of the germ is by far the
-most potent factor, and that the nature of the results of development
-depends on it in a much greater degree than on the external conditions,
-has long been known. The conditions, such as warmth, may vary
-within certain limits, and yet the frog's egg becomes a frog; though
-it does not follow that the result of development may not be modified
-through certain changes in the conditions. The interesting experiments
-made by Herbst with the eggs of sea-urchins have shown that,
-in artificially altered sea-water in which sodium-salts are to a slight
-extent replaced by lithium-salts, these eggs develop into larvæ which
-only remotely suggest the normal structure, and diverge widely from
-it both in external shape and in the form of the skeleton.</p>
-
-<p>Such larvæ are not able to survive, but soon perish; they
-are, however, of great interest from the point of view of our theory,
-for they show that determinants do not bring forth the same structure
-under all circumstances, but that, as I have already said, they are
-vital units of specific composition, which play a part in the course of<span class="pagenum"><a id="Page_384"></a>[Pg 384]</span>
-development, and give rise under normal external influences to normal
-parts, while under unusual influences, if these are not such as to
-prohibit development altogether, they may give rise to an abnormally
-formed part. It must not be forgotten that most composite parts&mdash;indeed,
-strictly speaking, all the parts&mdash;of an animal are not controlled
-by a single determinant, but by the many which successively determine
-the character of the cells and define the path of development of the
-part in question. There are no determinants of 'characters,' but only
-of parts; the germ-plasm no more contains the determinants of a
-'crooked nose' than it does those of a butterfly's tailed wing, but
-it contains a number of determinants which so control the whole
-cell-group in all its successive stages, leading on to the development
-of the nose, that ultimately the crooked nose must result, just as the
-butterfly's wing with all its veins, membranes, tracheæ, glandular
-cells, scales, pigment deposits, and pointed tail arises through the
-successive interposition of numerous determinants in the course of
-cell-multiplication.</p>
-
-<p>But in both processes we must presuppose <i>normal conditions of
-development</i>. In regard to the butterfly we know that abnormal
-conditions, such as cold during the pupal period, can cause considerable
-variation in the colour and marking of the wing, and in regard to the
-nose it can scarcely be doubted that, for instance, persistent pressure
-on the nasal region would result in a considerable deviation from the
-hereditary form.</p>
-
-<p>The case of the lithium-larvæ is similar. Here the chemical
-conditions of the first segmentation-cells are modified by the presence
-of the lithium-salts, and the determinants which make their way out
-of the nucleus in the first and in subsequent cell-generations find an
-unusual soil for their activity, which diverges further and further
-from the normal with each successive cell-generation. Thus the
-whole animal is abnormally formed. The process may perhaps be
-compared to a plant which is negatively geotropic and positively
-heliotropic, that is, the stem of which tends to grow straight upwards,
-while all its green parts grow towards the light. If a plant of this
-kind have light shed on it from one side only, the stem with its
-leaves will grow obliquely towards that side. If the plant be then
-turned round so that it receives light from the other side, the stem in
-its further growth will curve in a direction opposite to that which
-it took before, and so by continually changing the position of the
-plant in relation to the light one could&mdash;theoretically at least&mdash;produce
-a plant with a zigzag stem. But this would not furnish any evidence
-against the presence of determinants; there are no 'upright deter<span class="pagenum"><a id="Page_385"></a>[Pg 385]</span>minants'
-any more than there are 'zigzag determinants' or 'crooked
-nose determinants,' but there are determinants controlling the nature
-of the cells which give rise, under normal conditions of development,
-to the straight stem, under abnormal conditions to the zigzag stem, or
-to a flat nose instead of a crooked one, and so on.</p>
-
-<p>This consideration should make it clear that plant-galls are not
-in the remotest degree a stone of stumbling for the determinant
-theory, as some have supposed. Of course there can be no 'gall-determinants,'
-for galls are not transmissible adaptations of the plants
-on which they occur; they arise solely through the larvæ of the gall-insect
-which has laid its eggs within the tissues of the plant. But
-the specific nature of the different kinds of plant-cells, predetermined
-by their determinants, is such that, through the abnormal influences
-exercised upon them by the larvæ, they are compelled to a special
-reaction which results in the formation of galls. It is marvellous
-enough that these abnormal stimuli should be so precisely graded and
-adjusted that such a specifically definite structure should result, and in
-this case there is obviously a very different state of matters from that
-obtaining in most other processes of development, in which the chief
-determining factor is rather implied in the nature of the idioplasm,
-that is, of the determinants, than in the nature of the external
-influences. Here, however, the specific structure of the gall depends
-mainly on the quality, variety, and successive effects of the external
-influences or stimuli. In discussing the influences of surroundings
-I shall return once more to the galls.</p>
-
-<p>My determinants have generally been regarded as if they were
-like grains of seed, from which either nothing may arise, under unfavourable
-conditions, or just the particular kind of plant from which
-the seed itself originated.</p>
-
-<p>This simile is, however, to be taken <i>cum grano salis</i>. The whole
-ovum is certainly comparable to a grain of seed, but single determinants
-or groups of determinants will always be able to adapt
-themselves to different influences, and to remain active even under
-slightly abnormal conditions, though in that case the resulting
-structures may be somewhat divergent. This relative plasticity is
-indispensable even in relation to the ceaseless mutual adaptations of
-the growing parts of the organism. Not only do the cells which live
-beside each other at the same time influence each other mutually, but
-the influence extends to the whole cell-lineage. No cell or group
-of cells develops independently of all the others in the body, but each
-has its ancestral series of cells on whose determinants it is so far
-dependent, since these have taken part in determining its own nature,<span class="pagenum"><a id="Page_386"></a>[Pg 386]</span>
-in, so to speak, supplying the soil in which ultimately its own determinants
-will be sown from the nucleus, and whose influence modifies
-these last according to its quality. We might therefore say that
-every part is determined by all the determinants of its cell-ancestors.</p>
-
-<p>If there be urged against the doctrine of determinants the
-undoubted fact of the dependence of individual development on external
-conditions, or the capacity that organisms have of functional adaptation,
-or especially the power that some parts of the organism have of taking
-a different form in response to different stimuli, I can only say that
-I see no reason why certain cells and masses of cells should not be
-adapted from the first for responding differently to different stimuli.</p>
-
-<p>Therefore I see no contradiction of the determinant theory when,
-for instance, among the higher vertebrates, the cells of the connective
-tissue exhibit a great diversity of form, becoming a loose 'filling'
-connective tissue in one place, a tense fascia, ligament, or tendon tissue
-in another, according as they are subjected to slight pressure on all
-sides or to stronger pressure on one side. I see no difficulty in the
-fact that this connective tissue forms in one case bone-tissue with the
-most accurate adaptation of its microscopic structure to the conditions
-of stress and pressure which affect the relevant spot, or in another
-case cartilaginous tissue, when the cells are exposed to varying
-pressure (as on the surface of joints), or even that it gives rise to
-blood-vessels when the pressure of the circulating blood and the
-tension of the surrounding tissues supply the necessary stimulus.
-It is easy to see how important, indeed how necessary, the many-sidedness
-of these cells is for the organism, even leaving out of
-account such violent interference as the breaking of a bone, the
-irregular healing of broken ends of bones, new joint formation, and
-so on, and thinking only of the normal phenomena of growth. While
-the bone grows it is continually breaking up in the inside and
-forming anew on the surface, and this occurs through the power of
-the connective tissue-cells to form different tissues under different
-influences or stimuli.</p>
-
-<p>We must therefore assume that there are side by side in the
-connective cells of higher vertebrates determinants of bone, of
-cartilage, of connective tissue in the narrower sense, and of blood-vessels,
-and that one or other of these is liberated to activity
-according to the stimulus affecting it. Phenomena occur also in the
-development of lower animals which lead us to the same assumption.</p>
-
-<p>Among these is the remarkable behaviour of the primary
-mesoderm-cells in the young embryo (gastrula) of the Echinoderms
-(<a href="#f96">Fig. 92</a>). At the point where the primitive gut or archenteron<span class="pagenum"><a id="Page_387"></a>[Pg 387]</span>
-invaginates into the interior of the hitherto single-layered blastula
-(Fig. 92, <i>A</i>), some cells are separated off (<i>M</i>), and move independently,
-constantly multiplying the while, into the clear gelatinous fluid (<i>G</i>)
-which fills the cavity of the larva, and there they fix themselves,
-some on the outer ectodermic layer, others to the various regions
-and outgrowths of the archenteron (<i>Ms</i>). According as these cells
-have established themselves at one or another point, they become
-connective tissue, muscle, or skeleton cells of the dermis, or contribute
-to the muscular layer of the food-canal and water-vascular system,
-or, finally, become skeleton-forming cells of the calcareous ring which
-surrounds the gullet of the sea-cucumber. In all this there is nothing
-to indicate a determination of the cells in one direction; on the
-contrary it seems as if the fate of the individual cells depended on
-the chance conditions which may lead them to one place or to
-another.</p>
-
-<div class="figcenter" id="f96">
-<img src="images/fig96.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 92.</span> Echinoderm-larvæ. <i>A</i>, blastula-stage; the primary mesoderm-cells
-(<i>M</i>) are being formed at the subsequent invagination-area of the endoderm
-(<i>Ent</i>). <i>Ekt</i>, the ectoderm. <i>B</i>, gastrula-stage; the archenteron (<i>UD</i>) has been
-invaginated (<i>Ent</i>), and between it and the ectoderm (<i>Ekt</i>) the mesoderm-cells
-(<i>Ms</i>) migrate into the gelatinous fluid which fills this cavity. There they
-attach themselves partly to the ectoderm, and partly to the endoderm. After
-Selenka.</p>
-</div>
-
-<p>There are thus three possibilities of development, three kinds of
-reaction, implied in these cells, which are all outwardly alike, and we
-can only understand their rôle in the building up of this very
-symmetrical animal if we assume that of these three only one is
-in each case liberated, by the specific stimulus exerted by the
-immediate surroundings of the cell, so that it may become, according
-to the chance position it takes up after its migration, either a skin-cell,
-a muscle-cell, or a skeleton-forming cell.</p>
-
-<p><span class="pagenum"><a id="Page_388"></a>[Pg 388]</span></p>
-
-<p>This case may be compared in some respects with the permanent
-colour-adaptation of those caterpillars, in regard to which Poulton
-demonstrated that they become almost black if they are reared on
-blackish-brown bark, light brown on light bark, and green if they
-are kept among leaves, and in all cases permanently so. In this case
-also the implicated pigment-cells of the skin may develop in three
-ways, according to whether this or that quality of the light releases
-this or that determinant.</p>
-
-<p>But in many cases we do not know the quality of the liberating
-stimulus, and must content ourselves with imagining it. This is so
-in the case of dimorphism of the sexes. It is clear that in the males
-of a species the germ-cells develop quite otherwise than they do in
-the females, that different determining elements attain to activity in
-each sex, and since the primary constituents of both sexes must be
-contained in most animals in the ovum and in the spermatozoon,
-we must assume that in both there are at once 'ovogenic' and
-'spermogenic' determinants, of which, however, only <i>one</i> kind becomes
-active in a given individual. There are, however, both among plants
-and animals hermaphrodite individuals, in which both kinds of sexual
-products are developed simultaneously or successively.</p>
-
-<p>It is not only the primary sexual characters, however, that
-compel us to the assumption of double determinants in the germ-plasm,
-the secondary sexual characters do so too. We know very
-well in relation to ourselves that 'the beautiful soprano voice of the
-mother may be transmitted through the son to the grand-daughter,
-and that the black beard of the father may pass through the daughter
-to the grandson.' Thus both kinds of sexual characters <i>must be
-present in every sexually differentiated being</i>, some visible, others
-latent. In animals the determinants are sometimes handed on from
-germ-plasm to germ-plasm through several generations in a latent
-state, and only make their appearance again in a subsequent
-generation. This is the case in the water-fleas (Daphnids) and the
-plant-lice (Aphides), in which several exclusively female generations
-succeed one another, and only in the last of them do males occur
-again side by side with the females.</p>
-
-<p>The germ-plasm of the ovum which is ripe for development must
-thus contain not only the determinants of the specific ova and sperms
-of the species, but also those of all the male and female sexual
-characters, which we discussed at length in the section on sexual
-selection. I then showed that these secondary sexual characters
-differ greatly in range and in strength, that among lower animals
-they are almost entirely absent, and that among higher forms, such<span class="pagenum"><a id="Page_389"></a>[Pg 389]</span>
-Crustaceans, Insects, and Birds, they attain to very different grades
-of development even among the same species. Thus the birds of
-Paradise are in most species brilliantly coloured and adorned with
-decorative feathers only in the male sex, while the females are
-simply blackish-grey, but there is a single species in which the males
-are almost as soberly coloured as the females. Conversely, too, we
-find that in parrots both sexes are usually coloured alike, but a few
-species exhibit a totally different colouring in the two sexes. In the
-same way the secondary sex differences may affect only a few parts
-of the animal or many, while in a few species the sexes are so
-divergent in structure that almost everything about them may be
-called different. Examples of this are the dwarf males of most
-Rotifers, and the males, more minute still in proportion to the females,
-of the marine worm <i>Bonellia viridis</i> (<a href="#Page_227">p. 227</a>).</p>
-
-<p>We have now to inquire what theoretical explanation of these
-facts we can arrive at in accordance with the germ-plasm theory.
-That double determinants, male and female, for the differently formed
-parts of the two sexes must be assumed to exist in the germ-plasm
-has been already said, and we have to suppose that the same stimulus&mdash;usually
-unknown to us&mdash;which incites the determinants of the
-primary sexual characters to activity also liberates those of the
-secondary characters. But we may safely go a step further and
-conclude that there are male and female <i>ids</i>, that is, that the male
-and female determinants belong to different ids. I infer this from
-the fact that in some groups, such as the Rotifers and certain plant-lice,
-the ova are sexually differentiated even at the time of their
-origin. Males and females of these animals arise from different
-kinds of eggs, which are even externally recognizable. Both develop
-parthenogenetically, so that fertilization has nothing to do with it;
-from the first, therefore, they must contain ids which consist of determinants
-of one sex alone.</p>
-
-<p>If this conclusion be correct, then the sexual equipment of the
-determinants of the sexual characters must have taken place in
-the course of phylogeny in such a way that each id was affected
-in one direction only, and we should thus have to assume male and
-female ids, even before the separation of the sexes as males and
-females, and the same conclusion must be extended to the primary
-sexual characters. Only in this way can we understand the fact
-that differences between the sexes, at first small, have increased in the
-course of phylogeny to such complete divergence of structure as is
-now exhibited in the forms we have named, <i>Bonellia</i>, the Rotifers,
-and some parasitic worms.</p>
-
-<p><span class="pagenum"><a id="Page_390"></a>[Pg 390]</span></p>
-
-<p>But there is not only sexual dimorphism, there is also dimorphism
-of larvæ, e.g. green and brown caterpillars in certain species
-of hawk-moth (<i>Sphinx</i>), and there are sometimes not only two but
-three or more forms of a species; and in all these cases determinants of
-the differential parts must be represented twice, thrice, or several times
-in each germ-plasm, in each fertilized ovum, at least in all cases in
-which the different forms live together on the same area. In
-discussing mimicry we spoke of species of butterfly which were
-everywhere alike or nearly so in the male sex, while the females
-were not only quite different from the males, but differed greatly
-in many respects among themselves. Three different forms of females
-of <i>Papilio merope</i> occur in the same region of Cape Colony, each of
-these resembling a protected model. All three forms have been
-obtained from the eggs of one female. In this case the female ids of
-the germ-plasm must be represented by three different sets, one
-of which, when it is in the majority in the fertilized ovum, gives rise
-to the <i>Danais</i>-form, the second to the <i>Niavius</i>-form, and the third to
-the <i>Echeria</i>-form of the species. Phylogenetically considered, it is
-probable that each of these three kinds of ids originated by itself, on
-a more limited area on which the protected model lived in abundance;
-but with a wider distribution the different female ids mingled
-together, were united through the males into a single germ-plasm,
-and now occasionally exhibit all three forms on the same area.
-I doubt whether there is any other theory that can offer an interpretation
-of these facts, and I regard them, therefore, as affording
-further evidence of the real existence of ids.</p>
-
-<p>The polymorphism of social insects must be thought of as
-similarly based in the germ-plasm.</p>
-
-<p>In bees there are in addition to the males and females the so-called
-workers, and this can only depend on the existence of special
-kinds of ids. Those of the workers were originally truly female, but
-as many of their determinants underwent variations advantageous for
-the maintenance of the species, they were modified into special
-'worker-ids.' I postpone for the present any inquiry into the causes
-by which these ids come to dominate the ontogeny; obviously it
-cannot be by the mere fact of being in a majority over the rest
-of the ids, as I indicated in the case of the butterflies with polymorphic
-females.</p>
-
-<p>In many ants the division of labour goes further still; there are
-two kinds of workers in the colony, the ordinary workers and the
-so-called 'soldiers,' and in this case the worker-id must have developed
-in two different directions in the course of phylogeny, and have<span class="pagenum"><a id="Page_391"></a>[Pg 391]</span>
-separated into two kinds of ids, so that the germ-plasm of these
-species must contain four kinds of ids.</p>
-
-<p>I might cite many more cases in regard to which the assumption
-of two or more kinds of determinants seems imperative, but I believe
-that what has been said is enough to enable any one to think out
-other cases for himself.</p>
-<hr class="full" />
-
-<div class="chapter">
-<p><span class="pagenum"><a id="Page_392"></a>[Pg 392]</span></p>
-
-<h2 class="nobreak" id="LECTURE_XIX">LECTURE XIX</h2>
-</div>
-
-<p class="c">THE GERM-PLASM THEORY (<i>continued</i>)</p>
-
-<div class="blockquot">
-
-<p>Co-operation of the determinants to form an organ: insect appendages&mdash;Venation
-of the insect-wing&mdash;Deformities in Man&mdash;Apex of the fly's leg&mdash;Proofs of the
-existence of determinants&mdash;Claws and adhesive lobes&mdash;Difference between a theory
-of development and a theory of heredity&mdash;Metamorphosis of the food-canal in insects&mdash;Delage's
-theory&mdash;Reinke's theory of the organism-machine&mdash;Fechner's views&mdash;Apparent
-contradiction by the facts of developmental mechanics&mdash;Formation of the
-germ-cells&mdash;Displacement of the germinal areas in the hydro-medusoid polyps, a proof
-of the existence of germ-tracks.</p></div>
-
-
-<p><span class="smcap">It</span> would be futile to attempt to guess at the arrangement of the
-determinants in the germ-plasm, but so much at least we may say,
-that the determinants do not lie beside each other in the same
-disposition as their determinates exhibit in the fully-formed
-organism. This may be inferred from the complex formative processes
-of embryogenesis in which many groups of cells, which in their
-origin were far apart, combine together to form an organ. Thus the
-arrangement of the determinants in the germ-plasm does not correspond
-to the subsequent arrangement of the whole animal, nor are
-primary constituents of the <i>complete</i> organs contained within the
-germ-plasm. The organ is undoubtedly <i>predetermined</i> in the germ-plasm,
-but it is not <i>preformed</i> as such.</p>
-
-<p>Here, again, the history of development gives us a certain basis
-of fact from which to work. Let us consider, for instance, the origin
-of the appendages in those insects which in the larval state possess
-neither legs nor wings, but exhibit a gradual emergence of these
-structures from concealment underneath the integumentary skeleton.
-In these cases, as I have already shown in regard to the wings,
-the development of the limbs arises from definite groups of cells in
-the skin. These must therefore be regarded as the formative, and
-therefore as the most important and indispensable, parts of the
-rudiments, and may be designated the imaginal disks, as I many years
-ago proposed<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a> (Fig. 89, <i>ui</i> and <i>oi</i>).</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> <i>Die Entwicklung der Dipteren</i>, Leipzig, 1864.</p>
-
-</div>
-
-<p>But these disks of cells do not contain the <i>whole</i> leg, but only the
-<span class="pagenum"><a id="Page_393"></a>[Pg 393]</span>skin-layer of it, the 'hypodermis,' which, however, in this case
-undoubtedly determines the form. But the internal parts of the leg,
-especially the nerves, tracheæ, and probably also the muscles, are
-formed from other cell-groups and grow into the imaginal disk from
-outside. Something similar probably takes place in the case of all
-organs which are made up of many parts; they are, so to speak, shot
-together from several points of origin, from various primordia; and
-determinants are brought into co-operation whose relative value in
-determining the form and function of the organ may be very diverse.</p>
-
-<div class="figright" id="f93a">
-<img src="images/fig93.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 89.</span> Anterior region of the larva<br />
-of a Midge (<i>Corethra plumicornis</i>). <i>K</i>, head.<br />
-<i>Th</i>, thorax. <i>ui</i>, inferior imaginal disks.<br />
-<i>oi</i>, superior imaginal disks. <i>ui</i><sup>1</sup>, <i>ui</i><sup>2</sup>, and<br />
-<i>ui</i><sup>3</sup>, the primordia of the limbs. <i>oi</i><sup>2</sup><br />
-and <i>oi</i><sup>3</sup>, the primordia of the wings and<br />
-'balancers.' <i>g</i>, brain. <i>bg</i>, chain of ventral<br />
-ganglia with nerves which enter the<br />
-imaginal disks. <i>trb</i>, tracheal vesicle.<br />
-Enlarged about 15 times.</p>
-</div>
-
-<p>For it is undoubtedly a very different matter whether a cell bears
-within it the elements which compel it in the course of growth to
-develop an organ, for instance a leg, of quite definite size, sculpture,
-number of joints, and so on, or
-whether it only bears the somewhat
-vague power of determining that
-connective tissue or fatty tissue is
-to be produced. In the first case
-it controls the whole formation of
-the part, in the second it only fills
-up gaps or lays down fat or other
-substances within itself if these
-be presented to it. Between
-these two extremes of determining
-power there are many intermediate
-stages. Cells which contain
-the determinants of blood-vessels,
-tracheæ, or nerves need
-not be so definitely determined
-that they always give rise to
-precisely the same blood-vessels, the same branching of the tracheæ, or
-the same bifurcation of nerves; they may probably possess no more
-than the general tendency to the formation of such parts, and the
-special form taken by the nerves, tracheæ, or blood-vessels may be
-essentially determined by their environment. Thus in the morbid
-tumours of Man, nerves, and especially blood-vessels, may develop in
-a quite characteristic manner, which was certainly not determined in
-advance, but has been called forth by the stimulus, the pressure,
-and other influences of the cellular basis of the tumour. In short,
-the cells were only determined to this extent, that they contained
-the tendency to give rise to blood-vessels under particular
-influences.</p>
-
-<p>It would be a mistake, however, to think of the primary con<span class="pagenum"><a id="Page_394"></a>[Pg 394]</span>stituents
-of all cell-groups as so indefinite. Let us call to mind, for
-instance, the venation of the insect wing. It is well known that this
-is not only quite different in beetles, bugs, and Diptera from that in
-the Hymenoptera, and different again in the butterflies, but that it is
-quite characteristic in every individual family of butterflies, and
-indeed in every genus. We cannot conceive of the absolute certainty
-of development of these very characteristic and constant branchings as
-having its roots elsewhere than in the determinants of the germ-plasm,
-which, lying within certain series of cells, ultimately cause particular
-cell-series of the wing-rudiment to become the wing-veins. If this
-were not so, how would it be possible to understand the fact that
-every minute deviation in the course of these veins is repeated in
-exactly the same way in all the individuals of a genus, while in all
-the individuals of an allied genus the venation turns out slightly
-different with equal constancy.</p>
-
-<p>But it is quite certain that all determinations are in some degree
-susceptible to modifying influences, that they are in very different
-degrees capable of variation.</p>
-
-<p>Many deformities of particular parts in Man and the higher
-animals may be referred to imperfect or inhibited nutrition of the part
-in question during embryonic development; the determinants alone
-cannot make the part, they must have a supply of formative material,
-and according as this material is afforded more abundantly or more
-scantily the part will turn out larger or smaller. In the same way
-the pressure conditions of the surrounding parts must in many cases
-have a furthering or inhibiting influence, or may even determine
-the shape. But it is quite possible, indeed even probable, that other
-specific influences are exerted by the cells or cell-aggregates surrounding
-an organ which is in process of being formed, just as the
-stake on which a twining plant is growing may prompt it to coil. If
-the stake be absent, the predetermined twining of the plant cannot
-attain to more than very imperfect expression, if indeed it finds any.
-The spirally coiled sheath of muscle-cells which occurs so often around
-blood-vessels in worms, Echinoderms, and Vertebrates is probably due
-to similar processes, that is, on the one hand, to a specific mode of
-reaction characteristic of these cells, and predetermined from the germ;
-on the other hand, to the external influence of the cell-surroundings
-without which the determination of the muscle-cell is not liberated,
-that is, is not excited to activity.</p>
-
-<div class="figcenter" id="f97">
-<img src="images/fig97.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 93.</span> The development of a limb in the pupa of a Fly (<i>Sarcophaga
-carnaria</i>). <i>A</i>, apex of the limb from a pupa four days old; the jointing is hinted
-at; <i>hy</i>, hypodermis; <i>ps</i>, pupal sheath; <i>ph</i>, phagocytes; <i>tr</i>, tracheal branch. <i>B</i>,
-the same on the fifth day; the lumen of the limb is quite filled with phagocytes
-(<i>ph</i>); the last tarsal joint (<i>t</i><sup>5</sup>) is beginning to show a bifid apex. <i>C</i>, the same on
-the seventh day; the claws (<i>Kr</i>) and the adhesive lobes (<i>hl</i>) are formed.</p>
-</div>
-
-<p>But even if every determinant requires a stimulus to liberate it,
-whether this stimulus consists in currents of particular nutritive fluids,
-in contact with other cells, or, conversely, on the removal of some
-<span class="pagenum"><a id="Page_395"></a>[Pg 395]</span>pressure previously exerted on the cell by its surroundings, the
-material cause of a structure is to be sought for not in these conditions
-of its appearance, but in the primary constituents which have
-been handed on to the relevant cell or cell-group from the germ, in
-other words, through its determinants. How, for instance, could the
-blunt rounded knob of the rough and clumsily jointed sac of cells
-which represents the insect's leg at the beginning of the pupal period
-(<a href="#f97">Fig. 93</a>, <i>A</i>) be incited to thicken, to constrict at the root (<i>B</i>), and
-to form a joint-surface, to broaden out at the end, and produce
-two sharply cut points (<i>C</i>), which become incurved and form claws
-(<i>kr</i>), while beneath these a broad flat lobe (<i>hl</i>) grows forward, and
-with its regularly disposed cells gradually forms the characteristic
-adhesive organ of the fly&mdash;how could all this happen if there were not
-contained within these cells special formative forces which determine
-them not only in their form and the rest of their constitution, but
-above all in their power of multiplication? No special external
-stimulus affects the still unfinished knob of the fly's leg unless it be
-the removal of pressure; but this operates regularly, and cannot be<span class="pagenum"><a id="Page_396"></a>[Pg 396]</span>
-the cause of the growth, at definite places, of claws and adhesive lobes
-with all their characteristically placed hairs.</p>
-
-<p>We require to assume that each of the cells composing the
-primary rudiment of the limb possessed a determining power which
-made it grow and multiply under the given conditions of nutrition
-and pressure in a prescribed manner and at a prescribed rate; and
-we must make the same assumption in regard to all the daughter
-and grand-daughter-cells, and so on. The strictest regulation of the
-power of multiplication of each of the implicated cells is a necessary
-condition of the constant production of the same two claws and
-adhesive lobes, the same form of tarsal joint, the same regular
-covering of hair, and so on. This exact determination of the cells
-can only take place through material vital particles, and it is these
-which I call determinants.</p>
-
-<p>I have already said so much about the assumed 'determinants'
-of the germ-plasm that it might perhaps be supposed that we have
-now exhausted the topic; but the assumption of such 'primary
-constituents' is so fundamental, not only for my own germ-plasm
-theory of to-day and to-morrow, but also&mdash;unless I am much mistaken&mdash;for
-all future theories of development and inheritance. In
-point of fact, the conception of determinants has as yet penetrated
-so little into the consciousness of biologists, that I cannot remain
-content with what I have already said, but must endeavour to test
-and to corroborate my thesis by additional illustrations.</p>
-
-<p>As far as I am aware, only a few zoologists have expressly and
-unconditionally agreed with the assumption of determinants; on the
-other hand, several biologists have rejected it as fanciful and untenable,
-while others have set it aside as a useless playing with ideas.
-The last, I am inclined to believe, have not taken the trouble to
-think out what the idea is. It has even been objected that there can be
-no determinants because we can see nothing of them, and that they
-must therefore be pure figments of the imagination, invented to
-explain facts which could be explained much more easily and simply
-in some other way. From the very first I have stated emphatically
-that they have not been, and never will be seen, because they lie
-far below the limit of visibility, and thus can at best only become visible
-when they are collected in large aggregates like chromatin granules.
-Nor have I any objections to make if any one chooses to describe all
-the details of their activity as mere hypotheses, such, for instance,
-as their distribution during development, their 'maturation,' their
-migration from the nucleus, and the manner in which they control
-the cell. All this is really an imaginative picture which may be<span class="pagenum"><a id="Page_397"></a>[Pg 397]</span>
-correct to a certain degree, but may also be erroneous; no formal
-proof of it can be obtained at present; and I am content if it be
-simply admitted to be possible. On the other hand, the existence of
-determinants seems to me to be, in the sense indicated, indubitable
-and demonstrable.</p>
-
-<p>Let us return for a moment to the claws and adhesive lobes which
-are developed on the foot of the fly. It may perhaps be thought
-that it is possible to do without the assumption of determinants for
-these parts, by assuming that although 'external' influences in the
-ordinary sense could not possibly have determined that certain cells of
-the apex of the leg should form claws and others adhesive lobes, the
-result might be due to the differences of intercellular pressure within the
-apical knob; these may have been stronger in one direction, weaker in
-another, thus prompting the cells to grow here into claws and there
-into adhesive lobes. If we had merely to explain from the constitution
-of the germ-plasm the ontogeny or development of these parts in an
-individual fly there might perhaps be no radical objection to this view,
-though it would hardly be possible to explain the assumed differences
-in pressure otherwise than as due to a different intensity of growth in
-the cells in the various regions of the limb-apex, which again would
-have to be referred to differences in the germ-plasm. But when we
-reflect that these parts vary hereditarily and independently of other
-parts, and owe their present form to their power of doing so, and that
-they are differently formed in every genus and species, we see at once
-that they must be represented in the germ-plasm by particular vital
-particles, which are the roots of their transmissible variability, that
-is, which must have previously undergone a corresponding variation
-if the relevant parts themselves are to vary. Without previous variation
-of the determinants of the germ no transmissible independent
-deviation on the part of the claws or adhesive lobes of the animal
-is conceivable.</p>
-
-<p>All the opponents of my theory have overlooked this fact; both
-Oscar Hertwig and Kassowitz have forgotten that a theory of development
-is not a theory of heredity; they only aim at the former,
-and they therefore dispute the logical necessity for an assumption
-of determinants.</p>
-
-<p>But as this is the very foundation of the theory, let me further
-submit the following considerations in its favour.</p>
-
-<p>In insects which undergo metamorphosis, not only the external
-but the internal parts of the caterpillar or larva go through a more
-or less complete transformation. In the flies (Muscidæ), for instance,
-the whole intestinal tract of the larva is reconstructed in the pupa;<span class="pagenum"><a id="Page_398"></a>[Pg 398]</span>
-in fact it breaks up into a loose, flocculent, dead, but still coherent
-mass of tissue. Within this there arises a new intestine, as I have
-shown in an early work (1864); and Kowalewsky and Van Rees
-have since made us aware of the interesting details of this reconstruction,
-showing that the new intestine arises from definite cells
-of the old one, which are present in the larval gut at certain fairly
-wide distances, and which do not share in the general destruction,
-but remain alive, grow, and multiply, and form islands of cells
-in the dead mass. These living islands, continually extending,
-ultimately come into contact and again form a closed intestinal canal
-which differs entirely from that of the larva in its form, in its various
-areas, and in its differentiation. In this case those formative cells
-of the imago-intestine must have contained the elements which
-determined their descendants in number, power of multiplication,
-arrangement, and histological differentiation. In other words, each
-of these cells must contain the determinants of a particular limited
-section of the intestine of the imago. The other cells of the intestinal
-epithelium could not do this, even though they were under
-exactly the same conditions, were included in the same intimate
-cell-aggregate, and had the same nutritional opportunities. They
-break up when the formative cells begin to be active, for till then
-the latter had remained inactive, and had not multiplied, although
-they lay regularly distributed among the other cells. Whence, then,
-could the entire difference in the behaviour of these two sets of cells
-arise, if it does not depend on the <i>nature of the cells themselves</i>, and
-how could this difference of nature have developed during the racial
-history of insect-metamorphosis if determinants did not reach the cell
-from the germ-plasm&mdash;determinants which conditioned that some
-cells should be hereditarily modified into the cells of the imago-intestine
-and others into the larval intestine? Quite similar processes
-have been recently demonstrated in regard to the reconstruction
-of the larval intestine in other insect-groups. Deegener has done
-this, for instance, for the water-beetle (<i>Hydrophilus piceus</i>); and it is
-certain that all these reconstructions start from particular cells, which
-lie indifferently between the active cells during the larval period,
-and contain the primary constituents for the formation of a section
-of the intestine, but which only become active when their hitherto
-living neighbours die and break up.</p>
-
-<p>The whole of the reconstruction of the external form of the fly
-takes place in a similar manner. Not only the limb, the head, the
-stigmata, but the skin itself is formed anew from imaginal disks.
-In each of the abdominal segments three pairs of little cell-islands<span class="pagenum"><a id="Page_399"></a>[Pg 399]</span>
-are formed during larval life, and these only enter on the stage of
-formative activity after pupation, when they multiply rapidly and grow
-together to form a segment, whose size, form, and external nature
-is determined by them. But it is well known that the abdominal
-segments of the fly differ from those of the larva very markedly
-and in every respect, so that each cell-island must contain determinants
-which are quite different from those in the skin-cells of
-the corresponding larval segments. These last break up at the
-beginning of pupahood, while the former begin to grow vigorously,
-and to spread themselves out. The most remarkable fact about the
-whole business, and it seems to me also the most instructive, is that
-these imaginal disks frequently appear for the first time during larval
-life, as I found in the case of a midge, <i>Coretha plumicornis</i>, in regard
-to the disks of the thorax, and as Bruno Wahl<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a> has recently demonstrated
-in the case of the abdominal cell-islands. Since in the young
-larva the position of the subsequent imaginal disks is occupied by cells
-which apparently in no way differ from the rest of the skin-cells, and
-are also exposed to precisely the same external and internal influences,
-the origination of the imaginal cells from these can only depend on
-differential cell-division; the primordial cell of each imaginal disk
-must have separated at the beginning of disk-formation into a larval
-and an imaginal skin-cell.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> Bruno Wahl, <i>Ueber die Entwickelung der hypodermalen Imaginalscheiben im Thorax und
-Abdomen der Larve von 'Eristalis' L., Zeitschr. f. wiss. Zool.</i>, Bd. lxx. 1901.</p>
-
-</div>
-
-<p>In insects in which the larva and the imago differ widely, the
-perfect insect, as regards all its principal parts, is already represented
-in the larva, namely, in particular cells which lie among those of the
-corresponding larval parts, and do not visibly differ from these,
-although they are equipped with quite different determinants, and
-consequently enter on their formative activity much later, and give
-rise to quite different structures. As the determinants of the whole
-animal with all its parts are contained in the ovum, so those of the
-parts of its imaginal phase are contained in these cells of the imaginal
-disks.</p>
-
-<p>In addition to all this, we have incontrovertible evidence in
-favour of the theory of determinants in the independent phyletic
-variations of the individual stages of development, on which depends
-the whole phenomenon of 'metamorphosis' which we have just been
-considering. How could the larval stage have become so different
-from the imago-stage, if the one were not alterable by variation
-arising in the germ without the other being affected? If this absolute
-independence of the transmissible variability of the individual stages<span class="pagenum"><a id="Page_400"></a>[Pg 400]</span>
-were not an indispensable assumption in the explanation of metamorphosis
-and other phenomena of development, I should regard an
-attempt at a theory of development without determinants as justifiable.
-But I am forced to see in this fact alone an invalidation of all
-epigenetic theories of development, that is, of all theories which
-assume a germ-substance without primary constituents, which can
-produce the complicated body solely by varying step by step under
-the influence of external influences, both extra- and intra-somatic.
-It is possible to conceive of an ovum in which the living substance
-is of such a kind that it must vary in a definite manner under the
-influence of warmth, air, pressure, and so on, that it must divide into
-similar, and subsequently also into dissimilar parts, which then interact
-upon each other in diverse ways and give rise to further variations,
-which in their turn result in differentiations and variations, till
-ultimately we have the whole complicated organic machine complete
-and 'finished' in every detail. Certainly no mortal could make any
-pronouncement as to the constitution of such a substance, but even if
-we assume it, for the nonce, as possible, how can we account for the
-transmissible variation of the individual parts and developmental
-stages, on which the whole phylogenetic evolution depends?</p>
-
-<p>As the development of the butterfly exhibits the three main
-stages of caterpillar, pupa, and perfect insect, each of which is independently
-and hereditarily variable, and therefore implies a something
-in the germ, whose variation brings about a change in the one
-stage only, so the ontogeny of every higher animal is made up of
-numerous stages, which are all capable of independent and transmissible
-variation. How else should we human beings, in our
-embryonic phase, still possess the gill-arches of our fish-like ancestors,
-although much modified and without the gills? Truly, he who would
-seek to deny that the stages of individual development are capable of
-independent and transmissible variation must know very little about
-embryology. But if the facts are as stated, how can they be reconciled
-with the conception of a germinal substance developing in epigenetic
-fashion? Every variation in this substance would affect not
-only the whole <i>succession of stages, but the whole organism with all
-its parts</i>. In this way too, then, we are driven to the conclusion that
-there must be something in the germ whose variation causes variation
-only in a particular part of a particular stage. This something we
-define in our conception of the 'primary constituents' (<i>Anlagen</i>)&mdash;the
-determinants. These are not to be thought of either as 'miniature
-models,' or even as the 'seeds' of the parts; they alone cannot produce
-the part which they determine, but they effect changes in the<span class="pagenum"><a id="Page_401"></a>[Pg 401]</span>
-cell in which they become active, causing it to vary in such a manner
-that the formation of the relevant part results. While I conceive of
-development as a continuous process, I supplement this with the idea
-that from within, namely, from the nuclear substance, new, directive,
-'determining' influences are continually being exerted on the developing
-cells.</p>
-
-<p>I can hardly think of a better proof of the necessity of this
-assumption than that furnished by Delage, one of the most acute
-biologists of France, who, in his comprehensive book on <i>Heredity</i>, has
-striven to replace the theory of determinants by something simpler.
-Delage rejects all 'primary constituents' (<i>Anlagen</i>) in the germ, all
-'particules représentatives,' as much too complicated an assumption,
-and thinks it possible to work with the conception of a germ-plasm
-which is about as simple as the cell-substance of a Rhizopod, that is
-to say, a protoplasm of definite chemico-physical constitution and
-composition. Leaving out of account the consideration that the
-protoplasm of an amœba is scarcely of such extreme simplicity, but
-is certainly made up of numerous differentiated and definitely
-arranged biophors, how could such an extremely simple ('éminemment
-simple') constitution of the ovum as is here assumed give rise to such
-a complicated organism, the individual parts of which are capable of
-independent and transmissible variation? According to Delage it
-does so because the ovum, though not containing 'all the factors
-requisite for its ultimate resultant,' does contain 'un certain nombre
-des facteurs nécessaires à la détermination de chaque partie et de
-chaque caractère de l'organisme futur'! Determinants after all, it
-may be said, but that is far from the truth! It is not primary constituents
-that the germ contains, according to Delage, it is chemical
-substances, for instance muscle substances, probably 'les substances
-caractéristiques des principales catégories de cellules, c'est-à-dire,
-celles qui, dans ces cellules, sont la condition principale de leur fonctionnement.'
-All these must be contained in the ovum. How they
-are to reach their proper place in the organism, how the 'characteristic
-chemical substance' of a mole is to land just behind the right
-or left ear of the fully formed man, is not stated. But apart from
-this, there is a much deeper error in this assumption of specific
-chemical substances in the ovum as an explanation of the phenomena
-of local hereditary variation, and I have already touched upon it:
-chemical substances are not vital units, which feed and reproduce,
-which assimilate and which bear a charm against the assimilating
-power of the surrounding protoplasm. They would necessarily be
-modified and displaced in the course of ontogeny, and would therefore&mdash;no matter
-<span class="pagenum"><a id="Page_402"></a>[Pg 402]</span>where they had been placed at first&mdash;be incapable of
-performing all that Delage ascribes to them. Either the germ contains
-'living' primary constituents, or it is, as Delage maintains,
-determined chemico-physically; but in the latter case there is no
-scope for hereditary local variation. Delage must either renounce
-the attempt to explain this, or he must transform his 'substances
-chimiques' into real and actually living determinants.</p>
-
-<p>Thus from all sides we are forced to the conclusion that the
-germ-substance on the whole owes its marvellous power of development
-not only to its chemico-physical constitution, whether that be
-eminently simple or marvellously complex, but to the fact that it
-consists of many and different kinds of 'primary constituents'
-(<i>Anlagen</i>), that is, of groups of vital units equipped with the forces of
-life, and capable of interposing actively and in a specific manner, but
-also capable of remaining latent in a passive state, until they are
-affected by a liberating stimulus, and on this account able to interpose
-successively in development. The germ-cell cannot be merely a simple
-organism, it must be a fabric made up of many different organisms or
-units, a microcosm.</p>
-
-<p>Yet another train of thought leads us to the same idea, and this
-has its roots in the extraordinary complexity of the machine which
-we call the organism.</p>
-
-<p>The botanist Reinke has recently called attention once again to
-the fact that machines cannot be directly made up of primary
-physico-chemical forces or energies, but that, as Lotze said, forces of
-a superior order are indispensable, which so dispose the fundamental
-chemico-physical forces that they must act in the way aimed at by the
-purpose of the machine. To produce a watch it is not enough to bring
-together brass, steel, gold, and stones; to produce a piano it is not enough
-to lay wood, iron, leather, ivory, steel, &amp;c., side by side, but these stuffs
-must be brought together in a definite form and combination. In the
-same way, the mere juxtaposition of carbon and water does not result
-in a carbohydrate like sugar or illuminating gas; the component
-elements only yield what is desired when they are placed in a particular
-and absolutely definite relation to each other, in which they so act
-upon and with one another that sugar or illuminating gas results, and
-the same is true of the component elements of a watch or of a piano.
-In the watch and in the piano this relation is arranged by human
-intelligence, by the workmen who form the different materials and
-put them together in the proper manner. In this case, then, human
-intelligence is, as Reinke says, the 'superior force' which compels the
-energies to work together in a particular way.</p>
-
-<p><span class="pagenum"><a id="Page_403"></a>[Pg 403]</span></p>
-
-<p>But organisms also are machines which perform a particular and
-purposeful kind of work, and they are only capable of doing so
-because the energies which perform the work are forced into definite
-paths by superior forces; these superior forces are thus 'the steersmen
-of the energies.' There is undoubtedly a kernel of truth in this view,
-and I shall return to it. Reinke, however, uses it in a way which
-I cannot follow; that is, he infers from it a 'cosmic intelligence'
-which puts these superior forces into the organisms, and thus controls
-these machines to purposeful work, as the watchmaker puts 'superior
-forces' into the watch by means of wheels, cylinders, and levers. In
-one case it is human intelligence which controls the 'superior forces,'
-in the other 'cosmic' intelligence. I cannot regard this reasoning
-from analogy as convincing, because, in the first place, these 'superior
-forces' are not 'forces' at all. They are constellations of energy,
-co-ordinations of matter and the energies immanent therein under
-complex and precisely defined conditions, and it is a matter of
-indifference whether chance or human intelligence has brought them
-together. If we take Reinke's own example of carbohydrates it is
-certain that our coal-gas is due to the intelligence of man, which
-brings together the carbon and the water in such a way that coal-gas
-must arise. The 'superior forces' must here be looked for in the
-arrangements of the coke-stove, and, in the second place, in the
-intelligence of man. But when decaying plants in the marsh form
-another carbon-compound, marsh-gas, where do the directing 'superior
-forces' come in? Surely only in the fortuitous concomitance of the
-necessary materials and the necessary conditions. Or may 'cosmic'
-intelligence have established this laboratory in the marsh? If not,
-what can compel us to refer the formation of dextrin or starch in the
-cells of the green leaves of plants to 'superior forces' which are placed
-in them by 'cosmic' intelligence? I am far from believing that the
-great and deep problem here touched upon can be put aside in any
-off-hand manner, but I feel sure that it will never be solved by word-play
-about energies and 'superior forces.'</p>
-
-<p>Let us return to the kernel of truth in Reinke's thesis; it lies in
-this, that, while the working of a machine does really depend on the
-forces or energies which are bound up with the stuffs of which it
-consists, it also depends on a particular combination of these stuffs
-and forces, on a particular 'constellation' of them, as Fechner expressed
-it. In the watch these 'constellations' are the springs, the
-wheels, &amp;c., and their position in relation to each other; but in the
-organism they are the organs, down to the cells and cell-parts; for
-the cell too is a machine, indeed a very complex one, as its functions<span class="pagenum"><a id="Page_404"></a>[Pg 404]</span>
-prove. There are thousands of kinds of 'constellations' of elementary
-substances and forces which condition the activity of the living machine,
-and only when all these constellations are present in the proper
-manner and in the proper relations to each other can the functions
-of the organism be properly discharged.</p>
-
-<p>But the living machine differs essentially from other machines in
-the fact that it constructs itself; it arises by development from a cell,
-by going through numerous 'stages of development.' But none of
-these stages is a dead thing, each is itself a living organism whose
-chief function is to give rise to the next stage. Thus each stage of
-the development may be compared to a machine whose function
-consists in producing a similar but more complex machine. Each
-stage is thus composed, just like the complete organism, of a number
-of such 'constellations' of elementary substances and elementary forces,
-whose number in the beginning is relatively small, but increases rapidly
-with each new stage.</p>
-
-<p>But whence come these 'constellations' or, to keep to our metaphor,
-the levers, wheels, and cranks of each successive stage in the making
-of the organic machine? The epigenetic theory of a germ-plasm
-without primary constituents answers by pointing to internal and
-external influences which cause the germ-plasm, originally homogeneous,
-to differentiate gradually more and more, bringing it into
-the most diverse 'constellations.' But how can such influences
-introduce new springs, levers, and wheels of a quite specific kind, as
-must be the case if apparently similar germinal substances are to
-give rise to two such different animals as a domestic duck and
-a teal? The cause must lie in the invisible differences in the protoplasm,
-opponents will answer, and we with them. But our studies up
-to this point have shown us that the differences cannot be merely
-elementary differences, cannot be merely of a physico-chemical nature
-depending on the composition of the raw material and the implicated
-energies; they must depend on the definite co-ordination of substances
-and energies, in other words, on the occurrence of 'constellations' of
-these. Thus the germ-plasm must be composed of definite and very
-diverse combinations of living units, which are themselves bound up in
-a higher 'constellation,' so that they act as a living machine at the
-first stage of development, and liberate into activity the already
-existing constellations of the second stage. The second stage in the
-series of living machines which arise successively from each other
-liberates the sleeping 'constellations' for the third, and so on.</p>
-
-<p>These 'constellations' of matter and energy are the biophors, the
-determinants, and the 'groups of determinants' which we may think<span class="pagenum"><a id="Page_405"></a>[Pg 405]</span>
-of as disposed in a manifold overlapping series. That they do not
-enter into activity all at once, but successively take their part in
-development, seems to me a necessary consequence of their successive
-origin in the phylogeny; and the ontogeny, as we shall see later,
-arises through a modified condensation of the phylogeny. Now since
-every new determinant that arises in the course of phylogeny can
-only develop by division and subsequent variation from the determinants
-which were previously active at the same place in the
-organism, it is quite intelligible that later on, when the phylogeny
-has been condensed in the ontogeny, they should not enter upon their
-active stage at the same time as their phyletic predecessors, but after
-them. The theory of Oscar Hertwig, who starts from a germ-plasm
-without primary constituents, that all parts of the germ-plasm become
-active at the same time, seems to me quite untenable. How could the
-wheels, levers, and springs of the complete vital machine, which arose
-so very slowly in the course of phylogeny, arise to-day in the ontogeny
-in such rapid succession unless they were already present in the
-germ-plasm and only required to be incited to activity, that is,
-liberated by the stage preceding them? Even Fechner supported this
-view when he supposed that the interaction and mutual influences of
-the parts in the organism, that is, of the 'constellations,' gave rise of
-themselves to the succeeding stage, that is to say, to the new constellations
-peculiar to the succeeding stage. To this Reinke reasonably
-objected that it was like expecting the window frames of a house in
-process of building to produce the panes of glass. The panes in the
-organism only develop in the window frames if their determinants
-have been present in the germ-plasm from the beginning, and are
-liberated by the development of the frames, just as the activity of the
-glazier is liberated by the sight of the completed frames. Neither
-new panes nor new determinants could be produced rapidly; the
-former must be manufactured in the glass factory, the latter in the
-developmental workshop of the form of life in question, which
-workshop we call its phylogeny. But just as it is unnecessary to
-erect a new glass factory for each new house that is built, so the
-development of each individual does not require the establishment
-each time of those numberless life-factories&mdash;the constellations&mdash;whose
-business it is to produce anew the wheels, levers, springs, and
-cylinders of the developmental machinery at each stage, for they
-are all provided for in the germ-plasm, and it is only on this account
-that they are capable of hereditary variation.</p>
-
-<p>I have already directed attention to some embryological facts
-which seem to be contradictory, if not to the germ-plasm theory itself,<span class="pagenum"><a id="Page_406"></a>[Pg 406]</span>
-at least to the assumption it makes that the germ-plasm is analysed out
-during the ontogeny; and something more must be said on this head.
-I refer to the numerous facts brought to light through the science of
-developmental mechanics founded by Wilhelm Roux, and particularly
-to the investigations as to the prospective significance of the segmentation-cells
-of the animal ovum.</p>
-
-<p>Among these investigations we find experiments in compressing
-certain eggs (sea-urchin's) in the early stages of segmentation. The
-blastomeres are prevented by artificial pressure from grouping
-themselves in the normal manner; they are compelled to spread out
-side by side in the <i>same plane</i>. If the pressure is removed, they
-change their grouping, and yield a normal embryo. I will not here
-discuss whether these results can only be interpreted as showing that
-each segmentation-cell has the same prospective significance, and
-that it is only its relative position which decides what part of the
-embryo is to be formed from it; this could not be done without going
-into great detail; I therefore assume it to be true, and confine my
-survey to the second group of experiments, those on isolated segmentation-cells.</p>
-
-<p>It has been shown that in the eggs of the most diverse animals,
-for instance in the sea-urchin once more, each of the two first blastomeres,
-if separated from one another, can develop into a complete larva.
-Indeed, in the eggs of sea-urchin and some other animals each of the
-first four, or any of the first eight, blastomeres, and indeed any segmentation-cell
-during the earlier stages, possesses the power of developing
-to a certain point, namely, as far as the so-called 'blastula-larva.'
-This seems to contradict a theory which assumes that the primary
-constituents become separated in the successive stages of ontogeny.
-But in the first place the blastomeres of all animals do not behave in
-this way, and, moreover, the facts can be quite well explained without
-entirely renouncing the assumption of the segregation of the determinant-complexes.
-It is only necessary to assume that the segmentation-cells,
-which develop in the isolated condition as if they were
-intact eggs, still contain the complete germ-plasm, and that the
-differential segregation into groups of determinants with dissimilar
-hereditary tendencies takes place later. This would certainly load
-the theory with further complications, and I shall not enter into the
-question here, since the facts which we should have to consider are as
-yet by no means undisputed.</p>
-
-<p>But in any case the facts of developmental mechanics referred to,
-which we owe to numerous excellent observers of the last decade,&mdash;I
-need only name W. Roux, O. Hertwig, Chun, Driesch, Barfurth,<span class="pagenum"><a id="Page_407"></a>[Pg 407]</span>
-Morgan, Conklin, Wilson, Crampton, and Fischel&mdash;not only leave
-the essential part of the germ-plasm theory untouched, but rather
-strengthen than endanger its more subordinate points, such as the
-assumption of a segregation of the components of the germ-plasm in
-the course of ontogenesis.</p>
-
-<p>As to the fundamental ideas expressed in the theory, I have
-already shown that these remain unaltered, even if we do not assume
-a disintegration or segregation of the germ-plasm, but think of all
-the developing cells as equipped with the complete germ-plasm. In
-that case the determinants would be liberated to activity solely by
-specific stimuli. But in regard to the assumption of disintegration,
-it must be noted that the facts cited relative to the sea-urchin's ova
-do not by any means hold true of the eggs of all animals.</p>
-
-<p>In various animal types each of the first two segmentation-cells,
-when separated from its neighbour, produces only a half-embryo, and
-any one of the first four cells a quarter-embryo. This 'fractional
-embryo' is, however, in some cases able later to develop into a whole
-embryo (to 'postgenerate' itself, as W. Roux says). The isolated
-blastomere shows, to begin with, an activity of only a half of the
-primary constituents of the animal, as was first established by
-W. Roux and maintained conclusively, in spite of many attacks, until
-it was established beyond doubt by the detailed corroboratory investigations
-of Endres. The secondary completion of the embryo,
-which, however, is still disputed, must be regarded as a regeneration,
-and, to explain it, a co-operation of the complete but not yet wholly
-active germ-plasm in both segmentation-cells must therefore be
-assumed.</p>
-
-<p>It would carry us too far if I were to deal in detail even with
-the most important of the numerous facts that the last decade has
-brought to light; I shall restrict myself to the most essential.</p>
-
-<p>That isolated segmentation-cells have the capacity of developing
-into embryos which are complete but correspondingly smaller in size
-has been demonstrated in animals of various groups, though it does
-not seem to go to the same length in all. In the Medusæ we find
-that not only one of the first two, but one of the first four, eight, and
-even sixteen segmentation-cells may develop a whole larva when
-isolated (Zoja). In the sea-urchin at least any one of the first eight
-blastomeres may do so. And Driesch's experiments in cutting up the
-young larvæ at the blastula-stage (a single-layered ball of cells) leads
-us to assume that each of these cells still possesses the complete germ-plasm.
-Beyond that stage, however, the primary constituents
-obviously divide into those of the ectoderm and those of the endo<span class="pagenum"><a id="Page_408"></a>[Pg 408]</span>derm,
-for the subsequent two-layered stage in the sea-urchin's development,
-the gastrula, does not complete itself if it be artificially
-divided into fragments which consist only of cells from the outer, or
-only of cells from the inner layer. In corroboration of this experiment
-made by Barfurth, Samassa was able to demonstrate in regard
-to the egg of the frog that, even after the third division of the ovum,
-the segmentation-cells are so different from each other in respect of
-their primary constituents that they were not able to replace each
-other mutually. When this investigator killed the ectoderm-cells
-alone by means of an induction current, or the endoderm-cells alone,
-the dead half could not be replaced by the half which remained alive,
-and the whole ovum perished.</p>
-
-<p>If these facts may be adduced in favour of a separation of the
-primary constituents at an earlier or later stage, we find even stronger
-proofs among the Ctenophores, Gastropods, Bivalves, and Annelids. In
-the last-named group Wilson has shown it to be probable that development
-is really a 'mosaic work,' as Roux and I had assumed. The older
-observations made by Chun at an earlier date on the Ctenophora,
-and the more recent experiments of Fischel on the same animals,
-prove the same thing for this group. In this case complete larvæ are
-easily distinguished from mere 'partial developments' by the number
-of the characteristic 'ciliated meridional rows' or ribs, which extend
-from one pole of the larva to another. In the complete larva there
-are eight of these, but in larvæ from one of the first two blastomeres
-(isolated) there are only four, and in those which have arisen from
-one of the first four blastomeres there are only two. If an ovum at the
-eight-cell stage can be successfully divided into separate blastomeres,
-each of these will form an 'eighth larva,' always with only one
-ciliated rib. Even in the succeeding sixteen-cell stage it could still be
-demonstrated that the substance responsible for the formation of the
-ribs only lies in particular places and always suffices only for eight
-ribs. The sixteen-cell stage consists of eight large cells and eight
-small ones, the 'macromeres' and the 'micromeres'; if an ovum at
-this stage be cut so that one piece contains five macromeres and five
-micromeres, a partial larva will develop which possesses only five ribs,
-while the larva from the other portion will have only three. But the
-localizing of the rib-determinants can be followed still further, for in
-larvæ in which individual micromeres have been displaced from their
-normal position there is a correlated displacement of the corresponding
-ribs, and a dislocation of their ciliated comb-plates. The
-determinants of the ribs must therefore lie in the micromeres, and we
-must conclude that at the antecedent division they were only imparted<span class="pagenum"><a id="Page_409"></a>[Pg 409]</span>
-to one daughter-nucleus, while the other, that of the macromere, did
-not receive this kind of determinant. Here then we have an example
-of dissimilar or differential division. Those who oppose this theory of
-qualitative division will hardly be likely to admit this, but will rather
-seek to maintain that 'external influences,' such as relative position,
-determine which cells are to give rise to the ciliated ribs and which
-are not. But the fact that artificial displacement of the micromeres
-leads to a disarrangement of the ciliated comb-plates, of which the
-ribs are made up, invalidates this suggestion, and at the same time
-overthrows the interpretation that it may be the cells which lie on
-particular meridians that are determined by this position to the
-production of ciliated plates. Obviously, the converse of this is
-true; those cells which contain the rib-determinants come to lie in the
-regular course of development in these eight meridians, and the cells
-lying between them, though of the same descent (from micromeres),
-contain no such determinants and therefore form no ribs. But if those
-cells which are equipped with rib-determinants be artificially displaced,
-then they give rise to swimming-plates elsewhere than on the
-aforesaid meridians.</p>
-
-<p>The experiments made by Crampton on a marine Gastropod,
-<i>Ilyanassa</i>, likewise go to prove that a disintegration or segregation of
-the primary constituents does occur in the course of development. In
-this case, when the first two or first four segmentation-cells were
-artificially separated from each other, they developed exactly as if
-they still belonged to the complete ovum, that is, each isolated
-segmentation-cell yielded, respectively, a half or a quarter-embryo.
-And these 'partial embryos' were not able in this case to give rise
-subsequently to the missing parts or to form complete embryos.</p>
-
-<p>There are thus two contrasted groups of animals, in one of which
-a segregation of the mass of primary constituents apparently takes
-place at the very beginning, while in the other it does not take place
-in the first stages of development, but apparently occurs later on.
-We may distinguish these two groups, with Heider, as those having
-'regulation ova' and those having 'mosaic ova.' But I do not see
-that this affords any reason why we should give up our conception of
-the successive segregation of the germ-plasm into its determinants,
-even although, as I said before, I may modify it so far as to say that
-the segregation does not necessarily take place in all groups and
-species of animals at the same time, but occurs earlier in some and
-later in others.</p>
-
-<p>Now that I have shown how the germ-plasm theory may be
-brought into harmony with the phenomena of ontogeny, I wish to go<span class="pagenum"><a id="Page_410"></a>[Pg 410]</span>
-on to show what the theory can accomplish in clarifying our understanding
-of the phenomena of reproduction and heredity. I shall at
-the same time give a brief exposition of some of the most important
-of these phenomena.</p>
-
-<p>First, a few words in regard to the development of the reproductive
-cells. We may leave aside in the meantime the question
-whether they are sexually differentiated or not; we are only concerned
-just now with the main problem: How is it possible for the organism
-to produce germ-cells, that is, cells which contain the complete germ-plasm
-with all its determinants, when the building up of the body
-in ontogeny, according to our theory, involves a disintegration or
-segregation of the determinant-architecture into smaller and smaller
-groups? It is impossible that specific determinants should arise <i>de
-novo</i>, just as an animal cannot arise otherwise than from its germ, nor
-a cell otherwise than from a cell, nor a nucleus otherwise than from
-an already existing nucleus. If vital units ever originate <i>de novo</i> at
-all, it is only conceivable in the case of the very simplest biophors, as
-we shall see later when we come to speak of 'Spontaneous Generation.'
-Specific biophors and the determinants composed of them have behind
-them a phylogeny, a history, which conditions that they shall arise only
-from their like.</p>
-
-<p>Thus we see that germ-cells can only arise where all the determinants
-of the relevant species arranged as ids are already present.
-If we could assume that the ovum, just beginning to develop, divides
-at its first cleavage into two cells, one of which gives rise to the
-whole body (soma) and the other only to the germ-cells lying in this
-body, the matter would be theoretically simple. We should say, the
-germ-plasm of the ovum first doubles itself by growth, as the nuclear
-substance does at every nuclear division, and then divides into two
-similar halves, one of which, lying in the primordial somatic cell,
-becomes at once active and breaks up into smaller and smaller groups
-of determinants corresponding to the building up of the body, while
-the germ-plasm in the other remains in a more or less 'bound' or 'set'
-condition, and is only active to the extent of gradually stamping as
-germ-cells the cells which arise from the primordial germ-cell.</p>
-
-<p>As yet, however, only one group of animals is known to behave
-demonstrably in this manner, the Diptera among insects; in all others
-the cell from which the germ-cells exclusively arise, the 'primordial
-germ-cell,' makes its appearance later in development, usually during
-embryogenesis and often very early in it, after the first few divisions
-of the ovum, but sometimes not till long after the end of embryogenesis,
-and not even in the individual which arises from the ovum,<span class="pagenum"><a id="Page_411"></a>[Pg 411]</span>
-but in descendants which arise from it by budding. This last case
-occurs especially in the colonial hydroid polyps, which multiply by
-budding. Here the primordial germ-cell is separated from the ovum
-by a long series of cell-generations, and the sole possibility of
-explaining the presence of germ-plasm in this primordial cell is to be
-found in the assumption that in the divisions of the ovum the whole
-of the germ-plasm originally contained in it was not broken up into
-determinant groups, but that a part, perhaps the greater part, was
-handed on in a latent state from cell to cell, till sooner or later
-it reached a cell which it stamped as the primordial germ-cell.
-Theoretically it makes no difference whether these 'germ-tracks,'
-that is, the cell-generations which lead from the ovum to the primordial
-germ-cell, are short or very long, whether they consist of three or six
-or sixteen cells, or of hundreds and thousands of cells. That all the
-cells of the germ-track do not take on the character of germ-cells
-must, in accordance with our conception of the 'maturing' of determinants,
-be referred to the internal conditions of the cells and of the
-germ-plasm, perhaps in part also to an associated quantum of somatic
-idioplasm which is only overpowered in the course of the cell-divisions.</p>
-
-<p>This splitting up of the substance of the ovum into a somatic
-half, which directs the development of the individual, and a propagative
-half, which reaches the germ-cells and there remains inactive,
-and later gives rise to the succeeding generation, constitutes <i>the theory
-of the continuity of the germ-plasm</i>, which I first stated in a work
-which appeared in the year 1885. Its fundamental idea had already
-been expressed much earlier by Francis Galton (1872), without however
-being fully appreciated at the time or having any influence on
-the course of science, and the same is true with the later theoretical
-views of Jäger, Rauber, and Nussbaum, all of whom reached the
-same idea quite independently of each other, and sought to elaborate
-it more or less fully.</p>
-
-<p>The hypothesis does not depend for support merely on a recognition
-of its theoretical necessity; on the contrary, there is a whole
-series of facts which may be adduced as strongly in its favour.</p>
-
-<p>Thus, even the familiar fact that the excision of the reproductive
-organs in all animals produces sterility proves that no other cells of
-the body are able to give rise to germ-cells; germ-plasm cannot be
-produced <i>de novo</i>. An unmistakable corroboration of this, it seems
-to me, is to be found in the conditions of germ-cell formation in the
-medusoids and hydroid polyps, for here it is apparent that the birthplace
-of the germs, that is, the place at which the germ-cells of the
-<span class="pagenum"><a id="Page_412"></a>[Pg 412]</span>animal are formed, has been shifted backwards in the course of
-phylogenetic evolution, that is, has been moved nearer to the starting-point
-of development. This shifting has exactly followed the 'germ-tracks.'
-as we shall see, although in some cases it would have been
-more advantageous if the birthplace of the germ-cells could have lain
-outside of these. Obviously, then, it is only the existing cell-generations
-of the germ-track which were able to give rise to germ-cells, or,<span class="pagenum"><a id="Page_413"></a>[Pg 413]</span>
-in other words, they alone contained the indispensable germ-plasm.
-With the help of Figs. 94 and 95 I hope to be able to make this
-matter clear.</p>
-
-<div class="figcenter" id="f98">
-<img src="images/fig98.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 94.</span> Diagram to illustrate the phylogenetic shifting back of the
-origins of the germ-cells in medusoids and hydroids. A composite picture.
-<i>A</i>, branch of a polyp colony. <i>P</i>, polyp-head with mouth (<i>m</i>) and tentacles.
-<i>St</i>, stalk of the polyp. <i>M</i>, medusoid-bud with the bell (<i>Gl</i>). <i>T</i>, marginal tentacle.
-<i>m</i>, mouth. <i>Mst</i>, manubrium. <i>GphK</i>, a gonophore-bud. <i>GH</i>, gastric cavity. <i>ekt</i>,
-ectoderm. <i>ent</i>, endoderm. <i>st</i>, supporting lamella. The germ-cells (<i>kz</i>) arise
-in the medusoid in the ectoderm of the manubrium&mdash;first phyletic stage&mdash;where
-they also attain maturity. In the gonophore-bud (<i>GphK</i>) they arise in the
-ectoderm (<i>kz´</i>), or further down in the stalk of the polyp at <i>kz´´</i>&mdash;third phyletic
-stage, or in the ectoderm of the branch from which the polyp has arisen, at
-<i>kz´´´</i>&mdash;fourth phyletic stage of the shunting of the originative area of the
-germ-cells. In the two last cases the germ-cells migrate until they reach their
-primitive place of origination in the medusoid, or in the corresponding layer
-of the medusoid gonophore, as may be more clearly seen in Fig. 95. Drawn
-from my sketch by Dr. Petrunkewitsch.</p>
-</div>
-
-<p>In the hydroid polyps and their medusoids the germ-cells always
-arise in the ectoderm; in species which produce sexual medusoids by
-budding, the germ-cells arise in the ectoderm of the manubrium of
-these medusoids (Fig. 94, <i>M</i>, <i>kz</i>). But in many species these sexual
-stages have degenerated in the course of phylogeny into so-called
-gonophores, that is, to medusoids which still exhibit more or less
-complete bells, but neither mouth (<i>m</i>) nor marginal tentacles (<i>T</i>), and
-which no longer break away from the colony to swim freely about,
-to feed independently, and to produce and ripen germ-cells. The
-degeneration of the 'gonophores' often goes even further; in many
-the medusoid bell is represented only by a thin layer of cells, and in
-some even this token of descent from medusoid ancestry is absent,
-and they are mere single-layered closed brood-sacs (<a href="#f99">Fig. 95</a>, <i>Gph</i>).</p>
-
-<p>The adherence of the sexual animal to the hydroid colony has,
-however, made a more rapid ripening of the germ-cells possible, and
-nature has taken advantage of this possibility in all the cases known
-to me, for the germ-cells no longer arise in the manubrium of the
-mature degenerate medusoid, that is, of the gonophore, but <i>earlier</i>,
-before the bud which becomes a gonophore possesses a manubrium.
-The birthplace of the germ-cells is thus shifted back from the manubrium
-of the medusoid to the young gonophore-bud (Fig. 94, <i>M</i>, <i>kz</i>).
-The same thing occurs in species in which the medusoids are liberated,
-but live only for a short time, for instance, in the genus <i>Podocoryne</i>.
-Although perfect medusoids are formed, these have their germ-cells
-fully developed at the time of their liberation from the hydroid
-colony. But in species in which the medusoid-buds have really
-degenerated and are no longer liberated, the birthplace of the germ-cells
-is shifted <i>even further back</i>, and in the first place into the stalk
-(<i>St</i>, <i>kz´´</i>) of the polyp from the gonophore-buds. This is the case
-in the genus <i>Hydractinia</i>. In the further course of the process
-the birthplace of the germ-cells has shifted as far back as to the
-branch from which the polyp has grown out (<a href="#f98">Fig. 94</a>, <i>A</i>, <i>kz´´´</i>); and
-finally, in the cases in which the medusoid has degenerated to a mere
-brood-sac (Fig. 95, <i>Gph</i>), even to the generation of polyps immediately
-before, that is, into the polyp-stem from which the branch arises that
-bears the polyps producing the gonophore-bud (Fig. 95, <i>kz´´´</i>). Then
-we find the birthplace of the germ-cells <i>still</i> further back (Fig. 95, <i>kz´´´´</i>),
-for the egg and sperm-cells arise in the stem of the principal polyps
-(the main stem of the colony). The advantage of this arrangement<span class="pagenum"><a id="Page_414"></a>[Pg 414]</span>
-is easily seen, for the principal polyp is present earlier than those of
-the secondary branches, and these again earlier than the polyp which
-bears the sexual buds, and this, finally, earlier than the sexual bud
-which it bears. Thus this shunting backwards of the birthplace of
-the germ-cells means an earlier origin of the primordium (<i>Anlage</i>)
-of the germ-cells, and consequently an earlier maturing of these.</p>
-
-<div class="figcenter" id="f99">
-<img src="images/fig99.jpg" alt=""/>
-<p class="caption"><span class="smcap">Fig. 95.</span> Diagram to illustrate the migration of the germ-cells in hydro-medusæ
-from their remotely shunted place of origin to their primitive place
-of origin in the gonophore, in which they attain to maturity. The state of
-affairs in Eudendrium is taken as the basis of the diagram. <i>HP</i>, one of the
-principal polyps. <i>mu</i>, mouth. <i>ma</i>, gut-cavity. <i>t</i>, tentacle. <i>Sta</i>, its stem. <i>A</i>, a
-branch of the polyp colony. <i>SP</i>, lateral polyp. <i>Gph</i>, a medusoid-bud completely
-degenerated into a mere gonophore. <i>Ei</i>, ovum. <i>GH</i>, gastric cavity. <i>st</i>,
-supporting lamella. The originative area of the germ-cells lies in the stem
-of the principal polyp at <i>kz´´´´</i>, whence the germ-cells first migrate into the
-endoderm of the branch (<i>A</i>) at <i>kz´´´</i>, creeping within which they reach <i>kz´´</i> in
-the lateral polyp (blastostyle), finally reaching the gonophore (<i>kz</i>) and passing
-again into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.</p>
-</div>
-
-<p>But none of all these germ-cells come to maturity in the birthplace
-to which they have been shifted, for they migrate independently
-from it to the place at which they primitively arose, namely, into the<span class="pagenum"><a id="Page_415"></a>[Pg 415]</span>
-manubrium of the medusoid, which is still present even when great
-degeneration has occurred, or even&mdash;in the most extreme cases of
-degeneration&mdash;into the ectoderm of the brood-sac. This is the case
-in the genus <i>Eudendrium</i>, of which Fig. 95 gives a diagrammatic
-representation.</p>
-
-<p>The most interesting feature of this migration of the germ-cells
-is that the cells invariably arise in the ectoderm (<i>kz´´´´</i>), then pierce
-through the supporting lamella (<i>st</i>) into the endoderm (<i>kz´´´</i>), and
-then creep along it to their maturing-place. Once there they break
-through again to the outer layer of cells, the ectoderm (<i>kz</i>), and come
-to maturity (<i>Ei</i>). That they make their way through the endoderm
-is probably to be explained by the fact that they are there in direct
-proximity to the food-stream which flows through the colony (<i>GH</i> =
-gastric cavity), and they are thus more richly nourished there than
-in the ectoderm. But although this is the case, they never arise in
-the endoderm; in no single case is the birthplace of the germ-cells to
-be found in the endoderm, but always in the ectoderm, no matter
-how far back it may have been shunted. Even when the germ-cells
-migrate through the endoderm, their first recognizable appearance
-is invariably in the ectoderm, as, for instance, in <i>Podocoryne</i> and
-<i>Hydractinia</i>. The course of affairs is thus exactly what it would
-necessarily be if our supposition were correct, that only definite
-cell-generations&mdash;in this case the ectoderm-cells&mdash;contain the complete
-germ-plasm. If the endoderm-cells also contained germ-plasm it
-would be hard to understand why the germ-cells never arise from
-them, since their situation offers much better conditions for their
-further development than that of the ectoderm-cells. It would also
-be hard to understand why such a circuitous route was chosen as
-that exhibited by the migration of the young germ-cells into the
-endoderm. Something must be lacking in the endoderm that is
-necessary to make a cell into a germ-cell: that something is the
-germ-plasm.</p>
-
-<p>If we accept the theory of the continuity of the germ-plasm as
-in the main correct, it appears that higher animals and plants are
-constructed of two kinds of elements, the somatic cells and the germ-cells;
-both owe their being to the germ-plasm of the ovum, but the
-former do not contain it complete but only in individual determinants<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a>,
-<span class="pagenum"><a id="Page_416"></a>[Pg 416]</span>and therefore can never give rise again to the rank of germ-cells;
-the others contain the latent germ-plasm intact, and can therefore
-produce not only cells like themselves for a certain time by
-division, but have also the power, when they are mature and the
-necessary conditions have been fulfilled, of bringing forth a new
-individual of the same species. The former have only a limited
-length of life, they die&mdash;they must necessarily die&mdash;when the life of
-the individual to which they belong is at an end; the latter are
-potentially immortal, like the unicellular organisms, that is, they can
-in favourable circumstances give rise to the germ-cells of a new
-individual, and so on for all time, as far as we can see. The germ-plasm
-of a species is thus never formed <i>de novo</i>, but it grows and
-increases ceaselessly; it is handed on from one generation to another
-like a long root creeping through the earth, from which at regular
-distances shoots grow up and become plants, the individuals of the
-successive generations. If these conditions be considered from the
-point of view of reproduction, the germ-cells appear the most important
-part of the individual, for they alone maintain the species,
-and the body sinks down almost to the level of a mere cradle for the
-germ-cells, a place in which they are formed, and under favourable
-conditions are nourished, multiply, and attain to maturity. But the
-matter can also be looked at in an opposite light, and then the endless
-root of the germ-plasm, with its germ-cells ever forming new
-individuals, may be regarded as the means by which alone nature
-was able to create multicellular organisms, individuals of higher and
-higher differentiation and capacity, able to adapt themselves to all
-possible conditions, and to make the fullest use of all the possibilities
-of life.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> Boveri has recently made an observation upon the thread-worm of the horse,
-which points to the correctness of the conception of the germ-plasm. The two first
-segmentation-cells both receive the four chromosomes of the species, but, in one of the
-two, a portion of the chromatin breaks off and degenerates, or dissolves, at least as far
-as can be seen. The other cell retains the whole mass of chromatin, and from this
-there arise later the primitive genital-cells. In the germ-track, therefore&mdash;so we must
-interpret it&mdash;the whole of the germ-plasm is retained, while a part of it is withdrawn
-from the soma. I have only partly described the process, and I do not wish to enter
-in detail on an interpretation of it, since it seems to me obscure and to require
-further observations before an interpretation can be attempted with any confidence.</p>
-
-</div>
-
-<hr class="full" />
-
-<div class="chapter">
-
-<p class="ph2">MR. EDWARD ARNOLD'S</p>
-</div>
-
-<p class="c little">LIST OF</p>
-
-<p class="c oldeng xxxlarge">Scientific and Technical Books.</p>
-
-<div class="figcenter">
-<img src="images/fig100.jpg" alt=""/>
-</div>
-
-
-<div class="blockquot">
-
-<p><b>FOOD AND THE PRINCIPLES OF DIETETICS.</b> By <span class="smcap">Robert
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