initial commit of ebook 28775HEADmain

1 files changed, 5077 insertions, 0 deletions

diff --git a/28775.txt b/28775.txt
new file mode 100644
index 0000000..de76cca
--- /dev/null
+++ b/28775.txt
@@ -0,0 +1,5077 @@
+The Project Gutenberg eBook, Mendelism, by Reginald Crundall Punnett
+
+
+This eBook is for the use of anyone anywhere 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
+
+
+
+
+
+Title: Mendelism
+       Third Edition
+
+
+Author: Reginald Crundall Punnett
+
+
+
+Release Date: May 18, 2009  [eBook #28775]
+
+Language: English
+
+Character set encoding: ISO-646-US (US-ASCII)
+
+
+***START OF THE PROJECT GUTENBERG EBOOK MENDELISM***
+
+
+E-text prepared by Paul Hollander, Malcolm Farmer, Keith Edkins, and the
+Project Gutenberg Online Distributed Proofreading Team
+(http://www.pgdp.net)
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 28775-h.htm or 28775-h.zip:
+      (http://www.gutenberg.org/files/28775/28775-h/28775-h.htm)
+      or
+      (http://www.gutenberg.org/files/28775/28775-h.zip)
+
+
+Transcriber's note:
+
+      A few typographical errors have been corrected: they are listed
+      at the end of the text.
+
+      Fig. 8 has been re-mastered to match the text (the Black boxes
+      were shown as Albino and the heterozygous Albinos as Black).
+
+      Superscripted numbers are indicated by a carat character
+      followed by the superscript. For example, 2^4 denotes 2 raised
+      to the fourth power and 2^(10) denotes 2 raised to the tenth
+      power.
+
+      Subscripted numbers are indicated by an underscore followed by
+      the subscript. For example, in the expression "F_1" the 1 is
+      a subscript.
+
+      Page numbers in this text file are enclosed in curly brackets.
+      This enables the reader to use the index by searching for the
+      page number. To find page 35, search for {35}.
+
+
+
+
+
+[Illustration]
+
+MENDELISM
+
+by
+
+R. C. PUNNETT
+
+Fellow of Gonville and Caius College
+Professor of Biology in the University Of Cambridge
+
+THIRD EDITION
+Entirely Rewritten and Much Enlarged
+
+
+
+
+
+
+
+New York
+The MacMillan Company
+1911
+
+All rights reserved
+
+Copyright, 1911,
+by The MacMillan Company.
+
+Set up and electrotyped. Published May, 1911.
+
+Norwood Press
+J. S. Cushing Co.--Berwick & Smith Co.
+Norwood, Mass., U.S.A.
+
+
+
+{v}
+
+PREFACE
+
+A few years ago I published a short sketch of Mendel's discovery in
+heredity, and of some of the recent experiments which had arisen from it.
+Since then progress in these studies has been rapid, and the present
+account, though bearing the same title, has been completely rewritten. A
+number of illustrations have been added, and here I may acknowledge my
+indebtedness to Miss Wheldale for the two coloured plates of sweet peas, to
+the Hon. Walter Rothschild for the butterflies figured on Plate VI., to
+Professor Wood for photographs of sheep, and to Dr. Drinkwater for the
+figures of human hands. To my former publishers also, Messrs. Bowes and
+Bowes, I wish to express my thanks for the courtesy with which they
+acquiesced in my desire that the present edition should be published
+elsewhere.
+
+As the book is intended to appeal to a wide audience, I have not attempted
+to give more experimental instances than were necessary to illustrate the
+story, nor have I burdened it with bibliographical reference. The reader
+who desires further information may be referred to Mr. Bateson's
+indispensable Volume on _Mendel's {vi} Principles of Heredity_ (Cambridge,
+1909), where a full account of these matters is readily accessible. Neither
+have I alluded to recent cytological work in so far as it may bear upon our
+problems. Many of the facts connected with the division of the chromosomes
+are striking and suggestive, but while so much difference of opinion exists
+as to their interpretation they are hardly suited for popular treatment.
+
+In choosing typical examples to illustrate the growth of our ideas it was
+natural that I should give the preference to those with which I was most
+familiar. For this reason the book is in some measure a record of the work
+accomplished by the Cambridge School of Genetics, and it is not unfair to
+say that under the leadership of William Bateson the contributions of this
+school have been second to none. But it should not be forgotten that
+workers in other European countries, and especially in America, have
+amassed a large and valuable body of evidence with which it is impossible
+to deal in a small volume of this scope.
+
+It is not long since the English language was enriched by two new
+words--Eugenics and Genetics--and their similarity of origin has sometimes
+led to confusion between them on the part of those who are innocent of
+Greek. Genetics is the term applied to the experimental study of heredity
+and variation in animals and plants, and the main concern of its students
+is the establishing of law and order among the phenomena {vii} there
+encountered. Eugenics, on the other hand, deals with the improvement of the
+human race under existing conditions of law and sentiment. The Eugenist has
+to take into account the religious and social beliefs and prejudices of
+mankind. Other issues are involved besides the purely biological one,
+though as time goes on it is coming to be more clearly recognised that the
+Eugenic ideal is sharply circumscribed by the facts of heredity and
+variation, and by the laws which govern the transmission of qualities in
+living things. What these facts, what these laws are, in so far as we at
+present know them, I have endeavoured to indicate in the following pages;
+for I feel convinced that if the Eugenist is to achieve anything solid it
+is upon them that he must primarily build. Little enough material, it is
+true, exists at present, but that we now see to be largely a question of
+time and means. Whatever be the outcome, whatever the form of the structure
+which is eventually to emerge, we owe it first of all to Mendel that the
+foundations can be well and truly laid.
+
+R. C. P.
+
+CAMBRIDGE, _March, 1911_.
+
+       *       *       *       *       *
+
+
+{ix}
+
+                          CONTENTS
+
+                          CHAPTER I              PAGE
+  The Problem                                      1
+
+                          CHAPTER II
+  Historical                                       8
+
+                          CHAPTER III
+  Mendel's Work                                   17
+
+                          CHAPTER IV
+  The Presence and Absence Theory                 29
+
+                          CHAPTER V
+  Interaction of Factors                          42
+
+                          CHAPTER VI
+  Reversion                                       59
+
+                          CHAPTER VII
+  Dominance                                       68
+
+  {x}
+                          CHAPTER VIII
+  Wild Forms and Domestic Varieties               79
+
+                          CHAPTER IX
+  Repulsion and Coupling of Factors               88
+
+                          CHAPTER X
+  Sex                                             99
+
+                          CHAPTER XI
+  Sex (_continued_)                              115
+
+                          CHAPTER XII
+  Intermediates                                  125
+
+                          CHAPTER XIII
+  Variation and Evolution                        135
+
+                          CHAPTER XIV
+  Economical                                     153
+
+                          CHAPTER XV
+  Man                                            170
+
+  APPENDIX                                       187
+
+  INDEX                                          191
+
+
+
+       *       *       *       *       *
+
+
+{xi}
+
+ILLUSTRATIONS
+
+                                  PLATES
+
+  PLATE                                                          PAGE
+
+   Gregor Mendel                                        _Frontispiece_
+
+    I. Rabbits                                           _To face_ 60
+
+   II. Sweet Peas                                            "     64
+
+  III. Sheep                                                 "     78
+
+   IV. Sweet Peas                                            "     80
+
+    V. Fowls                                                 "    107
+
+  VI. Butterflies                                            "    146
+
+                          FIGURES IN TEXT
+
+  FIG.
+
+   1. Scheme of Inheritance in simple Mendelian Case               21
+
+   2. Feathers of Silky and Common Fowl                            30
+
+   3. Single and Double Primulas                                   31
+
+   4. Fowls' Combs                                                 32
+
+   5. Diagram of Inheritance of Fowls' Combs                       37
+
+   6. Fowls' Combs                                                 39
+
+   7. Diagram of F_2 Generation resulting from Cross between
+       two White Sweet Peas                                        46
+
+   8. Diagram illustrating 9 : 3 : 4 Ratio in Mice                 52
+
+   9. Sections of Primulas                                         55
+
+  {xii}
+  10. Small and Large-eyed Primulas                                56
+
+  11. Diagram illustrating Reversion in Pigeons                    67
+
+  12. _Primula sinensis_ x _Primula stellata_                      68
+
+  13. Diagram illustrating Cross between Dominant and
+        Recessive White Fowls                                      72
+
+  14. Bearded and Beardless Wheat                                  75
+
+  15. Feet of Fowls                                                76
+
+  16. Scheme of Inheritance of Horns in Sheep                      76
+
+  17. _Abraxas grossulariata_ and var. _lacticolor_                99
+
+  18. Scheme of Inheritance in _Abraxas_                          102
+
+  19. Scheme of Inheritance of Silky Hen x Brown Leghorn
+        Cock                                                      105
+
+  20. Scheme of Inheritance of Brown Leghorn Hen x Silky
+        Cock                                                      106
+
+  21. Scheme of F_1 (ex Brown Leghorn x Silky Cock) crossed
+        with pure Brown Leghorn                                   107
+
+  22. Scheme for Silky Hen x Brown Leghorn Cock                   108
+
+  23. Scheme for Brown Leghorn Hen x Silky Cock                   109
+
+  24. Diagram illustrating Nature of Offspring from Brown Leghorn
+        Hen x F_1 Cock                                            110
+
+  25. Scheme to illustrate Heterozygous Nature of Brown Leghorn
+        Hen                                                       111
+
+  26. Scheme of Inheritance of Colour-blindness                   117
+
+  27. Single and Double Stocks                                    122
+
+  28. F_2 Generation ex Silky Hen x Brown Leghorn Cock            127
+
+  29. Pedigree of Eurasian Family                                 130
+
+  30. Curve illustrating Influence of Selection                   159
+
+  {xiii}
+  31. Curve illustrating Conception of pure Lines                 162
+
+  32. Brachydactylous and Normal Hands                            170
+
+  33. Radiograph of Brachydactylous Hand                          170
+
+  34. Pedigree of Brachydactylous Family                          173
+
+  35. Pedigree of Haemophilic Family                              175
+
+
+
+       *       *       *       *       *
+
+
+{xiv}
+
+    For although it be a more new and difficult way, to find out the nature
+    of things, by the things themselves; then by reading of Books, to take
+    our knowledge upon trust from the opinions of Philosophers: yet must it
+    needs be confessed, that the former is much more open, and lesse
+    fraudulent, especially in the Secrets relating to _Natural Philosophy_.
+
+      WILLIAM HARVEY,
+      _Anatomical Exercitations_, 1653.
+
+       *       *       *       *       *
+
+
+{1}
+
+CHAPTER I
+
+THE PROBLEM
+
+A curious thing in the history of human thought so far as literature
+reveals it to us is the strange lack of interest shown in one of the most
+interesting of all human relationships. Few if any of the more primitive
+peoples seem to have attempted to define the part played by either parent
+in the formation of the offspring, or to have assigned peculiar powers of
+transmission to them, even in the vaguest way. For ages man must have been
+more or less consciously improving his domesticated races of animals and
+plants, yet it is not until the time of Aristotle that we have clear
+evidence of any hypothesis to account for these phenomena of heredity. The
+production of offspring by man was then held to be similar to the
+production of a crop from seed. The seed came from the man, the woman
+provided the soil. This remained the generally accepted view for many
+centuries, and it was not until the recognition of woman as more than a
+passive agent that the physical basis of heredity became established. That
+recognition was effected by the microscope, for only with its advent was
+actual {2} observation of the minute sexual cells made possible. After more
+than a hundred years of conflict lasting until the end of the eighteenth
+century, scientific men settled down to the view that each of the sexes
+makes a definite material contribution to the offspring produced by their
+joint efforts. Among animals the female contributes the ovum and the male
+the spermatozoon; among plants the corresponding cells are the ovules and
+pollen grains.
+
+As a general rule it may be stated that the reproductive cells produced by
+the female are relatively large and without the power of independent
+movement. In addition to the actual living substance which is to take part
+in the formation of a new individual, the ova are more or less heavily
+loaded with the yolk substance that is to provide for the nutrition of the
+developing embryo during the early stages of its existence. The size of the
+ova varies enormously in different animals. In birds and reptiles where the
+contents of the egg form the sole resources of the developing young they
+are very large in comparison with the size of the animal which lays them.
+In mammals, on the other hand, where the young are parasitic upon the
+mother during the earlier stages of their growth, the eggs are minute and
+only contain the small amount of yolk that enables them to reach the stage
+at which they develop the processes for attaching themselves to the wall of
+the maternal uterus. But whatever the differences in the size and
+appearance of the ova produced by different {3} animals, they are all
+comparable in that each is a distinct and separate sexual cell which, as a
+rule, is unable to develop into a new individual of its species unless it
+is fertilised by union with a sexual cell produced by the male.
+
+The male sexual cells are always of microscopic size and are produced in
+the generative gland or testis in exceedingly large numbers. In addition to
+their minuter size they differ from the ova in their power of active
+movement. Animals present various mechanisms by which the sexual elements
+may be brought into juxtaposition, but in all cases some distance must be
+traversed in a fluid or semifluid medium (frequently within the body of the
+female parent) before the necessary fusion can occur. To accomplish this
+latter end of its journey the spermatozoon is endowed with some form of
+motile apparatus, and this frequently takes the form of a long flagellum,
+or whip-like process, by the lashing of which the little creature propels
+itself much as a tadpole with its tail.
+
+In plants as in animals the female cells or ovules are larger than the
+pollen grains, though the disparity in size is not nearly so marked. Still
+they are always relatively minute cells since the circumstances of their
+development as parasites upon the mother plant render it unnecessary for
+them to possess any great supply of food yolk. The ovules are found
+surrounded by maternal tissue in the ovary, but through the stigma and down
+the pistil a {4} potential passage is left for the male cell. The majority
+of flowers are hermaphrodite, and in many cases they are also
+self-fertilising. The anthers burst and the contained pollen grains are
+then shed upon the stigma. When this happens, the pollen cell slips through
+a little hole in its coat and bores its way down the pistil to reach an
+ovule in the ovary. Complete fusion occurs, and the minute embryo of a new
+plant immediately results. But for some time it is incapable of leading a
+separate existence, and, like the embryo mammal, it lives as a parasite
+upon its parent. By the parent it is provided with a protective wrapping,
+the seed coat, and beneath this the little embryo swells until it reaches a
+certain size, when as a ripe seed it severs its connection with the
+maternal organism. It is important to realise that the seed of a plant is
+not a sexual cell but a young individual which, except for the coat that it
+wears, belongs entirely to the next generation. It is with annual plants in
+some respects as with many butterflies. During one summer they are
+initiated by the union of two sexual cells and pass through certain stages
+of larval development--the butterfly as a caterpillar, the plant as a
+parasite upon its mother. As the summer draws to a close each passes into a
+resting-stage against the winter cold--the butterfly as a pupa and the
+plant as a seed, with the difference that while the caterpillar provides
+its own coat, that of the plant is provided by its mother. With the advent
+of spring both butterfly and {5} plant emerge, become mature, and
+themselves ripen germ cells which give rise to a new generation.
+
+Whatever the details of development, one cardinal fact is clear. Except for
+the relatively rare instances of parthenogenesis a new individual, whether
+plant or animal, arises as the joint product of two sexual cells derived
+from individuals of different sexes. Such sexual cells, whether ovules or
+ova, spermatozoa or pollen grains, are known by the general term of
+GAMETES, or marrying cells, and the individual formed by the fusion or
+yoking together of two gametes is spoken of as a ZYGOTE. Since a zygote
+arises from the yoking together of two separate gametes, the individual so
+formed must be regarded throughout its life as a double structure in which
+the components brought in by each of the gametes remain intimately fused in
+a form of partnership. But when the zygote in its turn comes to form
+gametes, the partnership is broken and the process is reversed. The
+component parts of the dual structure are resolved, with the formation of a
+set of single structures, the gametes.
+
+The life cycle of a species from among the higher plants or animals may be
+regarded as falling into three periods: (1) a period of isolation in the
+form of gametes, each a living unit incapable of further development
+without intimate association with another produced by the opposite sex; (2)
+a period of association in which two gametes become yoked together into a
+zygote and react upon one {6} another to give rise by a process of cell
+division to what we ordinarily term an individual with all its various
+attributes and properties; and (3) a period of dissociation when the single
+structured gametes separate out from that portion of the double structured
+zygote which constitutes its generative gland. What is the relation between
+gamete and zygote, between zygote and gamete? how are the properties of the
+zygote represented in the gamete, and in what manner are they distributed
+from the one to the other?--these are questions which serve to indicate the
+nature of the problem underlying the process of heredity.
+
+Owing to their peculiar power of growth and the relatively large size to
+which they attain, many of the properties of zygotes are appreciable by
+observation. The colour of an animal or of a flower, the shape of a seed,
+or the pattern on the wings of a moth are all zygotic properties, and all
+capable of direct estimation. It is otherwise with the properties of
+gametes. While the difference between a black and a white fowl is
+sufficiently obvious, no one by inspection can tell the difference between
+the egg that will hatch into a black and that which will hatch into a
+white. Nor from a mass of pollen grains can any one to-day pick out those
+that will produce white from those that will produce coloured flowers.
+Nevertheless, we know that in spite of apparent similarity there must exist
+fundamental differences among the gametes, even {7} among those that spring
+from the same individual. At present our only way of appreciating those
+differences is to observe the properties of the zygotes which they form.
+And as it takes two gametes to form a zygote, we are in the position of
+attempting to decide the properties of two unknowns from one known.
+Fortunately the problem is not entirely one of simple mathematics. It can
+be attacked by the experimental method, and with what measure of success
+will appear in the following pages.
+
+       *       *       *       *       *
+
+
+{8}
+
+CHAPTER II
+
+HISTORICAL
+
+To Gregor Mendel, monk and abbot, belongs the credit of founding the modern
+science of heredity. Through him there was brought into these problems an
+entirely new idea, an entirely fresh conception of the nature of living
+things. Born in 1822 of Austro-Silesian parentage, he early entered the
+monastery of Bruenn, and there in the seclusion of the cloister garden he
+carried out with the common pea the series of experiments which has since
+become so famous. In 1865 after eight years' work he published the results
+of his experiments in the _Proceedings of the Natural History Society of
+Bruenn_, in a brief paper of some forty pages. But brief as it is the
+importance of the results and the lucidity of the exposition will always
+give it high rank among the classics of biological literature. For
+thirty-five years Mendel's paper remained unknown, and it was not until
+1900 that it was simultaneously discovered by several distinguished
+botanists. The causes of this curious neglect are not altogether without
+interest. Hybridisation experiments before Mendel there had been in plenty.
+The classificatory work of {9} Linnaeus in the latter half of the
+eighteenth century had given a definite significance to the word species,
+and scientific men began to turn their attention to attempting to discover
+how species were related to one another. And one obvious way of attacking
+the problem was to cross different species together and see what happened.
+This was largely done during the earlier half of the nineteenth century,
+though such work was almost entirely confined to the botanists. Apart from
+the fact that plants lend themselves to hybridisation work more readily
+than animals, there was probably another reason why zoologists neglected
+this form of investigation. The field of zoology is a wider one than that
+of botany, presenting a far greater variety of type and structure. Partly
+owing to their importance in the study of medicine, and partly owing to
+their smaller numbers, the anatomy of the vegetable was far better known
+than that of the animal kingdom. It is, therefore, not surprising that the
+earlier part of the nineteenth century found the zoologists, under the
+influence of Cuvier and his pupils, devoting their entire energies to
+describing the anatomy of the new forms of animal life which careful search
+at home and fresh voyages of discovery abroad were continually bringing to
+light. During this period the zoologist had little inclination or
+inducement to carry on those investigations in hybridisation which were
+occupying the attention of some botanists. Nor did the efforts of the
+botanists afford much {10} encouragement to such work, for in spite of the
+labour devoted to these experiments, the results offered but a confused
+tangle of facts, contributing in no apparent way to the solution of the
+problem for which they had been undertaken. After half a century of
+experimental hybridisation the determination of the relation of species and
+varieties to one another seemed as remote as ever. Then in 1859 came the
+_Origin of Species_, in which Darwin presented to the world a consistent
+theory to account for the manner in which one species might have arisen
+from another by a process of gradual evolution. Briefly put, that theory
+was as follows: In any species of plant or animal the reproductive capacity
+tends to outrun the available food supply, and the resulting competition
+leads to an inevitable struggle for existence. Of all the individuals born,
+only a portion, and that often a very small one, can survive to produce
+offspring. According to Darwin's theory, the nature of the surviving
+portion is not determined by chance alone. No two individuals of a species
+are precisely alike, and among the variations that occur some enable their
+possessors to cope more successfully with the competitive conditions under
+which they exist. In comparison with their less favoured brethren they have
+a better chance of surviving in the struggle for existence and consequently
+of leaving offspring. The argument is completed by the further assumption
+of a principle of heredity, in virtue of which offspring tend to {11}
+resemble their parents more than other members of the species. Parents
+possessing a favourable variation tend to transmit that variation to their
+offspring, to some in greater, to others in less degree. Those possessing
+it in greater degree will again have a better chance of survival, and will
+transmit the favourable variation in even greater degree to some of their
+offspring. A competitive struggle for existence working in combination with
+certain principles of variation and heredity results in a slow and
+continuous transformation of species through the operation of a process
+which Darwin termed natural selection.
+
+The coherence and simplicity of the theory, supported as it was by the
+great array of facts which Darwin had patiently marshalled together,
+rapidly gained the enthusiastic support of the great majority of
+biologists. The problem of the relation of species at last appeared to be
+solved, and for the next forty years zoologists and botanists were busily
+engaged in classifying by the light of Darwin's theory the great masses of
+anatomical facts which had already accumulated and in adding and
+classifying fresh ones. The study of comparative anatomy and embryology
+received a new stimulus, for with the acceptance of the theory of descent
+with modification it became incumbent upon the biologist to demonstrate the
+manner in which animals and plants differing widely in structure and
+appearance could be conceivably related to one another. Thenceforward the
+energies of both {12} botanists and zoologists have been devoted to the
+construction of hypothetical pedigrees suggesting the various tracks of
+evolution by which one group of animals or plants may have arisen from
+another through a long continued process of natural selection. The result
+of such work on the whole may be said to have shown that the diverse forms
+under which living things exist to-day, and have existed in the past so far
+as palaeontology can tell us, are consistent with the view that they are
+all related by the community of descent which the accepted theory of
+evolution demands, though as to the exact course of descent for any
+particular group of animals there is often considerable diversity of
+opinion. It is obvious that all this work has little or nothing to do with
+the manner in which species are formed. Indeed, the effect of Darwin's
+_Origin of Species_ was to divert attention from the way in which species
+originate. At the time that it was put forward his explanation appeared so
+satisfying that biologists accepted the notions of variation and heredity
+there set forth and ceased to take any further interest in the work of the
+hybridisers. Had Mendel's paper appeared a dozen years earlier it is
+difficult to believe that it could have failed to attract the attention it
+deserved. Coming as it did a few years after the publication of Darwin's
+great work, it found men's minds set at rest on the problems that he raised
+and their thoughts and energies directed to other matters. {13}
+
+Nevertheless one interesting and noteworthy attempt to give greater
+precision to the term heredity was made about this time. Francis Galton, a
+cousin of Darwin, working upon data relating to the breeding of Basset
+hounds, found that he could express on a definite statistical scheme the
+proportion in which the different colours appeared in successive
+generations. Every individual was conceived of as possessing a definite
+heritage which might be expressed as unity. Of this, 1/2 was on the average
+derived from the two parents (_i.e._ 1/4 from each parent), 1/4 from the
+four grandparents, 1/8 from the eight great-grandparents, and so on. _The
+Law of Ancestral Heredity_, as it was termed, expresses with fair accuracy
+some of the statistical phenomena relating to the transmission of
+characters in a mixed population. But the problem of the way in which
+characters are distributed from gamete to zygote and from zygote to gamete
+remained as before. Heredity is essentially a physiological problem, and
+though statistics may be suggestive in the initiation of experiment, it is
+upon the basis of experimental fact that progress must ultimately rest. For
+this reason, in spite of its ingenuity and originality, Galton's theory and
+the subsequent statistical work that has been founded upon it failed to
+give us any deeper insight into the nature of the hereditary process.
+
+While Galton was working in England the German zoologist August Weismann
+was elaborating the complicated {14} theory of heredity which eventually
+appeared in his work on _The Germplasm_ (1885), a book which will be
+remembered for one notable contribution to the subject. Until the
+publication of Weismann's work it had been generally accepted that the
+modifications brought about in the individual during its lifetime, through
+the varying conditions of nutrition and environment, could be transmitted
+to the offspring. In this biologists were but following Darwin, who held
+that the changes in the parent resulting from increased use or disuse of
+any part or organ were passed on to the children. Weismann's theory
+involved the conception of a sharp cleavage between the general body
+tissues or somatoplasm and the reproductive glands or germplasm. The
+individual was merely a carrier for the essential germplasm whose
+properties had been determined long before he was capable of leading a
+separate existence. As this conception ran counter to the possibility of
+the inheritance of "acquired characters," Weismann challenged the evidence
+upon which it rested and showed that it broke down wherever it was
+critically examined. By thus compelling biologists to revise their ideas as
+to the inherited effects of use and disuse, Weismann rendered a valuable
+service to the study of genetics and did much to clear the way for
+subsequent research.
+
+A further important step was taken in 1895, when Bateson once more drew
+attention to the problem of the origin {15} of species, and questioned
+whether the accepted ideas of variation and heredity were after all in
+consonance with the facts. Speaking generally, species do not grade
+gradually from one to the other, but the differences between them are sharp
+and specific. Whence comes this prevalence of discontinuity if the process
+by which they have arisen is one of accumulation of minute and almost
+imperceptible differences? Why are not intermediates of all sorts more
+abundantly produced in nature than is actually known to be the case?
+Bateson saw that if we are ever to answer this question we must have more
+definite knowledge of the nature of variation and of the nature of the
+hereditary process by which these variations are transmitted. And the best
+way to obtain that knowledge was to let the dead alone and to return to the
+study of the living. It was true that the past record of experimental
+breeding had been mainly one of disappointment. It was true also that there
+was no tangible clue by which experiments might be directed in the present.
+Nevertheless in this kind of work alone there seemed any promise of
+ultimate success.
+
+A few years later appeared the first volume of de Vries' remarkable book on
+_The Mutation Theory_. From a prolonged study of the evening primrose
+(_Oenothera_) de Vries concluded that new varieties suddenly arose from
+older ones by sudden sharp steps or mutations, and not by any process
+involving the gradual accumulation of minute {16} differences. The number
+of striking cases from among widely different plants which he was able to
+bring forward went far to convincing biologists that discontinuity in
+variation was a more widespread phenomenon than had hitherto been
+suspected, and not a few began to question whether the account of the mode
+of evolution so generally accepted for forty years was after all the true
+account. Such in brief was the outlook in the central problem of biology at
+the time of the rediscovery of Mendel's work.
+
+       *       *       *       *       *
+
+
+{17}
+
+CHAPTER III
+
+MENDEL'S WORK
+
+The task that Mendel set before himself was to gain some clear conception
+of the manner in which the definite and fixed varieties found within a
+species are related to one another, and he realised at the outset that the
+best chance of success lay in working with material of such a nature as to
+reduce the problem to its simplest terms. He decided that the plant with
+which he was to work must be normally self-fertilising and unlikely to be
+crossed through the interference of insects, while at the same time it must
+possess definite fixed varieties which bred true to type. In the common pea
+(_Pisum sativum_) he found the plant he sought. A hardy annual, prolific,
+easily worked, _Pisum_ has a further advantage in that the insects which
+normally visit flowers are unable to gather pollen from it and so to bring
+about cross fertilisation. At the same time it exists in a number of
+strains presenting well-marked and fixed differences. The flowers may be
+purple, or red, or white; the plants may be tall or dwarf; the ripe seeds
+may be yellow or green, round or wrinkled--such are a few of the characters
+in which the various races of peas differ from one another. {18}
+
+In planning his crossing experiments Mendel adopted an attitude which
+marked him off sharply from the earlier hybridisers. He realised that their
+failure to elucidate any general principle of heredity from the results of
+cross fertilisation was due to their not having concentrated upon
+particular characters or traced them carefully through a sequence of
+generations. That source of failure he was careful to avoid, and throughout
+his experiments he crossed plants presenting sharply contrasted characters,
+and devoted his efforts to observing the behaviour of these characters in
+successive generations. Thus in one series of experiments he concentrated
+his attention on the transmission of the characters tallness and dwarfness,
+neglecting in so far as these experiments were concerned any other
+characters in which the parent plants might differ from one another. For
+this purpose he chose two strains of peas, one of about 6 feet in height,
+and another of about 1-1/2 feet. Previous testing had shown that each
+strain bred true to its peculiar height. These two strains were
+artificially crossed[1] with one another, and it was found to make no
+difference which was used as the pollen parent and which was used as the
+ovule parent. In either case the result was the same. The result of
+crossing tall with dwarf was in every case nothing but talls, as tall or
+even a little taller than the tall parent. For this reason Mendel termed
+tallness the DOMINANT and {19} dwarfness the RECESSIVE character. The next
+stage was to collect and sow the seeds of these tall hybrids. Such seeds in
+the following year gave rise to a mixed generation consisting of talls and
+dwarfs _but no intermediates_. By raising a considerable number of such
+plants Mendel was able to establish the fact that the number of talls which
+occurred in this generation was almost exactly three times as great as the
+number of the dwarfs. As in the previous year, seed were carefully
+collected from this, the second hybrid generation, and in every case _the
+seeds from each individual plant were harvested separately and separately
+sown in the following year_. By this respect for the individuality of the
+different plants, however closely they resembled one another, Mendel found
+the clue that had eluded the efforts of all his predecessors. The seeds
+collected from the dwarf recessives bred true, giving nothing but dwarfs.
+And this was true for every dwarf tested. But with the talls it was quite
+otherwise. Although indistinguishable in appearance, some of them bred
+true, while others behaved like the original tall hybrids, giving a
+generation consisting of talls and dwarfs in the proportion of three of
+{20} the former to one of the latter. Counting showed that the number of
+the talls which gave dwarfs was double that of the talls which bred true.
+
+                  T x D --------------------- P
+                    |
+                   T(D) --------------------- F_1
+                    |
+  +-----------+-----------+-------------+
+  T          T(D)        T(D)           D --- F_2
+  |   +---+----+--+   +--+----+---+     |
+  T   T T(D) T(D) D   T T(D) T(D) D     D --- F_3
+  |                                     |
+  T                                     D --- F_4
+
+If we denote a dwarf plant as D, a true breeding tall plant as T, and a
+tall which gives both talls and dwarfs in the ratio 3 : 1 as T(D), the
+result of these experiments may be briefly summarised in the foregoing
+scheme.[2]
+
+Mendel experimented with other pairs of contrasted characters and found
+that in every instance they followed the same scheme of inheritance. Thus
+coloured flowers were dominant to white, in the ripe seeds yellow was
+dominant to green, and round shape was dominant to wrinkled, and so on. In
+every case where the inheritance of an alternative pair of characters was
+concerned the effect of the cross in successive generations was to produce
+three and only three different sorts of individuals, viz. dominants which
+bred true, dominants which gave both dominant and recessive offspring in
+the ratio 3 : 1, and recessives which always bred true. Having determined a
+general scheme of inheritance which experiment showed to hold good for each
+of the seven pairs of alternative characters with which he worked, Mendel
+set himself to providing a theoretical interpretation of this scheme which,
+as he clearly realised, must be in terms of germ cells. He {21} conceived
+of the gametes as bearers of something capable of giving rise to the
+characters of the plant, but he regarded any individual gamete as being
+able to carry one and one only of any alternative pair of characters. A
+given gamete could carry tallness _or_ dwarfness, but not both. The two
+were mutually exclusive so far as the gamete was concerned. It must be pure
+for one or the other of such a pair, and this conception of the purity of
+the gametes is the most essential part of Mendel's theory.
+
+[Illustration: FIG. 1.
+
+Scheme of inheritance in the cross of tall with dwarf pea. Gametes
+represented by small and zygotes by larger circles.]
+
+We may now proceed with the help of the accompanying scheme (Fig. 1) to
+deduce the results that should flow from Mendel's conception of the nature
+of the gametes, and to see how far they are in accordance with the facts.
+Since the original tall plant belonged to a strain which bred true, all the
+gametes produced by it must bear the tall character. Similarly all the
+gametes of the original dwarf plant must bear the dwarf character. A cross
+between these two means the union of {22} a gamete containing tallness with
+one bearing dwarfness. Owing to the completely dominant nature of the tall
+character, such a plant is in appearance indistinguishable from the pure
+tall, but it differs markedly from it in the nature of the gametes to which
+it gives rise. When the formation of the gametes occurs, the elements
+representing dwarfness and tallness SEGREGATE from one another, so that
+half of the gametes produced contain the one, and half contain the other of
+these two elements. For on hypothesis every gamete must be pure for one or
+other of these two characters. And this is true for the ovules as well as
+for the pollen grains. Such hybrid F_1 plants, therefore, must produce a
+series of ovules consisting of those bearing tallness and those bearing
+dwarfness, and must produce them in equal numbers. And similarly for the
+pollen grains. We may now calculate what should happen when such a series
+of pollen grains meets such a series of ovules, _i.e._ the nature of the
+generation that should be produced when the hybrid is allowed to fertilise
+itself. Let us suppose that there are 4x ovules so that 2x are "tall" and
+2x are "dwarf." These are brought in contact with a mass of pollen grains
+of which half are "tall" and half are "dwarf." It is obvious that a "tall"
+ovule has an equal chance of being fertilised by a "tall" or a "dwarf"
+pollen grain. Hence of our 2x "tall" ovules, x will be fertilised by "tall"
+pollen grains and x will be fertilised by "dwarf" pollen grains. The former
+must give rise to tall {23} plants, and since the dwarf character has been
+entirely eliminated from them they must in the future breed true. The
+latter must also give rise to tall plants, but since they carry also the
+recessive dwarf character they must when bred from produce both tails and
+dwarfs. Each of the 2x dwarf ovules, again, has an equal chance of being
+fertilised by a "tall" or by a "dwarf" pollen grain. Hence x will give rise
+to tall plants carrying the recessive dwarf character, while x will produce
+plants from which the tall character has been eliminated, _i.e._ to pure
+recessive dwarfs. Consequently from the 4x ovules of the self-fertilised
+hybrid we ought to obtain 3x tall and x dwarf plants. And of the 3x talls x
+should breed true to tallness, while the remaining 2x, having been formed
+like the original hybrid by the union of a "tall" and a "dwarf" gamete,
+ought to behave like it when bred from and give talls and dwarfs in the
+ratio 3 : 1. Now this is precisely the result actually obtained by
+experiment (cf. p. 17), and the close accord of the experimental results
+with those deduced on the assumption of the purity of the gametes as
+enunciated by Mendel affords the strongest of arguments for regarding the
+nature of the gametes and their relation to the characters of the zygotes
+in the way that he has done.
+
+It is possible to put the theory to a further test. The explanation of the
+3 : 1 ratio of dominants and recessives in the F_2 generation is regarded
+as due to the F_1 individuals producing equal numbers of gametes bearing
+the {24} dominant and recessive elements respectively. If now the F_1 plant
+be crossed with the pure recessive, we are bringing together a series of
+gametes consisting of equal numbers of dominants and recessives with a
+series consisting solely of recessives. We ought from such a cross to
+obtain equal numbers of dominant and recessive individuals, and further,
+the dominants so produced ought all to give both dominants and recessives
+in the ratio 3 : 1 when they themselves are bred from. Both of these
+expectations were amply confirmed by experiment, and crossing with the
+recessive is now a recognised way of testing whether a plant or animal
+bearing a dominant character is a pure dominant, or an impure dominant
+which is carrying the recessive character. In the former case the offspring
+will be all of the dominant form, while in the latter they will consist on
+the average of equal numbers of dominants and recessives.
+
+So far we have been concerned with the results obtained when two
+individuals differing in a single pair of characters are crossed together
+and with the interpretation of those results. But Mendel also used plants
+which differed in more than a single pair of differentiating characters. In
+such cases he found that each pair of characters followed the same definite
+rule, but that the inheritance of each pair was absolutely independent of
+the other. Thus, for example, when a tall plant bearing coloured flowers
+was crossed with a dwarf plant {25} bearing white flowers the resulting
+hybrid was a tall plant with coloured flowers. For coloured flowers are
+dominant to white, and tallness is dominant to dwarfness. In the succeeding
+generation there are plants with coloured flowers and plants with white
+flowers in the proportion of 3 : 1, and at the same time tall plants and
+dwarf plants in the same proportion. Hence the chances that a tall plant
+will have coloured flowers are three times as great as its chance of having
+white flowers. And this is also true for the dwarf plants. As the result of
+this cross, therefore, we should expect an F_2 generation consisting of
+four classes, viz. coloured talls, white talls, coloured dwarfs, and white
+dwarfs, and we should further expect these four forms to appear in the
+ratio of 9 coloured talls, 3 white talls, 3 coloured dwarfs, and 1 white
+dwarf. For this is the only ratio which satisfies the conditions that the
+talls should be to the dwarfs as 3 : 1, and at the same time the coloured
+should be to the whites as 3 : 1. And these are the proportions that Mendel
+found to obtain actually in his experiments. Put in a more general form, it
+may be stated that when two individuals are crossed which differ in two
+pairs of differentiating characters the hybrids (F_1) are all of the same
+form, exhibiting the dominant character of each of the two pairs, while the
+F_2 generation produced by such hybrids consists on the average of 9
+showing both dominants, 3 showing one dominant and one recessive, {26} 3
+showing the other dominant and the other recessive, and 1 showing both
+recessive characters. And, as Mendel pointed out, the principle may be
+extended indefinitely. If, for example, the parents differ in three pair of
+characters A, B, and C, respectively dominant to a, b, and c, the F_1
+individuals will be all of the form ABC, while the F_2 generation will
+consists of 27 ABC, 9 ABc, 9 AbC, 9 aBC, 3 Abc, 3 aBc, 3 abC, and 1 abc.
+When individuals differing in a number of alternative characters are
+crossed together, the hybrid generation, provided that the original parents
+were of pure strains, consists of plants of the same form; but when these
+are bred from a redistribution of the various characters occurs. That
+redistribution follows the same definite rule for each character, and if
+the constitution of the original parents be known, the nature of the F_2
+generation, _i.e._ the number of possible forms and the proportions in
+which they occur, can be readily calculated. Moreover, as Mendel showed, we
+can calculate also the chances of any given form breeding true. To this
+point, however, we shall return later.
+
+Of Mendel's experiments with beans it is sufficient to say here that they
+corroborated his more ample work with peas. He is also known to have made
+experiments with many other plants, and a few of his results are
+incidentally given in his series of letters to Naegeli the botanist. To the
+breeding and crossing of bees he also devoted much {27} time and attention,
+but unhappily the record of these experiments appears to have been lost.
+The only other published work that we possess dealing with heredity is a
+brief paper on some crossing experiments with the hawkweeds (_Hieracium_),
+a genus that he chose for working with because of the enormous number of
+forms under which it naturally exists. By crossing together the more
+distinct varieties, he evidently hoped to produce some of these numerous
+wild forms, and so throw light upon their origin and nature. In this hope
+he was disappointed. Owing in part to the great technical difficulties
+attending the cross fertilisation of these flowers he succeeded in
+obtaining very few hybrids. Moreover, the behaviour of those which he did
+obtain was quite contrary to what he had found in the peas. Instead of
+giving a variety of forms in the F_2 generation, they bred true and
+continued to do so as long as they were kept under observation. More recent
+research has shown that this is due to a peculiar form of parthenogenesis
+(cf. p. 135), and not to any failure of the characters to separate clearly
+from one another in the gametes. Mendel, however, could not have known of
+this, and his inability to discover in _Hieracium_ any indication of the
+rule which he had found to hold good for both peas and beans must have been
+a source of considerable disappointment. Whether for this reason, or owing
+to the utter neglect of his work by the scientific world, Mendel gave up
+his experimental {28} researches during the latter part of his life. His
+closing years were shadowed with ill-health and embittered by a controversy
+with the Government on a question of the rights of his monastery. He died
+of Bright's disease in 1884.
+
+    _Note._--Shortly after the discovery of Mendel's paper a need was felt
+    for terms of a general nature to express the constitution of
+    individuals in respect of inherited characters, and Bateson accordingly
+    proposed the words homozygote and heterozygote. An individual is said
+    to be homozygous for a given character when it has been formed by two
+    gametes each bearing the character, and all the gametes of a homozygote
+    bear the character in respect of which it is homozygous. When, however,
+    the zygote is formed by two gametes of which one bears the given
+    character while the other does not, it is said to be heterozygous for
+    the character in question, and only half the gametes produced by such a
+    heterozygote bear the character. An individual may be homozygous for
+    one or more characters, and at the same time may be heterozygous for
+    others.
+
+       *       *       *       *       *
+
+
+{29}
+
+CHAPTER IV
+
+THE PRESENCE AND ABSENCE THEORY
+
+It was fortunate for the development of biological science that the
+rediscovery of Mendel's work found a small group of biologists deeply
+interested in the problems of heredity, and themselves engaged in
+experimental breeding. To these men the extraordinary significance of the
+discovery was at once apparent. From their experiments, undertaken in
+ignorance of Mendel's paper, de Vries, Correns, and Tschermak were able to
+confirm his results in peas and other plants, while Bateson was the first
+to demonstrate their application to animals. Thenceforward the record has
+been one of steady progress, and the result of ten years' work has been to
+establish more and more firmly the fundamental nature of Mendel's
+discovery. The scheme of inheritance, which he was the first to enunciate,
+has been found to hold good for such diverse things as height, hairiness,
+and flower colour and flower form in plants, the shape of pollen grains,
+and the structure of fruits; while among animals the coat colour of
+mammals, the form of the feathers and of the comb in poultry, the waltzing
+habit of Japanese mice, and eye {30} colour in man are but a few examples
+of the diversity of characters which all follow the same law of
+transmission. And as time went on many cases which at first seemed to fall
+without the scheme have been gradually brought into line in the light of
+fuller knowledge. Some of these will be dealt with in the succeeding
+chapters of this book. Meanwhile we may concern ourselves with the single
+modification of Mendel's original views which has arisen out of more ample
+knowledge.
+
+[Illustration: FIG. 2.
+
+A wing feather and a contour feather of an ordinary and a silky fowl. The
+peculiar ragged appearance of the silky feathers is due to the absence of
+the little hooks or barbules which hold the barbs together. The silky
+condition is recessive.]
+
+As we have already seen, Mendel considered that in the gamete there was
+either a definite something {31} corresponding to the dominant character or
+a definite something corresponding to the recessive character, and that
+these somethings whatever they were could not coexist in any single gamete.
+For these somethings we shall in future use the term FACTOR. The factor,
+then, is what corresponds in the gamete to the UNIT-CHARACTER that appears
+in some shape or other in the development of the zygote. Tallness in the
+pea is a unit-character, and the gametes in which it is represented are
+said to contain the factor for tallness. Beyond their existence in the
+gamete and their mode of transmission we make no suggestion as to the
+nature of these factors.
+
+[Illustration: FIG. 3.
+
+Two double and an ordinary single primula flower. This form of double is
+recessive to the single.]
+
+{32}
+
+[Illustration: FIG. 4.
+
+Fowls' combs. A, pea; B, rose; C, single; D, walnut.]
+
+On Mendel's view there was a factor corresponding to the dominant character
+and another factor corresponding to the recessive character of each
+alternative pair of unit-characters, and the characters were alternative
+because no gamete could carry more than one of the two factors belonging to
+the alternative pair. On the other hand, Mendel supposed that it always
+carried either one or the other of such a pair. As experimental work
+proceeded, {33} it soon became clear that there were cases which could not
+be expressed in terms of this conception. The nature of the difficulty and
+the way in which it was met will perhaps be best understood by considering
+a set of experiments in which it occurred. Many of the different breeds of
+poultry are characterised by a particular form of comb, and in certain
+cases the inheritance of these has been carefully worked out. It was shown
+that the rose comb (Fig. 4, B) with its flattened papillated upper surface
+and backwardly projecting pike was dominant in the ordinary way to the
+deeply serrated high single comb (Fig. 4, C) which is characteristic of the
+Mediterranean races. Experiment also showed that the pea comb (Fig. 4, A),
+a form with a low central and two well-developed lateral ridges, such as is
+found in Indian game, behaves as a simple dominant to the single comb. The
+interesting question arose as to what would happen when the rose and the
+pea, two forms each dominant to the same third form, were mated together.
+It seemed reasonable to suppose that things which were alternative to the
+same thing would be alternative to one another--that either rose or pea
+would dominate in the hybrids, and that the F_2 generation would consist of
+dominants and recessives in the ratio 3 : 1. The result of the experiment
+was, however, very different. The cross rose x pea led to the production of
+a comb quite unlike either of them. This, the so-called walnut comb (Fig.
+4, D), {34} from its resemblance to the half of a walnut, is a type of comb
+which is normally characteristic of the Malay fowl. Moreover, when these
+F_1 birds were bred together, a further unlooked-for result was obtained.
+As was expected, there appeared in the F_2 generation the three forms
+walnut, rose, and pea. But there also appeared a definite proportion of
+single-combed birds, and among many hundreds of chickens bred in this way
+the proportions in which the four forms walnut, rose, pea, and single
+appeared was 9 : 3 : 3 : 1.
+
+            Rose x Pea
+                 |
+            +----+----+
+          Walnut x Walnut
+                 |
+    +--------+---+---+--------+
+  Walnut   Rose     Pea    Single
+   (9)      (3)     (3)      (1)
+
+Now this, as Mendel showed, is the ratio found in an F_2 generation when
+the original parents differ in two pairs of alternative characters, and
+from the proportions in which the different forms of comb occur we must
+infer that the walnut contains both dominants, the rose and the pea one
+dominant each, while the single is pure for both recessive characters. This
+accorded with subsequent breeding experiments, for the singles bred
+perfectly true as soon as they had once made their appearance. So far the
+case is clear. The difficulty comes when we attempt to define these two
+pairs of characters. How are we to express the fact that while single
+behaves as a simple recessive to either pure rose, or to pure pea, it can
+yet appear in F_2 from a cross {35} between these two pure forms, though
+neither of them should, on Mendel's view, contain the single? An
+explanation which covers the facts in a simple way is that which has been
+termed the "Presence and Absence" theory. On this theory the dominant
+character of an alternative pair owes its dominance to the presence of a
+factor which is absent in the recessive. The tall pea is tall owing to the
+presence in it of the factor for tallness, but in the absence of this
+factor the pea remains a dwarf. All peas are dwarf, but the tall is a dwarf
+plus a factor which turns it into a tall. Instead of the characters of an
+alternative pair being due to two separate factors, we now regard them as
+the expression of the only two possible states of a single factor, viz. its
+presence or its absence. The conception will probably become clearer if we
+follow its application in detail to the case of the fowl's combs. In this
+case we are concerned with the transmission of the two factors, rose (R)
+and pea (P), the presence of each of which is alternative to its absence.
+The rose-combed bird contains the factor for rose but not that for pea, and
+the pea-combed bird contains the factor for pea but not that for rose. When
+both factors are present in a bird, as in the hybrid made by crossing rose
+with pea, the result is a walnut. For convenience of argument we may denote
+the presence of a given factor by a capital letter and its absence by the
+corresponding small letter. The use of the small letter is merely a
+symbolic way of intimating {36} that a particular factor is absent in a
+gamete or zygote. Represented thus the zygotic constitution of a pure
+rose-combed bird is RRpp; for it has been formed by the union of two
+gametes both of which contained R but not P. Similarly we may denote the
+pure pea-combed bird as rrPP. On crossing the rose with the pea union
+occurs between a gamete Rp and a gamete rP, resulting in the formation of a
+heterozygote of the constitution RrPp. The use of the small letters here
+informs us that such a zygote contains only a single dose of each of the
+factors R and P, although, of course, it is possible for a zygote, if made
+in a suitable way, to have a double dose of any factor. Now when such a
+bird comes to form gametes a separation takes place between the part of the
+zygotic cell containing R and the part which does not contain it (r). Half
+of its gametes, therefore, will contain R and the other half will be
+without it (r). Similarly half of its gametes will contain P and the other
+half will be without it (p). It is obvious that the chances of R being
+distributed to a gamete with or without P are equal. Hence the gametes
+containing R will be of two sorts, RP and Rp, and these will be produced in
+equal numbers. Similarly the gametes without R will also be of two sorts,
+rP and rp, and these, again, will be produced in equal numbers. Each of the
+hybrid walnut-combed birds, therefore, gives rise to a series consisting of
+equal numbers of gametes of the four different types RP, Rp, rP, and rp;
+and the breeding {37} together of such F_1 birds means the bringing
+together of two such series of gametes. When this happens an ovum of any
+one of the four types has an equal chance of being fertilised by a
+spermatozoon of any one of the four types. A convenient and simple method
+of demonstrating what happens under such circumstances is the method
+sometimes termed the "chessboard" method. For two series each consisting of
+four different types of gamete we require a square divided up into 16
+parts. The four terms of the gametic series are first written horizontally
+across the four sets of four squares, so that the series is repeated four
+times. It is then written vertically four times, care being taken to keep
+to the same order. In this simple mechanical way all the possible
+combinations are represented and in their proper proportions.
+
+  +-------+-------+-------+-------+
+  |RP     |RP     |RP     |RP     |
+  |RP     |Rp     |rP     |rp     |
+  |       |       |       |       |
+  | Walnut| Walnut| Walnut| Walnut|
+  +-------+-------+-------+-------+
+  |Rp     |Rp     |Rp     |Rp     |
+  |RP     |Rp     |rP     |rp     |
+  |       |       |       |       |
+  | Walnut|   Rose| Walnut|   Rose|
+  +-------+-------+-------+-------+
+  |rP     |rP     |rP     |rP     |
+  |RP     |Rp     |rP     |rp     |
+  |       |       |       |       |
+  | Walnut| Walnut|    Pea|    Pea|
+  +-------+-------+-------+-------+
+  |rp     |rp     |rp     |rp     |
+  |RP     |Rp     |rP     |rp     |
+  |       |       |       |       |
+  | Walnut|   Rose|    Pea| Single|
+  +-------+-------+-------+-------+
+
+    FIG. 5.
+
+    Diagram to illustrate the nature of the F_2 generation from the cross
+    of rose comb x pea comb.
+
+Fig. 5 shows the result of applying this method to our series RP, Rp, rP,
+rp, and the 16 squares represent the different kinds of zygotes formed and
+the proportions in which they occur. As {38} the figure shows, 9 zygotes
+contain both R and P, having a double or a single dose of either or both of
+these factors. Such birds must be all walnut combed. Three out of the 16
+zygotes contain R but not P, and these must be rose-combed birds. Three,
+again, contain P but not R and must be pea-combed birds. Finally one out of
+the 16 contains neither R nor P. It cannot be rose--it cannot be pea. It
+must, therefore, be something else. As a matter of fact it is single. Why
+it should be single and not something else follows from what we already
+know about the behaviour of these various forms of comb. For rose is
+dominant to single; therefore on the Presence and Absence theory a rose is
+a single plus a factor which turns the single into a rose. If we could
+remove the "rose" factor from a rose-combed bird the underlying single
+would come into view. Similarly a pea comb is a single plus a factor which
+turns the single into a pea, and a walnut is a single which possesses two
+additional modifying factors. Singleness, in fact, underlies all these
+combs, and if we write their zygotic constitution in full we must denote a
+walnut as RRPPSS, a rose as RRppSS, a pea as rrPPSS, and a single as
+rrppSS. The crossing of rose with pea results in a reshuffling of the
+factors concerned, and in accordance with the principle of segregation some
+zygotes are formed in which neither of the modifying factors R and P are
+present, and the single character can then become manifest. {39}
+
+The Presence and Absence theory is to-day generally accepted by students of
+these matters. Not only does it afford a simple explanation of the
+remarkable fact that in all cases of Mendelian inheritance we should be
+able to express our unit-characters in terms of alternative pairs, but, as
+we shall have occasion to refer to later, it suggests a clue as to the
+course by which the various domesticated varieties of plants and animals
+have arisen from their wild prototypes.
+
+[Illustration: FIG. 6.
+
+Fowls' combs. A and B, F_1 hen from rose x Breda; C, an F_1 cock from the
+cross of single x Breda; D, head of Breda cock.]
+
+Before leaving this topic we may draw attention to some experiments which
+offer a pretty confirmation of the view that the rose comb is a single to
+which a modifying factor for roseness has been added. It was argued that if
+we could find a type of comb in which the factor for singleness was absent,
+then on crossing such a comb with a rose we ought, if singleness really
+underlies rose, to obtain some single combs in F_2 from such a cross. Such
+a comb we had the good fortune to find in the Breda fowl, a breed largely
+used in Holland. This fowl is usually spoken of as combless, for the place
+of the comb is taken by a covering of short bristlelike feathers (Fig. 6,
+D). In reality it possesses the vestige of a comb in the form of two minute
+lateral knobs of comb tissue. Characteristic also of this breed is the high
+development of the horny nostrils, a feature probably correlated with the
+almost complete absence of comb. The first step in the experiment was to
+prove the absence of the factor for singleness in the Breda. {40} On
+crossing Breda with single the F_1 birds exhibit a large comb of the form
+of a double single comb in which the two portions are united anteriorly,
+but diverge from one another towards the back of the head (Fig. 6, C). The
+Breda contains an element of duplicity which is dominant to the simplicity
+of the ordinary single comb. But it cannot contain the factor for the
+single comb, because as soon as that is put into it by crossing with a
+single the comb {41} assumes a large size, and is totally distinct in
+appearance from its almost complete absence in the pure Breda. Now when the
+Breda is crossed with the rose duplicity is dominant to simplicity, and
+rose is dominant to lack of comb, and the F_1 generation consists of birds
+possessing duplex rose combs (Fig. 6, A and B). On breeding such birds
+together we obtain a generation consisting of Bredas, duplex roses, roses,
+duplex singles, and singles. From our previous experiment we know that the
+singles could not have come from the Breda, since a Breda comb to which the
+factor for single has been added no longer remains a Breda. Therefore it
+must have come from the rose, thus confirming our view that the rose is in
+reality a single comb which contains in addition a dominant modifying
+factor (R) whose presence turns it into a rose. We shall take it,
+therefore, that there is good experimental evidence for the Presence and
+Absence theory, and we shall express in terms of it the various cases which
+come up for discussion in succeeding chapters.
+
+             Rose x Breda
+                  |
+        +---------+---------+
+        |                   |
+     Duplex       x      Duplex
+      Rose        |       Rose
+   +-------+------+-------+-------+
+   |       |      |       |       |
+  Duplex  Rose  Duplex  Single  Breda
+   Rose         Single         (Duplex
+                              and Simplex)
+
+       *       *       *       *       *
+
+
+{42}
+
+CHAPTER V
+
+INTERACTION OF FACTORS
+
+We have now reached a point at which it is possible to formulate a definite
+conception of the living organism. A plant or animal is a living entity
+whose properties may in large measure be expressed in terms of
+unit-characters, and it is the possession of a greater or lesser number of
+such unit-characters renders it possible for us to draw sharp distinctions
+between one individual and another. These unit-characters are represented
+by definite factors in the gamete which in the process of heredity behave
+as indivisible entities, and are distributed according to a definite
+scheme. The factor for this or that unit-character is either present in the
+gamete or it is not present. It must be there in its entirety or completely
+absent. Such at any rate is the view to which recent experiment has led us.
+But as to the nature of these factors, the conditions under which they
+exist in the gamete, and the manner in which they produce their specific
+effects in the zygote, we are at present almost completely in the dark.
+
+The case of the fowls' combs opens up the important question of the extent
+to which the various factors can {43} influence one another in the zygote.
+The rose and the pea factors are separate entities, and each when present
+alone produces a perfectly distinct and characteristic effect upon the
+single comb, turning it into a rose or a pea as the case may be. But when
+both are present in the same zygote their combined effect is to produce the
+walnut comb, a comb which is quite distinct from either and in no sense
+intermediate between them. The question of the influence of factors upon
+one another did not present itself to Mendel because he worked with
+characters which affected different parts of the plant. It was unlikely
+that the factor which led to the production of colour in the flower would
+affect the shape of the pod, or that the height of the plant would be
+influenced by the presence or absence of the factor that determined the
+shape of the ripe seed. But when several factors can modify the same
+structure it is reasonable to suppose that they will influence one another
+in the effects which their simultaneous presence has upon the zygote. By
+themselves the pea and the rose factors each produce a definite
+modification of the single comb, but when both are present in the zygote,
+whether as a single or double dose, the modification that results is quite
+different to that produced by either when present alone. Thus we are led to
+the conception of characters which depend for their manifestation on more
+than one factor in the zygote, and in the present chapter we may consider a
+few of the {44} phenomena which result from such interaction between
+separate and distinct factors.
+
+  White x White
+        |
+       Red -------------- F_1
+        |
+   +----------+
+  Red       White ------- F_2
+  (9)        (7)
+
+One of the most interesting and instructive cases in which the interaction
+between separate factors has been demonstrated is a case in the sweet pea.
+All white sweet peas breed true to whiteness. And generally speaking the
+result of crossing different whites is to produce nothing but whites,
+whether in F_1 or in succeeding generations. But there are certain strains
+of white sweet peas which when crossed together produce only coloured
+flowers. The colour may be different in different cases, though for our
+present purpose we may take a case in which the colour is red. When such
+reds are allowed to self-fertilise themselves in the normal way and the
+seeds sown, the resulting F_2 generation consists of reds and whites, the
+former being rather more numerous than the latter in the proportion of
+9 : 7. The raising of a further generation from the seeds of these F_2
+plants shows that the whites always breed true to whiteness, but that
+different reds may behave differently. Some breed true, others give reds
+and whites in the ratio 3 : 1, while others, again, give reds and whites in
+the ratio 9 : 7. As in the case of the fowls' combs, this case may be
+interpreted in terms of the presence and absence of two factors. {45}
+
+             White        White
+             AAbb         aaBB
+             / \          / \
+            /   \        /   \
+          Ab  Ab   aB  aB  gametes of parents
+                 `-------'
+                  Red F_1
+                    AaBb
+                    / \
+                   /   \
+                  AB   AB
+  Female gametes  Ab   Ab  Male gametes
+  of F_1          aB   aB  of F_1
+                  ab   ab
+
+Red in the sweet pea results from the interaction of two factors, and
+unless these are both present the red colour cannot appear. Each of the
+white parents carried one of the two factors whose interaction is necessary
+for the production of the red colour, and as a cross between them brings
+these two complementary factors together the F_1 plants must all be red. As
+this case is of considerable importance for the proper understanding of
+much that is to follow, and as it has been completely worked out, we shall
+consider it in some detail. Denoting these two colour factors by A and B
+respectively we may proceed to follow out the consequences of this cross.
+Since all the F_1 plants were red the constitution of the parental whites
+must have been AAbb and aaBB respectively, and their gametes consequently
+Ab and aB. The constitution of the F_1 plants must, therefore, be AaBb.
+Such a plant being heterozygous for two factors produces a series of
+gametes of the four kinds AB, Ab, aB, ab, and produces them in equal
+numbers (cf. p. 36). To obtain the various types of zygotes which are
+produced when such {46} a series of pollen grains meets a similar series of
+ovules we may make use of the same "chessboard" system which we have
+already adopted in the case of the fowls' combs.
+
+  +------+------+------+------+
+  |AB....|AB....|AB....|AB....|
+  |AB....|Ab....|aB....|ab....|
+  |......|......|......|......|
+  +------+------+------+------+
+  |Ab....|Ab    |Ab....|Ab    |
+  |AB....|Ab    |aB....|ab    |
+  |......|      |......|      |
+  +------+------+------+------+
+  |aB....|aB....|aB    |aB    |
+  |AB....|Ab....|aB    |ab    |
+  |......|......|      |      |
+  +------+------+------+------+
+  |ab....|ab    |ab    |ab    |
+  |AB....|Ab    |aB    |ab    |
+  |......|      |      |      |
+  +------+------+------+------+
+
+    FIG. 7.
+
+    Diagram to illustrate the nature of the F_2 generation from the two
+    white sweet peas which give a coloured F_1.
+
+An examination of this figure (Fig. 7) shows that 9 out of the 16 squares
+contain both A and B, while 7 contain either A or B alone, or neither. In
+other words, on this view of the nature of the two white sweet peas we
+should in the F_2 generation look for the appearance of coloured and white
+flowers in the ratio 9 : 7. And this, as we have already seen, is what was
+actually found by experiment. Further examination of the figure shows that
+the coloured plants are not all of the same constitution, but are of four
+kinds with respect to their zygotic constitution, viz. AABB, AABb, AaBB,
+and AaBb. Since AABB is homozygous for both A and B, all the gametes which
+it produces must contain both of these factors, and such a plant must
+therefore breed true to the red colour. A plant of the {47} constitution
+AABb is homozygous for the factor A, but heterozygous for B. All of its
+gametes will contain A, but only one-half of them will contain B, _i.e._ it
+produces equal numbers of gametes AB and Ab. Two such series of gametes
+coming together must give a generation consisting of x AABB, 2x AABb, and x
+AAbb, that is, reds and whites in the ratio 3 : 1. Lastly the red zygotes
+of the constitution AaBb have the same constitution as the original red
+made from the two whites, and must therefore when bred from give reds and
+whites in the ratio 9 : 7. The existence of all these three sorts of reds
+was demonstrated by experiment, and the proportions in which they were met
+with tallied with the theoretical explanation.
+
+The theory was further tested by an examination into the properties of the
+various F_2 whites which come from a coloured plant that has itself been
+produced by the mating of two whites. As Fig. 7 shows, these are, in
+respect of their constitution, of five different kinds, viz. AAbb, Aabb,
+aaBB, aaBb, and aabb. Since none of them produce anything but whites on
+self-fertilisation it was found necessary to test their properties in
+another way, and the method adopted was that of crossing them together. It
+is obvious that when this is done we should expect different results in
+different cases. Thus the cross between two whites of the constitution AAbb
+and aaBB should give nothing but coloured plants; for these two whites are
+of {48} the same constitution as the original two whites from which the
+experiment started. On the other hand, the cross between a white of the
+constitution aabb and any other white can never give anything but whites.
+For no white contains both A and B, or it would not be white, and a plant
+of the constitution aabb cannot supply the complementary factor necessary
+for the production of colour. Again, two whites of the constitution Aabb
+and aaBb when crossed should give both coloured and white flowers, the
+latter being three times as numerous as the former. Without going into
+further detail it may be stated that the results of a long series of
+crosses between the various F_2 whites accorded closely with the
+theoretical explanation.
+
+From the evidence afforded by this exhaustive set of experiments it is
+impossible to resist the deduction that the appearance of colour in the
+sweet pea depends upon the interaction of two factors which are
+independently transmitted according to the ordinary scheme of Mendelian
+inheritance. What these factors are is still an open question. Recent
+evidence of a chemical nature indicates that colour in a flower is due to
+the interaction of two definitive substances: (1) a colourless "chromogen,"
+or colour basis; and (2) a ferment which behaves as an activator of the
+chromogen, and by inducing some process of oxidation, leads to the
+formation of a coloured substance. But whether these two bodies exist as
+such {49} in the gametes or whether in some other form we have as yet no
+means of deciding.
+
+Since the elucidation of the nature of colour in the sweet pea phenomena of
+a similar kind have been witnessed in other plants, notably in stocks,
+snapdragons, and orchids. Nor is this class of phenomena confined to
+plants. In the course of a series of experiments upon the plumage colour of
+poultry, indications were obtained that different white breeds did not
+always owe their whiteness to the same cause. Crosses were accordingly made
+between the white Silky fowl and a pure white strain derived from the white
+Dorking. Each of these had been previously shown to behave as a simple
+recessive to colour. When the two were crossed only fully coloured birds
+resulted. From analogy with the case of the sweet pea it was anticipated
+that such F_1 coloured birds when bred together would produce an F_2
+generation consisting of coloured and white birds in the ratio 9 : 7, and
+when the experiment was made this was actually shown to be the case. With
+the growth of knowledge it is probable that further striking parallels of
+this nature between the plant and animal worlds will be met with.
+
+Before quitting the subject of these experiments attention may be drawn to
+the fact that the 9 : 7 ratio is in reality a 9 : 3 : 3 : 1 ratio in which
+the last three terms are indistinguishable owing to the special
+circumstances that neither factor can produce a visible effect without {50}
+the co-operation of the other. And we may further emphasise the fact that
+although the two factors thus interact upon one another they are
+nevertheless transmitted quite independently and in accordance with the
+ordinary Mendelian scheme.
+
+
+        Agouti x Agouti
+               |
+       +---------------+
+     Agouti    x    Agouti
+               |
+     +---------+---------+
+  Agouti     Black     Albino
+    (9)       (3)        (4)
+
+One of the earliest sets of experiments demonstrating the interaction of
+separate factors was that made by the French zoologist Cuenot on the coat
+colours of mice. It was shown that in certain cases agouti, which is the
+colour of the ordinary wild grey mouse, behaves as a dominant to the albino
+variety, _i.e._ the F_2 generation from such a cross consists of agoutis
+and albinos in the ratio 3 : 1. But in other cases the cross between albino
+and agouti gave a different result. In the F_1 generation appeared only
+agoutis as before, but the F_2 generation consisted of three distinct
+types, viz. agoutis, albinos, _and blacks_. Whence the sudden appearance of
+the new type? The answer is a simple one. The albino parent was really a
+black. But it lacked the factor without which the colour is unable to
+develop, and consequently it remained an albino. If we denote this factor
+by C, then the constitution of an albino must be cc, while that of a
+coloured animal may be CC or Cc, according as to whether it breeds true to
+colour or can {51} throw albinos. Agouti was previously known to be a
+simple dominant to black, _i.e._ an agouti is a black rabbit plus an
+additional greying factor which modifies the black into agouti. This factor
+we will denote by G, and we will use B for the black factor. Our original
+agouti and albino parents we may therefore regard as in constitution GGCCBB
+and ggccBB respectively. Both of the parents are homozygous for black. The
+gametes produced by the two parents are GCB, and gcB, and the constitution
+of the F_1 animals must be GgCcBB. Being heterozygous for two factors they
+will produce four kinds of gametes in equal numbers, viz. GCB, GcB, gCB,
+and gcB. The results of the mating of two such similar series of gametes
+when the F_1 animals are bred together we may determine by the usual
+"chessboard" method (Fig. 8). Out of the 16 squares 9 contain both C and G
+in addition to B. Such animals must be agoutis. Three squares contain C but
+not G. Such animals must be coloured, but as they do not contain the
+modifying agouti factor their colour will be black. The remaining four
+squares do not contain C, and in the absence of this colour-developing
+factor they must all be albinos. Theory demands that the three classes
+agouti, black, and albino should appear in F_2 in the ratio 9 : 3 : 4;
+experiment has shown that these are the only classes that appear, and that
+the proportions in which they are produced accord closely with the
+theoretical expectation. Put briefly, then, the explanation {52} of this
+case is that all the animals are black, and that we are dealing with the
+presence and absence of two factors, a colour developer (C), and a colour
+modifier (G), both acting, as it were, upon a substratum of black. The F_2
+generation really consists of the four classes agoutis, blacks, albino
+agoutis, and albino blacks in the ratio 9 : 3 : 3 : 1. But since in the
+absence of the colour developer C the colour modifier G can produce no
+visible result, the last two classes of the ratio are indistinguishable,
+and our F_2 generation comes to consist of three classes in the ratio
+9 : 3 : 4, instead of four classes in the ratio 9 : 3 : 3 : 1.
+
+  +-------+-------+-------+-------+
+  |GCB....|GCB....|GCB....|GCB....|
+  |GCB....|GcB....|gCB....|gcB....|
+  |.......|.......|.......|.......|
+  |.Agouti|.Agouti|.Agouti|.Agouti|
+  +-------+-------+-------+-------+
+  |GcB....|GcB    |GcB....|GcB    |
+  |GCB....|GcB    |gCB....|gcB    |
+  |.......|       |.......|       |
+  |.Agouti| Albino|.Agouti| Albino|
+  +-------+-------+-------+-------+
+  |gCB....|gCB....|gCB####|gCB####|
+  |GCB....|GcB....|gCB####|gcB####|
+  |.......|.......|#######|#######|
+  |.Agouti|.Agouti|##BLACK|##BLACK|
+  +-------+-------+-------+-------+
+  |gcB....|gcB    |gcB####|gcB    |
+  |GCB....|GcB    |gCB####|gcB    |
+  |.......|       |#######|       |
+  |.Agouti| Albino|##BLACK| Albino|
+  +-------+-------+-------+-------+
+
+    FIG. 8.
+
+    Diagram to illustrate the nature of the F_2 generation which may arise
+    from the mating of agouti with albino in mice or rabbits.
+
+This explanation was further tested by experiments with the albinos. In an
+F_2 family of this nature there ought to be three kinds, viz. albinos
+homozygous for G (GGccBB), albinos heterozygous for G (GgccBB), and albinos
+without G (ggccBB). These albinos are, as it were, like photographic plates
+exposed but undeveloped. {53} Their potentialities may be quite different,
+although they all look alike, but this can only be tested by treating them
+with a colour developer. In the case of the mice and rabbits the
+potentiality for which we wish to test is the presence or absence of the
+factor G, and in order to develop the colour we must introduce the factor
+C. Our developer, therefore, must contain C but not G. In other words, it
+must be a homozygous black mouse or rabbit, ggCCBB. Since such an animal is
+pure for C it must, when mated with any of the albinos, produce only
+coloured offspring. And since it does not contain G the appearance of
+agoutis among its offspring must be attributed to the presence of G in the
+albino. Tested in this way the F_2 albinos were proved, as was expected, to
+be of three kinds: (1) those which gave only agouti, _i.e._ which were
+homozygous for G; (2) those which gave agoutis and blacks in approximately
+equal numbers, _i.e._ which were heterozygous for G; and (3) those which
+gave only blacks, and therefore did not contain G.
+
+Though albinos, whether mice, rabbits, rats, or other animals, breed true
+to albinism, and though albinism behaves as a simple recessive to colour,
+yet albinos may be of many different sorts. There are in fact just as many
+kinds of albinos as there are coloured forms--neither more nor less. And
+all these different kinds of albinos may breed together, transmitting the
+various colour factors according to the Mendelian scheme of inheritance,
+{54} and yet the visible result will be nothing but albinos. Under the mask
+of albinism is all the while occurring that segregation of the different
+colour factors which would result in all the varieties of coloured forms,
+if only the essential factor for colour development were present. But put
+in the developer by crossing with a pure coloured form and their variety of
+constitution can then at last become manifest.
+
+So far we have dealt with cases in which the production of a character is
+dependent upon the interaction of two factors. But it may be that some
+characters require the simultaneous presence of a greater number of factors
+for their manifestation, and the experiments of Miss Saunders have shown
+that there is a character in stocks which is unable to appear except
+through the interaction of three distinct factors. Coloured stocks may be
+either hoary, with the leaves and stem covered by small hairs, or they may
+lack the hairy covering, in which case they are termed glabrous. Hoariness
+is dominant to glabrousness; that is to say, there is a definite factor
+which can turn the glabrous into a hoary plant when it is present. But in
+families where coloured and white stocks occur the white are always
+glabrous, while the coloured plants may or may not be hoary. Now colour in
+the stock as in the sweet pea has been proved to be dependent upon the
+interaction of two separate factors. Hence hoariness depends upon three
+separate factors, and a stock cannot be hoary unless {55} it contains the
+hoary factor in addition to the two colour factors. It requires the
+presence of all these three factors to produce the hoary character, though
+how this comes about we have not at present the least idea.
+
+[Illustration: FIG. 9.
+
+Sections of primula flowers. The anthers are shown as black. A, "pin" form
+with long style and anthers set low down; B, "thrum" form with short style
+and anthers set higher up; C, homostyle form with anthers set low down as
+in "pin," but with short style. This form only occurs with the large eye.]
+
+[Illustration: FIG. 10.
+
+Two primula flowers showing the extent of the small and of the large eye.]
+
+A somewhat different and less usual form of interaction between factors may
+be illustrated by a case in primulas recently worked out by Bateson and
+Gregory. Like the common primrose, the primula exhibits both pin-eyed and
+thrum-eyed varieties. In the former the style is long, and the centre of
+the eye is formed by the end of the stigma which more or less plugs up the
+opening of the corolla (cf. Fig. 9, A); in the latter the style is short
+and hidden by the four anthers which spring from higher up in the corolla
+and form the centre of the eye (cf. Fig. 9, B). The greater part of the
+"eye" is formed by the greenish-yellow patches on each petal just at the
+opening {56} of the corolla. In most primulas the eye is small, but there
+are some in which it is large and extends as a flush over a considerable
+part of the petals (Fig. 10). Experiments showed that these two pairs of
+characters behave in simple Mendelian fashion, short style ( = "thrum")
+being dominant to long style (= "pin") and small eye dominant to large.
+Besides the normal long and short styled forms, there occurs a third form,
+which has been termed homostyle. In this form the anthers are placed low
+down in the corolla tube as they are in the long-styled form, but the style
+remains short instead of reaching up to the corolla opening (Fig. 9, C). In
+the course of their experiments Bateson and Gregory crossed a large-eyed
+homostyle plant with a small-eyed thrum ( = short style). The F_1 plants
+were all short styled with small eyes. {57} On self-fertilisation these
+gave an F_2 generation consisting of four types, viz. short styled with
+small eyes, short styled with large eyes, _long styled_ with small eyes,
+and _homostyled_ with large eyes. The notable feature of this generation is
+the appearance of long-styled plants, which, however, occur only in
+association with the small eye. The proportions in which these four types
+appeared shows that the presence or absence of but two factors is
+concerned, and at the same time provides the key to the nature of the
+homostyled plants. These are potentially long styled, and the position of
+the anthers is that of normal long-styled plants, but owing to some
+interaction between the factors the style itself is unable to reach its
+full development unless the factor for the small eye is present. For this
+reason long-styled plants with the large eye are always of the homostyle
+form. What the connecting-link between these apparently unrelated
+structures may be we cannot yet picture to ourselves, any more than we can
+picture the relation between flower {58} colour and hairiness in stocks. It
+is evident, however, that the conception of the interaction of factors,
+besides clearing up much that is paradoxical in heredity, promises to
+indicate lines of research which may lead to valuable extensions in our
+knowledge of the way in which the various parts of the living organism are
+related to one another.
+
+      Short style }   { Homo style
+      small eye   } x { large eye
+                    |
+               Short style
+               small eye
+                    |
+      +-------------+----------+-----------+
+  Short style  Short style  Long style  Homo style
+  small eye    large eye      ("pin")   large eye
+     (9)          (3)           (3)       (1)
+
+       *       *       *       *       *
+
+
+{59}
+
+CHAPTER VI
+
+REVERSION
+
+As soon as the idea was grasped that characters in plants and animals might
+be due to the interaction of complementary factors, it became evident that
+this threw clear light upon the hitherto puzzling phenomenon of reversion.
+We have already seen that in certain cases the cross between a black mouse
+or rabbit and an albino, each belonging to true breeding strains, might
+produce nothing but agoutis. In other words, the cross between the black
+and the white in certain instances results in a complete reversion to the
+wild grey form. Expressed in Mendelian terms, the production of the agouti
+was the necessary consequence of the meeting of the factors C and G in the
+same zygote. As soon as they are brought together, no matter in what way,
+the reversion is bound to occur. Reversion, therefore, in such cases we may
+regard as the bringing together of complementary factors which had somehow
+in the course of evolution become separated from one another. In the
+simplest cases, such as that of the black and the white rabbit, only two
+factors are concerned, and one of them is brought in from each of the {60}
+two parents. But in other cases the nature of the reversion may be more
+complicated owing to a larger number of factors being concerned, though the
+general principle remains the same. Careful breeding from the reversions
+will enable us in each case to determine the number and nature of the
+factors concerned, and in illustration of this we may take another example
+from rabbits. The Himalayan rabbit is a well-known breed. In appearance it
+is a white rabbit with pink eyes, but the ears, paws, and nose are black
+(Pl. I., 2). The Dutch rabbit is another well-known breed. Generally
+speaking, the anterior portion of the body is white, and the posterior part
+coloured. Anteriorly, however, the eyes are surrounded by coloured patches
+extending up to the ears, which are entirely coloured. At the same time the
+hind paws are white (cf. Pl. I., 1). Dutch rabbits exist in many varieties
+of colour, though in each one of these the distribution of colour and white
+shows the same relations. In the experiments about to be described a yellow
+Dutch rabbit was crossed with a Himalaya. The result was a reversion to the
+wild agouti colour (Pl. I., 3). Some of the F_1 individuals showed white
+patches, while others were self-coloured. On breeding from the F_1 animals
+a series of coloured forms resulted in F_2. These were agoutis, blacks,
+yellows, and sooty yellows, the so-called tortoise shells of the fancy (Pl.
+I., 4-7).
+
+[Illustration: PLATE I.
+
+1, Yellow Dutch Rabbit; 2, Himalayan; 3, Agouti ( = grey) F_1 reversion;
+4-8, F_2 types, viz.: 4, Agouti; 5, Yellow; 6, Black; 7, Tortoiseshell; 8,
+Himalayan.]
+
+{61}
+
+             Yellow x Himalayan
+                    |
+             +-------------+
+          Agouti    x    Agouti
+                    |
+    +--------+------+-------+----------+
+  Agouti  Yellow  Black  Tortoise  Himalayan
+                           Shell
+   (27)    (9)     (9)     (3)       (16)
+
+In addition to these appeared Himalayans with either black points or with
+lighter brownish ones, and the proportions in which they came showed the
+Himalayan character to be a simple recessive. A certain number of the
+coloured forms exhibited the Dutch marking to a greater or less extent, but
+as its inheritance in this set of experiments is complicated and has not
+yet been worked out, we may for the present neglect it and confine our
+attention to the coloured types and to the Himalayans. The proportion in
+which the four coloured types appeared in F_2 was very nearly 9 agoutis, 3
+blacks, 3 yellows, and 1 tortoiseshell. Evidently we are here dealing with
+two factors: (1) the grey factor (G), which modifies black into agouti, or
+tortoiseshell into yellow; and (2) an intensifying factor (I), which
+intensifies yellow into agouti and tortoiseshell into black. It may be
+mentioned here that other experiments confirmed the view that the yellow
+rabbit is a dilute agouti, and the tortoiseshell a dilute black. The
+Himalayan pattern behaves as a recessive to self-colour. It is a
+self-coloured black rabbit lacking a factor that allows the colour to
+develop except in the points. That factor we may denote {62} by X, and as
+far as it is concerned the Himalayan is constitutionally xx. The Himalayan
+contains the intensifying factor, for such pigment as it possesses in the
+points is full coloured. At the same time it is black, _i.e._ lacking in
+the factor G. With regard to these three factors, therefore, the
+constitution of the Himalayan is ggIIxx. The last character which we have
+to consider in this cross is the Dutch character. This was found by Hurst
+to behave as a recessive to self-colour (S), and for our present purpose we
+will regard it as differing from a self-coloured rabbit in the lack of this
+factor.[3] The Himalayan is really a self-coloured animal, which, however,
+is unable to show itself as a full black owing to its not possessing the
+factor X. The results of breeding experiments then suggest that we may
+denote the Himalayan by the formula ggIIxxSS and the yellow Dutch by
+GGiiXXss. Each lacks two of the factors upon the full complement of which
+the agouti colour depends. By crossing them the complete series GIXS is
+brought into the same zygote, and the result is a reversion to the colour
+of the wild rabbit.
+
+             Bush x Cupid
+                  |
+                 Tall -------------------------- F_1
+                  |
+   +----------+---+------+----------+
+  Tall      Bush       Cupid      Cupid -------- F_2
+                   (procumbent)   (erect)
+
+Most of the instances of reversion yet worked out are those in which colour
+characters are concerned. The sweet pea, however, supplies us with a good
+example of reversion in structural characters. A dwarf variety known as the
+"Cupid" has been extensively grown for {63} some years. In these little
+plants the internodes are very short and the stems are few in number, and
+attain to a length of only 9-10 inches. In course of growth they diverge
+from one another, and come to lie prostrate on the ground (Pl. II., 2).
+Curiously enough, although the whole plant is dwarfed in other respects,
+this does not seem to affect the size of the flower, which is that of a
+normal sweet pea. Another though less-known variety is the "Bush" sweet
+pea. Its name is derived from its habit of growth. The numerous stems do
+not diverge from one another, but all grow up side by side, giving the
+plant the appearance of a compact bush (Pl. II., 1). Under ordinary
+conditions it attains a height of 3-1/2-4 feet. A number of crosses were
+made between the Bush and Cupid varieties, with the somewhat unexpected
+result that in every instance the F_1 plants showed complete reversion to
+the size and habit of the ordinary tall sweet pea (Pl. II., 3), which is
+the form of the wild plant as it occurs in Sicily to-day. The F_2
+generation from these reversionary talls consisted of four different types,
+viz. {64} talls, bushes, Cupids of the procumbent type like the original
+Cupid parent, and Cupids with the compact upright Bush habit (Pl. II., 4).
+These four types appeared in the ratio 9 : 3 : 3 : 1, and this, of course,
+provided the clue to the nature of the case. The characters concerned are
+(1) long internode of stem between the leaves which is dominant to short
+internode, and (2) the creeping procumbent habit which is dominant to the
+erect bush-like habit. Of these characters length of internode was carried
+by the Bush, and the procumbent habit by the original Cupid parent. The
+bringing of them together by the cross resulted in a procumbent plant with
+long internodes. This is the ordinary tall sweet pea of the wild Sicilian
+type, reversion here, again, being due to the bringing together of two
+complementary factors which had somehow become separated in the course of
+evolution.
+
+To this interpretation it may be objected that the ordinary sweet pea is a
+plant of upright habit. This, however, is not true. It only appears so
+because the conventional way of growing it is to train it up sticks. In
+reality it is of procumbent habit, with divergent stems like the ordinary
+Cupid, a fact which can easily be observed by anyone who will watch them
+grow without the artificial aid of prepared supports.
+
+[Illustration: PLATE II.
+
+1, Bush Sweet Pea; 2, Cupid Sweet Pea; 3, F_1 reversionary Tall; 4, Erect
+Cupid Sweet Pea; 5, Purple Invincible; 6, Painted Lady; 7, Duke of
+Westminster (hooded standard).]
+
+{65}
+
+The cases of reversion with which we have so far dealt have been cases in
+which the reversion occurs as an immediate result of a cross, _i.e._ in the
+F_1 generation. This is perhaps the commonest mode of reversion, but
+instances are known in which the reversion that occurs when two pure types
+are crossed does not appear until the F_2 generation. Such a case we have
+already met with in the fowls' combs. It will be remembered that the cross
+between pure pea and pure rose gave walnut combs in F_1, while in the F_2
+generation a definite proportion, 1 in 16, of single combs appeared (cf. p.
+32). Now the single comb is the form that is found in the wild jungle fowl,
+which is generally regarded as the ancestor of the domestic breeds. If this
+is so, we have a case of reversion in F_2; and this in the _absence_ of the
+two factors brought together by the rose-comb and pea-comb parents. Instead
+of the reversion being due to the bringing together of two complementary
+factors, we must regard it here as due to the association of two
+complementary absences. To this question, however, we shall revert later in
+discussing the origin of domesticated varieties.
+
+  Black Barb x White Fantail   Black Barb x Spot[4]
+             |                            |
+           Dark              x           Dark
+             Among the offspring one very similar
+                to the wild blue rock.
+
+
+
+                    Black   White
+                    Barb  x  Fantail
+                          |
+            +------------------------+
+          Black           x        Black
+      (White Splashed)    |     (White Splashed)
+                          |
+        +--------+--------+---------+-----------+
+      Black    Black    Blue      Blue        White
+  (White Splashed)    (White Splashed)
+  \--------------/     \-------------/
+        (9)                 (3)                (4)
+
+There is one other instance of reversion to which we must allude. This is
+Darwin's famous case of the occasional appearance of pigeons reverting to
+the wild blue rock (_Columba livia_), when certain domesticated races are
+crossed together. As is well known, Darwin made use of this as an argument
+for regarding all the domesticated varieties as having arisen from the same
+wild species. The original experiment is somewhat complicated, and is shown
+in the accompanying scheme. Essentially it lay in {66} following the
+results flowing from crosses between blacks and whites. Experiments
+recently made by Staples-Browne have shown that this case of reversion also
+can be readily interpreted in Mendelian terms. In these experiments the
+cross was made between black barbs and white fantails. The F_1 birds were
+all black with some white splashes, evidently due to a separate factor
+introduced by the fantail. On breeding these blacks together they gave an
+F_2 generation, consisting of blacks (with or without white splashes),
+blues (with or without white splashes), and whites in the ratio 9 : 3 : 4.
+The factors concerned are colour (C), in the absence of {67} which a bird
+is white, and a black modifier (B), in the absence of which a coloured bird
+is blue. The original black barb contained both of these factors, being in
+constitution CCBB. The fantail, however, contained neither, and was
+constitutionally ccbb. The F_1 birds produced by crossing were in
+constitution CcBb, and being heterozygous for two factors produced in equal
+numbers the four sorts of gametes CB, Cb, cB, cb. The results of two such
+series of gametes being brought together are shown in the usual way in Fig.
+11. A blue is a bird containing the colour factor but lacking the black
+modifier, _i.e._ of the constitution CCbb, or Ccbb, and such birds as the
+figure shows appear in the F_2 generation on the average three times out of
+sixteen. Reversion here comes about in F_2, when the redistribution of the
+factors leads to the formation of zygotes containing one of the two factors
+but not the other.
+
+  +-------+-------+-------+-------+
+  |CB#####|CB#####|CB#####|CB#####|
+  |CB#####|Cb#####|cB#####|cb#####|
+  |#######|#######|#######|#######|
+  |##BLACK|##BLACK|##BLACK|##BLACK|
+  +-------+-------+-------+-------+
+  |Cb#####|Cb.....|Cb#####|Cb.....|
+  |CB#####|Cb.....|cB#####|cb.....|
+  |#######|.......|#######|.......|
+  |##BLACK|...Blue|##BLACK|...Blue|
+  +-------+-------+-------+-------+
+  |cB#####|cB#####|cB     |cB     |
+  |CB#####|Cb#####|cB     |cb     |
+  |#######|#######|       |       |
+  |##BLACK|##BLACK|       |       |
+  +-------+-------+-------+-------+
+  |cb#####|cb.....|cb     |cb     |
+  |CB#####|Cb.....|cB     |cb     |
+  |#######|.......|       |       |
+  |##BLACK|...Blue|       |       |
+  +-------+-------+-------+-------+
+
+    FIG. 11.
+
+    Diagram to illustrate the appearance of the reversionary blue pigeon in
+    F_2 from the cross of black with white.
+
+       *       *       *       *       *
+
+
+{68}
+
+CHAPTER VII
+
+DOMINANCE
+
+[Illustration: FIG. 12.
+
+Primula flowers to illustrate the intermediate nature of the F_1 flower
+when _sinensis_ is crossed with _stellata_.]
+
+            Sinensis x Stellata
+                     |
+                Intermediate -------------------------- F_1
+                     |
+      +---------+----+-------+-----------+
+  Sinensis    Inter.       Inter.    Stellata --------- F_2
+     |          |                        |
+  Sinensis      |                    Stellata --------- F{3}
+     |       +-----+-----+-----+         |
+  Sinensis  Sin.  Int.  Int. Stell.  Stellata --------- F{4}
+
+In the cases which we have hitherto considered the presence of a factor
+produces its full effect whether it is introduced by both of the gametes
+which go to form the zygote, or by one of them alone. The heterozygous tall
+pea or the heterozygous rose-combed fowl cannot be distinguished from the
+homozygous form by mere inspection, however close. Breeding tests alone can
+decide which is the heterozygous and which the homozygous form. Though this
+is true for the majority of characters yet investigated, there are cases
+known in which the heterozygous form differs in appearance from either
+parent. Among plants such a case has been met with in the primula. The
+ordinary Chinese primula (_P. sinensis_) (Fig. 12) has large rather wavy
+petals much crenated at the edges. In the Star Primula (_P. stellata_) the
+flowers are much smaller, while the petals are flat and present only a
+terminal notch instead of the numerous crenations of _P. sinensis_. The
+heterozygote produced by crossing these forms is intermediate in size and
+appearance. When self-fertilised such plants behave in simple Mendelian
+fashion, {69} giving a generation consisting of _sinensis_, intermediates,
+and _stellata_ in the ratio 1 : 2 : 1. Subsequent breeding from these
+plants showed that both the _sinensis_ and _stellata_ which appeared in the
+F_2 generation bred true, while the intermediates always gave all three
+forms again in the same proportion. But though there is no dominance of the
+character of either parent in such a case as this, the Mendelian principle
+of segregation could hardly have a better illustration.
+
+{70}
+
+                Blue  x  Blue
+                      |
+    +-------------+--------+------------+
+  Black         Blue  x  Blue         White
+    |                 |                 |
+    |       +------+--+--+-----+        |
+  Black   Black  Blue  Blue  White    White
+    |                                   |
+  Black ------------- x ------------- White
+                      |
+                    Blue
+                    (all)
+
+Among birds a case of similar nature is that of the Blue Andalusian fowl.
+Fanciers have long recognised the difficulty of getting this variety to
+breed true. Of a slaty blue colour itself with darker hackles and with
+black lacing on the feathers of the breast, it always throws "wasters" of
+two kinds, viz. blacks, and whites splashed with black. Careful breeding
+from the blues shows that the three sorts are always produced in the same
+definite {71} proportions, viz., one black, two blues, one splashed white.
+This at once suggests that the black and the splashed white are the two
+homozygous forms, and that the blues are heterozygous, _i.e._, producing
+equal numbers of "black" and "white splashed" gametes. The view was tested
+by breeding the "wasters" together--black with black, and splashed white
+with splashed white--and it was found that each bred true to its respective
+type. But when the black and the splashed white were crossed they gave, as
+was expected, nothing but blues. In other words, we have the seeming
+paradox of the black and the splashed white producing twice as many blues
+as do the blues when bred together. The black and the splashed white
+"wasters" are in reality the pure breeds, while the "pure" Blue Andalusian
+is a mongrel which no amount of selection will ever be able to fix.
+
+In such cases as this it is obvious that we cannot speak of dominance. And
+with the disappearance of this phenomenon we lose one criterion for
+determining which of the two parent forms possesses the additional factor.
+Are we, for example, to regard the black Andalusian as a splashed white to
+which has been added a double dose of a colour-intensifying factor, or are
+we to consider the white splashed bird as a black which is unable to show
+its true pigmentation owing to the possession of some inhibiting factor
+which prevents the manifestation of the black. Either interpretation fits
+the facts equally well, {72} and until further experiments have been
+devised and carried out it is not possible to decide which is the correct
+view.
+
+Besides these comparatively rare cases where the heterozygote cannot be
+said to bear a closer resemblance to one parent more than to the other,
+there are cases in which it is often possible to draw a visible distinction
+between the heterozygote and the pure dominant. There are certain white
+breeds of poultry, notably the White Leghorn, in which the white behaves as
+a dominant to colour. But the heterozygous whites made by crossing the
+dominant white birds with a pure coloured form (such as the Brown Leghorn)
+almost invariably show a few coloured feathers or "ticks" in their plumage.
+The dominance of white is not quite complete, and renders it possible to
+distinguish the pure from the impure dominant without recourse to breeding
+experiments.
+
+  +------+------+------+------+
+  |CI    |CI    |CI    |CI    |
+  |CI    |Ci    |cI    |ci    |
+  |      |      |      |      |
+  |      |      |      |      |
+  +------+------+------+------+
+  |Ci    |Ci....|Ci    |Ci....|
+  |CI    |Ci....|cI    |ci....|
+  |      |......|      |......|
+  |      |......|      |......|
+  +------+------+------+------+
+  |cI    |cI    |cI    |cI    |
+  |CI    |Ci    |cI    |ci    |
+  |      |      |      |      |
+  |      |      |      |      |
+  +------+------+------+------+
+  |ci    |ci....|ci    |ci    |
+  |CI    |Ci....|cI    |ci    |
+  |      |......|      |      |
+  |      |......|      |      |
+  +------+------+------+------+
+
+    FIG. 13.
+
+    Diagram to illustrate the nature of the F_2 generation from the cross
+    between dominant white and recessive white fowls.
+
+This case of the dominant white fowl opens up another interesting problem
+in connection with dominance. By accepting the "Presence and Absence"
+hypothesis we are committed to the view that the dominant form possesses an
+extra factor as compared with the recessive. The natural way of looking at
+this case of the fowl is to regard white as the absence of colour. But were
+this so, colour should be dominant to white, which is not the case. We are
+therefore forced to suppose that the absence of colour in this instance is
+due to the presence of a factor whose {73} property is to inhibit the
+production of colour in what would otherwise be a pure coloured bird. On
+this view the dominant white fowl is a coloured bird plus a factor which
+inhibits the development of the colour. The view can be put to the test of
+experiment. We have already seen that there are other white fowls in which
+white is recessive to colour, and that the whiteness of such birds is due
+to the fact that they lack a factor for the development of colour. If we
+denote this factor by C and our postulated inhibitor factor in the dominant
+white bird by I, then we must write the constitution of the recessive white
+as ccii, and the dominant white as CCII. We may now work out the results we
+ought to obtain when a cross is made between these two pure white breeds.
+The constitution of the F_1 bird must be CcIi. Such birds being
+heterozygous for the inhibitor factor, should be whites showing some
+coloured "ticks." Being heterozygous for both of the two factors C and I,
+they will produce in equal numbers the four different sorts of gametes CI,
+Ci, cI, ci. The result of bringing two such similar series of gametes
+together is shown in Fig. 13. Out of the sixteen squares, twelve contain I;
+these will be white birds either with or without a few coloured ticks.
+Three contain C but not I: these must be coloured birds. One contains
+neither C nor I; this must be a white. From such a mating we ought,
+therefore, to obtain both white and coloured birds in the ratio 13 : 3. The
+results thus theoretically {74} deduced were found to accord with the
+actual facts of experiment. The F_1 birds were all "ticked" whites, and in
+the F_2 generation came white and coloured birds in the expected ratio.
+There seems, therefore, little reason to doubt that the dominant white is a
+coloured bird in which the absence of colour is due to the action of a
+colour-inhibiting factor, though as to the nature of that factor we can at
+present make no surmise. It is probable that other facts, which at first
+sight do not appear to be in agreement with the "Presence and Absence"
+hypothesis, will eventually be brought into line through the action of
+inhibitor factors. Such a case, for instance, is that of bearded and
+beardless wheats. Though the beard is obviously the additional character,
+the bearded condition is recessive to the beardless. Probably we ought to
+regard the beardless as a bearded wheat in which there is an inhibitor that
+stops the beard from growing. It is not unlikely that as time goes on we
+shall {75} find many more such cases of the action of inhibitor factors,
+and we must be prepared to find that the same visible effect may be
+produced either by the addition or by the omission of a factor. The
+dominant and recessive white poultry are indistinguishable in appearance.
+Yet the one contains a factor more and the other a factor less than the
+coloured bird.
+
+[Illustration: FIG. 14.
+
+Ears of beardless and bearded wheat. The beardless condition is dominant to
+the bearded.]
+
+{76}
+
+A phenomenon sometimes termed irregularity of dominance has been
+investigated in a few cases. In certain breeds of poultry such as Dorkings
+there occurs an extra toe directed backwards like the hallux (cf. Fig. 15).
+In some families this character behaves as an ordinary dominant to the
+normal, giving the expected 3 : 1 ratio in F_2. But in other families
+similarly bred the proportions of birds with and without the extra toe
+appear to be unusual. It has been shown that in such a family some of the
+birds without the extra toe may nevertheless transmit the peculiarity when
+mated with birds belonging to strains in which the extra toe never occurs.
+Though the external appearance of the bird generally affords some
+indication of the nature of the gametes which it is carrying, this is not
+always the case. Nevertheless we have reason to suppose that the character
+segregates in the gametes, though the nature of these cannot always be
+decided from the appearance of the bird which bears them.
+
+[Illustration: FIG. 15.
+
+Fowls' feet. On the right a normal and on the left one with an extra toe.]
+
+[Illustration: FIG. 16.
+
+Scheme to illustrate the inheritance of horns in sheep. Heterozygous males
+shown dark with a white spot, heterozygous females light with a dark spot
+in the centre.]
+
+There are cases in which an apparent irregularity of dominance has been
+shown to depend upon another character, as in the experiments with sheep
+carried out by Professor Wood. In these experiments two breeds were
+crossed, of which one, the Dorset, is horned in both sexes, while the
+other, the Suffolk, is without horns in either sex. Whichever way the cross
+was made the resulting F_1 generation was similar; the rams were horned,
+and {77} the ewes were hornless. In the F_2 generation raised from these
+F_1 animals both horned and hornless types appeared in both sexes but in
+very different proportions. While the horned rams were about three times as
+numerous as the hornless, this relation was reversed among the females, in
+which the horned formed only about one-quarter of the total. The simplest
+explanation of this interesting case is to suppose that the dominance of
+the horned character depends upon the sex of the animal--that it is
+dominant in the male but recessive in the female. A pretty experiment was
+devised for putting this view to the test. If it is true, equal numbers of
+gametes with and without the horned factor must be produced by the F_1
+ewes, while the factor should be lacking in all the gametes of the hornless
+F_2 rams. A {78} hornless ram, therefore, put to a flock of F_1 ewes should
+give rise to equal numbers of zygotes which are heterozygous for the horned
+character, and of zygotes in which it is completely absent. And since the
+heterozygous males are horned, while the heterozgyous females are hornless,
+we should expect from this mating equal numbers of horned and hornless
+rams, but only hornless ewes. The result of the experiment confirmed this
+expectation. Of the ram lambs 9 were horned and 8 were hornless, while all
+the 11 ewe lambs were completely destitute of horns.
+
+[Illustration: PLATE III.
+
+Sheep]
+
+       *       *       *       *       *
+
+
+{79}
+
+CHAPTER VIII
+
+WILD FORMS AND DOMESTIC VARIETIES
+
+In discussing the phenomena of reversion we have seen that in most cases
+such reversion occurs when the two varieties which are crossed each contain
+certain factors lacking in the other, of which the full complement is
+necessary for the production of the reversionary wild form. This at once
+suggests the idea that the various domestic forms of animals and plants
+have arisen by the omission from time to time of this factor or of that. In
+some cases we have clear evidence that this is the most natural
+interpretation of the relation between the cultivated and the wild forms.
+Probably the species in which it is most evident is the sweet pea
+(_Lathyrus odoratus_). We have already seen reason to suppose that as
+regards certain structural features the Bush variety is a wild lacking the
+factor for the procumbent habit, that the Cupid is a wild without the
+factor for the long inter-node, and that the Bush Cupid is a wild minus
+both these factors. Nor is the evidence less clear for the many colour
+varieties. In illustration we may consider in more detail a case in which
+the cross between two whites resulted {80} in a complete reversion to the
+purple colour characteristic of the wild Sicilian form (Pl. IV.). In this
+particular instance subsequent breeding from the purples resulted in the
+production of six different colour forms in addition to whites. The
+proportion of the coloured forms to the whites was 9 : 7 (cf. p. 44), but
+it is with the relation of the six coloured forms that we are concerned
+here. Of these six forms three were purples and three were reds. The three
+purple forms were (1) the wild bicolor purple with blue wings known in
+cultivation as the Purple Invincible (Pl. IV., 4); (2) a deep purple with
+purple wings (Pl. IV., 5); and (3) a very dilute purple known as the
+Picotee (Pl. IV., 6). Corresponding to these three purple forms were three
+reds: (1) a bicolor red known as Painted Lady (Pl. IV., 7); (2) a deep red
+with red wings known as Miss Hunt (Pl. IV., 8); and (3) a very pale red
+which we have termed Tinged White[5] (Pl. IV., 9). In the F_2 generation
+the total number of purples bore to the total number of reds the ratio
+3 : 1, and this ratio was maintained for each of the corresponding classes.
+Purple, therefore, is dominant to red, and each of the three classes of red
+differs from its corresponding purple in not possessing the blue factor (B)
+which turns it into purple.
+
+[Illustration: PLATE IV.
+
+1, 2, Emily Henderson; 3, F_1 reversionary Purple; 4-10, Various F_2 forms:
+4, Purple; 5, Deep Purple; 6, Picotee; 7, Painted Lady; 8, Miss Hunt; 9,
+Tinged White; 10, White.]
+
+{81} Again, the proportion in which the three classes of purples appeared
+was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, concerned
+here with the operation of two factors: (1) a light wing factor, which
+renders the bicolor dominant to the dark winged form; and (2) a factor for
+intense colour, which occurs in the bicolor and in the deep purple, but is
+lacking in the dilute picotee. And here it should be mentioned that these
+conclusions rest upon an exhaustive set of experiments involving the
+breeding of many thousands of plants. In this cross, therefore, we are
+concerned with the presence or absence of five factors, which we may denote
+as follows:--
+
+  A colour base, R.
+  A colour developer, C.
+  A purple factor, B.
+  A light wing factor, L.
+  A factor for intense colour, I.
+
+On this notation our six coloured forms are:--
+
+  (1) Purple bicolor                        CRBLI.[6]
+  (2) Deep purple                           CRBlI.
+  (3) Picotee                               CRBLi or CRBli.
+  (4) Red bicolor ( = Painted Lady)         CRbLI.
+  (5) Deep red ( = Miss Hunt)               CRblI.
+  (6) Tinged white                          CRbLi or CRbli.
+
+It will be noticed in this series that the various coloured {82} forms can
+be expressed by the omission of one or more factors from the purple bicolor
+of the wild type. With the complete omission of each factor a new colour
+type results, and it is difficult to resist the inference that the various
+cultivated forms of the sweet pea have arisen from the wild by some process
+of this kind. Such a view tallies with what we know of the behaviour of the
+wild form when crossed by any of the garden varieties. Wherever such
+crossing has been made the form of the hybrid has been that of the wild,
+thus supporting the view that the wild contains a complete set of all the
+differentiating factors which are to be found in the sweet pea.
+
+Moreover, this view is in harmony with such historical evidence as is to be
+gleaned from botanical literature, and from old seedsmen's catalogues. The
+wild sweet pea first reached England in 1699, having been sent from Sicily
+by the monk Franciscus Cupani as a present to a certain Dr. Uvedale in the
+county of Middlesex. Somewhat later we hear of two new varieties, the red
+bicolor, or Painted Lady, and the white, each of which may be regarded as
+having "sported" from the wild purple by the omission of the purple factor,
+or of one of the two colour factors. In 1793 we find a seedsman offering
+also what he called black and scarlet varieties. It is probable that these
+were our deep purple and Miss Hunt varieties, and that somewhere about this
+time the factor for the {83} light wing (L) was dropped out in certain
+plants. In 1860 we have evidence that the pale purple or Picotee, and with
+it doubtless the Tinged White, had come into existence. This time it was
+the factor for intense colour which had dropped out. And so the story goes
+on until the present day, and it is now possible to express by the same
+simple method the relation of the modern shades, of purple and reds, of
+blues and pinks, of hooded and wavy standards, to one another and to the
+original wild form. The constitution of many of these has now been worked
+out, and to-day it would be a simple though perhaps tedious task to denote
+all the different varieties by a series of letters indicating the factors
+which they contain, instead of by the present system of calling them after
+kings and queens, and famous generals, and ladies more or less well known.
+
+From what we know of the history of the various strains of sweet peas one
+thing stands out clearly. The new character does not arise from a
+pre-existing variety by any process of gradual selection, conscious or
+otherwise. It turns up suddenly complete in itself, and thereafter it can
+be associated by crossing with other existing characters to produce a gamut
+of new varieties. If, for example, the character of hooding in the standard
+(cf. Pl. II., 7) suddenly turned up in such a family as that shown on Plate
+IV. we should be able to get a hooded form corresponding to each of the
+forms with the erect {84} standard; in other words, the arrival of the new
+form would give us the possibility of fourteen varieties instead of seven.
+As we know, the hooded character already exists. It is recessive to the
+erect standard, and we have reason to suppose that it arose as a sudden
+sport by the omission of the factor in whose presence the standard assumes
+the erect shape characteristic of the wild flower. It is largely by keeping
+his eyes open and seizing upon such sports for crossing purposes that the
+horticulturist "improves" the plants with which he deals. How these sports
+or _mutations_ come about we can now surmise. They must owe their origin to
+a disturbance in the processes of cell division through which the gametes
+originate. At some stage or other the normal equal distribution of the
+various factors is upset, and some of the gametes receive a factor less
+than others. From the union of two such gametes, provided that they are
+still capable of fertilisation, comes the zygote which in course of growth
+develops the new character.
+
+Why these mutations arise: what leads to the surmised unequal division of
+the gametes: of this we know practically nothing. Nor until we can induce
+the production of mutations at will are we likely to understand the
+conditions which govern their formation. Nevertheless there are already
+hints scattered about the recent literature of experimental biology which
+lead us to hope that we may know more of these matters in the future. {85}
+
+In respect of the evolution of its now multitudinous varieties, the story
+of the sweet pea is clear and straightforward. These have all arisen from
+the wild by a process of continuous loss. Everything was there in the
+beginning, and as the wild plant parted with factor after factor there came
+into being the long series of derived forms. Exquisite as are the results
+of civilization, it is by the degradation of the wild that they have been
+brought about. How far are we justified in regarding this as a picture of
+the manner in which evolution works?
+
+There are certainly other species in which we must suppose that this is the
+way that the various domesticated forms have arisen. Such, for example, is
+the case in the rabbit, where most of the colour varieties are recessive to
+the wild agouti form. Such also is the case in the rat, where the black and
+albino varieties and the various pattern forms are also recessive to the
+wild agouti type. And with the exception of a certain yellow variety to
+which we shall refer later, such is also the case with the many fancy
+varieties of mice.
+
+Nevertheless there are other cases in which we must suppose evolution to
+have proceeded by the interpolation of characters. In discussing reversion
+on crossing, we have already seen that this may not occur until the F_2
+generation, as, for example, in the instance of the fowls' combs (cf. p.
+65). The reversion to the single comb occurred as the result of the removal
+of the two factors {86} for rose and pea. These two domesticated varieties
+must be regarded as each possessing an additional factor in comparison with
+the wild single-combed bird. During the evolution of the fowl, these two
+factors must be conceived of as having been interpolated in some way. And
+the same holds good for the inhibitory factor on which, as we have seen,
+the dominant white character of certain poultry depends. In pigeons, too,
+if we regard the blue rock as the ancestor of the domesticated breeds, we
+must suppose that an additional melanic factor has arisen at some stage.
+For we have already seen that black is dominant to blue, and the characters
+of F_1, together with the greater number of blacks than blues in F_2,
+negatives the possibility that we are here dealing with an inhibitory
+factor. The hornless or polled condition of cattle, again, is dominant to
+the horned condition, and if, as seems reasonable, we regard the original
+ancestors of domestic cattle as having been horned, we have here again the
+interpolation of an inhibitory factor somewhere in the course of evolution.
+
+On the whole, therefore, we must be prepared to admit that the evolution of
+domestic varieties may come about by a process of addition of factors in
+some cases and of subtraction in others. It may be that what we term
+additional factors fall into distinct categories from the rest. So far,
+experiment seems to show that they are either of the nature of melanic
+factors, or of inhibitory {87} factors, or of reduplication factors as in
+the case of the fowls' combs. But while the data remain so scanty,
+speculation in these matters is too hazardous to be profitable.
+
+       *       *       *       *       *
+
+
+{88}
+
+CHAPTER IX
+
+REPULSION AND COUPLING OF FACTORS
+
+Although different factors may act together to produce specific results in
+the zygote through their interaction, yet in all the cases we have hitherto
+considered the heredity of each of the different factors is entirely
+independent. The interaction of the factors affects the characters of the
+zygote, but makes no difference to the distribution of the separate
+factors, which is always in strict accordance with the ordinary Mendelian
+scheme. Each factor in this respect behaves as though the other were not
+present.
+
+A few cases have been worked out in which the distribution of the different
+factors to the gametes is affected by their simultaneous presence in the
+zygote. And the influence which they are able to exert upon one another in
+such cases is of two kinds. They may repel one another, refusing, as it
+were, to enter into the same zygote, or they may attract one another, and,
+becoming linked together, pass into the same gamete, as it were by
+preference. For the moment we may consider these two sets of phenomena
+apart. {89}
+
+One of the best illustrations of repulsion between factors occurs in the
+sweet pea. We have already seen that the loss of the blue or purple factor
+(B) from the wild bicolor results in the formation of the red bicolor known
+as Painted Lady (Pl. IV., 7). Further, we have seen that the hooded
+standard is recessive to the ordinary erect standard. The omission of the
+factor for the erect standard (E) from the purple bicolor (Pl. II., 5)
+results in a hooded purple known as Duke of Westminster (Pl. II., 7). And
+here it should be mentioned that in the corresponding hooded forms the
+difference in colour between the wings and standard is not nearly so marked
+as in the forms with the erect standard, but the difference in structure
+appears to affect the colour, which becomes nearly uniform. This may be
+readily seen by comparing the picture of the purple bicolor on Plate II.
+with that of the Duke of Westminster flower.
+
+Now when a Duke of Westminster is mated with a Painted Lady the factor for
+erect standard (E) is brought in by the red, and that for blue (B) by the
+Duke, and the offspring are consequently all purple bicolors. Purples so
+formed are all heterozygous for these two factors, and were the case a
+simple one, such as those which have already been discussed, we should
+expect the F_2 generation to consist of the four forms: erect purple,
+hooded purple, erect red, and hooded red in the ratio 9 : 3 : 3 : 1. Such,
+however, is not the case. The F_2 generation {90} actually consists of only
+three forms, viz. erect red, erect purple, and hooded purple, and the ratio
+in which these three forms occur is 1 : 2 : 1. No hooded red has been known
+to occur in such a family. Moreover further breeding shows that while the
+erect reds and the hooded purples always breed true, the erect purples in
+such families _never_ breed true, but always behave like the original F_1
+plant, giving the three forms again in the ratio 1 : 2 : 1. Yet we know
+that there is no difficulty in getting purple bicolors to breed true from
+other families; and we know also that hooded red sweet peas exist in other
+strains.
+
+   Painted Lady   x   Duke of Westminster
+   (erect red)    |    (hooded purple)
+                  |
+          Purple Invincible
+            (erect purple)
+                  |
+    +-------------+-----------------+
+    |             |                 |
+  Painted   Purple Invincible    Duke of
+   Lady                        Westminster
+   (1)           (2)               (1)
+
+
+
+                   EEbb                    eeBB        Parents
+                    /\                      /\
+                   /  \                    /  \
+                  /    \                  /    \
+                Eb      Eb              eB      eB     gametes
+                          \------------/
+                               EeBb                      F_2
+                          ____/    \____
+                         /              \
+  Fem. gametes of F_1  Eb ---> EEbb <--- Eb Male gametes of F_1
+                       Eb ---> EeBb <--- eB
+                       eB ---> EeBb <--- Eb
+                       eB ---> eeBB <--- eB
+                              \----/
+                          F_2 generation
+
+On the assumption that there exists a repulsion between the factors for
+erect standard and blue in a plant which is heterozygous for both, this
+peculiar case receives a simple explanation. The constitutions of the erect
+red and the hooded purple are EEbb and eeBB respectively and that of the
+F_1 erect purple is EeBb. Now let us suppose that in such a zygote there
+exists a repulsion {91} between E and B, such that when the plant forms
+gametes these two factors will not go into the same gamete. On this view it
+can only form two kinds of gametes, viz. Eb and eB, and these, of course,
+will be formed in equal numbers. Such a plant on self-fertilisation must
+give the zygotic series EEbb + 2 EeBb + eeBB, _i.e._ 1 erect red, 2 erect
+purples, and 1 hooded purple. And because the erect reds and the hooded
+purples are respectively homozygous for E and B, they must thenceforward
+breed true. The erect purples, on the other hand, being always formed by
+the union of a gamete Eb with a gamete eB, are always heterozygous for both
+of these factors. They can, consequently, never breed true, but must always
+give erect reds, erect purples, and hooded purples in the ratio 1 : 2 : 1.
+The experimental facts are readily explained on the assumption of repulsion
+between the two {92} factors B and E during the formation of the gametes in
+a plant which is heterozygous for both.
+
+Other similar cases of factorial repulsion have been demonstrated in the
+sweet pea, and two of these are also concerned with the two factors with
+which we have just been dealing. Two distinct varieties of pollen grains
+occur in this species, viz. the ordinary oblong form and a rather smaller
+rounded grain. The former is dominant to the latter.[7] When a cross is
+made between a purple with round pollen and a red with long pollen the F_1
+plant is a long pollened purple. But the F_2 generation consists of purples
+with round pollen, purples with long pollen, and reds with long pollen in
+the ratio 1 : 2 : 1. No red with round pollen appears in F_2 owing to
+repulsion between the factors for purple (B) and for long pollen (L).
+Similarly plants produced by crossing a red hooded long with a red round
+having an erect standard give in F_1 long pollened reds with an erect
+standard, and these in F_2 produce the three types, round pollened erect,
+long pollened erect, and long pollened hooded, in the ratio 1 : 2 : 1. The
+repulsion here is between the long pollen factor (L) and the factor for the
+erect standard (E).
+
+{93}
+
+Yet another similar case is known in which we are concerned with quite
+different factors. In some sweet peas the axils whence the leaves and
+flower stalks spring from the main stem are of a deep red colour. In others
+they are green. The dark pigmented axil is dominant to the light one.
+Again, in some sweet peas the anthers are sterile, setting no pollen, and
+this condition is recessive to the ordinary fertile condition. When a
+sterile plant with a dark axil is crossed by a fertile plant with a light
+axil, the F_1 plants are all fertile with dark axils. But such plants in
+F_2 give fertiles with light axils, fertiles with dark axils, and steriles
+with dark axils in the ratio 1 : 2 : 1. No light axilled steriles appear
+from such a cross owing to the repulsion between the factor for dark axil
+(D) and that for the fertile anther (F).
+
+These four cases have already been found in the sweet pea, and similar
+phenomena have been met with by Gregory in primulas. To certain seemingly
+analogous cases in animals where sex is concerned we shall refer later.
+
+Now all of these four cases present a common feature which probably has not
+escaped the attention of the reader. In all of them _the original cross was
+such as to introduce one of the repelling factors with each of the two
+parents_. If we denote our two factors by A and B, the crosses have always
+been of the nature AAbb x aaBB. Let us now consider what happens when both
+of the {94} factors, which in these cases repel one another, are introduced
+by one of the parents, and neither by the other parent. And in particular
+we will take the case in which we are concerned with purple and red flower
+colour, and with long and round pollen, _i.e._ with the factors B and L.
+When a purple long (BBLL) is crossed with a red round (bbll) the F_1 (BbLl)
+is a purple with long pollen, identical in appearance with that produced by
+crossing the long pollened red with the round pollened purple. But the
+nature of the F_2 generation is in some respects very different. The ratio
+of purples to reds and of longs to rounds is in each case 3 : 1, as before.
+But instead of an association between the red and the long pollen
+characters the reverse is the case. The long pollen character is now
+associated with purple and the round pollen with red. The association,
+however, is not quite complete, and the examination of a large quantity of
+similarly bred material shows that the purple longs are about twelve times
+as numerous as the purple rounds, while the red rounds are rather more than
+three times as many as the red longs. Now this peculiar result could be
+brought about if the gametic series produced by the F_1 plant consisted of
+7 BL + 1 Bl + 1 bL + 7 bl out of every 16 gametes. Fertilization between
+two such similar series of 16 gametes would result in 256 plants, of which
+177 would be purple longs, 15 purple rounds, 15 red longs, and 49 red
+rounds--a proportion of the four different kinds very close to {95} that
+actually found by experiment. It will be noticed that in the whole family
+the purples are to the reds as 3 : 1, and the longs are also three times as
+numerous as the rounds. The peculiarity of the case lies in the
+distribution of these two characters with regard to one another. In some
+way or other the factors for blue and for long pollen become linked
+together in the cell divisions that give rise to the gametes, but the
+linking is not complete. This holds good for all the four cases in which
+repulsion between the factors occurs when one of the two factors is
+introduced by each of the parents. _When both of the factors are brought
+into the cross by the same parent we get coupling between them instead of
+repulsion._ The phenomena of repulsion and coupling between separate
+factors are intimately related, though hitherto we have not been able to
+suggest why this should be so.
+
+Nor for the present can we suggest why certain factors should be linked
+together in the peculiar way that we have reason to suppose that they are
+during the process of the formation of the gametes. Nevertheless the
+phenomena are very definite, and it is not unlikely that a further study of
+them may throw important light on the architecture of the living cell.
+
+APPENDIX TO CHAPTER IX
+
+As it is possible that some readers may care, in spite of its complexity,
+to enter rather more fully into the peculiar phenomenon {96} of the
+coupling of characters, I have brought together some further data in this
+Appendix. In the case we have already considered, where the factors for
+blue colour and long pollen are concerned, we have been led to suppose that
+the gametes produced by the heterozygous plant are of the nature 7 BL : 1
+Bl : 1 bL : 7 bl. Such a series of ovules fertilised by a similar series of
+pollen grains will give a generation of the following composition:--
+
+  49 BBLL + 7 BBLl + 7 BbLL + 49 BbLl
+          + 7 BBLl + 7 BbLL +    BbLl
+                            +    BbLl
+                            + 49 BbLl
+  \---------------------------------/
+           177 purple, long
+
+  + BBll + 7 Bbll + bbLL + 7 bbLl + 49 bbll
+         + 7 Bbll        + 7 bbLl
+  \-------------/   \-----------/   \-----/
+     15 purple,         15 red,     49 red,
+       round             long        round
+
+and as this theoretical result fits closely with the actual figures
+obtained by experiment we have reason for supposing that the heterozygous
+plant produces a series of gametes in which the factors are coupled in this
+way. The intensity of the coupling, however, varies in different cases.
+Where we are dealing with another, viz. fertility (F) and the dark axil
+(D), the experimental numbers accord with the view that the gametic series
+is here 15 FD : 1 Fd : 1 fD : 15 fd. The coupling is in this instance more
+intense. In the case of the erect standard (E) and blueness (B) the
+coupling is even more intense, and the experimental evidence available at
+present points to the gametic series here being 63 Eb : 1 EB : 1 eB : 63
+eb. There is evidence also for supposing that the intensity of the coupling
+may vary in different families for the same pair of factors. The coupling
+between blue and long pollen is generally on the 7 : 1 : 1 : 7 {97} basis,
+but in some cases it may be on the 15 : 1 : 1 : 15 basis. But though the
+intensity of the coupling may vary it varies in an orderly way. If A and B
+are the two factors concerned, the results obtained in F_2 are explicable
+on the assumption that the ratio of the four sorts of gametes produced is a
+term of the series--
+
+   3 AB + Ab + aB +  3 ab
+   7 AB + Ab + aB +  7 ab
+  15 AB + Ab + aB + 15 ab, etc., etc.
+
+In such a series the number of gametes containing A is equal to the number
+lacking A, and the same is true for B. Consequently the number of zygotes
+formed containing A is three times as great as the number of zygotes which
+do not contain A; and similarly for B. The proportion of dominants to
+recessives in each case is 3 : 1. It is only in the distribution of the
+characters with relation to one another that these cases differ from a
+simple Mendelian case.
+
+As the study of these series presents another feature of some interest, we
+may consider it in a little more detail. In the accompanying table are set
+out the results produced by these different series of gametes. The series
+marked by an asterisk have already been demonstrated experimentally. The
+first term in the series, {98} in which all the four kinds of gametes are
+produced in equal numbers is, of course, that of a simple Mendelian case
+where no coupling occurs.
+
+  +-------+------------------+---------+---------------------------------+
+  |No. of | Distribution of  | No. of  |                                 |
+  |Gametes|Factors in Gametic| Zygotes |    Form of F_2 Generation.      |
+  |  in   |      Series      |produced.|                                 |
+  |series.|                  |         |                                 |
+  +-------+------------------+---------+---------------------------------+
+  |       |  AB. Ab. aB. ab. |         |      AB.     Ab.   aB.    ab.   |
+  |   4   |    1: 1: 1:  1   |    16   |       9       3     3      1    |
+  |   8   |    3: 1: 1:  3   |    64   |      49       7     7      9    |
+  |  16   |    7: 1: 1:  7   |   256   |     177      15    15     49*   |
+  |  32   |   15: 1: 1: 15   |  1024   |     737      31    31    225*   |
+  |  64   |   31: 1: 1: 31   |  4096   |    3009      63    63    961    |
+  | 128   |   63: 1: 1: 63   | 16384   |   12161     127   127   3969*   |
+  |  2n   |(n-1): 1: 1:(n-1) |  4n^2   |3n^2-(2n-1) 2n-1  2n-1 n^2-(2n-1)|
+  +-------+------------------+---------+---------------------------------+
+
+Now, as the table shows, it is possible to express the gametic series by a
+general formula (n + 1) AB + Ab + aB + (n - 1) ab, where 2n is the total
+number of the gametes in the series. A plant producing such a series of
+gametes gives rise to a family of zygotes in which 3n^2 - (2n - 1) show
+both of the dominant characters and n^2 - (2n - 1) show both of the
+recessive characters, while the number of the two classes which each show
+one of the two dominants is (2n - 1). When in such a series the coupling
+becomes closer the value of n increases, but in comparison with n^2 its
+value becomes less and less. The larger n becomes the more negligible is
+its value relatively to n^2. If, therefore, the coupling were very close,
+the series 3n^2 - (2n - 1) : (2n - 1) : (2n - 1) : n^2 - (2n - 1) would
+approximate more and more to the series 3n^2 : n^2, _i.e._ to a simple
+3 : 1 ratio. Though the point is probably of more theoretical than
+practical interest, it is not impossible that some of the cases which have
+hitherto been regarded as following a simple 3 : 1 ratio will turn out on
+further analysis to belong to this more complicated scheme.
+
+       *       *       *       *       *
+
+
+{99}
+
+CHAPTER X
+
+SEX
+
+[Illustration: FIG. 17.
+
+_Abraxas grossulariata_, the common currant moth, and (on the right) its
+paler lacticolor variety.]
+
+In their simplest expression the phenomena exhibited by Mendelian
+characters are sharp and clean cut. Clean cut and sharp also are the
+phenomena of sex. It was natural, therefore, that a comparison should have
+been early instituted between these two sets of phenomena. As a general
+rule, the cross between a male and a female results in the production of
+the two sexes in approximately equal numbers. The cross between a
+heterozygous dominant and a recessive also leads to equal numbers of
+recessives and of heterozygous dominants. Is it not, therefore, possible
+that one of the sexes is heterozygous for a factor which is lacking in the
+other, and that the presence or absence of this factor determines the sex
+of the zygote? The results of some recent experiments would appear to
+justify this interpretation, at any rate in particular cases. Of these, the
+simplest is that of the common currant moth (_Abraxas grossulariata_), of
+which there exists a pale variety (Fig. 17) known as _lacticolor_. The
+experiments of Doncaster and Raynor showed that the variety behaved as a
+simple recessive to the normal form. But the distribution of the dominants
+and {100} recessives [Illustration]with with regard to the sexes was
+peculiar. The original cross was between a _lacticolor_ female and a normal
+male. All the F_1 moths of both sexes were of the normal _grossulariata_
+type. The F_1 insects were then paired together and gave a generation
+consisting of 3 normals : 1 _lacticolor_. But all the _lacticolor_ were
+females, and all the males were of the normal pattern. It was, however,
+found possible to obtain the _lacticolor male_ by mating a _lacticolor_
+female with the F_1 male. The family resulting from this cross consisted of
+normal males and normal females, _lacticolor_ males and _lacticolor_
+females, and the {101} four sorts were produced in approximately equal
+numbers. In such a family there was no special association of either of the
+two colour varieties with one sex rather than the other. But the reverse
+cross, F_1 female by _lacticolor_ male, gave a very different result. As in
+the previous cross such families contained equal numbers of the normal form
+and of the recessive variety. But all of the normal _grossulariata_ were
+males, while all the _lacticolor_ were females. Now this seemingly complex
+collection of facts is readily explained if we make the following three
+assumptions:--
+
+[Illustration]
+
+(1) The _grossulariata_ character (G) is dominant to the lacticolor
+character (g). This is obviously justified by the experiments, for, leaving
+the sex distribution out of account, we get the expected 3 : 1 ratio from
+F_1 x F_1, and also the expected ratio of equality when the heterozygote is
+crossed with the recessive.
+
+(2) The female is heterozygous for a dominant factor (F) which is lacking
+in the male. The constitution of a female is consequently Ff, and of a male
+ff. This assumption is in harmony with the fact that the sexes are produced
+in approximately equal numbers.
+
+(3) There exists repulsion between the factors G and F in a zygote which is
+heterozygous for them both. Such zygotes (FfGg) must always be females, and
+on this assumption will produce gametes Fg and fG in equal numbers. {102}
+
+[Illustration: FIG. 18.
+
+Scheme of inheritance in the F_1 and F_2 generations resulting from the
+cross of _lacticolor_ female with _grossulariata_ male. The character of
+each individual is represented by the sex signs in brackets, the black
+being _grossulariata_ in appearance and the light ones _lacticolor_.]
+
+We may now construct a scheme for comparison with that on page 100 to show
+how these assumptions explain the experimental results. The original
+parents were _lacticolor_ female and _grossulariata_ male, which on our
+assumptions must be Ffgg and ffGG respectively in constitution. Since the
+female is always heterozygous for F, her gametes must be of two kinds, viz.
+Fg and fg, while those of the pure _grossulariata_ male must be all fG.
+When an ovum Fg is fertilised by a spermatozoon fG, the resulting zygote,
+FfGg, is heterozygous for both F and G, and in appearance is a female
+_grossulariata_. The zygote resulting from the fertilisation of an ovum fg
+by a spermatozoon fG is heterozygous for G, but does not contain F, and
+therefore is a male _grossulariata_. Such a male being in constitution
+{103} ffGg must produce gametes of two kinds, fG and fg, in equal numbers.
+And since we are assuming repulsion between F and G, the F_1 female being
+in constitution FfGg, must produce equal numbers of gametes Fg and fG. For
+on our assumption F and G cannot enter into the same gamete. The series of
+gametes produced by the F_1 moths, therefore, are fG, fg by the male and
+Fg, fG by the female. The resulting F_2 generation consequently consists of
+the four classes of zygotes Ffgg, FfGg, ffGg, and ffGG in equal numbers. In
+other words, the sexes are produced in equal numbers, the proportion of
+normal grossulariata to _lacticolor_ is 3 : 1, and all of the _lacticolor_
+are females; that is to say, the results worked out on our assumptions
+accord with those actually produced by experiment. We may now turn to the
+results which should be obtained by crossing the F_1 moths with the
+_lacticolor_ variety. And first we will take the cross _lacticolor_ female
+x F_1 male. The gametes produced by the lacticolor female we have already
+seen to be Fg and fg, while those produced by the F_1 male are fG and fg.
+The bringing together of these two series of gametes must result in equal
+numbers of the four kinds of zygotes FfGg, Ffgg, ffGg, and ffgg, _i.e._ of
+female _grossulariata_ and _lacticolor_, and of male _grossulariata_ and
+_lacticolor_ in equal numbers. Here, again, the calculated results accord
+with those of experiment. Lastly, we may examine what should happen when
+the F_1 female is crossed with the _lacticolor_ {104} male. The F_1 female,
+owing to the repulsion between F and G, produces only the two kinds of ova
+Fg and fG, and produces them in equal numbers. Since the _lacticolor_ male
+can contain neither F nor G, all of its spermatozoa must be fg. The results
+of such a cross, therefore, should be to produce equal numbers of the two
+kinds of zygote Ffgg and ffGg, _i.e._ of _lacticolor_ females and of
+_grossulariata_ males. And this, as we have already seen, is the actual
+result of such a cross.
+
+Before leaving the currant moth we may allude to an interesting discovery
+which arose out of these experiments. The _lacticolor_ variety in Great
+Britain is a southern form and is not known to occur in Scotland. Matings
+were made between wild Scotch females and _lacticolor_ males. The families
+resulting from such matings were precisely the same as those from
+_lacticolor_ males and F_1 females, viz. _grossulariata_ males and
+_lacticolor_ females only. We are, therefore, forced to regard the
+constitution of the wild _grossulariata_ female as identical with that of
+the F_1 female, _i.e._ as heterozygous for the _grossulariata_ factor as
+well as for the factor for femaleness. Though from a region where
+_lacticolor_ is unknown, the "pure" wild _grossulariata_ female is
+nevertheless a permanent mongrel, but it can never reveal its true colours
+unless it is mated with a male which is either heterozygous for G or pure
+_lacticolor_. And as all the wild northern males are {105} pure for the
+_grossulariata_ character this can never happen in a state of nature.
+
+[Illustration: FIG. 19.
+
+Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn
+cock. Black sex signs denote deeply pigmented birds, and light sex signs
+those without pigmentation. The light signs with a black dot in the centre
+denote birds with a small amount of pigment.]
+
+An essential feature of the case of the currant moth lies in the different
+results given by reciprocal crosses. _Lacticolor_ female x _grossulariata_
+male gives _grossulariata_ alone of both sexes. But _grossulariata_ female
+x _lacticolor_ male gives only _grossulariata_ males and _lacticolor_
+females. Such a difference between reciprocal crosses has also been found
+in other animals, and the experimental results, though sometimes more
+complicated, are explicable on the same lines. An interesting case in which
+three factors are concerned has been recently worked out in poultry. The
+Silky breed of fowls is characterised among other peculiarities by a
+remarkable abundance of melanic pigment. The skin is dull black, while the
+comb and wattles are of a deep purple colour contrasting sharply with the
+white plumage (Pl. V., 3). Dissection shows that this black pigment is
+widely spread throughout the body, being especially marked in such
+membranes as the mesenteries, the periosteum, and the pia mater surrounding
+the brain. It also occurs in the connective tissues among the muscles. In
+the Brown Leghorn, on the other hand, this pigment is not found. Reciprocal
+crosses between these two breeds gave a remarkable difference in result. A
+cross between the Silky hen and the Brown Leghorn cock produced F_1 birds
+in which both sexes exhibited only traces of the pigment. On casual
+observation they might have {106} passed for unpigmented birds, for with
+the exception of an occasional fleck of pigment their skin, comb and
+wattles were as clear as in the Brown Leghorn (Pl. V., 1 and 4). Dissection
+revealed the presence of a slight amount of internal pigment. Such birds
+bred together gave some offspring with the full pigmentation of the Silky,
+some without any pigment, and others showing different degrees of pigment.
+None of the F_2 male birds, however, showed the full deep pigmentation of
+the Silky.
+
+[Illustration: FIG. 20.
+
+Scheme illustrating the result of crossing a Brown Leghorn hen with a Silky
+cock (cf. Fig. 19).]
+
+When, however, the cross was made the other way, viz. Brown Leghorn hen x
+Silky cock, the result was different. While the F_1 male birds were almost
+destitute of pigment as in the previous cross, the F_1 hens, on the other
+hand, were nearly as deeply pigmented as the pure Silky {107} (Pl. V., 2).
+The male Silky transmitted the pigmentation, but only to his daughters.
+Such birds bred together gave an F_2 generation containing chicks with the
+full deep pigment, chicks without pigment, and chicks with various grades
+of pigmentation, all the different kinds in both sexes.
+
+[Illustration: FIG. 21.
+
+Scheme to illustrate the result of crossing F_1 birds (_e.g._ Brown Leghorn
+x Silky) with the pure Brown Leghorn.]
+
+In analysing this complicated case many other different crosses were made,
+but for the present it will be sufficient to mention but one of these, viz.
+that between the F_1 birds and the pure Brown Leghorn. The cross between
+the F_1 hen and the Brown Leghorn cock produced only birds with a slight
+amount of pigment and birds without pigment. And this was true for both the
+deeply pigmented and the slightly pigmented types of F_1 hen. But when the
+F_1 cock was mated to a Brown Leghorn hen, a definite proportion of the
+chicks, one in eight, was deeply pigmented, and _these deeply pigmented
+birds were always females_ (cf. Fig. 21). And in this respect all the F_1
+males behaved alike, whether they were from the Silky hen or from the Silky
+cock. We have, therefore, the paradox that the F_1 hen, though herself
+deeply pigmented, cannot transmit this condition to any of her offspring
+when she is mated to the unpigmented Brown Leghorn, but that, when
+similarly mated, the F_1 cock can transmit this pigmented condition to a
+quarter of his female offspring though he himself is almost devoid of
+pigment.
+
+[Illustration: PLATE V.
+
+1, 2, F_1 Cock and Hen, ex Brown Leghorn Hen x Silky Cock; 3, Silky Cock;
+4, Hen ex Silky Hen x Brown Leghorn Cock.]
+
+{108}
+
+[Illustration: FIG. 22.
+
+Scheme to illustrate the nature of the F_1 generation from the Silky hen
+and Brown Leghorn cock (cf. Fig. 23).]
+
+Now all these apparently complicated results, as well as many others to
+which we have not alluded, can be expressed by the following simple scheme.
+There are three factors affecting pigment, viz. (1) a pigmentation factor
+(P); (2) a factor which inhibits the production of pigment (I); and (3) a
+factor for femaleness (F), for which the female birds are heterozygous, but
+which is not present in the males. Further, we make the assumptions (a)
+that there is repulsion between F and I in the female zygote (FfIi), and
+(b) that the male Brown Leghorn is homozygous for the inhibitor factor (I),
+but that the hen Brown Leghorn is always heterozygous for this factor just
+in the same way as the female of the currant moth is always heterozygous
+for the _grossulariata_ factor. We may now proceed to show how this
+explanation fits the experimental facts which we have given.
+
+The Silky is pure for the pigmentation factor, but does not contain the
+inhibitor factor. The Brown Leghorn, on the other hand, contains the
+inhibitor factor, but not the {109} pigmentation factor. In crossing a
+Silky hen with a Brown Leghorn cock we are mating two birds of the
+constitution FfPPii and ffppII, and all the F_1 birds are consequently
+heterozygous for both P and I. In such birds the pigment is almost but not
+completely suppressed, and as both sexes are of the same constitution with
+regard to these two factors they are both of similar appearance.
+
+[Illustration: FIG. 23.
+
+Scheme to illustrate the nature of the F_1 generation from the Brown
+Leghorn hen and Silky cock (cf. Fig. 22).]
+
+In the reciprocal cross, on the other hand, we are mating a Silky male
+(ffPPii) with a Brown Leghorn hen which on our assumption is heterozygous
+for the inhibitor factor (I), and in constitution therefore is FfppIi.
+Owing to the repulsion between F and I the gametes produced by such a bird
+are Fpi and fpI in equal numbers. All the gametes produced by the Silky
+cock are fPi. Hence the constitution of the F_1 male birds produced by this
+cross is ffPpIi as before, but the female birds must be all of the
+constitution FfPpii. The Silky cock transmits the fully pigmented condition
+to his daughters, because the gametes of the Brown Leghorn hen which
+contain the factor for femaleness do not contain the {110} inhibitory
+factor owing to the repulsion between these factors. The nature of the F_2
+generation in each case is in harmony with the above scheme. As, however,
+it serves to illustrate certain points in connection with intermediate
+forms we shall postpone further consideration of it till we discuss these
+matters, and for the present shall limit ourselves to the explanation of
+the different behaviour of the F_1 males and females when crossed with the
+Brown Leghorn. And, first, the cross of Brown Leghorn female by F_1 male.
+The Brown Leghorn hen is on our hypothesis FfppIi, and produces gametes Fpi
+and fpI. The F_1 cock is on our hypothesis ffPpIi, and produces in equal
+numbers the four kinds of gametes fPI, fPi, fpI, fpi. The result of the
+meeting of these two series of gametes is given in Fig. 24. Of the eight
+different kinds of zygote formed only one contains P in the absence of I,
+and this is a female. The result, as we have already seen, is in accordance
+with the experimental facts.
+
+[Illustration: FIG. 24.
+
+Diagram showing the nature of the offspring from a Brown Leghorn hen and an
+F_1 cock bred from Silky hen x Brown Leghorn cock, or _vice versa_.]
+
+On the other hand, the Brown Leghorn cock is on our hypothesis ffppII. All
+his gametes consequently contain the inhibitor factor, and when he is mated
+with an F_1 {111} hen all the zygotes produced must contain I. None of his
+offspring, therefore, can be fully pigmented, for this condition only
+occurs in the absence of the inhibitor factor among zygotes which are
+either homozygous or heterozygous for P.
+
+[Illustration: FIG. 25.
+
+Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen.
+For explanation see text.]
+
+The interpretation of this case turns upon the constitution of the Brown
+Leghorn hen, upon her heterozygous condition with regard to the two factors
+F and I, and upon the repulsion that occurs between them when the gametes
+are formed. Through an independent set of experiments this view of the
+nature of the Brown Leghorn hen has been confirmed in an interesting way.
+There are fowls which possess neither the factor for pigment nor the
+inhibitory factor, which are in constitution ppii. Such birds when crossed
+with the Silky give dark pigmented birds of both sexes in F_1, and the F_2
+generation consists of pigmented and unpigmented in the ratio 3 : 1. Now a
+cock of such a strain crossed with a Brown Leghorn hen should give only
+completely unpigmented birds. But if, as we have supposed, the Brown
+Leghorn hen is producing gametes Fpi and fpI, the male birds produced by
+such a cross should be heterozygous for I, {112} _i.e._ in constitution
+ffppIi, while the hen birds, though identical in appearance so far as
+absence of pigmentation goes, should not contain this factor but should be
+constitutionally Ffppii. Crossed with the pure Silky, the F_1 birds of
+opposite sexes should give an entirely different result. For while the hens
+should give only deeply pigmented birds of both sexes, the cocks should
+give equal numbers of deeply pigmented and slightly pigmented birds (cf.
+Fig. 25). These were the results which the experiment actually gave, thus
+affording strong confirmation of the view which we have been led to take of
+the Brown Leghorn hen. Essentially the poultry case is that of the currant
+moth. It differs in that the factor which {113} repels femaleness produces
+no visible effect, and its presence or absence can only be determined by
+the introduction of a third factor, that for pigmentation.
+
+This conception of the nature of the Brown Leghorn hen leads to a curious
+paradox. We have stated that the Silky cock transmits the pigmented
+condition, but transmits it to his daughters only. Apparently the case is
+one of unequal transmission by the father. Actually, as our analysis has
+shown, it is one of unequal transmission by the mother, the father's
+contribution to the offspring being identical for each sex. The mother
+transmits to the daughters her dominant quality of femaleness, but to
+balance this, as it were, she transmits to her sons another quality which
+her daughters do not receive. It is a matter of common experience among
+human families that in respect to particular qualities the sons tend to
+resemble their mothers more than the daughters do, and it is not improbable
+that such observations have a real foundation for which the clue may be
+provided by the Brown Leghorn hen.
+
+Nor is this the only reflection that the Brown Leghorn suggests. Owing to
+the repulsion between the factors for femaleness and for pigment
+inhibition, it is impossible by any form of mating to make a hen which is
+homozygous for the inhibitor factor. She has bartered away for femaleness
+the possibility of ever receiving a double dose of this factor. We know
+that in some cases, as, for example, {114} that of the blue Andalusian
+fowl, the qualities of the individual are markedly different according as
+to whether he or she has received a single or a double dose of a given
+factor. It is not inconceivable that some of the qualities in which a man
+differs from a woman are founded upon a distinction of this nature. Certain
+qualities of intellect, for example, may depend upon the existence in the
+individual of a double dose of some factor which is repelled by femaleness.
+If this is so, and if woman is bent upon achieving the results which such
+qualities of intellect imply, it is not education or training that will
+help her. Her problem is to get the factor on which the quality depends
+into an ovum that carries also the factor for femaleness.
+
+       *       *       *       *       *
+
+
+{115}
+
+CHAPTER XI
+
+SEX (_continued_)
+
+The cases which we have considered in the last chapter belong to a group in
+which the peculiarities of inheritance are most easily explained by
+supposing that the female is heterozygous for some factor that is not found
+in the male. Femaleness is an additional character superposed upon a basis
+of maleness, and as we imagine that there is a separate factor for each the
+full constitutional formula for a female is FfMM, and for a male ffMM. Both
+sexes are homozygous for the male element, and the difference between them
+is due to the presence or absence of the female element F.
+
+There are, however, other cases for which the explanation will not suffice,
+but can be best interpreted on the view that the male is heterozygous for a
+factor which is not found in the female. Such a case is that recently
+described by Morgan in America for the pomace fly (_Drosophila
+ampelophila_). Normally this little insect has a red eye, but white eyed
+individuals are known to occur as rare sports. Red eye is dominant to
+white. In their relation to sex the eye colours of the pomace fly {116} are
+inherited on the same lines as the _grossulariata_ and _lacticolor_
+patterns of the currant moth, but with one essential difference. The factor
+which repels the red-eye factor is in this case to be found in the male,
+and here consequently it is the male which must be regarded as heterozygous
+for a sex factor that is lacking in the female.
+
+[Illustration]
+
+In order to bring these cases and others into line an interesting
+suggestion has recently been put forward by Bateson. On this suggestion
+each sex is heterozygous for its own sex factor only, and does not contain
+the factor proper to the opposite sex. The male is of the constitution,
+Mmff and the female Ffmm. Each sex produces two sorts of gametes, Mf and mf
+in the case of the male, and Fm, fm in that of the female. But on this view
+a further supposition is necessary. If each of the two kinds of spermatozoa
+were capable of fertilising each of the two kinds of ova, we should get
+individuals of the constitution MmFf and mmff, as well as the normal males
+and females, Mmff and Ffmm. As the facts of ordinary bisexual reproduction
+afford us no grounds for assuming the existence of these two classes of
+individuals, whatever they may be, we must suppose that fertilisation. is
+productive only between the spermatozoa carrying M and the ova without F,
+or between the spermatozoa {117} without M and the ova containing F. In
+other words we must on this view suppose that fertilisations between
+certain forms of gametes, even if they can occur, are incapable of giving
+rise to zygotes with the capacity for further development. If we admit this
+supposition, the scheme just given will cover such cases as those of the
+currant moth and the fowl, equally as well as that of the pomace fly. In
+the former there is repulsion between either the _grossulariata_ factor and
+F, or else between the pigment inhibitor factor and F, while in the latter
+there is repulsion between the factor for red eye and M.
+
+[Illustration: FIG. 26.
+
+Scheme to illustrate the probable mode of inheritance of colour-blindness.
+The dark signs represent affected individuals. A black dot in the centre
+denotes an unaffected female who is capable of transmitting the condition
+to her sons.]
+
+Whatever the merits or demerits of such a scheme it certainly does offer an
+explanation of a peculiar form of sex limited inheritance in man. It has
+long been a matter of common knowledge that colour-blindness is much more
+common among men than among women, and also that unaffected women can
+transmit it to their sons. At first sight the case is not unlike that of
+the sheep, where the horned character is apparently dominant in the male
+but recessive in the female. The hypothesis that the colour-blind condition
+is due to the presence of an extra factor as compared with the normal, and
+that a single dose of it will produce {118} colour-blindness in the male
+but not in the female, will cover a good many of the observed facts (cf.
+Fig. 26). Moreover, it serves to explain the remarkable fact that _all_ the
+sons of colour-blind women are also colour-blind. For a woman cannot be
+colour-blind unless she is homozygous for the colour-blind factor, in which
+case all her children must get a single dose of it even if she marries a
+normal male. And this is sufficient to produce colour-blindness in the
+male, though not in the female.
+
+But there is one notable difference in this case as compared with that of
+the sheep. When crossed with pure hornless ewes the heterozygous horned ram
+transmits the horned character to half his male offspring (cf. p. 71). But
+the heterozygous colour-blind man does not behave altogether like a sheep,
+for he apparently does not transmit the colour-blind condition to any of
+his male offspring. If, however, we suppose that the colour-blind factor is
+repelled by the factor for maleness, the amended scheme will cover the
+observed facts. For, denoting the colour-blind factor by X, the gametes
+produced by the colour-blind male are of two sorts only, viz. Mfx and mfX.
+If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova
+fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to
+give females which are heterozygous for the colour-blind factor. These
+daughters are themselves normal, but transmit the condition to about half
+their sons. {119}
+
+The attempt to discover a simple explanation of the nature of sex has led
+us to assume that certain combinations between gametes are incapable of
+giving rise to zygotes which can develop further. In the various cases
+hitherto considered there is no reason to suppose that anything of the sort
+occurs, or that the different gametes are otherwise than completely fertile
+one with another. One peculiar case, however, has been known for several
+years in which some of the gametes are apparently incapable of uniting to
+produce offspring. Yellow in the mouse is dominant to agouti, but hitherto
+a homozygous yellow has never been met with. The yellows from families
+where only yellows and agoutis occur produce, when bred together, yellows
+and agoutis in the ratio 2 : 1. If it were an ordinary Mendelian case the
+ratio should be 3 : 1, and one out of every three yellows so bred should be
+homozygous and give only yellows when crossed with agouti. But Cuenot and
+others have shown that _all_ of the yellows are heterozygous, and when
+crossed with agoutis give both yellows and agoutis. We are led, therefore,
+to suppose that an ovum carrying the yellow factor is unproductive if
+fertilised by a spermatozoon which also bears this factor. In this way
+alone does it seem possible to explain the deficiency of yellows and the
+absence of homozygous ones in the families arising from the mating of
+yellows together. At present, however, it remains the only definite
+instance among animals in which we have {120} grounds for assuming that
+anything in the nature of unproductive fertilisation takes place.[8]
+
+If we turn from animals to plants we find a more complicated state of
+affairs. Generally speaking, the higher plants are hermaphrodite, both
+ovules and pollen grains occurring on the same flower. Some plants, however
+like most animals, are of separate sexes, a single plant bearing only male
+or female flowers. In other plants the separate flowers are either male or
+female, though both are borne on the same individual. In others, again, the
+conditions are even more complex, for the same plant may bear flowers of
+three kinds, viz. male, female, and hermaphrodite. Or it may be that these
+three forms occur in the same species but in different individuals--female
+and hermaphrodites in one species; males, females, and hermaphrodites in
+another. One case, however, must be mentioned as it suggests a possibility
+which we have not hitherto encountered. In the common English bryony
+(_Bryonia dioica_) the sexes are separate, some plants having only male and
+others only female flowers. In another European species, _B. alba_, both
+male and female flowers occur on the same plant. Correns crossed these two
+species reciprocally, and also fertilised _B. dioica_ by its own male with
+the following results:--
+
+{121}
+
+  dioica [female] x dioica [male] gave [female] [female] and [male] [male]
+    "             x alba   [male]  "   [female] [female] only
+  alba   [female] x dioica [male]  "   [female] [female] and [male] [male].
+
+The point of chief interest lies in the striking difference shown by the
+reciprocal crosses between _dioica_ and _alba_. Males appear when _alba_ is
+used as the female parent but not when the female _dioica_ is crossed by
+male _alba_. It is possible to suggest more than one scheme to cover these
+facts, but we may confine ourselves here to that which seems most in accord
+with the general trend of other cases. We will suppose that in _dioica_
+femaleness is dominant to maleness, and that the female is heterozygous for
+this additional factor. In this species, then, the female produces equal
+numbers of ovules with and without the female factor, while this factor is
+absent in all the pollen grains. _Alba_ [female] x _dioica_ [male] gives
+the same result as _dioica_ [female] x _dioica_ [male], and we must
+therefore suppose that alba produces male and female ovules in equal
+numbers. _Alba_ [male] x _dioica_ [female], however, gives nothing but
+females. Unless, therefore, we assume that there is selective fertilisation
+we must suppose that all the pollen grains of alba carry the female
+factor--in other words, that so far as the sex factors are concerned there
+is a difference between the ovules and pollen grains borne by the same
+plant. Unfortunately further investigation of this case is rendered
+impossible owing to the complete sterility of the F_1 plants. {122}
+
+[Illustration: FIG. 27.
+
+Single and double stocks raised from the same single parent.]
+
+That the possibility of a difference between the ovules and pollen grains
+of the same individual must be taken into account in future work there is
+evidence from quite a different source. The double stock is an old
+horticultural favourite, and for centuries it has been known that of itself
+it sets no seed, but must be raised from special strains of the single
+variety. "You must understand withall," wrote John Parkinson of his
+gilloflowers,[9] "that those plants that beare double flowers, doe beare no
+seed at all ... but the onely way to have double flowers any yeare is to
+save the seedes of those plants of this kinde that beare single flowers,
+for from that seede will rise some that will beare single, and some double
+flowers." With regard to the nature of these double-throwing strains of
+singles, Miss Saunders has recently brought out some interesting facts. She
+crossed the double-throwing singles with pure singles belonging to strains
+in which doubles never occur. The cross was made both ways, and in both
+cases all the F_1 plants were single. A distinction, however, appeared when
+a further generation was raised from the F_1 plants. All the F_1 plants
+from the pollen of the double-throwing single behaved like double-throwing
+singles, but of the F_1 plants from the ovules of the double throwers some
+behaved as double throwers, and some as pure singles. We are led to infer,
+therefore, that the ovules and pollen grains {123} of the double throwers,
+though both produced by the same plant, differ in their relation to the
+factor (or factors) for doubleness. Doubleness is apparently carried by all
+the pollen grains of such plants, but only by some of the ovules. Though
+the nature of doubleness in stocks is not yet clearly understood, the facts
+discovered by Miss Saunders suggest strongly that the ovules and pollen
+grains of the same plant may differ in their transmitting properties,
+probably owing to some process of segregation in the growing plant which
+leads to an unequal distribution of some or other factors to the cells
+which give rise to the ovules as compared with those from which {124} the
+pollen grains eventually spring. Whether this may turn out to be the true
+account or not, the possibility must not be overlooked in future work.
+
+                    Single
+                      |
+                      +-------------+
+                    Single         Double
+                   /      \
+  Pollen of    x Ovule    Pollen x Ovule of
+  pure single  |                 |  pure single
+               |                 |
+    +----------+                 |
+  Single    Single            Single
+    |          |                 |
+    |          +-------+         +---------+
+  Single    Single   Double   Single     Double
+    |          |                 |
+    |          +-------+         +---------+
+  Single    Single   Double   Single     Double
+
+From all this it is clear enough that there is much to be done before the
+problem of sex is solved even so far as the biologist can ever expect to
+solve it. The possibilities are many, and many a fresh set of facts is
+needed before we can hope to decide among them. Yet the occasional glimpses
+of clear-cut and orderly phenomena, which Mendelian spectacles have already
+enabled us to catch, offer a fair hope that some day they may all be
+brought into focus, and assigned their proper places in a general scheme
+which shall embrace them all. Then, though not till then, will the problem
+of the nature of sex pass from the hands of the biologist into those of the
+physicist and the chemist.
+
+       *       *       *       *       *
+
+
+{125}
+
+CHAPTER XII
+
+INTERMEDIATES
+
+So far as we have gone we have found it possible to express the various
+characters of animals and plants in terms of definite factors which are
+carried by the gametes, and are distributed according to a definite scheme.
+Whatever may be the nature of these factors it is possible for purposes of
+analysis to treat them as indivisible entities which may or may not be
+present in any given gamete. When the factor is present it is present as a
+whole. The visible properties developed by a zygote in the course of its
+growth depend upon the nature and variety of the factors carried in by the
+two gametes which went to its making, and to a less degree upon whether
+each factor was brought in by both gametes or by one only. If the given
+factor is brought in by one gamete only, the resulting heterozygote may be
+more or less intermediate between the homozygous form with a double dose of
+the factor and the homozygous form which is entirely destitute of the
+factor. Cases in point are those of the primula flowers and the Andalusian
+fowls. Nevertheless these intermediates produce only pure gametes, as is
+{126} shown by the fact that the pure parental types appear in a certain
+proportion of their offspring. In such cases as these there is but a single
+type of intermediate, and the simple ratio in which this and the two
+homozygous forms appear renders the interpretation obvious. But the nature
+of the F_2 generation may be much more complex, and, where we are dealing
+with factors which interact upon one another, may even present the
+appearance of a series of intermediate forms grading from the condition
+found in one of the original parents to that which occurred in the other.
+As an illustration we may consider the cross between the Brown Leghorn and
+Silky fowls which we have already dealt with in connection with the
+inheritance of sex. The offspring of a Silky hen mated with a Brown Leghorn
+are in both sexes birds with but a trace of the Silky pigmentation. But
+when such birds are bred together they produce a generation consisting of
+chicks as deeply pigmented as the original Silky parent, chicks devoid of
+pigment like the Brown Leghorn, and chicks in which the pigmentation shows
+itself in a variety of intermediate stages. Indeed from a hundred chicks
+bred in this way it would be possible to pick out a number of individuals
+and arrange them in an apparently continuous series of gradually increasing
+pigmentation, with the completely unpigmented at one end and the most
+deeply pigmented at the other. Nevertheless, the case is one in which
+complete segregation of the different factors takes {127}
+[Illustration]place, place, and the apparently continuous series of
+intermediates is the result of the interaction of the different factors
+upon one another. The constitution of the F_1 [male] is a ffPpIi, and such
+a bird produces in equal numbers the four sorts of gametes fPI, fPi, fpI,
+fpi. The constitution of the F_1 [female] in this case is FfPpIi. Owing to
+the repulsion between F and I she produces the four kinds of gametes FPi,
+Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing
+two such series of gametes together is shown in Fig. 28. Out of the sixteen
+types of zygote formed one (FfPPii) is homozygous for the pigmentation
+factor, and does not contain the inhibitor factor. Such a bird is as deeply
+pigmented as the pure Silky parent. Two, again, contain a single dose of P
+in the absence of I. These are nearly as dark as the pure Silky. Four
+zygotes are destitute of P, though they may or may not contain I. These
+birds are completely devoid of pigment like the Brown Leghorn. The
+remaining nine zygotes show {128} various combinations of the two factors P
+and I, being either PPIi, PPII, PpII, or PpIi, and in each of these cases
+the pigment is more or less intense according to the constitution of the
+bird. Thus a bird of the constitution PPIi approaches in pigmentation a
+bird of the constitution Ppii, while a bird of the constitution PpII has
+but little more pigment than the unpigmented bird. In this way we have
+seven distinct grades of pigmentation, and the series is further
+complicated by the fact that these various grades exhibit a rather
+different amount of pigmentation according as they occur in a male or a
+female bird, for, generally speaking, the female of a given grade exhibits
+rather more pigment than the corresponding male. The examination of a
+number of birds bred in this way might quite well suggest that in this case
+we were dealing with a character which could break up, as it were, to give
+a continuous series of intergrading forms between the two extremes. With
+the constant handling of large numbers it becomes possible to recognise
+most of the different grades, though even so it is possible to make
+mistakes. Nevertheless, as breeding tests have amply shown, we are dealing
+with but two interacting factors which segregate cleanly from one another
+according to the strict Mendelian rule. The approach to continuity in
+variation exhibited by the F_2 generation depends upon the fact that these
+two factors interact upon one another, and to different degrees according
+as the zygote is for one {129} or other or both of them in a homozygous or
+a heterozygous state. Moreover, certain of these intermediates will breed
+true to an intermediate condition of the pigmentation. A male of the
+constitution ffPPII when bred with females of the constitution FfPPIi will
+produce only males like itself and females like the maternal parent. We
+have dealt with this case in some detail, because the existence of families
+showing a series of intermediate stages between two characters has
+sometimes been brought forward in opposition to the view that the
+characters of organisms depend upon specific factors which are transmitted
+according to the Mendelian rule. But, as this case from poultry shows
+clearly, neither the existence of such a continuous series of
+intermediates, nor the fact that some of them may breed true to the
+intermediate condition, are incompatible with the Mendelian principle of
+segregation.
+
+[Illustration: FIG. 28.
+
+Diagram to illustrate the nature and composition of the F_2 generations
+arising from the cross of Silky hen with Brown Leghorn cock.]
+
+In connection with intermediates a more cogent objection to the Mendelian
+view is the case of the first cross between two definite varieties
+thenceforward breeding true. The case that will naturally occur to the
+reader is that of the mulatto, which results from the cross between the
+negro and the white. According to general opinion, these mulattos, of
+intermediate pigmentation, continue to produce mulattos. Unfortunately this
+interesting case has never been critically investigated, and the statement
+that the mulatto breeds true rests almost entirely upon {130} information
+that is general and often vague. It may be that the inheritance of skin
+pigmentation in this instance is a genuine exception to the normal rule,
+but at the same time it must not be forgotten that it may be one in which
+several interacting factors are concerned, and that the pure white and the
+pure black are the result of combinations which from their rarity are apt
+to be overlooked. But until we are in possession of accurate information it
+is impossible to pronounce definitely upon the nature of the inheritance in
+this case.
+
+[Illustration: FIG. 29.
+
+Pedigree of a family which originated from a cross between a Hindu and a
+European. Black signs denote individuals as dark as average Hindus. Plain
+signs denote quite-fair members, while those with a dot in the centre are
+intermediate.]
+
+{131}
+
+On the other hand, from the cross between the darkly pigmented Eastern
+races and the white segregation seems to occur in subsequent generations.
+Families are to be found in which one parent is a pure white, while the
+other has arisen from the cross between the dark and light in the first or
+some subsequent generation. Such families may contain children
+indistinguishable from pure blonds as well as children of very dark and of
+intermediate shades. As an example, I may give the following pedigree,
+which was kindly communicated to me by an Anglo-Indian friend (Fig. 29).
+The family had resided in England for several generations, so that in this
+case there was no question of a further admixture of black. Most noticeable
+is the family produced by a very dark lady who had married a white man.
+Some of the children were intermediate in colour, but two were fair whites
+and two were dark as dark Hindus. This sharp segregation or splitting out
+of blacks and whites in addition to intermediates strongly suggests that
+the nature of the inheritance is Mendelian, though it may be complicated by
+the existence of several factors which may also react upon one another. Nor
+must it be forgotten that in so far as these different factors are
+concerned the whites themselves may differ in constitution without showing
+any trace of it in their appearance. Before the case can be regarded as
+settled all these different possibilities will have to be definitely
+tested. With the dark Eastern races as with the negro we cannot {132} hope
+to come to any conclusion until we have evidence collected by critical and
+competent observers.
+
+Though for the present we must regard the case of the negro as not proven,
+there are nevertheless two others in which the heredity would appear not to
+follow the Mendelian rule. Castle in America crossed the lop-eared rabbit
+with the normal form, and found that the F_1 animals were intermediate with
+respect to their ears. And subsequent experiment showed that, on the whole,
+they bred true to this intermediate condition. The other case relates to
+Lepidoptera. The speckled wood butterfly (_Pararge egeria_) has a southern
+form which differs from the northern one in the greater brightness and
+depth of its yellow-brown markings. The northern form is generally
+distinguished as var. _egeriades_. Bateson crossed the southern form from
+the south of France with the paler British form, and found that the
+offspring were more or less intermediate in colour, and that in subsequent
+generations the parental types did not recur. These cases at present stand
+alone. It is possible that further research may reveal complications which
+mask or interfere with an underlying process of segregation. Or it may be
+that segregation does not occur owing to some definite physiological reason
+which at present we do not understand.
+
+And here it is impossible not to recall Mendel's own experiences with the
+Hawkweeds (_Hieracium_). This {133} genus of plants exhibits an
+extraordinary profusion of forms differing from one another sometimes in a
+single feature, sometimes in several. The question as to how far these
+numerous forms were to be classified as distinct species, how far as
+varieties, and how far as products of chance hybridisation, was even at
+that time a source of keen controversy among botanists. There is little
+doubt that Mendel undertook his experiments on the Hawkweeds in the hope
+that the conception of unit-characters so brilliantly demonstrated for the
+pea would serve to explain the great profusion of forms among the
+Hieraciums. Owing to the minute size of their florets, these plants offer
+very considerable technical difficulties in the way of cross fertilisation.
+By dint of great perseverance and labour, however, Mendel succeeded in
+obtaining a few crosses between different forms. These hybrids were reared
+and a further generation produced from them, and, no doubt somewhat to
+Mendel's chagrin, every one of them proved to breed true. There was a
+complete absence of that segregation of characters which he had shown to
+exist in peas and beans, and had probably looked forward with some
+confidence to finding in _Hieracium_. More than thirty years passed before
+the matter was cleared up. To-day we know that the peculiar behaviour of
+the hybrid Hieraciums is due to the fact that they normally produce seed by
+a peculiar process of parthenogenesis. It is possible to take an unopened
+flower and to shear off with a {134} razor all the male organs together
+with the stigmata through which the pollen reaches the ovules. The flower,
+nevertheless, sets perfectly good seed. But the cells from which the seeds
+develop are not of the same nature as the normal ovules of a plant. They
+are not gametes but retain the double structure of the maternal cells. They
+are rather to be regarded as of the nature of buds which early become
+detached from the parent stock to lead an independent existence, and, like
+buds, they reproduce exactly the maternal characteristics. The discovery of
+the true nature of this case was only rendered possible by the development
+of the study of cytology, and it was not given to Mendel to live long
+enough to learn why his hybrid Hieraciums all bred true.
+
+       *       *       *       *       *
+
+
+{135}
+
+CHAPTER XIII
+
+VARIATION AND EVOLUTION
+
+Through the facts of heredity we have reached a new conception of the
+individual. Hitherto we have been accustomed to distinguish between the
+members of a family of rabbits like that illustrated on Plate I. by
+assigning to each an individuality, and by making use of certain external
+features, such as the coat colour or the markings, as convenient outward
+signs to express our idea that the individuality of these different animals
+is different. Apart from this, our notions as to what constituted the
+individuality in each case were at best but vague. Mendelian analysis has
+placed in our hands a more precise method of estimating and expressing the
+variations that are to be found between one individual and another. Instead
+of looking at the individual as a whole, which is in some vague way endowed
+with an individuality marking it off from its fellows, we now regard it as
+an organism built up of definite characters superimposed on a basis beyond
+which for the moment our analysis will not take us. We have begun to
+realise that each individual has a definite architecture, and that this
+architecture depends {136} primarily upon the number and variety of the
+factors that existed in the two gametes that went to its building. Now most
+species exhibit considerable variation and exist in a number, often very
+large, of more or less well-defined varieties. How far can this great
+variety be explained in terms of a comparatively small number of factors if
+the number of possible forms depends upon the number of the factors which
+may be present or absent?
+
+In the simple case where the homozygous and heterozygous conditions are
+indistinguishable in appearance the number of possible forms is 2, raised
+to the power of the number of factors concerned. Thus where one factor is
+concerned there are only 2^1 = 2 possible forms, where ten factors are
+concerned there are 2^{10} = 1024 possible forms differing from one another
+in at most ten and at least one character. Where the factors interact upon
+one another this number will, of course, be considerably increased. If the
+heterozygous form is different in appearance from the homozygous form,
+there are three possible forms connected with each factor; for ten such
+factors the possible number of individuals would be 3^{10} = 59,049; for
+twenty such factors the possible number of different individuals would be
+3^{20} = 3,486,784,401. The presence or absence of a comparatively small
+number of factors in a species carries with it the possibility of an
+enormous range of individual variation. But every one of these individuals
+has a perfectly definite constitution which can {137} be determined in each
+case by the ordinary methods of Mendelian analysis. For in every instance
+the variation depends upon the presence or absence of definite factors
+carried in by the gametes from whose union the individual results. And as
+these factors separate out cleanly in the gametes which the individual
+forms, such variations as depend upon them are transmitted strictly
+according to the Mendelian scheme. Provided that the constitution of the
+gametes is unchanged, the heredity of such variation is independent of any
+change in the conditions of nutrition or environment which may operate upon
+the individual producing the gametes.
+
+But, as everybody knows, an individual organism, whether plant or animal,
+reacts, and often reacts markedly, to the environmental conditions under
+which its life is passed. More especially is this to be seen where such
+characters as size or weight are concerned. More sunlight or a richer soil
+may mean stronger growth in a plant, better nutrition may result in a finer
+animal, superior education may lead to a more intelligent man. But although
+the changed conditions produce a direct effect upon the individual, we have
+no indisputable evidence that such alterations are connected with
+alterations in the nature of the gametes which the individual produces. And
+without this such variations cannot be perpetuated through heredity, but
+the conditions which produce the effect must always be renewed in each
+{138} successive generation. We are led, therefore, to the conclusion that
+two sorts of variations exist, those which are due to the presence of
+specific factors in the organism and those which are due to the direct
+effect of the environment during its lifetime. The former are known as
+_mutations_, and are inherited according to the Mendelian scheme; the
+latter have been termed _fluctuations_, and at present we have no valid
+reason for supposing that they are ever inherited. For though instances may
+be found in which effects produced during the lifetime of the individual
+would appear to affect the offspring, this is not necessarily due to
+heredity. Thus plants which are poorly nourished and grown under adverse
+conditions may set seed from which come plants that are smaller than the
+normal although grown under most favorable conditions. It is natural to
+attribute the smaller size of the offspring to the conditions under which
+the parents were grown, and there is no doubt that we should be quite right
+in doing so. Nevertheless, it need have nothing to do with heredity. As we
+have already pointed out, the seed is a larval plant which draws its
+nourishment from the mother. The size of the offspring is affected because
+the poorly nourished parent offered a bad environment to the young plant,
+and not because the gametes of the parent were changed through the adverse
+conditions under which it grew. The parent in this case is not only the
+producer of gametes, but also a part of the environment of the young {139}
+plant, and it is in this latter capacity that it affects its offspring.
+Wherever, as in plants and mammals, the organism is parasitic upon the
+mother during its earlier stages, the state of nutrition of the latter will
+almost certainly react upon it, and in this way a semblance of transmitted
+weakness or vigour is brought about. Such a connection between mother and
+offspring is purely one of environment, and it cannot be too strongly
+emphasised that it has nothing to do with the ordinary process of heredity.
+
+The distinction between these two kinds of variation, so entirely different
+in their causation, renders it possible to obtain a clearer view of the
+process of evolution than that recently prevalent. As Darwin long ago
+realised, any theory of evolution must be based upon the facts of heredity
+and variation. Evolution only comes about through the survival of certain
+variations and the elimination of others. But to be of any moment in
+evolutionary change a variation must be inherited. And to be inherited it
+must be represented in the gametes. This, as we have seen, is the case for
+those variations which we have termed mutations. For the inheritance of
+fluctuations, on the other hand, of the variations which result from the
+direct action of the environment upon the individual, there is no
+indisputable evidence. Consequently we have no reason for regarding them as
+playing any part in the production of that succession of temporarily stable
+forms which we term evolution. In {140} the light of our present knowledge
+we must regard the mutation as the basis of evolution--as the material upon
+which natural selection works. For it is the only form of variation of
+whose heredity we have any certain knowledge.
+
+It is evident that this view of the process of evolution is in some
+respects at variance with that generally held during the past half century.
+There we were given the conception of an abstract type representing the
+species, and from it most of the individuals diverged in various
+directions, though, generally speaking, only to a very small extent. It was
+assumed that any variation, however small, might have a selection value,
+that is to say, could be transmitted to the offspring. Some of these would
+possess it in a less and some in a greater degree than the parent. If the
+variation were a useful one, those possessing to a rather greater extent
+would be favoured through the action of natural selection at the expense of
+their less fortunate brethren, and would leave a greater number of
+offspring, of whom some possessed it in an even more marked degree than
+themselves. And so it would go on. The process was a cumulative one. The
+slightest variation in a favourable direction gave natural selection a
+starting-point to work on. Through the continued action of natural
+selection on each successive generation the useful variation was gradually
+worked up, until at last it reached the magnitude of a specific {141}
+distinction. Were it possible in such a case to have all the forms before
+us, they would present the appearance of a long series imperceptibly
+grading from one extreme to the other.
+
+Upon this view are made two assumptions not unnatural in the absence of any
+exact knowledge of the nature of heredity and variation. It was assumed, in
+the first place that variation was a continuous process, and, second, that
+any variation could be transmitted to the offspring. Both of these
+assumptions have since been shown to be unjustified. Even before Mendel's
+work became known Bateson had begun to call attention to the prevalence of
+discontinuity in variation, and a few years later this was emphasised by
+the Dutch botanist Hugo de Vries in his great work on _The Mutation
+Theory_. The ferment of new ideas was already working in the solution, and
+under the stimulus of Mendel's work they have rapidly crystallised out.
+With the advent of heredity as a definite science we have been led to
+revise our views as to the nature of variation, and consequently in some
+respects as to the trend of evolution. Heritable variation has a definite
+basis in the gamete, and it is to the gamete, therefore, not to the
+individual, that we must look for the initiation of this process. Somewhere
+or other in the course of their production is added or removed the factor
+upon whose removal or addition the new variation owes its existence. The
+new variation springs into being by a {142} sudden step, not by a process
+of gradual and almost imperceptible augmentation. It is not continuous but
+discontinuous, because it is based upon the presence or absence of some
+definite factor or factors--upon discontinuity in the gametes from which it
+sprang. Once formed, its continued existence is subject to the arbitrament
+of natural selection. If of value in the struggle for existence natural
+selection will decide that those who possess it shall have a better chance
+of survival and of leaving offspring than those who do not possess it. If
+it is harmful to the individual natural selection will soon bring about its
+elimination. But if the new variation is neither harmful nor useful there
+seems no reason why it should not persist.
+
+In this way we avoid a difficulty that beset the older view. For on that
+view no new character could be developed except by the piling up of minute
+variations through the action of natural selection. Consequently any
+character found in animals and plants must be supposed to be of some
+definite use to the individual. Otherwise it could not have developed
+through the action of natural selection. But there are plenty of characters
+to which it is exceedingly difficult to ascribe any utility, and the
+ingenuity of the supporters of this view has often been severely taxed to
+account for their existence. On the more modern view this difficulty is
+avoided. The origin of a new variation is independent of natural {143}
+selection, and provided that it is not directly harmful there is no reason
+why it should not persist. In this way we are released from the burden of
+discovering a utilitarian motive behind all the multitudinous characters of
+living organisms. For we now recognise that the function of natural
+selection is selection and not creation. It has nothing to do with the
+formation of the new variation. It merely decides whether it is to survive
+or to be eliminated.
+
+One of the arguments made use of by supporters of the older view is that
+drawn from the study of adaptation. Animals and plants are as a rule
+remarkably well adapted to living the life which their surroundings impose
+upon them, and in some cases this adaptation is exceedingly striking.
+Especially is this so in the many instances of what is called protective
+coloration, where the animal comes to resemble its surroundings so closely
+that it may reasonably be supposed to cheat even the keenest sighted enemy.
+Surely, we are told, such perfect adaptation could hardly have arisen
+through the mere survival of chance sports. Surely there must be some
+guiding hand moulding the species into the required shape. The argument is
+an old one. For John Ray that guiding hand was the superior wisdom of the
+Creator: for the modern Darwinian it is Natural Selection controlling the
+direction of variation. Mendelism certainly offers no suggestion of any
+such controlling force. It interprets the {144} variations of living forms
+in terms of definite physiological factors, and the diversity of animal and
+plant life is due to the gain or loss of these factors, to the origination
+of new ones, or to fresh combinations among those already in existence. Nor
+is there any valid reason against the supposition that even the most
+remarkable cases of resemblance, such as that of the leaf insect, may have
+arisen through a process of mutation. Experience with domestic plants and
+animals shows that the most bizarre forms may arise as sports and
+perpetuate themselves. Were such forms, arising under natural conditions,
+to be favoured by natural selection owing to a resemblance to something in
+their environment we should obtain a striking case of protective
+adaptation. And here it must not be forgotten that those striking cases to
+which our attention is generally called are but a very small minority of
+the existing forms of life.
+
+For that special group of adaptation phenomena classed under the head of
+Mimicry, Mendelism seems to offer an interpretation simpler than that at
+present in vogue. This perhaps may be more clearly expressed by taking a
+specific case. There is in Africa a genus of Danaine butterflies known as
+_Amauris_, and there are reasons for considering that the group to which it
+belongs possesses properties which render it unpalatable to vertebrate
+enemies such as birds or monkeys. In the same region is also found the
+genus _Euralia_ belonging to the entirely {145} different family of the
+Nymphalidae, to which there is no evidence for assigning the disagreeable
+properties of the Danaines. Now the different species of _Euralia_ show
+remarkably close resemblances to the species of _Amauris_, which are found
+flying in the same region, and it is supposed that by "mimicking" the
+unpalatable forms they impose upon their enemies and thereby acquire
+immunity from attack. The point at issue is the way in which this seemingly
+purposeful resemblance has been brought about.
+
+One of the species of _Euralia_ occurs in two very distinct forms (Pl.
+VI.), which were previously regarded as separate species under the names
+_E. wahlbergi_ and _E. mima_. These two forms respectively resemble
+_Amauris dominicanus_ and _A. echeria_. For purposes of argument we will
+assume _A. echeria_ to be the more recent form of the two. On the modern
+Darwinian view certain individuals of _A. dominicanus_ gradually diverged
+from the _dominicanus_ type and eventually reached the _echeria_ type,
+though why this should have happened does not appear to be clear. At the
+same time those specimens which tended to vary in the direction of _A.
+echeria_ in places where this species was more abundant than _A.
+dominicanus_ were encouraged by natural selection, and under its guiding
+hand the form _mima_ eventually arose from _wahlbergi_.
+
+According to Mendelian views, on the other hand, {146} _A. echeria_ arose
+suddenly from _A. dominicanus_ (or _vice versa_), and similarly _mima_
+arose suddenly from _wahlbergi_. If _mima_ occurred where _A. echeria_ was
+common and _A. dominicanus_ was rare, its resemblance to the more plentiful
+distasteful form would give it the advantage over _wahlbergi_ and allow it
+to establish itself in place of the latter. On the modern Darwinian view
+natural selection gradually shapes _wahlbergi_ into the _mima_ form owing
+to the presence of _A. echeria_; on the Mendelian view natural selection
+merely conserves the _mima_ form when once it has arisen. Now this case of
+mimicry is one of especial interest, because we have experimental evidence
+that the relation between _mima_ and _wahlbergi_ is a simple Mendelian one,
+though at present it is uncertain which is the dominant and which the
+recessive form. The two have been proved to occur in families bred from the
+same female without the occurrence of any intermediates, and the fact that
+the two segregate cleanly is strong evidence in favour of the Mendelian
+view. On this view the genera _Amauris_ and _Euralia_ contain a similar set
+of pattern factors, and the conditions, whatever they may be, which bring
+about mutation in the former lead to the production of a similar mutation
+in the latter. Of the different forms of _Euralia_ produced in any region
+that one has the best chance of survival, through the operation of natural
+selection, which resembles the most plentiful _Amauris_ form. Mimetic
+resemblance is a true phenomenon, but natural selection plays the part of a
+conservative, not of a formative agent.
+
+[Illustration: PLATE VI.]
+
+{147}
+
+It is interesting to recall that in earlier years Darwin was inclined to
+ascribe more importance to "sports" as opposed to continuous minute
+variation, and to consider that they might play a not inconsiderable part
+in the formation of new varieties in nature. This view, however, he gave up
+later, because he thought that the relatively rare sport or mutation would
+rapidly disappear through the swamping effects of crossing with the more
+abundant normal form, and so, even though favoured by natural selection,
+would never succeed in establishing itself. Mendel's discovery has
+eliminated this difficulty. For suppose that the sport differed from the
+normal in the loss of a factor and were recessive. When mated with the
+normal this character would seem to disappear, though, of course, half of
+the gametes of its progeny would bear it. By continual crossing with
+normals a small proportion of heterozygotes would eventually be scattered
+among the population, and as soon as any two of these mated together the
+recessive sport would appear in one quarter of their offspring.
+
+A suggestive contribution to this subject was recently made by G. H. Hardy.
+Considering the distribution of a single factor in a mixed population
+consisting of the heterozygous and the two homozygous forms he showed that
+such a population breeding at random rapidly fell into a {148} stable
+condition with regard to the proportion of these three forms, whatever may
+have been the proportion of the three forms to start with. Let us suppose
+for instance, that the population consists of p homozygotes of one kind, r
+homozygotes of the other kind, and 2 q heterozygotes. Hardy pointed out
+that, other things being equal, such a population would be in equilibrium
+for this particular factor so long as the condition q^2 = pr was fulfilled.
+If the condition is fulfilled to start with, the population remains in
+equilibrium. If the condition is not fulfilled to start with, Hardy showed
+that a position of equilibrium becomes established after a single
+generation, and that this position is thereafter maintained. The
+proportions of the three classes which satisfy the equation q^2 = pr are
+exceedingly numerous, and populations in which they existed in the
+proportions shown in the appended table would remain in stable equilibrium
+generation after generation:--
+
+     p.     2q.           r.
+     1       2            1
+     1       4            4
+     1       6            9
+     1       8           16
+     1     20,000    100,000,000
+     1       2n          n^2
+
+This, of course, assumes that all three classes are equally fertile, and
+that no form of selection is taking place to the {149} benefit of one class
+more than of another. Moreover, it makes no difference whether p represents
+the homozygous dominants or whether it stands for the recessives. A
+population containing a very small proportion of dominants and one
+containing a similar proportion of recessives are equally stable. The term
+dominant is in some respects apt to be misleading, for a dominant character
+cannot in virtue of its dominance establish itself at the expense of a
+recessive one. Brown eyes in man are dominant to blue, but there is no
+reason to suppose that as years go on the population of these islands will
+become increasingly brown eyed. Given equality of conditions both are on an
+equal footing. If, however, either dominant or recessive be favoured by
+selection the conditions are altered, and it can be shown that even a small
+advantage possessed by the one will rapidly lead to the elimination of the
+other. Even with but a 5 per cent selection advantage in its favour it can
+be shown that a rare sport will oust the normal form in a few hundred
+generations. In this way we are freed from a difficulty inherent in the
+older view that varieties arose through a long-continued process involving
+the accumulation of very slight variations. On that view the establishing
+of a new type was of necessity a very long and tedious business, involving
+many thousands of generations. For this reason the biologist has been
+accustomed to demand a very large supply of time, often a great deal more
+than the physicist is {150} disposed to grant, and this has sometimes led
+him to expostulate with the latter for cutting off the supply. On the newer
+views, however, this difficulty need not arise, for we realise that the
+origin and establishing of a new form may be a very much more rapid process
+than has hitherto been deemed possible.
+
+One last question with regard to evolution. How far does Mendelism help us
+in connection with the problem of the origin of species? Among the plants
+and animals with which we have dealt we have been able to show that
+distinct differences, often considerable, in colour, size, and structure,
+may be interpreted in terms of Mendelian factors. It is not unlikely that
+most of the various characters which the systematist uses to mark off one
+species from another, the so-called specific characters, are of this
+nature. They serve as convenient labels, but are not essential to the
+conception of species. A systematist who defined the wild sweet pea could
+hardly fail to include in his definition such characters as the procumbent
+habit, the tendrils, the form of the pollen, the shape of the flower, and
+its purple colour. Yet all these and other characters have been proved to
+depend upon the presence of definite factors which can be removed by
+appropriate crossing. By this means we can produce a small plant a few
+inches in height with an erect habit of growth, without tendrils, with
+round instead of oblong pollen, and with colourless deformed flowers quite
+different {151} in appearance from those of the wild form. Such a plant
+would breed perfectly true, and a botanist to whom it was presented, if
+ignorant of its origin, might easily relegate it to a different genus.
+Nevertheless, though so widely divergent in structure, such a plant must
+yet be regarded as belonging to the species _Lathyrus odoratus_. For it
+still remains fertile with the many different varieties of sweet pea. It is
+not visible attributes that constitute the essential difference between one
+species and another. The essential difference, whatever it may be, is that
+underlying the phenomenon of sterility. The visible attributes are those
+made use of by the systematist in cataloguing the different forms of animal
+and plant life, for he has no other choice. But it must not be forgotten
+that they are often misleading. Until they were bred together _Euralia
+wahlbergi_ and _E. mima_ were regarded as perfectly valid species, and
+there is little doubt that numbers of recognised species will eventually
+fall to the ground in the same way as soon as we are in a position to apply
+the test of breeding. Mendelism has helped us to realise that specific
+characters may be but incidental to a species--that the true criterion of
+what constitutes a species is sterility, and that particular form of
+sterility which prevents two healthy gametes on uniting from producing a
+zygote with normal powers of growth and reproduction. For there are forms
+of sterility which are purely mechanical. The pollen of _Mirabilis jalapa_
+cannot fertilise _M._ {152} _longiflora_, because the pollen tubes of the
+former are not long enough to penetrate down to the ovules of the latter.
+Hybrids can nevertheless be obtained from the reciprocal cross. Nor should
+we expect offspring from a St. Bernard and a toy terrier without recourse
+to artificial fertilisation. Or sterility may be due to pathological causes
+which prevent the gametes from meeting one another in a healthy state. But
+in most cases it is probable that the sterility is due to some other cause.
+It is not inconceivable that definite differences in chemical composition
+render the protoplasm of one species toxic to the gametes of the other, and
+if this is so it is not impossible that we may some day be able to express
+these differences in terms of Mendelian factors. The very nature of the
+case makes it one of extreme difficulty for experimental investigation. At
+any rate, we realise more clearly than before that the problem of species
+is not one that can be resolved by the study of morphology or of
+systematics. It is a problem in physiology.
+
+       *       *       *       *       *
+
+
+{153}
+
+CHAPTER XIV
+
+ECONOMICAL
+
+Since heredity lies at the basis of the breeder's work, it is evident that
+any contribution to a more exact knowledge of this subject must prove of
+service to him, and there is no doubt that he will be able to profit by
+Mendelian knowledge in the conduct of his operations. Indeed, as we shall
+see later, these ideas have already led to striking results in the raising
+of new and more profitable varieties. In the first place, heredity is a
+question of individuals. Identity of appearance is no sure guide to
+reproductive qualities. Two individuals similarly bred and
+indistinguishable in outward form may nevertheless behave entirely
+differently when bred from. Take, for instance, the family of sweet peas
+shown on Plate IV. The F_2 generation here consists of seven distinct
+types, three sorts of purples, three sorts of reds, and whites. Let us
+suppose that our object is to obtain a true breeding strain of the pale
+purple picotee form. Now from the proportions in which they come we know
+that the dilute colour is due to the absence of the factor which
+intensifies the colour. Consequently the picotee cannot throw the {154} two
+deeper shades of red or purple. But it may be heterozygous for the purpling
+factor when it will throw the dilute red (tinged white), or it may be
+heterozygous for either or both of the two colour factors (cf. p. 44), in
+which case it will throw whites. Of the picotees which come in such a
+family, therefore, some will give picotees, tinged whites, and whites,
+others will give picotees and tinged whites only, others will give picotees
+and whites only, while others, again, and these the least numerous, will
+give nothing but picotees. The new variety is already fixed in a certain
+definite proportion of the plants; in this particular instance in 1 out of
+every 27. All that remains to be done is to pick out these plants. Since
+all the picotees look alike, whatever their breeding capacity, the only way
+to do this is to save the seed from a number of such plants _individually_,
+and to raise a further generation. Some of them will be found to breed
+true. The variety is then established, and may at once be put on the market
+with full confidence that it will hereafter throw none of the other forms.
+The all-important thing is to save and sow the seed of separate individuals
+separately. However alike they look, the seed from different individuals
+must on no account be mixed. Provided that due care is taken in this
+respect no long and tedious process of selection is required for the
+fixation of any given variety. Every possible variety arising from a cross
+appears in the F_2 generation if only a sufficient {155} number is raised,
+and of all these different varieties a certain proportion of each is
+already fixed. Heredity is a question of individuals, and the recognition
+of this will save the breeder much labour, and enable him to fix his
+varieties in the shortest possible time.
+
+Such cases as these of the sweet pea throw a fresh light upon another of
+the breeder's conceptions, that of purity of type. Hitherto the criterion
+of a "pure-bred" thing, whether plant or animal, has been its pedigree, and
+the individual was regarded as more or less pure bred for a given quality
+according as it could show a longer or shorter list of ancestors possessing
+this quality. To-day we realise that this is not essential. The pure-bred
+picotee appears in our F_2 family though its parent was a purple bicolor,
+and its remoter ancestors whites for generations. So also from the cross
+between pure strains of black and albino rabbits we may obtain in the F_2
+generation animals of the wild agouti colour which breed as true to type as
+the pure wild rabbit of irreproachable pedigree. The true test of the pure
+breeding thing lies not in its ancestry but in the nature of the gametes
+which have gone to its making. Whenever two similarly constituted gametes
+unite, whatever the nature of the parents from which they arose, the
+resulting individual is homozygous in all respects and must consequently
+breed true. In deciding questions of purity it is to the gamete, and not to
+ancestry, that our appeal must henceforth be made. {156}
+
+Improvement is after all the keynote to the breeder's operations. He is
+aiming at the production of a strain which shall combine the greatest
+number of desirable properties with the least number of undesirable ones.
+This good quality he must take from one strain, that from another, and that
+again from a third, while at the same time avoiding all the poor qualities
+that these different strains possess. It is evident that the Mendelian
+conception of characters based upon definite factors which are transmitted
+on a definite scheme must prove of the greatest service to him. For once
+these factors have been determined, their distribution is brought under
+control, and they can be associated together or dissociated at the
+breeder's will. The chief labour involved is that necessary for the
+determination of the factors upon which the various characters depend. For
+it often happens that what appears to be a simple character turns out when
+analysed to depend upon the simultaneous presence of several distinct
+factors. Thus the Malay fowl breeds true to the walnut comb, as does also
+the Leghorn to the single comb, and when pure strains are crossed all the
+offspring have walnut combs. At first sight it would be not unnatural to
+regard the difference as dependent upon the presence or absence of a single
+factor. Yet, as we have already seen, two other types of comb, the pea and
+the rose, make their appearance in the F_2 generation. Analysis shows that
+the difference between the walnut {157} and the single is a difference of
+two factors, and it is not until this has been determined that we can
+proceed with certainty to transfer the walnut character to a single-combed
+breed. Moreover, in his process of analysis the breeder must be prepared to
+encounter the various phenomena that we have described under the headings
+of interaction of factors, coupling and repulsion, and the recognition of
+these phenomena will naturally influence his procedure. Or again, his
+experiments may show him that one of the characters he wants, like the blue
+of the Andalusian fowl, is dependent upon the heterozygous nature of the
+individual which exhibits it, and if such is the case he will be wise to
+refrain from any futile attempt at fixing it. If it is essential it must be
+built up again in each generation, and he will recognise that the most
+economical way of doing this is to cross the two pure strains so that all
+the offspring may possess the desired character. The labour of analysis is
+often an intricate and tedious business. But once done it is done once for
+all. As soon as the various factors are determined, upon which the various
+characters of the individual depend, as soon as the material to be made use
+of has been properly analysed, the production and fixation of the required
+combinations becomes a matter of simple detail.
+
+An excellent example of the practical application of Mendelian principles
+is afforded by the experiments which Professor Biffen has recently carried
+out in Cambridge. {158} Taken as a whole English wheats compare favourably
+with foreign ones in respect of their cropping power. On the other hand,
+they have two serious defects. They are liable to suffer from the attacks
+of the fungus which causes rust, and they do not bake into a good loaf.
+This last property depends upon the amount of gluten present, and it is the
+greater proportion of this which gives to the "hard" foreign wheat its
+quality of causing the loaf to rise well when baked. For some time it was
+held that "hard" wheat with a high glutinous content could not be grown in
+the English climate, and undoubtedly most of the hard varieties imported
+for trial deteriorated greatly in a very short time. Professor Biffen
+managed to obtain a hard wheat which kept its qualities when grown in
+England. But in spite of the superior quality of its grain from the baker's
+point of view its cropping capacity was too low for it to be grown
+profitably in competition with English wheats. Like the latter, it was also
+subject to rust. Among the many varieties which Professor Biffen collected
+and grew for observation he managed to find one which was completely immune
+to the attacks of the rust fungus, though in other respects it had no
+desirable quality to recommend it. Now as the result of an elaborate series
+of investigations he was able to show that the qualities of heavy cropping
+capacity, "hardness" of grain, and immunity to rust can all be expressed in
+terms of Mendelian factors. Having once analysed his material {159} the
+rest was comparatively simple, and in a few years he has been able to build
+up a strain of wheat which combines the cropping capacity of the best
+English varieties with the hardness of the foreign kinds, and at the same
+time is completely immune to rust. This wheat has already been shown to
+keep its qualities unchanged for several years, and there is little doubt
+that when it comes to be grown in quantity it will exert an appreciable
+influence on wheat-growing in Great Britain.
+
+[Illustration: FIG. 30.
+
+Curves to illustrate the influence of selection.]
+
+It may be objected that it is often with small differences rather than with
+the larger and more striking ones that the breeder is mainly concerned. It
+does not matter much to him whether the colour of a pea flower is purple or
+pink or white. But it does matter whether the plant bears rather larger
+seeds than usual, or rather more of them. Even a small difference when
+multiplied by the {160} size of the crop will effect a considerable
+difference in the profit. It is the general experience of seedsmen and
+others that differences of this nature are often capable of being developed
+up to a certain point by a process of careful selection each generation. At
+first sight this appears to be something very like the gradual accumulation
+of minute variations through the continuous application of a selective
+process. Some recent experiments by Professor Johannsen of Copenhagen set
+the matter in a different light. One of his investigations deals with the
+inheritance of the weight of beans, but as an account of these experiments
+would involve us in the consideration of a large amount of detail we may
+take a simple imaginary case to illustrate the nature of the conclusions at
+which he arrived. If we weigh a number of seeds collected from a patch of
+plants such as Johannsen's beans we should find that they varied
+considerably in size. The majority would probably not diverge very greatly
+from the general average, and as we approached the high or low extreme we
+should find a constantly decreasing number of individuals with these
+weights. Let us suppose that the weight of our seed varied between 4 and 20
+grains, that the greatest number of seeds were of the mean weight, viz. 12
+grains, and that as we passed to either extreme at 4 and 20 the number
+became regularly less. The weight relation of such a collection of seeds
+can be expressed by the accompanying curve (Fig. 30). Now if we select for
+{161} sowing only that seed which weighs over 12 grains, we shall find that
+in the next generation the average weight of the seed is raised and the
+curve becomes somewhat shifted to the right as in the dotted line of Fig.
+30. By continually selecting we can shift our curve a little more to the
+right, _i.e._ we can increase the average weight of the seeds until at last
+we come to a limit beyond which further selection has no effect. This
+phenomenon has been long known, and it was customary to regard these
+variations as of a continuous nature, _i.e._ as all chance fluctuations in
+a homogeneous mass, and the effect of selection was supposed to afford
+evidence that small continuous variations could be increased by this
+process. But Johannsen's results point to another interpretation. Instead
+of our material being homogeneous it is probably a mixture of several
+strains each with its own average weight about {162} which the varying
+conditions of the environment cause it to fluctuate. Each of these strains
+is termed a PURE LINE. If we imagine that there are three such pure lines
+in our imaginary case, with average weights 10, 12, 14 grains respectively,
+and if the range of fluctuation of each of these pure lines is 12 grains,
+then our curve must be represented as made up of the three components
+
+  A fluctuating between 4 and 16 with a mean of 10
+  B     "          "    6  "  18   "     "      12
+  C     "          "    8  "  20   "     "      14
+
+[Illustration: FIG. 31.
+
+Curves to illustrate the conception of pure lines in a population.]
+
+as is shown in Fig. 31. A seed that weighs 12 grains may belong to any of
+these three strains. It may be an average seed of B, or a rather large seed
+of A, or a rather small seed of C. If it belongs to B its offspring will
+average 12 grains, if to A they will average 10 grains, and if to C they
+will average 14 grains. Seeds of similar weight may give a different result
+because they happen to be fluctuations of different pure lines. But within
+the pure line any seed, large or small, produces the average result for
+that line. Thus a seed of line C which weighs 20 grains will give
+practically the same result as one that weighs 10 grains.
+
+On this view we can understand why selection of the largest seed raises the
+average weight in the next generation. We are picking out more of C and
+less of A and B, and as this process is repeated the proportion of C
+gradually increases and we get the appearance of selection {163} acting on
+a continuously varying homogeneous material and producing a permanent
+effect. This is because the interval between the average weight of the
+different pure lines is small compared with the environmental fluctuations.
+None the less it is there, and the secret of separating and fixing any of
+these pure lines is again to breed from the individual separately. As soon
+as the pure line is separated further selection becomes superfluous.
+
+Since the publication of Darwin's famous work upon the effects of cross and
+self fertilisation, it has been generally accepted that the effect of a
+cross is commonly, though not always, to introduce fresh vigour into the
+offspring, though why this should be so we are quite at a loss to explain.
+Continued close inbreeding, on the contrary, eventually leads to
+deterioration, though, as in many self-fertilised plants, a considerable
+number of generations may elapse before it shows itself in any marked
+degree. The fine quality of many of the seedsman's choice varieties of
+vegetables probably depends upon the fact that they had resulted from a
+cross but a few generations back, and it is possible that they often oust
+the older kinds not because they started as something intrinsically better,
+but because the latter had gradually deteriorated through continuous
+self-fertilisation. Most breeders are fully alive to the beneficial results
+of a cross so far as vigour is concerned, but they often hesitate to embark
+upon it owing to what was held {164} to be the inevitably lengthy and
+laborious business of recovering the original variety and refixing it, even
+if in the process it was not altogether lost. That danger Mendelism has
+removed, and we now know that by working on these lines it is possible in
+three or four generations to recover the original variety in a fixed state
+with all the superadded vigour that follows from a cross.
+
+Nor is the problem one that concerns self-fertilised plants only. Plants
+that are reproduced asexually often appear to deteriorate after a few
+generations unless a sexual generation is introduced. New varieties of
+potato, for example, are frequently put upon the market, and their
+excellent qualities give them a considerable vogue. Much is expected of
+them, but time after time they deteriorate in a disappointing way and are
+lost to sight. It is not improbable that we are here concerned with a case
+in which the plants lose their vigour after a few asexual generations of
+reproduction from tubers, and can only recover it with the stimulus that
+results from the interpolation of a sexual generation. Unfortunately this
+generally means that the variety is lost, for owing to the haphazard way in
+which new kinds of potatoes are reproduced it is probable that most
+cultivated varieties are complex heterozygotes. Were the potato plant
+subjected to careful analysis and the various factors determined upon which
+its variations depend, we should be in a position to remake continually any
+good potato without {165} running the risk of losing it altogether, as is
+now so often the case.
+
+The application of Mendelian principles is likely to prove of more
+immediate service for plants than animals, for owing to the large numbers
+which can be rapidly raised from a single individual and the prevalence of
+self-fertilisation, the process of analysis is greatly simplified. Even
+apart from the circumstance that the two sexes may sometimes differ in
+their powers of transmission, the mere fact of their separation renders the
+analysis of their properties more difficult. And as the constitution of the
+individual is determined by the nature and quality of its offspring, it is
+not easy to obtain this knowledge where the offspring, as in most animals,
+are relatively few. Still, as has been abundantly shown, the same
+principles hold good here also, and there is no reason why the process of
+analysis, though more troublesome, should not be effectively carried out.
+At the same time, it affords the breeder a rational basis for some familiar
+but puzzling phenomena. The fact, for instance, that certain characters
+often "skip a generation" is simply the effect of dominance in F_1 and the
+reappearance of the recessive character in the following generation.
+"Reversion" and "atavism," again, are phenomena which are no longer
+mysterious, but can be simply expressed in Mendelian terms as we have
+already suggested in Chap. VI. The occasional appearance of a sport in a
+supposedly pure strain is {166} often due to the reappearance of a
+recessive character. Thus even in the most highly pedigreed strains of
+polled cattle such as the Aberdeen Angus, occasional individuals with horns
+appear. The polled character is dominant to the horned, and the occasional
+reappearance of the horned animal is due to the fact that some of the
+polled herd are heterozygous in this character. When two such individuals
+are mated, the chances are 1 in 4 that the offspring will be horned. Though
+the heterozygous individuals may be indistinguishable in appearance from
+the pure dominant, they can be readily separated by the breeding test. For
+when crossed by the recessive, in this case horned animals, the pure
+dominant gives only polled beasts, while the heterozygous individual gives
+equal numbers of polled and horned ones. In this particular instance it
+would probably be impracticable to test all the cows by crossing with a
+horned bull. For in each case it would be necessary to have several polled
+calves from each before they could with reasonable certainty be regarded as
+pure dominants. But to ensure that no horned calves should come, it is
+enough to use a bull which is pure for that character. This can easily be
+tested by crossing him with a dozen or so horned cows. If he gets no horned
+calves out of these he may be regarded as a pure dominant and thenceforward
+put to his own cows, whether horned or polled, with the certainty that all
+his calves will be polled. {167}
+
+Or, again, suppose that a breeder has a chestnut mare and wishes to make
+certain of a bay foal from her. We know that bay is dominant to chestnut,
+and that if a homozygous bay stallion is used a bay foal must result. In
+his choice of a sire, therefore, the breeder must be guided by the previous
+record of the animal, and select one that has never given anything but bays
+when put to either bay or chestnut mares. In this way he will assure
+himself of a bay foal from his chestnut mare, whereas if the record of the
+sire shows that he has given chestnuts he will be heterozygous, and the
+chances of his getting a bay or a chestnut out of a chestnut mare are
+equal.
+
+It is not impossible that the breeder may be unwilling to test his animals
+by crossing them with a different breed through fear that their purity may
+be thereby impaired, and that the influence of the previous cross may show
+itself in succeeding generations. He might hesitate, for instance, to test
+his polled cows by crossing them with a horned bull for fear of getting
+horned calves when the cows were afterwards put to a polled bull of their
+own breed. The belief in the power of a sire to influence subsequent
+generations, or telegony as it is sometimes called, is not uncommon even
+to-day. Nevertheless, carefully conducted experiments by more than one
+competent observer have failed to elicit a single shred of unequivocal
+evidence in favour of the view. Until we have evidence based upon
+experiments which are capable of {168} repetition, we may safely ignore
+telegony as a factor in heredity.
+
+Heterozygous forms play a greater part in the breeding of animals than of
+plants, for many of the qualities sought after by the breeder are of this
+nature. Such is the blue of the Andalusian fowl, and, according to
+Professor Wilson, the roan of the Shorthorn is similar, being the
+heterozygous form produced by mating red with white. The characters of
+certain breeds of canaries and pigeons again appear to depend upon their
+heterozygous nature. Such forms cannot, of course, ever be bred true, and
+where several factors are concerned they may when bred together produce but
+a small proportion of offspring like themselves. As soon, however, as their
+constitution has been analysed and expressed in terms of Mendelian factors,
+pure strains can be built up which when crossed will give nothing but
+offspring of the desired heterozygous form.
+
+The points with which the breeder is concerned are often fine ones, not
+very evident except to the practised eye. Between an ordinary Dutch rabbit
+and a winner, or between the comb of a Hamburgh that is fit to show and one
+that is not, the differences are not very apparent to the uninitiated.
+Whether Mendelism will assist the breeder in the production of these finer
+points is at present doubtful. It may be that these small differences are
+heritable, such as those that form the basis of Johannsen's pure lines. In
+this case the breeder's outlook is {169} hopeful. But it may be that the
+variations which he seeks to perpetuate are of the nature of fluctuations,
+dependent upon the earlier life conditions of the individual, and not upon
+the constitution of the gametes by which it was formed. If such is the
+case, he will get no help from the science of heredity, for we know of no
+evidence which might lead us to suppose that variations of this sort can
+ever become fixed and heritable.
+
+       *       *       *       *       *
+
+
+{170}
+
+CHAPTER XV
+
+MAN
+
+[Illustration: FIG. 32.
+
+Normal and brachydactylous hands placed together for comparison. (From
+Drinkwater.)]
+
+[Illustration: FIG. 33.
+
+Radiograph of a brachydactylous hand.]
+
+Though the interest attaching to heredity in man is more widespread than in
+other animals, it is far more difficult to obtain evidence that is both
+complete and accurate. The species is one in which the differentiating
+characters separating individual from individual are very numerous, while
+the number of the offspring is comparatively few, and the generations are
+far between. For these reasons, even if it were possible, direct
+experimental work with man would be likely to prove both tedious and
+expensive. There is, however, another method besides the direct one from
+which something can be learned. This consists in collecting all the
+evidence possible, arranging it in the form of pedigrees, and comparing it
+with standard cases already worked out in animals and plants. In this way
+it has been possible to demonstrate in man the existence of several
+characters showing simple Mendelian inheritance. As few besides medical men
+have hitherto been concerned practically with heredity, such records as
+exist are, for the most part, records of deformity or of disease. So it
+happens that most of the {171} pedigrees at present available deal with
+characters which are usually classed as abnormal. In some of these the
+inheritance is clearly Mendelian. One of the cases which has been most
+fully worked out is that of a deformity known as brachydactyly. In
+brachydactylous people the {172} whole of the body is much stunted, and the
+fingers and toes appear to have two joints only instead of three (cf. Figs.
+32 and 33). The inheritance of this peculiarity has been carefully
+investigated by Dr. Drinkwater, who collected all the data he was able to
+find among the members of a large family in which it occurred. The result
+is the pedigree shown on p. 173. It is assumed that all who are recorded as
+having offspring were married to normals. Examination of the pedigree
+brings out the facts (1) that all affected individuals have an affected
+parent; (2) that none of the unaffected individuals, though sprung from the
+affected, ever have descendants who are affected, and (3) that in families
+where both affected and unaffected {173} occur, the numbers of the two
+classes are, on the average, equal. (The sum of such families in the
+complete pedigree is thirty-nine affected and thirty-six normals.) It is
+obvious that these are the conditions which are fulfilled in a simple
+Mendelian case, and there is nothing in this pedigree to contradict the
+assertion that brachydactyly, whatever it may be due to, behaves as a
+simple dominant to the normal form, _i.e._ that it depends upon a factor
+which the normal does not contain. The recessive normals cannot transmit
+the affected condition whatever their ancestry. Once free they are always
+free, and can marry other normals with full confidence that none of their
+children will show the deformity.
+
+[Illustration: FIG. 34.
+
+Pedigree of Drinkwater's brachydactylous family. The affected members are
+indicated by black and the normals by light circles.]
+
+{174}
+
+The evidence available from pedigrees has revealed the simplest form of
+Mendelian inheritance in several human defects and diseases, among which
+may be mentioned presenile cataract of the eyes, an abnormal form of skin
+thickening in the palms of the hands and soles of the feet, known as
+tylosis, and epidermolysis bullosa, a disease in which the skin rises up
+into numerous bursting blisters.
+
+Among the most interesting of all human pedigrees is one recently built up
+by Mr. Nettleship from the records of a night-blind family living near
+Monpelier in the south of France. In night-blind people the retina is
+insensitive to light which falls below a certain intensity, and such people
+are consequently blind in failing daylight or in moonlight. As the
+Monpelier case had excited interest for some time, the records are
+unusually complete. They commence with a certain Jean Nougaret, who was
+born in 1637, and suffered from night-blindness, and they end for the
+present with children who are to-day but a few years of age. Particulars
+are known of over 2000 of the descendants of Jean Nougaret. Through ten
+generations and nearly three centuries the affection has behaved as a
+Mendelian dominant, and there is no sign that long-continued marriage with
+folk of normal vision has produced any amelioration of the night-blind
+state. {175}
+
+[Illustration: FIG. 35.
+
+Pedigree of a haemophilic family. Affected (all males) represented by
+black, and normals of both sexes by light circles. (From Stahel.)]
+
+Besides cases such as these where a simple form of Mendelian inheritance is
+obviously indicated, there are others which are more difficult to read. Of
+some it may be said that on the whole the peculiarity behaves as though it
+were an ordinary dominant; but that exceptions occur in which affected
+children are born to unaffected parents. It is not impossible that the
+condition may, like colour in the sweet pea, depend upon the presence or
+absence of more than one factor. In none of these cases, however, are the
+data sufficient for determining with certainty whether this is so or not.
+
+A group of cases of exceptional interest is that in which the incidence of
+disease is largely, if not absolutely, restricted to one sex, and so far as
+is hitherto known the burden is invariably borne by the male. In the
+inheritance of colour-blindness (p. 117) we have already discussed an
+instance in which the defect is rare, though not {176} unknown, in the
+female. Sex-limited inheritance of a similar nature is known for one or two
+ocular defects, and for several diseases of the nervous system. In the
+peculiarly male disease known as haemophilia the blood refuses to clot when
+shed, and there is nothing to prevent great loss from even a superficial
+scratch. In its general trend the inheritance of haemophilia is not unlike
+that of horns among sheep, and it is possible that we are here again
+dealing with a character which is dominant in one sex and recessive in the
+other. But the evidence so far collected points to a difference somewhere,
+for in haemophilic families the affected males, instead of being equal in
+number to the unaffected, show a considerable preponderance. The
+unfortunate nature of the defect, however, forces us to rely for our
+interpretation almost entirely upon the families produced by the unaffected
+females who can transmit it. Our knowledge of the offspring of "bleeding"
+males is as yet far too scanty, and until it is improved, or until we can
+find some parallel case in animals or plants, the precise scheme of
+inheritance for haemophilia must remain undecided.
+
+Though by far the greater part of the human evidence relates to abnormal or
+diseased conditions, a start has been made in obtaining pedigrees of normal
+characters. From the ease with which it can be observed, it was natural
+that eye-colour should be early selected as a subject of investigation, and
+the work of Hurst and others {177} has clearly demonstrated the existence
+of one Mendelian factor in operation here. Eyes are of many colours, and
+the colour depends upon the pigment in the iris. Some eyes have pigment on
+both sides of the iris--on the side that faces the retina as well as on the
+side that looks out upon the world. Other eyes have pigment on the retinal
+side only. To this class belong the blues and clear greys; while the eyes
+with pigment in front of the iris also are brown, hazel, or green in
+various shades according to the amount of pigment present. In albino
+animals the pigment is entirely absent, and as the little blood-vessels are
+not obscured the iris takes on its characteristic pinkish-red appearance.
+The condition in which pigment is present in front of the iris is dominant
+to that in which it is absent. Greens, browns, or hazels mated together
+may, if heterozygous, give the recessive blue, but no individuals of the
+brown class are to be looked for among the offspring of blues mated
+together. The blues, however, may carry factors which are capable of
+modifying the brown. Just as the pale pink-tinged sweet pea (Pl. IV., 9)
+when mated with a suitable white gives only deep purples, so an eye with
+very little brown pigment mated with certain blues produces progeny of a
+deep brown, far darker than either parent. The blue may carry a factor
+which brings about intensification of the brown pigment. There are
+doubtless other factors which modify the brown when present, but we do not
+yet know enough of the {178} inheritance of the various shades to justify
+any statement other than that the heredity of the pigment in front of the
+iris behaves as though it were due to a Mendelian factor.
+
+Even this fact is of considerable importance, for it at once suggests that
+the present systems of classification of eye-colours, to which some
+anthropologists attach considerable weight, are founded on a purely
+empirical and unsatisfactory basis. Intensity of colour is the criterion at
+present in vogue, and it is customary to arrange the eye-colours in a scale
+of increasing depth of shade, starting with pale greys and ending with the
+deepest browns. On this system the lighter greens are placed among the
+blues. But we now know that blues may differ from the deep browns in the
+absence of only a single factor, while, on the other hand, the difference
+between a blue and a green may be a difference dependent upon more than one
+factor. To what extent eye-colour may be valuable as a criterion of race it
+is at present impossible to say, but if it is ever to become so, it will
+only be after a searching Mendelian analysis has disclosed the factors upon
+which the numerous varieties depend.
+
+A discussion of eye-colour suggests reflections of another kind. It is
+difficult to believe that the markedly different states of pigmentation
+which occur in the same species are not associated with deep-seated
+chemical differences influencing the character and bent of the individual.
+{179} May not these differences in pigmentation be coupled with and so
+become in some measure a guide to mental and temperamental characteristics?
+In the National Portrait Gallery in London the pictures of celebrated men
+and women are largely grouped according to the vocations in which they have
+succeeded. The observant will probably have noticed that there is a
+tendency for a given type of eye-colour to predominate in some of the
+larger groups. It is rare to find anything but a blue among the soldiers
+and sailors, while among the actors, preachers, and orators the dark eye is
+predominant, although for the population as a whole it is far scarcer than
+the light. The facts are suggestive, and it is not impossible that future
+research may reveal an intimate connection between peculiarities of
+pigmentation and peculiarities of mind.
+
+The inheritance of mental characters is often elusive, for it is frequently
+difficult to appraise the effects of early environment in determining a
+man's bent. That ability can be transmitted there is no doubt, for this is
+borne out by general experience, as well as by the numerous cases of able
+families brought together by Galton and others. But when we come to inquire
+more precisely what it is that is transmitted we are baffled. A
+distinguished son follows in the footsteps of a distinguished father. Is
+this due to the inheritance of a particular mental aptitude, or is it an
+instance of general mental ability displayed in a field rendered attractive
+by early association? We have {180} at present very little definite
+evidence for supposing that what appear to be special forms of ability may
+be due to specific factors. Hurst, indeed, has brought forward some facts
+which suggest that musical sense sometimes behaves as a recessive
+character, and it is likely that the study of some clean-cut faculty such
+as the mathematical one would yield interesting results.
+
+The analysis of mental characters will no doubt be very difficult, and
+possibly the best line of attack is to search for cases where they are
+associated with some physical feature such as pigmentation. If an
+association of this kind be found, and the pigmentation factors be
+determined, it is evident that we should thereby obtain an insight into the
+nature of the units upon which mental conditions depend. Nor must it be
+forgotten that mental qualities, such as quickness, generosity,
+instability, etc.,--qualities which we are accustomed to regard as
+convenient units in classifying the different minds with which we are daily
+brought into contact,--are not necessarily qualities that correspond to
+heritable units. Effective mental ability is largely a matter of
+temperament, and this in turn is quite possibly dependent upon the various
+secretions produced by the different tissues of the body. Similar nervous
+systems associated with different livers might conceivably result in
+individuals upon whose mental ability the world would pass a very different
+judgment. Indeed, it is not at all impossible {181} that a particular form
+of mental ability may depend for its manifestation, not so much upon an
+essential difference in the structure of the nervous system, as upon the
+production by another tissue of some specific poison which causes the
+nervous system to react in a definite way. We have mentioned these
+possibilities merely to indicate how complex the problem may turn out to
+be. Though there is no doubt that mental ability is inherited, what it is
+that is transmitted, whether factors involving the quality and structure of
+the nervous system itself, or factors involving the production of specific
+poisons by other tissues, or both together, is at present uncertain.
+
+Little as is known to-day of heredity in man, that little is of
+extraordinary significance. The qualities of men and women, physical and
+mental, depend primarily upon the inherent properties of the gametes which
+went to their making. Within limits these qualities are elastic, and can be
+modified to a greater or lesser extent by influences brought to bear upon
+the growing zygote, provided always that the necessary basis is present
+upon which these influences can work. If the mathematical faculty has been
+carried in by the gamete, the education of the zygote will enable him to
+make the most of it. But if the basis is not there, no amount of education
+can transform that zygote into a mathematician. This is a matter of common
+experience. Neither is there any reason for supposing that the superior
+education of a {182} mathematical zygote will thereby increase the
+mathematical propensities of the gametes which live within him. For the
+gamete recks little of quaternions. It is true that there is progress of a
+kind in the world, and that this progress is largely due to improvements in
+education and hygiene. The people of to-day are better fitted to cope with
+their material surroundings than were the people of even a few thousand
+years ago. And as time goes on they are able more and more to control the
+workings of the world around them. But there is no reason for supposing
+that this is because the effects of education are inherited. Man stores
+knowledge as a bee stores honey or a squirrel stores nuts. With man,
+however, the hoard is of a more lasting nature. Each generation in using it
+sifts, adds, and rejects, and passes it on to the next a little better and
+a little fuller. When we speak of progress we generally mean that the hoard
+has been improved, and is of more service to man in his attempts to control
+his surroundings. Sometimes this hoarded knowledge is spoken of as the
+inheritance which a generation receives from those who have gone before.
+This is misleading. The handing on of such knowledge has nothing more to do
+with heredity in the biological sense than has the handing on from parent
+to offspring of a picture, or a title, or a pair of boots. All these things
+are but the transfer from zygote to zygote of something extrinsic to the
+species. Heredity, on the other hand, deals with the {183} transmission of
+something intrinsic from gamete to zygote and from zygote to gamete. It is
+the participation of the gamete in the process that is our criterion of
+what is and what is not heredity.
+
+Better hygiene and better education, then, are good for the zygote, because
+they help him to make the fullest use of his inherent qualities. But the
+qualities themselves remain unchanged in so far as the gamete is concerned,
+since the gamete pays no heed to the intellectual development of the zygote
+in whom he happens to dwell. Nevertheless, upon the gamete depend those
+inherent faculties which enable the zygote to profit by his opportunities,
+and, unless the zygote has received them from the gamete, the advantages of
+education are of little worth. If we are bent upon producing a permanent
+betterment that shall be independent of external circumstances, if we wish
+the national stock to become inherently more vigorous in mind and body,
+more free from congenital physical defect and feeble mentality, better able
+to assimilate and act upon the stores of knowledge which have been
+accumulated through the centuries, then it is the gamete that we must
+consult. The saving grace is with the gamete, and with the gamete alone.
+
+People generally look upon the human species as having two kinds of
+individuals, males and females, and it is for them that the sociologists
+and legislators frame their schemes. This, however, is but an imperfect
+view to {184} take of ourselves. In reality we are of four kinds, male
+zygotes and female zygotes, large gametes and small gametes, and heredity
+is the link that binds us together. If our lives were like those of the
+starfish or the sea-urchin, we should probably have realised this sooner.
+For the gametes of these animals live freely, and contract their marriages
+in the waters of the sea. With us it is different, because half of us must
+live within the other half or perish. Parasites upon the rest, levying a
+daily toll of nutriment upon their hosts, they are yet in some measure the
+arbiters of the destiny of those within whom they dwell. At the moment of
+union of two gametes is decided the character of another zygote, as well as
+the nature of the population of gametes which must make its home within
+him. The union once affected the inevitable sequence takes its course, and
+whether it be good, or whether it be evil, we, the zygotes, have no longer
+power to alter it. We are in the hands of the gamete; yet not entirely. For
+though we cannot influence their behaviour we can nevertheless control
+their unions if we choose to do so. By regulating their marriages, by
+encouraging the desirable to come together, and by keeping the undesirable
+apart we could go far towards ridding the world of the squalor and the
+misery that come through disease and weakness and vice. But before we can
+be prepared to act, except, perhaps, in the simplest cases, we must learn
+far more about them. At present we are woefully ignorant {185} of much,
+though we do know that full knowledge is largely a matter of time and
+means. One day we shall have it, and the day may be nearer than most
+suspect. Whether we make use of it will depend in great measure upon
+whether we are prepared to recognise facts, and to modify or even destroy
+some of the conventions which we have become accustomed to regard as the
+foundations of our social life. Whatever be the outcome, there can be
+little doubt that the future of our civilisation, perhaps even the
+possibility of a future at all, is wrapped up with the recognition we
+accord to those who live unseen and inarticulate within us--the fateful
+race of gametes so irrevocably bound to us by that closest of all ties,
+heredity.
+
+       *       *       *       *       * {187}
+
+
+APPENDIX
+
+As some readers may possibly care to repeat Mendel's experiments for
+themselves, a few words on the methods used in crossing may not be
+superfluous. The flower of the pea with its standard, wings, and median
+keel is too familiar to need description. Like most flowers it is
+hermaphrodite. Both male and female organs occur on the same flower, and
+are covered by the keel. The anthers, ten in number, are arranged in a
+circle round the pistil. As soon as they are ripe they burst and shed their
+pollen on the style. The pollen tubes then penetrate the stigma, pass down
+the style, and eventually reach the ovules in the lower part of the pistil.
+Fertilisation occurs here. Each ovule, which is reached by a pollen tube,
+swells up and becomes a seed. At the same time the fused carpels enclosing
+the ovules enlarge to form the pod. When this, the normal mode of
+fertilisation, takes place, the flower is said to be SELFED.
+
+In crossing, it is necessary to emasculate a flower on the plant chosen to
+be the female parent. For this purpose a young flower must be taken in
+which the anthers have not yet burst. The keel is depressed, and the
+stamens bearing the anthers are removed at their base by a {188} pair of
+fine forceps. It will probably be found necessary to tear the keel slightly
+in order to do this. The pistil is then covered up again with the keel, and
+the flower is enclosed in a bag of waxed paper until the following day. The
+stigma is then again exposed and dusted with ripe pollen from a flower of
+the plant selected as the male parent. This done, the keel is replaced, and
+the flower again enclosed in its bag to protect it from the possible
+attentions of insects until it has set seed. The bag may be removed in
+about a week after fertilisation. It is perhaps hardly necessary to add
+that strict biological cleanliness must be exercised during the fertilising
+operations. This is readily attained by sterilising fingers and forceps
+with a little strong spirit before each operation, thereby ensuring the
+death of any foreign pollen grains which may be present.
+
+The above method applies also to sweet peas, with these slight
+modifications. As the anthers ripen relatively sooner in this species,
+emasculation must be performed at a rather earlier stage. It is generally
+safe to choose a bud about three parts grown. The interval between
+emasculation and fertilisation must be rather longer. Two to three days is
+generally sufficient. Further, the sweet pea is visited by the leaf-cutter
+bee, _Megachile_, which, unlike the honey bee, is able to depress the keel
+and gather pollen. If the presence of this insect is suspected, it is
+desirable to guard against the risk of admixture of {189} foreign pollen by
+selecting for pollinating purposes a flower which has not quite opened. If
+the standard is not erected, it is unlikely to have been visited by
+_Megachile_. Lastly, it not infrequently happens that the little beetle
+_Meligethes_ is found inside the keel. Such flowers should be rejected for
+crossing purposes.
+
+       *       *       *       *       *
+
+
+{191}
+
+INDEX
+
+  _Abraxas grossulariata_, 99
+  "Acquired" characters, 14
+  Adaptation, 143
+  Agouti mice, 50
+  Albino mice, 50
+  Albinos, nature of, 53
+  _Amauris_, 144
+  Analysis of types, 156
+  Ancestral Heredity, Law of, 13
+  Andalusian fowls, 70
+  Axil colour in sweet peas, 93
+
+  Bateson, W., 14, 29, 55, 116, 132, 141
+  Biffen, R. H., 157
+  Blue Andalusian fowls, 71
+  Brachydactyly, 171
+  Bryony, 120
+  Bush sweet peas, 63
+
+  Castle, 132
+  Cattle, horns in, 86, 166
+  Colour, nature of, in flowers, 48
+  Colour-blindness, 117
+  Combs of fowls, 33, 43
+  Correns, C., 29, 120
+  Coupling of characters in gametes, 93
+  Cuenot, 50, 119
+  "Cupid" sweet peas, 62
+  Currant moth, 99
+
+  Darwin, C., 10, 65, 147, 163
+  De Vries, H., 15, 29, 141
+  Discontinuity in variation, 14
+  Dominant characters, 18
+  Doncaster, L., 99
+  Drinkwater, H., 172
+  Dutch rabbits, 60
+
+  Eggs, 2
+  Environment, influence of, 137
+  _Euralia_, 144
+  Evolution, 10, 85, 139
+  Eye, in primulas, 55
+  Eye-colour, in man, 176
+
+  Factor, definition of, 31
+  Factors, interaction of, 42
+  Fertilisation, 3
+  Fertilisation, self- and cross-, 163
+  Fixation of varieties, 153
+  Fluctuations, 138
+  Fowls, coloured from whites, 49, 73
+
+  Galton, 13, 179
+  Gametes, nature of, 6
+  Gregory, R. P., 55, 93
+
+  Haemophilia, 176
+  Hardy, G. H., 147
+  Heterozygote, definition of, 28
+  Heterozygote, of intermediate form, 68
+  _Hieracium_, 27, 132
+  Himalayan rabbits, 60
+  Homostyle primulas, 56
+  Homozygote, definition of, 28
+  Hooded sweet peas, 89
+  Horses, bay and chestnut in, 167
+  Hurst, C. C., 62, 176, 180
+
+  Immunity in wheat, 158
+  Individuality, 135
+  Inhibition, factors for, 74, 108
+  Intermediates, 125
+  {192}
+
+  Johannsen, W., 160
+
+  Lop-eared rabbits, 132
+
+  Mendel, 8, 17, 26, 132
+  Mental characters, 180
+  Mice, inheritance of coat colour in, 50
+  Mimicry, 143
+  _Mirabilis_, 151
+  Morgan, T. H., 116
+  Mulattos, 129
+  Mutation, 83, 138
+
+  Naegeli, C., 26
+  Natural selection, 11, 140, 142, 149
+  Nettleship, E., 175
+  Night-blindness, 175
+
+  _Pararge egeria_, 132
+  Parkinson, J., 122
+  Pea comb, 33
+  Peas, coloured flowers in, 24
+  Peas, tall and dwarf, 18
+  Pigeons, 86
+  Pin-eye in primulas, 55
+  _Pisum_, 17
+  Primulas, 31, 55, 68, 93
+  Pollen, 3
+  Pollen of sweet peas, 92
+  Pomace fly, 115
+  Population, inheritance of characters in a, 147
+  Presence and Absence theory, 35
+  Pure lines, 162
+  Purity of gametes, 24
+  Purity of type, 155
+
+  Rabbits, 53, 60
+  Ratios, Mendelian--
+    3 : 1, 20
+    9 : 3 : 3 : 1, 25, 34
+    9 : 3 : 4, 51
+    9 : 7, 49
+  Ray, John, 143
+  Recessive characters, 19
+  Repulsion between factors, 90
+  Reversion, 59, 165
+    in rabbits, 59
+    in sweet peas, 62
+    in fowls, 65
+    in pigeons, 65
+  Rose comb, 33
+
+  Saunders, E. R., 54, 122
+  Seeds, nature of, 4
+  Segregation, 22
+  Selection, 162
+  Sheep, horns in, 76
+  Silky fowls, 30, 105
+  Single comb, 32
+  Species, nature of, 150
+  Species, origin of, 11
+  Speckled wood butterfly, 132
+  Spermatozoa, 3
+  Sports, 147
+  Staples-Browne, R., 66
+  Sterility, 151
+  Sterility in sweet peas, 93
+  Stocks, double, 122
+  Stocks, hoariness in, 54
+  Sweet pea, colour in, 44, 79
+    history of, 82
+    inheritance of hood in, 89
+    inheritance of size in, 62
+
+  Telegony, 167
+  Thrum-eye in primulas, 55
+  Toe, extra toe in poultry, 76
+  Tschermak, E., 29
+
+  Unit-character, definition of, 31
+
+  Variation, 14, 137, 139
+
+  Walnut comb, 33
+  Weismann, A., 13
+  Wheat, beard in, 74
+    experiments with, 157
+  White, dominant in poultry, 72
+  Wilson, J., 168
+
+  Yellow mice, 119
+
+  Zygotes, nature of, 5
+
+       *       *       *       *       *
+
+
+Notes
+
+       *       *       *       *       *
+
+[1] Cf. note on p. 171.
+
+[2] It has been found convenient to denote the various generations
+resulting from a cross by the signs F_1, F_2, F_3, etc. F_1 on this system
+denotes the first filial generation, F_2 the second filial generation
+produced by two parents belonging to the F_1 generation, and so on.
+
+[3] Hurst's original cross was between a Belgian hare and an albina Angora,
+which turned out to be a masked Dutch.
+
+[4] The Spot is an almost white bird, the colour being confined to the tail
+and the characteristic spot on the head.
+
+[5] The reader who searches florists' catalogues for these varieties will
+probably experience disappointment. The sweet pea has been much "improved"
+in the past few years, and it is unlikely that the modern seedsman would
+list such unfashionable forms.
+
+[6] It is to be understood that wherever a given factor is present the
+plant may be homozygous or heterozygous for it without alteration in its
+colour.
+
+[7] It should be mentioned that as the shape of the pollen coat, like that
+of the seed coat, is a maternal character, all the grains of any given
+plant are either long or else round. The two kinds do not occur together on
+the same plant.
+
+[8] For the most recent discussion of this peculiar case the reader is
+referred to Professor Castle's paper in _Science_, December 16, 1910.
+
+[9] _Paradisus Terrestris_, London, 1629, p. 261.
+
+
+
+
+       *       *       *       *       *
+
+
+
+
+Transcriber's note:
+
+Corrections made to printed original.
+
+   Page 36, "two sorts, RP and Rp": 'PR and Rp' in original.
+
+   Page 51, "9 contain both C and G": 'c and G' in original.
+
+   Page 184, "in the simplest cases": 'simples' in original.
+
+   Footnote 3, "turned out to be a masked Dutch": 'turned
+   to out be' in original.
+
+
+
+***END OF THE PROJECT GUTENBERG EBOOK MENDELISM***
+
+
+******* This file should be named 28775.txt or 28775.zip *******
+
+
+This and all associated files of various formats will be found in:
+http://www.gutenberg.org/dirs/2/8/7/7/28775
+
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+http://www.gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere 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
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need are critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at http://www.gutenberg.org/fundraising/pglaf.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at http://www.gutenberg.org/about/contact
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit http://www.gutenberg.org/fundraising/donate
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including checks, online payments and credit card donations.
+To donate, please visit:
+http://www.gutenberg.org/fundraising/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart is the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+Most people start at our Web site which has the main PG search facility:
+
+     http://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
+