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diff --git a/old/64227-0.txt b/old/64227-0.txt deleted file mode 100644 index 4c7a391..0000000 --- a/old/64227-0.txt +++ /dev/null @@ -1,18331 +0,0 @@ -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 Euplœæ. 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 Euplœæ -(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 _Euplœæ_ 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 -_Euplœæ_--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 _Anœæ_, -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. - - - - -MR. EDWARD ARNOLD'S - -LIST OF - -Scientific and Technical Books. - - - =FOOD AND THE PRINCIPLES OF DIETETICS.= By ROBERT HUTCHISON, M.D. - Edin., F.R.C.P., Assistant Physician to the London Hospital. Seventh - and Revised Edition. Illustrated. 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