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+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-8859-1
+
+
+***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:
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+      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_ × _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 × Brown Leghorn
+        Cock                                                      105
+
+  20. Scheme of Inheritance of Brown Leghorn Hen × Silky
+        Cock                                                      106
+
+  21. Scheme of F_1 (ex Brown Leghorn × Silky Cock) crossed
+        with pure Brown Leghorn                                   107
+
+  22. Scheme for Silky Hen × Brown Leghorn Cock                   108
+
+  23. Scheme for Brown Leghorn Hen × Silky Cock                   109
+
+  24. Diagram illustrating Nature of Offspring from Brown Leghorn
+        Hen × 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 × 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 Hæmophilic 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 Brünn, 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
+Brünn_, 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, ½ was on the average
+derived from the two parents (_i.e._ ¼ from each parent), ¼ 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 1½ 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 × 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 Nägeli 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 × 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 × Pea
+                 |
+            +----+----+
+          Walnut × 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 × 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 × Breda; C, an F_1 cock from the
+cross of single × 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 × Breda
+                  |
+        +---------+---------+
+        |                   |
+     Duplex       ×      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 × 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 × Agouti
+               |
+       +---------------+
+     Agouti    ×    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 Cuénot 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   } × { 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 × Himalayan
+                    |
+             +-------------+
+          Agouti    ×    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 × 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 3½-4 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 × White Fantail   Black Barb × Spot[4]
+             |                            |
+           Dark              ×           Dark
+             Among the offspring one very similar
+                to the wild blue rock.
+
+
+
+                    Black   White
+                    Barb  ×  Fantail
+                          |
+            +------------------------+
+          Black           ×        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 × 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  ×  Blue
+                      |
+    +-------------+--------+------------+
+  Black         Blue  ×  Blue         White
+    |                 |                 |
+    |       +------+--+--+-----+        |
+  Black   Black  Blue  Blue  White    White
+    |                                   |
+  Black ------------- × ------------- 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   ×   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 × 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 × 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
+× 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 × _grossulariata_
+male gives _grossulariata_ alone of both sexes. But _grossulariata_ female
+× _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 ×
+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
+× 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 × Silky Cock; 3, Silky Cock;
+4, Hen ex Silky Hen × 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 × 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 Cuénot 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] × dioica [male] gave [female] [female] and [male] [male]
+    "             × alba   [male]  "   [female] [female] only
+  alba   [female] × 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] × _dioica_ [male] gives
+the same result as _dioica_ [female] × _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    × Ovule    Pollen × 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 hæmophilic 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 hæmophilia 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 hæmophilia 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 hæmophilic 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 hæmophilia 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
+  Cuénot, 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
+
+  Hæmophilia, 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
+
+  Nägeli, 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.
+
+
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+<h1>The Project Gutenberg eBook, Mendelism, by Reginald Crundall Punnett</h1>
+<pre>
+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 <a href = "http://www.gutenberg.org">www.gutenberg.org</a></pre>
+<p>Title: Mendelism</p>
+<p>       Third Edition</p>
+<p>Author: Reginald Crundall Punnett</p>
+<p>Release Date: May 18, 2009  [eBook #28775]</p>
+<p>Language: English</p>
+<p>Character set encoding: ISO-8859-1</p>
+<p>***START OF THE PROJECT GUTENBERG EBOOK MENDELISM***</p>
+<p>&nbsp;</p>
+<h3>E-text prepared by Paul Hollander, Malcolm Farmer, Keith Edkins,<br />
+    and the Project Gutenberg Online Distributed Proofreading Team<br />
+    (http://www.pgdp.net)</h3>
+<p>&nbsp;</p>
+<table border="0" cellpadding="10" style="background-color: #ccccff;">
+<tr>
+<td style="width:25%; vertical-align:top">
+Transcriber's note:
+</td>
+<td>
+
+  <p>A few typographical errors have been corrected. They appear in the
+  text <span class="correction" title="explanation will pop up">like
+  this</span>, and the explanation will appear when the mouse pointer is
+  moved over the marked passage.</p>
+
+  <p>Fig. 8 has been re-mastered to match the text (the Black boxes were
+  shown as Albino and the heterozygous Albinos as Black).</p>
+
+</td>
+</tr>
+</table>
+<p>&nbsp;</p>
+<hr class="pg" />
+<p>&nbsp;</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/001.jpg"><img style="width:100%" src="images/001t.jpg"
+      alt="Gregor Mendel." title="Gregor Mendel." /></a>
+  </div>
+<h1>MENDELISM</h1>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead">BY</p>
+
+<h2>R. C. PUNNETT</h2>
+
+<p class="cenhead">FELLOW OF GONVILLE AND CAIUS COLLEGE</p>
+
+<p class="cenhead">PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF CAMBRIDGE</p>
+
+  <p>&nbsp;</p>
+
+<h3><i>THIRD EDITION<br />
+ENTIRELY REWRITTEN AND MUCH ENLARGED</i></h3>
+
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+
+<h3>New York</h3>
+
+<h3>THE MACMILLAN COMPANY</h3>
+
+<h3>1911</h3>
+
+<p class="cenhead"><i>All rights reserved</i></p>
+
+<hr class="short" />
+
+<p class="cenhead"><span class="sc">Copyright, 1911,</span></p>
+
+<p class="cenhead"><span class="sc">By</span> THE MACMILLAN COMPANY.</p>
+
+<hr class="short" />
+
+<p class="cenhead">Set up and electrotyped. Published May, 1911.</p>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead">Norwood Press<br />
+J. S. Cushing Co.&mdash;Berwick &amp; Smith Co.<br />
+Norwood, Mass., U.S.A.</p>
+
+<hr class="full" />
+
+<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p>
+
+<h3>PREFACE</h3>
+
+  <p>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.</p>
+
+  <p>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 <i>Mendel's <!-- Page vi --><span
+  class="pagenum"><a name="pagevi"></a>{vi}</span>Principles of
+  Heredity</i> (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.</p>
+
+  <p>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.</p>
+
+  <p>It is not long since the English language was enriched by two new
+  words&mdash;Eugenics and Genetics&mdash;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
+  <!-- Page vii --><span class="pagenum"><a
+  name="pagevii"></a>{vii}</span>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.</p>
+
+  <p class="author">R. C. P.</p>
+
+  <p class="address"><span class="sc">Cambridge</span>, <i>March, 1911</i>.</p>
+
+<hr class="full" />
+
+<p><!-- Page ix --><span class="pagenum"><a name="pageix"></a>{ix}</span></p>
+
+<h3>CONTENTS</h3>
+
+<table class="nobctr" summary="Contents." title="Contents.">
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER I</td></tr>
+<tr><td class="spacsingle"> </td><td class="spacsingle" align="right"> <span class="scac">PAGE</span></td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">The Problem</span> </td><td class="spacsingle" align="right"> <a href="#page1">1</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER II</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Historical</span> </td><td class="spacsingle" align="right"> <a href="#page8">8</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER III</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Mendel's Work</span> </td><td class="spacsingle" align="right"> <a href="#page17">17</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER IV</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">The Presence and Absence Theory</span> </td><td class="spacsingle" align="right"> <a href="#page29">29</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER V</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Interaction of Factors</span> </td><td class="spacsingle" align="right"> <a href="#page42">42</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER VI</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Reversion</span> </td><td class="spacsingle" align="right"> <a href="#page59">59</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER VII</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Dominance</span> </td><td class="spacsingle" align="right"> <a href="#page68">68</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;">
+<!-- Page x --><span class="pagenum"><a name="pagex"></a>{x}</span>
+CHAPTER VIII</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Wild Forms and Domestic Varieties</span> </td><td class="spacsingle" align="right"> <a href="#page79">79</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER IX</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Repulsion and Coupling of Factors</span> </td><td class="spacsingle" align="right"> <a href="#page88">88</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER X</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Sex</span> </td><td class="spacsingle" align="right"> <a href="#page99">99</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XI</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Sex</span> (<i>continued</i>) </td><td class="spacsingle" align="right"> <a href="#page115">115</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XII</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Intermediates</span> </td><td class="spacsingle" align="right"> <a href="#page125">125</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XIII</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Variation and Evolution</span> </td><td class="spacsingle" align="right"> <a href="#page135">135</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XIV</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Economical</span> </td><td class="spacsingle" align="right"> <a href="#page153">153</a></td></tr>
+
+<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XV</td></tr>
+
+<tr><td class="spacsingle"> <span class="sc">Man</span> </td><td class="spacsingle" align="right"> <a href="#page170">170</a></td></tr>
+<tr><td class="spacsingle"> &nbsp;</td></tr>
+<tr><td class="spacsingle"> APPENDIX </td><td class="spacsingle" align="right"> <a href="#page187">187</a></td></tr>
+<tr><td class="spacsingle"> &nbsp;</td></tr>
+<tr><td class="spacsingle"> INDEX </td><td class="spacsingle" align="right"> <a href="#page191">191</a></td></tr>
+</table>
+
+  <p>&nbsp;</p>
+
+<hr class="full" />
+
+<p><!-- Page xi --><span class="pagenum"><a name="pagexi"></a>{xi}</span></p>
+
+<h3>ILLUSTRATIONS</h3>
+
+<table class="nobctr" summary="Illustrations." title="Illustrations.">
+<tr><td colspan="4" align="center">PLATES</td></tr>
+
+<tr><td class="qspcsingle" align="right"> <span class="scac">PLATE</span> </td><td colspan="2"> </td><td class="nspac" align="right"> <span class="scac">PAGE</span></td></tr>
+
+<tr><td class="qspcsingle" align="right"> </td><td class="qspcsingle"> Gregor Mendel </td><td class="nspac" colspan="2" align="right"><i>Frontispiece</i></td></tr>
+
+<tr><td class="qspcsingle" align="right"> I. </td><td class="qspcsingle"> Rabbits&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </td><td class="nspac" align="center"> <i>To face</i> </td><td class="nspac" align="right"> <a href="#page60">60</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> II. </td><td class="qspcsingle"> Sweet Peas </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page64">64</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> III. </td><td class="qspcsingle"> Sheep </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page78">78</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> IV. </td><td class="qspcsingle"> Sweet Peas </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page80">80</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> V. </td><td class="qspcsingle"> Fowls </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page107">107</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> VI. </td><td class="qspcsingle"> Butterflies </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page146">146</a></td></tr>
+
+<tr><td colspan="4" align="center">&nbsp;<br />FIGURES IN TEXT</td></tr>
+
+<tr><td class="qspcsingle" align="right"> <span class="scac">FIG.</span></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 1. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance in simple Mendelian Case </td><td class="nspac" align="right"> <a href="#page21">21</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 2. </td><td class="qspcsingle" colspan="2"> Feathers of Silky and Common Fowl </td><td class="nspac" align="right"> <a href="#page30">30</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 3. </td><td class="qspcsingle" colspan="2"> Single and Double Primulas </td><td class="nspac" align="right"> <a href="#page31">31</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 4. </td><td class="qspcsingle" colspan="2"> Fowls' Combs </td><td class="nspac" align="right"> <a href="#page32">32</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 5. </td><td class="qspcsingle" colspan="2"> Diagram of Inheritance of Fowls' Combs </td><td class="nspac" align="right"> <a href="#page37">37</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 6. </td><td class="qspcsingle" colspan="2"> Fowls' Combs </td><td class="nspac" align="right"> <a href="#page39">39</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 7. </td><td class="qspcsingle" colspan="2"> Diagram of F<sub>2</sub> Generation resulting from Cross between two White Sweet Peas </td><td class="nspac" align="right"> <a href="#page46">46</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 8. </td><td class="qspcsingle" colspan="2"> Diagram illustrating 9&nbsp;:&nbsp;3&nbsp;:&nbsp;4 Ratio in Mice </td><td class="nspac" align="right"> <a href="#page52">52</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 9. </td><td class="qspcsingle" colspan="2"> Sections of Primulas </td><td class="nspac" align="right"> <a href="#page55">55</a></td></tr>
+
+<tr><td class="qspcsingle" align="right">
+<!-- Page xii --><span class="pagenum"><a name="pagexii"></a>{xii}</span>
+10. </td><td class="qspcsingle" colspan="2"> Small and Large-eyed Primulas </td><td class="nspac" align="right"> <a href="#page55">55</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 11. </td><td class="qspcsingle" colspan="2"> Diagram illustrating Reversion in Pigeons </td><td class="nspac" align="right"> <a href="#page67">67</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 12. </td><td class="qspcsingle" colspan="2"> <i>Primula sinensis</i> × <i>Primula stellata</i> </td><td class="nspac" align="right"> <a href="#page68">68</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 13. </td><td class="qspcsingle" colspan="2"> Diagram illustrating Cross between Dominant and Recessive White Fowls </td><td class="nspac" align="right"> <a href="#page72">72</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 14. </td><td class="qspcsingle" colspan="2"> Bearded and Beardless Wheat </td><td class="nspac" align="right"> <a href="#page75">75</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 15. </td><td class="qspcsingle" colspan="2"> Feet of Fowls </td><td class="nspac" align="right"> <a href="#page76">76</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 16. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Horns in Sheep </td><td class="nspac" align="right"> <a href="#page76">76</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 17. </td><td class="qspcsingle" colspan="2"> <i>Abraxas grossulariata</i> and var. <i>lacticolor</i> </td><td class="nspac" align="right"> <a href="#page99">99</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 18. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance in <i>Abraxas</i> </td><td class="nspac" align="right"> <a href="#page102">102</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 19. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Silky Hen × Brown Leghorn Cock </td><td class="nspac" align="right"> <a href="#page105">105</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 20. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Brown Leghorn Hen × Silky Cock </td><td class="nspac" align="right"> <a href="#page106">106</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 21. </td><td class="qspcsingle" colspan="2"> Scheme of F<sub>1</sub> (ex Brown Leghorn × Silky Cock) crossed with pure Brown Leghorn </td><td class="nspac" align="right"> <a href="#page107">107</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 22. </td><td class="qspcsingle" colspan="2"> Scheme for Silky Hen × Brown Leghorn Cock </td><td class="nspac" align="right"> <a href="#page108">108</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 23. </td><td class="qspcsingle" colspan="2"> Scheme for Brown Leghorn Hen × Silky Cock </td><td class="nspac" align="right"> <a href="#page109">109</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 24. </td><td class="qspcsingle" colspan="2"> Diagram illustrating Nature of Offspring from Brown Leghorn Hen × F<sub>1</sub> Cock </td><td class="nspac" align="right"> <a href="#page111">111</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 25. </td><td class="qspcsingle" colspan="2"> Scheme to illustrate Heterozygous Nature of Brown Leghorn Hen </td><td class="nspac" align="right"> <a href="#page111">111</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 26. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Colour-blindness </td><td class="nspac" align="right"> <a href="#page117">117</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 27. </td><td class="qspcsingle" colspan="2"> Single and Double Stocks </td><td class="nspac" align="right"> <a href="#page122">122</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 28. </td><td class="qspcsingle" colspan="2"> F<sub>2</sub> Generation ex Silky Hen × Brown Leghorn Cock </td><td class="nspac" align="right"> <a href="#page127">127</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 29. </td><td class="qspcsingle" colspan="2"> Pedigree of Eurasian Family </td><td class="nspac" align="right"> <a href="#page131">131</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 30. </td><td class="qspcsingle" colspan="2"> Curve illustrating Influence of Selection </td><td class="nspac" align="right"> <a href="#page159">159</a></td></tr>
+
+<tr><td class="qspcsingle" align="right">
+<!-- Page xiii --><span class="pagenum"><a name="pagexiii"></a>{xiii}</span>
+31. </td><td class="qspcsingle" colspan="2"> Curve illustrating Conception of pure Lines </td><td class="nspac" align="right"> <a href="#page162">162</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 32. </td><td class="qspcsingle" colspan="2"> Brachydactylous and Normal Hands </td><td class="nspac" align="right"> <a href="#page170">170</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 33. </td><td class="qspcsingle" colspan="2"> Radiograph of Brachydactylous Hand </td><td class="nspac" align="right"> <a href="#page170">170</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 34. </td><td class="qspcsingle" colspan="2"> Pedigree of Brachydactylous Family </td><td class="nspac" align="right"> <a href="#page173">173</a></td></tr>
+
+<tr><td class="qspcsingle" align="right"> 35. </td><td class="qspcsingle" colspan="2"> Pedigree of Hæmophilic Family </td><td class="nspac" align="right"> <a href="#page175">175</a></td></tr>
+</table>
+
+  <p>&nbsp;</p>
+
+<hr class="full" />
+
+<p><!-- Page xiv --><span class="pagenum"><a name="pagexiv"></a>{xiv}</span></p>
+
+<blockquote class="b1n">
+
+  <p>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 <i>Natural
+  Philosophy</i>.</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p><span class="sc">William Harvey</span>,</p>
+      <p><i>Anatomical Exercitations</i>, 1653.</p>
+    </div>
+  </div>
+
+</blockquote>
+
+<hr class="full" />
+
+<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p>
+
+<h3>CHAPTER I</h3>
+
+<p class="cenhead">THE PROBLEM</p>
+
+  <p>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 <!-- Page 2 --><span class="pagenum"><a
+  name="page2"></a>{2}</span>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.</p>
+
+  <p>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 <!-- Page 3 --><span class="pagenum"><a
+  name="page3"></a>{3}</span>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.</p>
+
+  <p>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.</p>
+
+  <p>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 <!-- Page 4 --><span class="pagenum"><a
+  name="page4"></a>{4}</span>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&mdash;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&mdash;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 <!-- Page 5 --><span
+  class="pagenum"><a name="page5"></a>{5}</span>plant emerge, become
+  mature, and themselves ripen germ cells which give rise to a new
+  generation.</p>
+
+  <p>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 <b>gametes</b>, or marrying cells, and the individual
+  formed by the fusion or yoking together of two gametes is spoken of as a
+  <b>zygote</b>. 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.</p>
+
+  <p>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 <!-- Page 6 --><span
+  class="pagenum"><a name="page6"></a>{6}</span>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?&mdash;these are questions which serve to indicate the nature of
+  the problem underlying the process of heredity.</p>
+
+  <p>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 <!-- Page 7
+  --><span class="pagenum"><a name="page7"></a>{7}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 8 --><span class="pagenum"><a name="page8"></a>{8}</span></p>
+
+<h3>CHAPTER II</h3>
+
+<p class="cenhead">HISTORICAL</p>
+
+  <p>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 Brünn, 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 <i>Proceedings of
+  the Natural History Society of Brünn</i>, 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 <!-- Page 9
+  --><span class="pagenum"><a name="page9"></a>{9}</span>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 <!-- Page 10 --><span
+  class="pagenum"><a name="page10"></a>{10}</span>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 <i>Origin of Species</i>,
+  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 <!-- Page 11 --><span class="pagenum"><a
+  name="page11"></a>{11}</span>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.</p>
+
+  <p>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 <!-- Page 12 --><span class="pagenum"><a
+  name="page12"></a>{12}</span>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 <i>Origin of Species</i> 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. <!-- Page 13 --><span
+  class="pagenum"><a name="page13"></a>{13}</span></p>
+
+  <p>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, ½ was on the average
+  derived from the two parents (<i>i.e.</i> ¼ from each parent), ¼ from the
+  four grandparents, &#x215B; from the eight great-grandparents, and so on.
+  <i>The Law of Ancestral Heredity</i>, 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.</p>
+
+  <p>While Galton was working in England the German zoologist August
+  Weismann was elaborating the complicated <!-- Page 14 --><span
+  class="pagenum"><a name="page14"></a>{14}</span>theory of heredity which
+  eventually appeared in his work on <i>The Germplasm</i> (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.</p>
+
+  <p>A further important step was taken in 1895, when Bateson once more
+  drew attention to the problem of the origin <!-- Page 15 --><span
+  class="pagenum"><a name="page15"></a>{15}</span>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.</p>
+
+  <p>A few years later appeared the first volume of de Vries' remarkable
+  book on <i>The Mutation Theory</i>. From a prolonged study of the evening
+  primrose (<i>Oenothera</i>) 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 <!-- Page
+  16 --><span class="pagenum"><a name="page16"></a>{16}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 17 --><span class="pagenum"><a name="page17"></a>{17}</span></p>
+
+<h3>CHAPTER III</h3>
+
+<p class="cenhead">MENDEL'S WORK</p>
+
+  <p>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 (<i>Pisum sativum</i>) he
+  found the plant he sought. A hardy annual, prolific, easily worked,
+  <i>Pisum</i> 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&mdash;such are a few of the characters
+  in which the various races of peas differ from one another. <!-- Page 18
+  --><span class="pagenum"><a name="page18"></a>{18}</span></p>
+
+  <p>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 1½
+  feet. Previous testing had shown that each strain bred true to its
+  peculiar height. These two strains were artificially crossed<a
+  name="footnotetag1" href="#footnote1"><sup>[1]</sup></a> 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 <b>dominant</b> and
+  <!-- Page 19 --><span class="pagenum"><a
+  name="page19"></a>{19}</span>dwarfness the <b>recessive</b> 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 <i>but no intermediates</i>. 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 <i>the seeds from each individual plant
+  were harvested separately and separately sown in the following year</i>.
+  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 <!-- Page 20 --><span
+  class="pagenum"><a name="page20"></a>{20}</span>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.</p>
+
+  <div class="figcenter" style="width:32%;">
+      <a href="images/031.png"><img style="width:100%" src="images/031.png"
+      alt="Generations of cross of tall and dwarf peas." title="Generations of cross of tall and dwarf peas." /></a>
+  </div>
+  <p>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&nbsp;:&nbsp;1 as T(D), the
+  result of these experiments may be briefly summarised in the foregoing
+  scheme.<a name="footnotetag2" href="#footnote2"><sup>[2]</sup></a></p>
+
+  <p>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&nbsp;:&nbsp;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 <!-- Page 21
+  --><span class="pagenum"><a name="page21"></a>{21}</span>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 <i>or</i> 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.</p>
+
+  <div class="figright" style="width:31%;">
+      <a href="images/033.png"><img style="width:100%" src="images/033.png"
+      alt="Fig. 1. Scheme of inheritance for cross of tall and dwarf peas." title="Fig. 1. Scheme of inheritance for cross of tall and dwarf peas." /></a>
+    <span class="sc">Fig. 1.</span>
+
+    <p class="poem">Scheme of inheritance in the cross of tall with dwarf
+    pea. Gametes represented by small and zygotes by larger circles.</p>
+  </div>
+
+  <p>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 <!-- Page 22
+  --><span class="pagenum"><a name="page22"></a>{22}</span>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
+  <b>segregate</b> 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<sub>1</sub> 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>i.e.</i> the nature of
+  the generation that should be produced when the hybrid is allowed to
+  fertilise itself. Let us suppose that there are 4<i>x</i> ovules so that
+  2<i>x</i> are "tall" and 2<i>x</i> 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
+  2<i>x</i> "tall" ovules, <i>x</i> will be fertilised by "tall" pollen
+  grains and <i>x</i> will be fertilised by "dwarf" pollen grains. The
+  former must give rise to tall <!-- Page 23 --><span class="pagenum"><a
+  name="page23"></a>{23}</span>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 2<i>x</i> dwarf ovules, again, has an equal
+  chance of being fertilised by a "tall" or by a "dwarf" pollen grain.
+  Hence <i>x</i> will give rise to tall plants carrying the recessive dwarf
+  character, while <i>x</i> will produce plants from which the tall
+  character has been eliminated, <i>i.e.</i> to pure recessive dwarfs.
+  Consequently from the 4<i>x</i> ovules of the self-fertilised hybrid we
+  ought to obtain 3<i>x</i> tall and <i>x</i> dwarf plants. And of the
+  3<i>x</i> talls <i>x</i> should breed true to tallness, while the
+  remaining 2<i>x</i>, 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&nbsp;:&nbsp;1. Now this is precisely
+  the result actually obtained by experiment (cf. p. <a
+  href="#page17">17</a>), 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.</p>
+
+  <p>It is possible to put the theory to a further test. The explanation of
+  the 3&nbsp;:&nbsp;1 ratio of dominants and recessives in the F<sub>2</sub>
+  generation is regarded as due to the F<sub>1</sub> individuals producing
+  equal numbers of gametes bearing the <!-- Page 24 --><span
+  class="pagenum"><a name="page24"></a>{24}</span>dominant and recessive
+  elements respectively. If now the F<sub>1</sub> 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&nbsp;:&nbsp;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.</p>
+
+  <p>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 <!-- Page 25 --><span
+  class="pagenum"><a name="page25"></a>{25}</span>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&nbsp;:&nbsp;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<sub>2</sub> 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&nbsp;:&nbsp;1, and at the same time the coloured
+  should be to the whites as 3&nbsp;:&nbsp;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<sub>1</sub>)
+  are all of the same form, exhibiting the dominant character of each of
+  the two pairs, while the F<sub>2</sub> generation produced by such
+  hybrids consists on the average of 9 showing both dominants, 3 showing
+  one dominant and one recessive, <!-- Page 26 --><span class="pagenum"><a
+  name="page26"></a>{26}</span>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 <i>A</i>, <i>B</i>, and
+  <i>C</i>, respectively dominant to <i>a</i>, <i>b</i>, and <i>c</i>, the
+  F<sub>1</sub> individuals will be all of the form <i>ABC</i>, while the
+  F<sub>2</sub> generation will consists of 27 <i>ABC</i>, 9 <i>ABc</i>, 9
+  <i>AbC</i>, 9 <i>aBC</i>, 3 <i>Abc</i>, 3 <i>aBc</i>, 3 <i>abC</i>, and 1
+  <i>abc</i>. 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<sub>2</sub> generation, <i>i.e.</i> 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.</p>
+
+  <p>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 Nägeli the botanist. To
+  the breeding and crossing of bees he also devoted much <!-- Page 27
+  --><span class="pagenum"><a name="page27"></a>{27}</span>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
+  (<i>Hieracium</i>), 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<sub>2</sub>
+  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. <a
+  href="#page135">135</a>), 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 <i>Hieracium</i>
+  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 <!-- Page 28 --><span
+  class="pagenum"><a name="page28"></a>{28}</span>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.</p>
+
+<blockquote class="b1n">
+
+  <p><i>Note.</i>&mdash;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.</p>
+
+</blockquote>
+
+<hr class="full" />
+
+<p><!-- Page 29 --><span class="pagenum"><a name="page29"></a>{29}</span></p>
+
+<h3>CHAPTER IV</h3>
+
+<p class="cenhead">THE PRESENCE AND ABSENCE THEORY</p>
+
+  <p>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 <!-- Page 30 --><span
+  class="pagenum"><a name="page30"></a>{30}</span>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.</p>
+
+  <div class="figcenter" style="width:35%;">
+      <a href="images/042.jpg"><img style="width:100%" src="images/042t.jpg"
+      alt="Fig. 2. Feathers of an ordinary and a silky fowl." title="Fig. 2. Feathers of an ordinary and a silky fowl." /></a>
+    <span class="sc">Fig. 2.</span>
+
+    <p class="poem">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.</p>
+  </div>
+
+  <p>As we have already seen, Mendel considered that in the gamete there
+  was either a definite something <!-- Page 31 --><span class="pagenum"><a
+  name="page31"></a>{31}</span>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
+  <b>factor</b>. The factor, then, is what corresponds in the gamete to the
+  <b>unit-character</b> 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.</p>
+
+  <div class="figcenter" style="width:50%;">
+      <a href="images/043.jpg"><img style="width:100%" src="images/043t.jpg"
+      alt="Fig. 3. Two double and an ordinary single primula flower." title="Fig. 3. Two double and an ordinary single primula flower." /></a>
+    <span class="sc">Fig. 3.</span>
+
+    <p class="cenhead">Two double and an ordinary single primula flower.
+    This form of double is recessive to the single.</p>
+  </div>
+
+<p><!-- Page 32 --><span class="pagenum"><a name="page32"></a>{32}</span></p>
+
+  <div class="figright" style="width:47%;">
+      <a href="images/044.png"><img style="width:100%" src="images/044.png"
+      alt="Fig. 4. Fowls' combs." title="Fig. 4. Fowls' combs." /></a>
+    <span class="sc">Fig. 4.</span>
+
+    <p class="cenhead">Fowls' combs. A, pea; B, rose; C, single; D,
+    walnut.</p>
+  </div>
+
+  <p>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, <!-- Page 33 --><span class="pagenum"><a
+  name="page33"></a>{33}</span>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&mdash;that either rose or
+  pea would dominate in the hybrids, and that the F<sub>2</sub> generation
+  would consist of dominants and recessives in the ratio 3&nbsp;:&nbsp;1. The result
+  of the experiment was, however, very different. The cross rose × pea led
+  to the production of a comb quite unlike either of them. This, the
+  so-called walnut comb (Fig. 4, D), <!-- Page 34 --><span
+  class="pagenum"><a name="page34"></a>{34}</span>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<sub>1</sub> birds were bred
+  together, a further unlooked-for result was obtained. As was expected,
+  there appeared in the F<sub>2</sub> 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&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;1. <span class="figleft" style="width:25%;"><a
+  href="images/046.png"><img style="width:100%" src="images/046.png"
+  alt="Generations of Rose × Pea cross." title="Generations of Rose × Pea cross."
+  /></a></span> Now this, as Mendel showed, is the ratio found in an
+  F<sub>2</sub> 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<sub>2</sub> from
+  a cross <!-- Page 35 --><span class="pagenum"><a
+  name="page35"></a>{35}</span>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 (<i>R</i>) and pea (<i>P</i>), 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 <!--
+  Page 36 --><span class="pagenum"><a name="page36"></a>{36}</span>that a
+  particular factor is absent in a gamete or zygote. Represented thus the
+  zygotic constitution of a pure rose-combed bird is <i>RRpp</i>; for it
+  has been formed by the union of two gametes both of which contained
+  <i>R</i> but not <i>P</i>. Similarly we may denote the pure pea-combed
+  bird as <i>rrPP</i>. On crossing the rose with the pea union occurs
+  between a gamete <i>Rp</i> and a gamete <i>rP</i>, resulting in the
+  formation of a heterozygote of the constitution <i>RrPp</i>. The use of
+  the small letters here informs us that such a zygote contains only a
+  single dose of each of the factors <i>R</i> and <i>P</i>, 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
+  <i>R</i> and the part which does not contain it (<i>r</i>). Half of its
+  gametes, therefore, will contain <i>R</i> and the other half will be
+  without it (<i>r</i>). Similarly half of its gametes will contain
+  <i>P</i> and the other half will be without it (<i>p</i>). It is obvious
+  that the chances of <i>R</i> being distributed to a gamete with or
+  without <i>P</i> are equal. Hence the gametes containing <i>R</i> will be
+  of two sorts, <span class="correction" title="Original reads `PR'."
+  ><i>RP</i></span> and <i>Rp</i>, and these will be produced in equal
+  numbers. Similarly the gametes without <i>R</i> will also be of two
+  sorts, <i>rP</i> and <i>rp</i>, 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 <i>RP</i>, <i>Rp</i>, <i>rP</i>, and <i>rp</i>; and the
+  breeding <!-- Page 37 --><span class="pagenum"><a
+  name="page37"></a>{37}</span>together of such F<sub>1</sub> 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.
+  <span class="figright" style="width:31%;"><a href="images/049.png"><img
+  style="width:100%" src="images/049.png" alt="Fig. 5. Combs in F2."
+  title="Fig. 5. Combs in F2." /></a><span class="sc">Fig.</span> 5. <br
+  />Diagram to illustrate the nature of the F<sub>2</sub> generation from
+  the cross of rose comb × pea comb.</span> Fig. 5 shows the result of
+  applying this method to our series <i>RP</i>, <i>Rp</i>, <i>rP</i>,
+  <i>rp</i>, and the 16 squares represent the different kinds of zygotes
+  formed and the proportions in which they occur. As <!-- Page 38 --><span
+  class="pagenum"><a name="page38"></a>{38}</span>the figure shows, 9
+  zygotes contain both <i>R</i> and <i>P</i>, 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 <i>R</i> but not <i>P</i>,
+  and these must be rose-combed birds. Three, again, contain <i>P</i> but
+  not <i>R</i> and must be pea-combed birds. Finally one out of the 16
+  contains neither <i>R</i> nor <i>P</i>. It cannot be rose&mdash;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 <i>RRPPSS</i>, a rose as <i>RRppSS</i>, a
+  pea as <i>rrPPSS</i>, and a single as <i>rrppSS</i>. 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 <i>R</i> and <i>P</i> are present,
+  and the single character can then become manifest. <!-- Page 39 --><span
+  class="pagenum"><a name="page39"></a>{39}</span></p>
+
+  <p>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.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/052.png"><img style="width:100%" src="images/052.png"
+      alt="Fig. 6. Fowls' combs, rose and Breda." title="Fig. 6. Fowls' combs, rose and Breda." /></a>
+    <span class="sc">Fig. 6.</span>
+
+    <p class="poem">Fowls' combs. A and B, F<sub>1</sub> hen from rose ×
+    Breda; C, an F<sub>1</sub> cock from the cross of single × Breda; D,
+    head of Breda cock.</p>
+  </div>
+
+  <p>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<sub>2</sub> 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. <!-- Page 40
+  --><span class="pagenum"><a name="page40"></a>{40}</span>On crossing
+  Breda with single the F<sub>1</sub> 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 <!-- Page 41 --><span class="pagenum"><a
+  name="page41"></a>{41}</span><span class="figright" style="width:36%;"><a
+  href="images/053.png"><img style="width:100%" src="images/053.png"
+  alt="Breda and rose." title="Breda and rose." /></a></span>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<sub>1</sub> 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 (<i>R</i>) 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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 42 --><span class="pagenum"><a name="page42"></a>{42}</span></p>
+
+<h3>CHAPTER V</h3>
+
+<p class="cenhead">INTERACTION OF FACTORS</p>
+
+  <p>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.</p>
+
+  <p>The case of the fowls' combs opens up the important question of the
+  extent to which the various factors can <!-- Page 43 --><span
+  class="pagenum"><a name="page43"></a>{43}</span>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 <!-- Page 44 --><span
+  class="pagenum"><a name="page44"></a>{44}</span>phenomena which result
+  from such interaction between separate and distinct factors.</p>
+
+  <div class="figright" style="width:22%;">
+      <a href="images/056.png"><img style="width:100%" src="images/056.png"
+      alt="Generation of cross of red and white sweet peas." title="Generation of cross of red and white sweet peas." /></a>
+  </div>
+  <p>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<sub>1</sub> 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<sub>2</sub> generation consists of reds and whites, the
+  former being rather more numerous than the latter in the proportion of
+  9&nbsp;:&nbsp;7. The raising of a further generation from the seeds of these
+  F<sub>2</sub> 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&nbsp;:&nbsp;1, while others,
+  again, give reds and whites in the ratio 9&nbsp;:&nbsp;7. As in the case of the
+  fowls' combs, this case may be interpreted in terms of the presence and
+  absence of two factors. <!-- Page 45 --><span class="pagenum"><a
+  name="page45"></a>{45}</span><span class="figleft" style="width:33%;"><a
+  href="images/057.png"><img style="width:100%" src="images/057.png"
+  alt="Factors in red and white sweet peas." title="Factors in red and white sweet peas."
+  /></a></span>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<sub>1</sub> 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 <i>A</i> and <i>B</i> respectively
+  we may proceed to follow out the consequences of this cross. Since all
+  the F<sub>1</sub> plants were red the constitution of the parental whites
+  must have been <i>AAbb</i> and <i>aaBB</i> respectively, and their
+  gametes consequently <i>Ab</i> and <i>aB</i>. The constitution of the
+  F<sub>1</sub> plants must, therefore, be <i>AaBb</i>. Such a plant being
+  heterozygous for two factors produces a series of gametes of the four
+  kinds <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i>, and produces them in
+  equal numbers (cf. p. <a href="#page36">36</a>). To obtain the various
+  types of zygotes which are produced when such <!-- Page 46 --><span
+  class="pagenum"><a name="page46"></a>{46}</span><span class="figright"
+  style="width:29%;"><a href="images/058.png"><img style="width:100%"
+  src="images/058.png" alt="Fig. 7. Scheme of inheritance for red and white
+  sweet peas." title="Fig. 7. Scheme of inheritance for red and white sweet peas."
+  /></a><span class="sc">Fig. 7.</span><br />Diagram to illustrate the
+  nature of the F<sub>2</sub> generation from the two white sweet peas
+  which give a coloured F<sub>1</sub>.</span>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. An
+  examination of this figure (Fig. 7) shows that 9 out of the 16 squares
+  contain both <i>A</i> and <i>B</i>, while 7 contain either <i>A</i> or
+  <i>B</i> alone, or neither. In other words, on this view of the nature of
+  the two white sweet peas we should in the F<sub>2</sub> generation look
+  for the appearance of coloured and white flowers in the ratio 9&nbsp;:&nbsp;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. <i>AABB</i>, <i>AABb</i>, <i>AaBB</i>, and
+  <i>AaBb</i>. Since <i>AABB</i> is homozygous for both <i>A</i> and
+  <i>B</i>, 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 <!-- Page 47 --><span class="pagenum"><a
+  name="page47"></a>{47}</span>constitution <i>AABb</i> is homozygous for
+  the factor <i>A</i>, but heterozygous for <i>B</i>. All of its gametes
+  will contain <i>A</i>, but only one-half of them will contain <i>B</i>,
+  <i>i.e.</i> it produces equal numbers of gametes <i>AB</i> and <i>Ab</i>.
+  Two such series of gametes coming together must give a generation
+  consisting of <i>x</i> <i>AABB</i>, 2<i>x</i> <i>AABb</i>, and <i>x</i>
+  <i>AAbb</i>, that is, reds and whites in the ratio 3&nbsp;:&nbsp;1. Lastly the red
+  zygotes of the constitution <i>AaBb</i> 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&nbsp;:&nbsp;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.</p>
+
+  <p>The theory was further tested by an examination into the properties of
+  the various F<sub>2</sub> 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. <i>AAbb</i>, <i>Aabb</i>, <i>aaBB</i>, <i>aaBb</i>, and <i>aabb</i>.
+  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 <i>AAbb</i> and
+  <i>aaBB</i> should give nothing but coloured plants; for these two whites
+  are of <!-- Page 48 --><span class="pagenum"><a
+  name="page48"></a>{48}</span>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 <i>aabb</i> and any other white can
+  never give anything but whites. For no white contains both <i>A</i> and
+  <i>B</i>, or it would not be white, and a plant of the constitution
+  <i>aabb</i> cannot supply the complementary factor necessary for the
+  production of colour. Again, two whites of the constitution <i>Aabb</i>
+  and <i>aaBb</i> 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<sub>2</sub> whites accorded closely with
+  the theoretical explanation.</p>
+
+  <p>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 <!-- Page 49 --><span class="pagenum"><a
+  name="page49"></a>{49}</span>in the gametes or whether in some other form
+  we have as yet no means of deciding.</p>
+
+  <p>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<sub>1</sub> coloured birds when bred
+  together would produce an F<sub>2</sub> generation consisting of coloured
+  and white birds in the ratio 9&nbsp;:&nbsp;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.</p>
+
+  <p>Before quitting the subject of these experiments attention may be
+  drawn to the fact that the 9&nbsp;:&nbsp;7 ratio is in reality a 9&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;1
+  ratio in which the last three terms are indistinguishable owing to the
+  special circumstances that neither factor can produce a visible effect
+  without <!-- Page 50 --><span class="pagenum"><a
+  name="page50"></a>{50}</span>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.</p>
+
+  <div class="figright" style="width:24%;">
+      <a href="images/062.png"><img style="width:100%" src="images/062.png"
+      alt="Coat colours of mice." title="Coat colours of mice." /></a>
+  </div>
+  <p>One of the earliest sets of experiments demonstrating the interaction
+  of separate factors was that made by the French zoologist Cuénot 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>i.e.</i> the F<sub>2</sub> generation from such a
+  cross consists of agoutis and albinos in the ratio 3&nbsp;:&nbsp;1. But in other
+  cases the cross between albino and agouti gave a different result. In the
+  F<sub>1</sub> generation appeared only agoutis as before, but the
+  F<sub>2</sub> generation consisted of three distinct types, viz. agoutis,
+  albinos, <i>and blacks</i>. 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 <i>C</i>,
+  then the constitution of an albino must be <i>cc</i>, while that of a
+  coloured animal may be <i>CC</i> or <i>Cc</i>, according as to whether it
+  breeds true to colour or can <!-- Page 51 --><span class="pagenum"><a
+  name="page51"></a>{51}</span>throw albinos. Agouti was previously known
+  to be a simple dominant to black, <i>i.e.</i> an agouti is a black rabbit
+  plus an additional greying factor which modifies the black into agouti.
+  This factor we will denote by <i>G</i>, and we will use <i>B</i> for the
+  black factor. Our original agouti and albino parents we may therefore
+  regard as in constitution <i>GGCCBB</i> and <i>ggccBB</i> respectively.
+  Both of the parents are homozygous for black. The gametes produced by the
+  two parents are <i>GCB</i>, and <i>gcB</i>, and the constitution of the
+  F<sub>1</sub> animals must be <i>GgCcBB</i>. Being heterozygous for two
+  factors they will produce four kinds of gametes in equal numbers, viz.
+  <i>GCB</i>, <i>GcB</i>, <i>gCB</i>, and <i>gcB</i>. The results of the
+  mating of two such similar series of gametes when the F<sub>1</sub>
+  animals are bred together we may determine by the usual "chessboard"
+  method (Fig. 8). Out of the 16 squares 9 contain both <span
+  class="correction" title="Original reads (small) `c'."><i>C</i></span>
+  and <i>G</i> in addition to <i>B</i>. Such animals must be agoutis. Three
+  squares contain <i>C</i> but not <i>G</i>. 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 <i>C</i>, 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<sub>2</sub> in the ratio 9&nbsp;:&nbsp;3&nbsp;:&nbsp;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 <!-- Page 52 --><span class="pagenum"><a
+  name="page52"></a>{52}</span><span class="figright" style="width:30%;"><a
+  href="images/064.png"><img style="width:100%" src="images/064.png"
+  alt="Fig. 8. Scheme of inheritance for agouti and black mice."
+  title="Fig. 8. Scheme of inheritance for agouti and black mice."
+  /></a><span class="sc">Fig. 8.</span><br />Diagram to illustrate the
+  nature of the F<sub>2</sub> generation which may arise from the mating of
+  agouti with albino in mice or rabbits.</span>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 (<i>C</i>), and a colour modifier
+  (<i>G</i>), both acting, as it were, upon a substratum of black. The
+  F<sub>2</sub> generation really consists of the four classes agoutis,
+  blacks, albino agoutis, and albino blacks in the ratio 9&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;1. But
+  since in the absence of the colour developer <i>C</i> the colour modifier
+  <i>G</i> can produce no visible result, the last two classes of the ratio
+  are indistinguishable, and our F<sub>2</sub> generation comes to consist
+  of three classes in the ratio 9&nbsp;:&nbsp;3&nbsp;:&nbsp;4, instead of four classes in the
+  ratio 9&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;1. This explanation was further tested by experiments
+  with the albinos. In an F<sub>2</sub> family of this nature there ought
+  to be three kinds, viz. albinos homozygous for <i>G</i> (<i>GGccBB</i>),
+  albinos heterozygous for <i>G</i> (<i>GgccBB</i>), and albinos without
+  <i>G</i> (<i>ggccBB</i>). These albinos are, as it were, like
+  photographic plates exposed but undeveloped. <!-- Page 53 --><span
+  class="pagenum"><a name="page53"></a>{53}</span>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 <i>G</i>, and in order to develop the colour we
+  must introduce the factor <i>C</i>. Our developer, therefore, must
+  contain <i>C</i> but not <i>G</i>. In other words, it must be a
+  homozygous black mouse or rabbit, <i>ggCCBB</i>. Since such an animal is
+  pure for <i>C</i> it must, when mated with any of the albinos, produce
+  only coloured offspring. And since it does not contain <i>G</i> the
+  appearance of agoutis among its offspring must be attributed to the
+  presence of <i>G</i> in the albino. Tested in this way the F<sub>2</sub>
+  albinos were proved, as was expected, to be of three kinds: (1) those
+  which gave only agouti, <i>i.e.</i> which were homozygous for <i>G</i>;
+  (2) those which gave agoutis and blacks in approximately equal numbers,
+  <i>i.e.</i> which were heterozygous for <i>G</i>; and (3) those which
+  gave only blacks, and therefore did not contain <i>G</i>.</p>
+
+  <p>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&mdash;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, <!-- Page 54 --><span class="pagenum"><a
+  name="page54"></a>{54}</span>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.</p>
+
+  <p>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
+  <!-- Page 55 --><span class="pagenum"><a name="page55"></a>{55}</span>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.</p>
+
+  <div class="figcenter" style="width:49%;">
+      <a href="images/067.png"><img style="width:100%" src="images/067.png"
+      alt="Fig. 9. Sections of primula flowers." title="Fig. 9. Sections of primula flowers." /></a>
+    <span class="sc">Fig. 9.</span>
+
+    <p class="poem">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.</p>
+  </div>
+
+  <div class="figcenter" style="width:50%;">
+      <a href="images/068.jpg"><img style="width:100%" src="images/068t.jpg"
+      alt="Fig. 10. Two primula flowers." title="Fig. 10. Two primula flowers." /></a>
+    <span class="sc">Fig. 10.</span>
+
+    <p class="cenhead">Two primula flowers showing the extent of the small
+    and of the large eye.</p>
+  </div>
+
+  <p>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 <!-- Page 56 --><span class="pagenum"><a
+  name="page56"></a>{56}</span>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<sub>1</sub> plants were all
+  short styled with small eyes. <!-- Page 57 --><span class="pagenum"><a
+  name="page57"></a>{57}</span><span class="figright" style="width:35%;"><a
+  href="images/069.png"><img style="width:100%" src="images/069.png"
+  alt="Generations of primulas." title="Generations of primulas."
+  /></a></span>On self-fertilisation these gave an F<sub>2</sub> generation
+  consisting of four types, viz. short styled with small eyes, short styled
+  with large eyes, <i>long styled</i> with small eyes, and
+  <i>homostyled</i> 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 <!-- Page 58 --><span
+  class="pagenum"><a name="page58"></a>{58}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 59 --><span class="pagenum"><a name="page59"></a>{59}</span></p>
+
+<h3>CHAPTER VI</h3>
+
+<p class="cenhead">REVERSION</p>
+
+  <p>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 <i>C</i> and <i>G</i> 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 <!-- Page 60
+  --><span class="pagenum"><a name="page60"></a>{60}</span>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<sub>1</sub> individuals showed white patches, while others were
+  self-coloured. On breeding from the F<sub>1</sub> animals a series of
+  coloured forms resulted in F<sub>2</sub>. These were agoutis, blacks,
+  yellows, and sooty yellows, the so-called tortoise shells of the fancy
+  (Pl. I., 4-7).</p>
+
+  <div class="figcenter" style="width:75%;">
+      <a href="images/073.jpg"><img style="width:100%" src="images/073t.jpg"
+      alt="Plate I." title="Plate I." /></a>
+    <span class="sc">Plate</span> I.
+
+    <p class="cenhead">1, Yellow Dutch Rabbit; 2, Himalayan; 3, Agouti ( =
+    grey) F<sub>1</sub> reversion; 4-8, F<sub>2</sub> types, viz.: 4,
+    Agouti; 5, Yellow; 6, Black; 7, Tortoiseshell; 8, Himalayan.</p>
+  </div>
+
+<p><!-- Page 61 --><span class="pagenum"><a name="page61"></a>{61}</span></p>
+
+  <div class="figright" style="width:35%;">
+      <a href="images/074.png"><img style="width:100%" src="images/074.png"
+      alt="Generations of rabbits." title="Generations of rabbits." /></a>
+  </div>
+  <p>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<sub>2</sub> 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>I</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 <!-- Page 62 --><span class="pagenum"><a
+  name="page62"></a>{62}</span>by <i>X</i>, and as far as it is concerned
+  the Himalayan is constitutionally <i>xx</i>. 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>i.e.</i> lacking in the
+  factor <i>G</i>. With regard to these three factors, therefore, the
+  constitution of the Himalayan is <i>ggIIxx</i>. 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 (<i>S</i>), and for our
+  present purpose we will regard it as differing from a self-coloured
+  rabbit in the lack of this factor.<a name="footnotetag3"
+  href="#footnote3"><sup>[3]</sup></a> 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 <i>X</i>. The results of
+  breeding experiments then suggest that we may denote the Himalayan by the
+  formula <i>ggIIxxSS</i> and the yellow Dutch by <i>GGiiXXss</i>. Each
+  lacks two of the factors upon the full complement of which the agouti
+  colour depends. By crossing them the complete series <i>GIXS</i> is
+  brought into the same zygote, and the result is a reversion to the colour
+  of the wild rabbit.</p>
+
+  <div class="figright" style="width:35%;">
+      <a href="images/076.png"><img style="width:100%" src="images/076.png"
+      alt="Generations of Bush and Cupid sweet peas." title="Generations of Bush and Cupid sweet peas." /></a>
+  </div>
+  <p>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 <!-- Page 63 --><span
+  class="pagenum"><a name="page63"></a>{63}</span>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 3½-4 feet. A number of
+  crosses were made between the Bush and Cupid varieties, with the somewhat
+  unexpected result that in every instance the F<sub>1</sub> 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<sub>2</sub> generation from these reversionary talls
+  consisted of four different types, viz. <!-- Page 64 --><span
+  class="pagenum"><a name="page64"></a>{64}</span>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&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;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.</p>
+
+  <p>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.</p>
+
+  <div class="figcenter" style="width:50%;">
+      <a href="images/078.jpg"><img style="width:100%" src="images/078t.jpg"
+      alt="Plate II." title="Plate II." /></a>
+    <span class="sc">Plate II.</span>
+
+    <p class="cenhead">1, Bush Sweet Pea; 2, Cupid Sweet Pea; 3,
+    F<sub>1</sub> reversionary Tall; 4, Erect Cupid Sweet Pea; 5, Purple
+    Invincible; 6, Painted Lady; 7, Duke of Westminster (hooded
+    standard).</p>
+  </div>
+
+<p><!-- Page 65 --><span class="pagenum"><a name="page65"></a>{65}</span></p>
+
+  <p>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>i.e.</i> in the F<sub>1</sub> 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<sub>2</sub> 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<sub>1</sub>, while in the
+  F<sub>2</sub> generation a definite proportion, 1 in 16, of single combs
+  appeared (cf. p. <a href="#page32">32</a>). 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<sub>2</sub>; and this in the <i>absence</i> 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.</p>
+
+  <div class="figright" style="width:39%;">
+      <a href="images/080a.png"><img style="width:100%" src="images/080a.png"
+      alt="Darwin's case of reversion in pigeons." title="Darwin's case of reversion in pigeons." /></a>
+  </div>
+  <p>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 (<i>Columba livia</i>), when certain domesticated
+  races are crossed together.<a name="footnotetag4"
+  href="#footnote4"><sup>[4]</sup></a> 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 <!-- Page 66 --><span class="pagenum"><a
+  name="page66"></a>{66}</span>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. <span class="figleft" style="width:40%;"><a
+  href="images/080b.png"><img style="width:100%" src="images/080b.png"
+  alt="Reversion in pigeons." title="Reversion in pigeons."
+  /></a></span>The F<sub>1</sub> 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<sub>2</sub> generation,
+  consisting of blacks (with or without white splashes), blues (with or
+  without white splashes), and whites in the ratio 9&nbsp;:&nbsp;3&nbsp;:&nbsp;4. The factors
+  concerned are colour (<i>C</i>), in the absence of <!-- Page 67 --><span
+  class="pagenum"><a name="page67"></a>{67}</span>which a bird is white,
+  and a black modifier (<i>B</i>), in the absence of which a coloured bird
+  is blue. The original black barb contained both of these factors, being
+  in constitution <i>CCBB</i>. The fantail, however, contained neither, and
+  was constitutionally <i>ccbb</i>. The F<sub>1</sub> birds produced by
+  crossing were in constitution <i>CcBb</i>, and being heterozygous for two
+  factors produced in equal numbers the four sorts of gametes <i>CB</i>,
+  <i>Cb</i>, <i>cB</i>, <i>cb</i>. 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>i.e.</i> of the constitution <i>CCbb</i>, or <i>Ccbb</i>,
+  and such birds as the figure shows appear in the F<sub>2</sub> generation
+  on the average three times out of sixteen. Reversion here comes about in
+  F<sub>2</sub>, when the redistribution of the factors leads to the
+  formation of zygotes containing one of the two factors but not the
+  other.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/081.png"><img style="width:100%" src="images/081.png"
+      alt="Fig. 11. Scheme of inheritance for reversion in pigeons." title="Fig. 11. Scheme of inheritance for reversion in pigeons." /></a>
+    <span class="sc">Fig. 11.</span>
+
+    <p class="poem">Diagram to illustrate the appearance of the
+    reversionary blue pigeon in F<sub>2</sub> from the cross of black with
+    white.</p>
+  </div>
+
+<hr class="full" />
+
+<p><!-- Page 68 --><span class="pagenum"><a name="page68"></a>{68}</span></p>
+
+<h3>CHAPTER VII</h3>
+
+<p class="cenhead">DOMINANCE</p>
+
+  <div class="figcenter" style="width:50%;">
+      <a href="images/083.jpg"><img style="width:100%" src="images/083t.jpg"
+      alt="Fig. 11. Primula flowers." title="Fig. 11. Primula flowers." /></a>
+    <span class="sc">Fig. 12.</span>
+
+    <p class="cenhead">Primula flowers to illustrate the intermediate
+    nature of the F<sub>1</sub> flower when <i>sinensis</i> is crossed with
+    <i>stellata</i>.</p>
+  </div>
+
+  <div class="figright" style="width:40%;">
+      <a href="images/084a.png"><img style="width:100%" src="images/084a.png"
+      alt="Generations of Primula Sinensis × Stellata." title="Generations of Primula Sinensis × Stellata." /></a>
+  </div>
+  <p>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 (<i>P.
+  sinensis</i>) (Fig. 12) has large rather wavy petals much crenated at the
+  edges. In the Star Primula (<i>P. stellata</i>) the flowers are much
+  smaller, while the petals are flat and present only a terminal notch
+  instead of the numerous crenations of <i>P. sinensis</i>. The
+  heterozygote produced by crossing these forms is intermediate in size and
+  appearance. When self-fertilised such plants behave in simple Mendelian
+  fashion, <!-- Page 69 --><span class="pagenum"><a
+  name="page69"></a>{69}</span>giving a generation consisting of
+  <i>sinensis</i>, intermediates, and <i>stellata</i> in the ratio
+  1&nbsp;:&nbsp;2&nbsp;:&nbsp;1. Subsequent breeding from these plants showed that both the
+  <i>sinensis</i> and <i>stellata</i> which appeared in the F<sub>2</sub>
+  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.</p>
+
+<p><!-- Page 70 --><span class="pagenum"><a name="page70"></a>{70}</span></p>
+
+  <div class="figright" style="width:38%;">
+      <a href="images/084b.png"><img style="width:100%" src="images/084b.png"
+      alt="Generations of Blue Andalusian fowl." title="Generations of Blue Andalusian fowl." /></a>
+  </div>
+  <p>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 <!-- Page 71 --><span class="pagenum"><a
+  name="page71"></a>{71}</span>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>i.e.</i>, producing equal numbers of "black" and "white splashed"
+  gametes. The view was tested by breeding the "wasters"
+  together&mdash;black with black, and splashed white with splashed
+  white&mdash;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.</p>
+
+  <p>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, <!-- Page 72 --><span
+  class="pagenum"><a name="page72"></a>{72}</span>and until further
+  experiments have been devised and carried out it is not possible to
+  decide which is the correct view.</p>
+
+  <p>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.</p>
+
+  <div class="figright" style="width:31%;">
+      <a href="images/088.png"><img style="width:100%" src="images/088.png"
+      alt="Fig. 13. Scheme of inheritance for dominant and recessive white fowls." title="Fig. 13. Scheme of inheritance for dominant and recessive white fowls." /></a>
+    <span class="sc">Fig. 13.</span>
+
+    <p class="poem">Diagram to illustrate the nature of the F<sub>2</sub>
+    generation from the cross between dominant white and recessive white
+    fowls.</p>
+  </div>
+
+  <p>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 <!--
+  Page 73 --><span class="pagenum"><a name="page73"></a>{73}</span>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 <i>C</i> and our
+  postulated inhibitor factor in the dominant white bird by <i>I</i>, then
+  we must write the constitution of the recessive white as <i>ccii</i>, and
+  the dominant white as <i>CCII</i>. 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<sub>1</sub> bird must be <i>CcIi</i>. Such
+  birds being heterozygous for the inhibitor factor, should be whites
+  showing some coloured "ticks." Being heterozygous for both of the two
+  factors <i>C</i> and <i>I</i>, they will produce in equal numbers the
+  four different sorts of gametes <i>CI</i>, <i>Ci</i>, <i>cI</i>,
+  <i>ci</i>. The result of bringing two such similar series of gametes
+  together is shown in Fig. 13. Out of the sixteen squares, twelve contain
+  <i>I</i>; these will be white birds either with or without a few coloured
+  ticks. Three contain <i>C</i> but not <i>I</i>: these must be coloured
+  birds. One contains neither <i>C</i> nor <i>I</i>; this must be a white.
+  From such a mating we ought, therefore, to obtain both white and coloured
+  birds in the ratio 13&nbsp;:&nbsp;3. The results thus theoretically <!-- Page 74
+  --><span class="pagenum"><a name="page74"></a>{74}</span>deduced were
+  found to accord with the actual facts of experiment. The F<sub>1</sub>
+  birds were all "ticked" whites, and in the F<sub>2</sub> 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 <!-- Page 75 --><span class="pagenum"><a
+  name="page75"></a>{75}</span>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.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/089.jpg"><img style="width:100%" src="images/089t.jpg"
+      alt="Fig. 14. Ears of beardless and bearded wheat." title="Fig. 14. Ears of beardless and bearded wheat." /></a>
+    <span class="sc">Fig. 14.</span>
+
+    <p class="poem">Ears of beardless and bearded wheat. The beardless
+    condition is dominant to the bearded.</p>
+  </div>
+
+<p><!-- Page 76 --><span class="pagenum"><a name="page76"></a>{76}</span></p>
+
+  <p>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&nbsp;:&nbsp;1 ratio in F<sub>2</sub>.
+  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.</p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/091.png"><img style="width:100%" src="images/091.png"
+      alt="Fig. 15. Fowls' feet." title="Fig. 15. Fowls' feet." /></a>
+    <span class="sc">Fig. 15.</span>
+
+    <p class="cenhead">Fowls' feet. On the right a normal and on the left
+    one with an extra toe.</p>
+  </div>
+
+  <div class="figright" style="width:44%;">
+      <a href="images/092.png"><img style="width:100%" src="images/092.png"
+      alt="Fig. 16. Scheme of inheritance of horns in sheep." title="Fig. 16. Scheme of inheritance of horns in sheep." /></a>
+    <span class="sc">Fig. 16.</span>
+
+    <p class="poem">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.</p>
+  </div>
+
+  <p>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<sub>1</sub> generation was similar; the
+  rams were horned, and <!-- Page 77 --><span class="pagenum"><a
+  name="page77"></a>{77}</span>the ewes were hornless. In the F<sub>2</sub>
+  generation raised from these F<sub>1</sub> 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&mdash;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<sub>1</sub> ewes,
+  while the factor should be lacking in all the gametes of the hornless
+  F<sub>2</sub> rams. A <!-- Page 78 --><span class="pagenum"><a
+  name="page78"></a>{78}</span>hornless ram, therefore, put to a flock of
+  F<sub>1</sub> 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.</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/093.jpg"><img style="width:100%" src="images/093t.jpg"
+      alt="Plate III." title="Plate III." /></a>
+    <span class="sc">Plate III.</span>
+
+    <p class="cenhead">Sheep</p>
+  </div>
+
+<hr class="full" />
+
+<p><!-- Page 79 --><span class="pagenum"><a name="page79"></a>{79}</span></p>
+
+<h3>CHAPTER VIII</h3>
+
+<p class="cenhead">WILD FORMS AND DOMESTIC VARIETIES</p>
+
+  <p>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 (<i>Lathyrus odoratus</i>). 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 <!-- Page 80 --><span
+  class="pagenum"><a name="page80"></a>{80}</span>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&nbsp;:&nbsp;7 (cf. p. <a
+  href="#page44">44</a>), 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<a
+  name="footnotetag5" href="#footnote5"><sup>[5]</sup></a> (Pl. IV., 9). In
+  the F<sub>2</sub> generation the total number of purples bore to the
+  total number of reds the ratio 3&nbsp;:&nbsp;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 (<i>B</i>) which turns it into
+  purple.</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/096.jpg"><img style="width:100%" src="images/096t.jpg"
+      alt="Plate IV." title="Plate IV." /></a>
+    <span class="sc">Plate IV.</span>
+
+    <p class="cenhead">1, 2, Emily Henderson; 3, F<sub>1</sub> reversionary
+    Purple; 4-10, Various F<sub>2</sub> forms: 4, Purple; 5, Deep Purple;
+    6, Picotee; 7, Painted Lady; 8, Miss Hunt; 9, Tinged White; 10,
+    White.</p>
+  </div>
+
+<p><!-- Page 81 --><span class="pagenum"><a name="page81"></a>{81}</span></p>
+
+  <p>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:&mdash;</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>A colour base, <i>R</i>.</p>
+      <p>A colour developer, <i>C</i>.</p>
+      <p>A purple factor, <i>B</i>.</p>
+      <p>A light wing factor, <i>L</i>.</p>
+      <p>A factor for intense colour, <i>I</i>.</p>
+    </div>
+  </div>
+
+  <p>On this notation our six coloured forms are:&mdash;</p>
+
+<table class="nob" summary="Factors of sweet peas." title="Factors of sweet peas.">
+<tr><td class="spacsingle"> (1) Purple bicolor </td><td class="spacsingle"> <i>CRBLI</i>.<a name="footnotetag6" href="#footnote6"><sup>[6]</sup></a></td></tr>
+<tr><td class="spacsingle"> (2) Deep purple </td><td class="spacsingle"> <i>CRBlI</i>.</td></tr>
+<tr><td class="spacsingle"> (3) Picotee </td><td class="spacsingle"> <i>CRBLi</i> or <i>CRBli</i>.</td></tr>
+<tr><td class="spacsingle"> (4) Red bicolor ( = Painted Lady) </td><td class="spacsingle"> <i>CRbLI</i>.</td></tr>
+<tr><td class="spacsingle"> (5) Deep red ( = Miss Hunt) </td><td class="spacsingle"> <i>CRblI</i>.</td></tr>
+<tr><td class="spacsingle"> (6) Tinged white </td><td class="spacsingle"> <i>CRbLi</i> or <i>CRbli</i>.</td></tr>
+</table>
+
+  <p>It will be noticed in this series that the various coloured <!-- Page
+  82 --><span class="pagenum"><a name="page82"></a>{82}</span>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.</p>
+
+  <p>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 <!-- Page 83
+  --><span class="pagenum"><a name="page83"></a>{83}</span>light wing
+  (<i>L</i>) 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.</p>
+
+  <p>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 <!-- Page 84 --><span
+  class="pagenum"><a name="page84"></a>{84}</span>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 <i>mutations</i> 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.</p>
+
+  <p>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. <!-- Page 85 --><span class="pagenum"><a
+  name="page85"></a>{85}</span></p>
+
+  <p>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?</p>
+
+  <p>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.</p>
+
+  <p>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<sub>2</sub> generation, as, for example, in the instance of the
+  fowls' combs (cf. p. <a href="#page65">65</a>). The reversion to the
+  single comb occurred as the result of the removal of the two factors <!--
+  Page 86 --><span class="pagenum"><a name="page86"></a>{86}</span>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<sub>1</sub>, together with the greater number of blacks than blues in
+  F<sub>2</sub>, 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.</p>
+
+  <p>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 <!-- Page 87 --><span
+  class="pagenum"><a name="page87"></a>{87}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 88 --><span class="pagenum"><a name="page88"></a>{88}</span></p>
+
+<h3>CHAPTER IX</h3>
+
+<p class="cenhead">REPULSION AND COUPLING OF FACTORS</p>
+
+  <p>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.</p>
+
+  <p>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. <!-- Page 89 --><span class="pagenum"><a
+  name="page89"></a>{89}</span></p>
+
+  <p>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 (<i>B</i>) 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 (<i>E</i>) 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.</p>
+
+  <p>Now when a Duke of Westminster is mated with a Painted Lady the factor
+  for erect standard (<i>E</i>) is brought in by the red, and that for blue
+  (<i>B</i>) 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<sub>2</sub> generation to consist of
+  the four forms: erect purple, hooded purple, erect red, and hooded red in
+  the ratio 9&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;1. Such, however, is not the case. The
+  F<sub>2</sub> generation <!-- Page 90 --><span class="pagenum"><a
+  name="page90"></a>{90}</span>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&nbsp;:&nbsp;2&nbsp;:&nbsp;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 <i>never</i> breed true, but always behave like the original
+  F<sub>1</sub> plant, giving the three forms again in the ratio 1&nbsp;:&nbsp;2&nbsp;:&nbsp;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.</p>
+
+  <div class="figcenter" style="width:36%;">
+      <a href="images/106.png"><img style="width:100%" src="images/106.png"
+      alt="Generations of Painted Lady × Duke of Westminster cross." title="Generations of Painted Lady × Duke of Westminster cross." /></a>
+  </div>
+  <div class="figright" style="width:36%;">
+      <a href="images/107.png"><img style="width:100%" src="images/107.png"
+      alt="Factors in Painted Lady × Duke of Westminster cross." title="Factors in Painted Lady × Duke of Westminster cross." /></a>
+  </div>
+  <p>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 <i>EEbb</i> and <i>eeBB</i>
+  respectively and that of the F<sub>1</sub> erect purple is <i>EeBb</i>.
+  Now let us suppose that in such a zygote there exists a repulsion <!--
+  Page 91 --><span class="pagenum"><a name="page91"></a>{91}</span>between
+  <i>E</i> and <i>B</i>, 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. <i>Eb</i> and <i>eB</i>, and these, of course,
+  will be formed in equal numbers. Such a plant on self-fertilisation must
+  give the zygotic series <i>EEbb</i> + 2 <i>EeBb</i> + <i>eeBB</i>,
+  <i>i.e.</i> 1 erect red, 2 erect purples, and 1 hooded purple. And
+  because the erect reds and the hooded purples are respectively homozygous
+  for <i>E</i> and <i>B</i>, they must thenceforward breed true. The erect
+  purples, on the other hand, being always formed by the union of a gamete
+  <i>Eb</i> with a gamete <i>eB</i>, 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&nbsp;:&nbsp;2&nbsp;:&nbsp;1. The experimental facts are readily explained on the assumption
+  of repulsion between the two <!-- Page 92 --><span class="pagenum"><a
+  name="page92"></a>{92}</span>factors <i>B</i> and <i>E</i> during the
+  formation of the gametes in a plant which is heterozygous for both.</p>
+
+  <p>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.<a
+  name="footnotetag7" href="#footnote7"><sup>[7]</sup></a> When a cross is
+  made between a purple with round pollen and a red with long pollen the
+  F<sub>1</sub> plant is a long pollened purple. But the F<sub>2</sub>
+  generation consists of purples with round pollen, purples with long
+  pollen, and reds with long pollen in the ratio 1&nbsp;:&nbsp;2&nbsp;:&nbsp;1. No red with
+  round pollen appears in F<sub>2</sub> owing to repulsion between the
+  factors for purple (<i>B</i>) and for long pollen (<i>L</i>). Similarly
+  plants produced by crossing a red hooded long with a red round having an
+  erect standard give in F<sub>1</sub> long pollened reds with an erect
+  standard, and these in F<sub>2</sub> produce the three types, round
+  pollened erect, long pollened erect, and long pollened hooded, in the
+  ratio 1&nbsp;:&nbsp;2&nbsp;:&nbsp;1. The repulsion here is between the long pollen factor
+  (<i>L</i>) and the factor for the erect standard (<i>E</i>).</p>
+
+<p><!-- Page 93 --><span class="pagenum"><a name="page93"></a>{93}</span></p>
+
+  <p>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<sub>1</sub> plants are all fertile with
+  dark axils. But such plants in F<sub>2</sub> give fertiles with light
+  axils, fertiles with dark axils, and steriles with dark axils in the
+  ratio 1&nbsp;:&nbsp;2&nbsp;:&nbsp;1. No light axilled steriles appear from such a cross owing
+  to the repulsion between the factor for dark axil (<i>D</i>) and that for
+  the fertile anther (<i>F</i>).</p>
+
+  <p>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.</p>
+
+  <p>Now all of these four cases present a common feature which probably
+  has not escaped the attention of the reader. In all of them <i>the
+  original cross was such as to introduce one of the repelling factors with
+  each of the two parents</i>. If we denote our two factors by <i>A</i> and
+  <i>B</i>, the crosses have always been of the nature <i>AAbb</i> ×
+  <i>aaBB</i>. Let us now consider what happens when both of the <!-- Page
+  94 --><span class="pagenum"><a name="page94"></a>{94}</span>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>i.e.</i> with the factors <i>B</i> and
+  <i>L</i>. When a purple long (<i>BBLL</i>) is crossed with a red round
+  (<i>bbll</i>) the F<sub>1</sub> (<i>BbLl</i>) 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<sub>2</sub> generation is in some respects very different. The ratio of
+  purples to reds and of longs to rounds is in each case 3&nbsp;:&nbsp;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<sub>1</sub> plant consisted of 7 <i>BL</i> + 1 <i>Bl</i> + 1 <i>bL</i>
+  + 7 <i>bl</i> 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&mdash;a proportion of the four different kinds very close to <!--
+  Page 95 --><span class="pagenum"><a name="page95"></a>{95}</span>that
+  actually found by experiment. It will be noticed that in the whole family
+  the purples are to the reds as 3&nbsp;:&nbsp;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. <i>When both of the factors are
+  brought into the cross by the same parent we get coupling between them
+  instead of repulsion.</i> 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.</p>
+
+  <p>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.</p>
+
+<p class="cenhead">APPENDIX TO CHAPTER IX</p>
+
+  <p>As it is possible that some readers may care, in spite of its
+  complexity, to enter rather more fully into the peculiar phenomenon <!--
+  Page 96 --><span class="pagenum"><a name="page96"></a>{96}</span>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
+  <i>BL</i>&nbsp;: 1 <i>Bl</i>&nbsp;: 1 <i>bL</i>&nbsp;: 7 <i>bl</i>. Such a series of
+  ovules fertilised by a similar series of pollen grains will give a
+  generation of the following composition:&mdash;</p>
+
+<table class="nobctr" summary="Factors of blue colour and pollen." title="Factors of blue colour and pollen.">
+<tr><td class="qspcsingle"> 49 <i>BBLL</i> </td><td class="qspcsingle"> + 7 <i>BBLl</i> </td><td class="qspcsingle"> + 7 <i>BbLL</i> </td><td class="qspcsingle"> + 49 </td><td class="qspcsingle"> <i>BbLl</i> </td><td class="qspcsingle"> + <i>BBll</i> </td><td class="qspcsingle"> + 7 <i>Bbll</i> </td><td class="qspcsingle"> + <i>bbLL</i> </td><td class="qspcsingle"> + 7 <i>bbLl</i> </td><td class="qspcsingle"> + 49 <i>bbll</i></td></tr>
+<tr><td class="qspcsingle"> </td><td class="qspcsingle"> + 7 <i>BBLl</i> </td><td class="qspcsingle"> + 7 <i>BbLL</i> </td><td class="qspcsingle"> + </td><td class="qspcsingle"> <i>BbLl</i> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + 7 <i>Bbll</i> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + 7 <i>bbLl</i></td></tr>
+<tr><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + </td><td class="qspcsingle"> <i>BbLl</i></td></tr>
+<tr><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + 49 </td><td class="qspcsingle"> <i>BbLl</i></td></tr>
+<tr><td colspan="5" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:14em" alt="brace" /></a></td>
+  <td colspan="2" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:5em" alt="brace" /></a></td>
+<td colspan="2" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:5em" alt="brace" /></a></td>
+<td colspan="1" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:3em" alt="brace" /></a></td></tr>
+<tr><td colspan="5" align="center">177 purple, long</td>
+  <td colspan="2" align="center">15 purple, round</td>
+<td colspan="2" align="center">15 red, long</td>
+<td colspan="1" align="center">49 red, round</td></tr>
+</table>
+
+  <p>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 (<i>F</i>) and
+  the dark axil (<i>D</i>), the experimental numbers accord with the view
+  that the gametic series is here 15 <i>FD</i>&nbsp;: 1 <i>Fd</i>&nbsp;: 1
+  <i>fD</i>&nbsp;: 15 <i>fd</i>. The coupling is in this instance more intense.
+  In the case of the erect standard (<i>E</i>) and blueness (<i>B</i>) the
+  coupling is even more intense, and the experimental evidence available at
+  present points to the gametic series here being 63 <i>Eb</i>&nbsp;: 1
+  <i>EB</i>&nbsp;: 1 <i>eB</i>&nbsp;: 63 <i>eb</i>. 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&nbsp;:&nbsp;1&nbsp;:&nbsp;1&nbsp;:&nbsp;7 <!-- Page 97 --><span
+  class="pagenum"><a name="page97"></a>{97}</span>basis, but in some cases
+  it may be on the 15&nbsp;:&nbsp;1&nbsp;:&nbsp;1&nbsp;:&nbsp;15 basis. But though the intensity of the
+  coupling may vary it varies in an orderly way. If <i>A</i> and <i>B</i>
+  are the two factors concerned, the results obtained in F<sub>2</sub> are
+  explicable on the assumption that the ratio of the four sorts of gametes
+  produced is a term of the series&mdash;</p>
+
+<table class="nobctr" summary="Factors of blue colour and pollen." title="Factors of blue colour and pollen.">
+<tr><td class="qspcsingle" align="right"> 3 <i>AB</i>&nbsp; + </td><td class="qspcsingle" align="right"> <i>Ab</i>&nbsp; + </td><td class="qspcsingle" align="right"> <i>aB</i>&nbsp; + </td><td class="qspcsingle" align="right"> 3 </td><td class="qspcsingle"> <i>ab</i></td></tr>
+<tr><td class="qspcsingle" align="right"> 7 <i>AB</i>&nbsp; + </td><td class="qspcsingle" align="right"> <i>Ab</i>&nbsp; + </td><td class="qspcsingle" align="right"> <i>aB</i>&nbsp; + </td><td class="qspcsingle" align="right"> 7 </td><td class="qspcsingle"> <i>ab</i></td></tr>
+<tr><td class="qspcsingle" align="right"> 15 <i>AB</i>&nbsp; + </td><td class="qspcsingle" align="right"> <i>Ab</i>&nbsp; + </td><td class="qspcsingle" align="right"> <i>aB</i>&nbsp; + </td><td class="qspcsingle" align="right"> 15 </td><td class="qspcsingle"> <i>ab</i>, etc., etc.</td></tr>
+</table>
+
+  <p>In such a series the number of gametes containing <i>A</i> is equal to
+  the number lacking <i>A</i>, and the same is true for <i>B</i>.
+  Consequently the number of zygotes formed containing <i>A</i> is three
+  times as great as the number of zygotes which do not contain <i>A</i>;
+  and similarly for <i>B</i>. The proportion of dominants to recessives in
+  each case is 3&nbsp;:&nbsp;1. It is only in the distribution of the characters with
+  relation to one another that these cases differ from a simple Mendelian
+  case.</p>
+
+  <p>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, <!-- Page 98 --><span
+  class="pagenum"><a name="page98"></a>{98}</span>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.</p>
+
+<table class="allbctr" summary="Gametic series." title="Gametic series.">
+<tr><td class="allb" align="center"> No. of<br />Gametes<br />in series.
+</td><td class="allb" align="center"> Distribution of<br />Factors in Gametic<br />Series
+</td><td class="allb" align="center"> No. of<br />Zygotes<br />produced.
+</td><td class="allb" colspan="4" align="center"> Form of F<sub>2</sub> Generation.</td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> </td><td class="vertbsing" align="center" valign="bottom"> AB. Ab. aB. ab. </td><td class="vertbsing" align="right" valign="bottom"> </td><td class="qspcsingle" align="right" valign="bottom"> AB. </td><td class="qspcsingle" align="right" valign="bottom"> Ab.</td><td class="qspcsingle" align="right" valign="bottom"> aB.</td><td class="qspcsingle" align="right" valign="bottom"> ab. &nbsp; </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 4 </td><td class="vertbsing" align="center" valign="bottom"> &nbsp; 1 : 1 : 1 : &nbsp; 1 </td><td class="vertbsing" align="right" valign="bottom"> 16 </td><td class="qspcsingle" align="right" valign="bottom"> 9 </td><td class="qspcsingle" align="right" valign="bottom"> 3 </td><td class="qspcsingle" align="right" valign="bottom"> 3 </td><td class="qspcsingle" align="right" valign="bottom"> 1 &nbsp; </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 8 </td><td class="vertbsing" align="center" valign="bottom"> &nbsp; 3 : 1 : 1 : &nbsp; 3 </td><td class="vertbsing" align="right" valign="bottom"> 64 </td><td class="qspcsingle" align="right" valign="bottom"> 49 </td><td class="qspcsingle" align="right" valign="bottom"> 7 </td><td class="qspcsingle" align="right" valign="bottom"> 7 </td><td class="qspcsingle" align="right" valign="bottom"> 9 &nbsp; </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 16 </td><td class="vertbsing" align="center" valign="bottom"> &nbsp; 7 : 1 : 1 : &nbsp; 7 </td><td class="vertbsing" align="right" valign="bottom"> 256 </td><td class="qspcsingle" align="right" valign="bottom"> 177 </td><td class="qspcsingle" align="right" valign="bottom"> 15 </td><td class="qspcsingle" align="right" valign="bottom"> 15 </td><td class="qspcsingle" align="right" valign="bottom"> 49* </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 32 </td><td class="vertbsing" align="center" valign="bottom"> 15 : 1 : 1 : 15 </td><td class="vertbsing" align="right" valign="bottom"> 1024 </td><td class="qspcsingle" align="right" valign="bottom"> 737 </td><td class="qspcsingle" align="right" valign="bottom"> 31 </td><td class="qspcsingle" align="right" valign="bottom"> 31 </td><td class="qspcsingle" align="right" valign="bottom"> 225* </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 64 </td><td class="vertbsing" align="center" valign="bottom"> 31 : 1 : 1 : 31 </td><td class="vertbsing" align="right" valign="bottom"> 4096 </td><td class="qspcsingle" align="right" valign="bottom"> 3009 </td><td class="qspcsingle" align="right" valign="bottom"> 63 </td><td class="qspcsingle" align="right" valign="bottom"> 63 </td><td class="qspcsingle" align="right" valign="bottom"> 961 &nbsp; </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 128 </td><td class="vertbsing" align="center" valign="bottom"> 63 : 1 : 1 : 63 </td><td class="vertbsing" align="right" valign="bottom"> 16384 </td><td class="qspcsingle" align="right" valign="bottom"> 12161 </td><td class="qspcsingle" align="right" valign="bottom"> 127 </td><td class="qspcsingle" align="right" valign="bottom"> 127 </td><td class="qspcsingle" align="right" valign="bottom"> 3969* </td></tr>
+<tr><td class="vertbsing" align="right" valign="bottom"> 2<i>n</i> </td><td class="vertbsing" align="center" valign="bottom"> (<i>n</i>-1) : 1 : 1 : (<i>n</i>-1) </td><td class="vertbsing" align="right" valign="bottom"> 4<i>n</i><sup>2</sup> </td><td class="qspcsingle" align="right" valign="bottom"> 3<i>n</i><sup>2</sup>-(2<i>n</i>-1) </td><td class="qspcsingle" align="right" valign="bottom"> (2<i>n</i>-1) </td><td class="qspcsingle" align="right" valign="bottom"> (2<i>n</i>-1) </td><td class="qspcsingle" align="right" valign="bottom"> <i>n</i><sup>2</sup>-(2<i>n</i>-1)</td></tr>
+</table>
+
+  <p>Now, as the table shows, it is possible to express the gametic series
+  by a general formula (<i>n</i> + 1) <i>AB</i> + <i>Ab</i> + <i>aB</i> +
+  (<i>n</i> - 1) <i>ab</i>, where 2<i>n</i> 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 3<i>n</i><sup>2</sup> - (2<i>n</i> -
+  1) show both of the dominant characters and <i>n</i><sup>2</sup> -
+  (2<i>n</i> - 1) show both of the recessive characters, while the number
+  of the two classes which each show one of the two dominants is (2<i>n</i>
+  - 1). When in such a series the coupling becomes closer the value of
+  <i>n</i> increases, but in comparison with <i>n</i><sup>2</sup> its value
+  becomes less and less. The larger <i>n</i> becomes the more negligible is
+  its value relatively to <i>n</i><sup>2</sup>. If, therefore, the coupling
+  were very close, the series 3<i>n</i><sup>2</sup> - (2<i>n</i> - 1)&nbsp;:
+  (2<i>n</i> - 1)&nbsp;: (2<i>n</i> - 1)&nbsp;: <i>n</i><sup>2</sup> - (2<i>n</i> -
+  1) would approximate more and more to the series 3<i>n</i><sup>2</sup>&nbsp;:
+  <i>n</i><sup>2</sup>, <i>i.e.</i> to a simple 3&nbsp;:&nbsp;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&nbsp;:&nbsp;1 ratio will turn out on further analysis to
+  belong to this more complicated scheme.</p>
+
+<hr class="full" />
+
+<p><!-- Page 99 --><span class="pagenum"><a name="page99"></a>{99}</span></p>
+
+<h3>CHAPTER X</h3>
+
+<p class="cenhead">SEX</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/116.jpg"><img style="width:100%" src="images/116t.jpg"
+      alt="Fig. 17. Abraxas grossulariata varieties." title="Fig. 17. Abraxas grossulariata varieties." /></a>
+    <span class="sc">Fig.</span> 17.
+
+    <p class="cenhead"><i>Abraxas grossulariata</i>, the common currant
+    moth, and (on the right) its paler lacticolor variety.</p>
+  </div>
+
+  <p>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 (<i>Abraxas grossulariata</i>), of which there exists a pale variety
+  (Fig. 17) known as <i>lacticolor</i>. 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 <!-- Page 100
+  --><span class="pagenum"><a name="page100"></a>{100}</span>recessives
+  <span class="figright" style="width:52%;"><a href="images/116.png"><img
+  style="width:100%" src="images/116.png" alt="Results of crosses in
+  Abraxas grossulariata." title="Results of crosses in Abraxas grossulariata."
+  /></a></span>with regard to the sexes was peculiar. The original cross
+  was between a <i>lacticolor</i> female and a normal male. All the
+  F<sub>1</sub> moths of both sexes were of the normal <i>grossulariata</i>
+  type. The F<sub>1</sub> insects were then paired together and gave a
+  generation consisting of 3 normals&nbsp;: 1 <i>lacticolor</i>. But all the
+  <i>lacticolor</i> were females, and all the males were of the normal
+  pattern. It was, however, found possible to obtain the <i>lacticolor
+  male</i> by mating a <i>lacticolor</i> female with the F<sub>1</sub>
+  male. The family resulting from this cross consisted of normal males and
+  normal females, <i>lacticolor</i> males and <i>lacticolor</i> females,
+  and the <!-- Page 101 --><span class="pagenum"><a
+  name="page101"></a>{101}</span>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<sub>1</sub> female by <i>lacticolor</i> 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 <i>grossulariata</i> were males, while all the
+  <i>lacticolor</i> were females. Now this seemingly complex collection of
+  facts is readily explained if we make the following three
+  assumptions:&mdash;</p>
+
+  <p>(1) The <i>grossulariata</i> character (<i>G</i>) is dominant to the
+  lacticolor character (<i>g</i>). This is obviously justified by the
+  experiments, for, leaving the sex distribution out of account, we get the
+  expected 3&nbsp;:&nbsp;1 ratio from F<sub>1</sub> × F<sub>1</sub>, and also the
+  expected ratio of equality when the heterozygote is crossed with the
+  recessive.</p>
+
+  <p>(2) The female is heterozygous for a dominant factor (<i>F</i>) which
+  is lacking in the male. The constitution of a female is consequently
+  <i>Ff</i>, and of a male <i>ff</i>. This assumption is in harmony with
+  the fact that the sexes are produced in approximately equal numbers.</p>
+
+  <p>(3) There exists repulsion between the factors <i>G</i> and <i>F</i>
+  in a zygote which is heterozygous for them both. Such zygotes
+  (<i>FfGg</i>) must always be females, and on this assumption will produce
+  gametes <i>Fg</i> and <i>fG</i> in equal numbers. <!-- Page 102 --><span
+  class="pagenum"><a name="page102"></a>{102}</span></p>
+
+  <div class="figcenter" style="width:54%;">
+      <a href="images/118.png"><img style="width:100%" src="images/118.png"
+      alt="Fig. 18. Scheme of inheritance for Abraxas grossulariata." title="Fig. 18. Scheme of inheritance for Abraxas grossulariata." /></a>
+    <span class="sc">Fig.</span> 18.
+
+    <p class="poem">Scheme of inheritance in the F<sub>1</sub> and
+    F<sub>2</sub> generations resulting from the cross of <i>lacticolor</i>
+    female with <i>grossulariata</i> male. The character of each individual
+    is represented by the sex signs in brackets, the black being
+    <i>grossulariata</i> in appearance and the light ones
+    <i>lacticolor</i>.</p>
+  </div>
+
+  <p>We may now construct a scheme for comparison with that on page <a
+  href="#page100">100</a> to show how these assumptions explain the
+  experimental results. The original parents were <i>lacticolor</i> female
+  and <i>grossulariata</i> male, which on our assumptions must be
+  <i>Ffgg</i> and <i>ffGG</i> respectively in constitution. Since the
+  female is always heterozygous for <i>F</i>, her gametes must be of two
+  kinds, viz. <i>Fg</i> and <i>fg</i>, while those of the pure
+  <i>grossulariata</i> male must be all <i>fG</i>. When an ovum <i>Fg</i>
+  is fertilised by a spermatozoon <i>fG</i>, the resulting zygote,
+  <i>FfGg</i>, is heterozygous for both <i>F</i> and <i>G</i>, and in
+  appearance is a female <i>grossulariata</i>. The zygote resulting from
+  the fertilisation of an ovum <i>fg</i> by a spermatozoon <i>fG</i> is
+  heterozygous for <i>G</i>, but does not contain <i>F</i>, and therefore
+  is a male <i>grossulariata</i>. Such a male being in constitution <!--
+  Page 103 --><span class="pagenum"><a
+  name="page103"></a>{103}</span><i>ffGg</i> must produce gametes of two
+  kinds, <i>fG</i> and <i>fg</i>, in equal numbers. And since we are
+  assuming repulsion between <i>F</i> and <i>G</i>, the F<sub>1</sub>
+  female being in constitution <i>FfGg</i>, must produce equal numbers of
+  gametes <i>Fg</i> and <i>fG</i>. For on our assumption <i>F</i> and
+  <i>G</i> cannot enter into the same gamete. The series of gametes
+  produced by the F<sub>1</sub> moths, therefore, are <i>fG</i>, <i>fg</i>
+  by the male and <i>Fg</i>, <i>fG</i> by the female. The resulting
+  F<sub>2</sub> generation consequently consists of the four classes of
+  zygotes <i>Ffgg</i>, <i>FfGg</i>, <i>ffGg</i>, and <i>ffGG</i> in equal
+  numbers. In other words, the sexes are produced in equal numbers, the
+  proportion of normal grossulariata to <i>lacticolor</i> is 3&nbsp;:&nbsp;1, and all
+  of the <i>lacticolor</i> 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<sub>1</sub> moths with the <i>lacticolor</i> variety. And first we will
+  take the cross <i>lacticolor</i> female × F<sub>1</sub> male. The gametes
+  produced by the lacticolor female we have already seen to be <i>Fg</i>
+  and <i>fg</i>, while those produced by the F<sub>1</sub> male are
+  <i>fG</i> and <i>fg</i>. The bringing together of these two series of
+  gametes must result in equal numbers of the four kinds of zygotes
+  <i>FfGg</i>, <i>Ffgg</i>, <i>ffGg</i>, and <i>ffgg</i>, <i>i.e.</i> of
+  female <i>grossulariata</i> and <i>lacticolor</i>, and of male
+  <i>grossulariata</i> and <i>lacticolor</i> in equal numbers. Here, again,
+  the calculated results accord with those of experiment. Lastly, we may
+  examine what should happen when the F<sub>1</sub> female is crossed with
+  the <i>lacticolor</i> <!-- Page 104 --><span class="pagenum"><a
+  name="page104"></a>{104}</span>male. The F<sub>1</sub> female, owing to
+  the repulsion between <i>F</i> and <i>G</i>, produces only the two kinds
+  of ova <i>Fg</i> and <i>fG</i>, and produces them in equal numbers. Since
+  the <i>lacticolor</i> male can contain neither <i>F</i> nor <i>G</i>, all
+  of its spermatozoa must be <i>fg</i>. The results of such a cross,
+  therefore, should be to produce equal numbers of the two kinds of zygote
+  <i>Ffgg</i> and <i>ffGg</i>, <i>i.e.</i> of <i>lacticolor</i> females and
+  of <i>grossulariata</i> males. And this, as we have already seen, is the
+  actual result of such a cross.</p>
+
+  <p>Before leaving the currant moth we may allude to an interesting
+  discovery which arose out of these experiments. The <i>lacticolor</i>
+  variety in Great Britain is a southern form and is not known to occur in
+  Scotland. Matings were made between wild Scotch females and
+  <i>lacticolor</i> males. The families resulting from such matings were
+  precisely the same as those from <i>lacticolor</i> males and
+  F<sub>1</sub> females, viz. <i>grossulariata</i> males and
+  <i>lacticolor</i> females only. We are, therefore, forced to regard the
+  constitution of the wild <i>grossulariata</i> female as identical with
+  that of the F<sub>1</sub> female, <i>i.e.</i> as heterozygous for the
+  <i>grossulariata</i> factor as well as for the factor for femaleness.
+  Though from a region where <i>lacticolor</i> is unknown, the "pure" wild
+  <i>grossulariata</i> 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 <i>G</i> or pure <i>lacticolor</i>. And as all
+  the wild northern males are <!-- Page 105 --><span class="pagenum"><a
+  name="page105"></a>{105}</span>pure for the <i>grossulariata</i>
+  character this can never happen in a state of nature.</p>
+
+  <div class="figright" style="width:28%;">
+      <a href="images/122a.png"><img style="width:100%" src="images/122a.png"
+      alt="Fig. 19. Results of crossing Silky hen × Brown Leghorn cock." title="Fig. 19. Results of crossing Silky hen × Brown Leghorn cock." /></a>
+    <span class="sc">Fig. 19.</span>
+
+    <p class="poem">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.</p>
+  </div>
+
+  <p>An essential feature of the case of the currant moth lies in the
+  different results given by reciprocal crosses. <i>Lacticolor</i> female ×
+  <i>grossulariata</i> male gives <i>grossulariata</i> alone of both sexes.
+  But <i>grossulariata</i> female × <i>lacticolor</i> male gives only
+  <i>grossulariata</i> males and <i>lacticolor</i> 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<sub>1</sub>
+  birds in which both sexes exhibited only traces of the pigment. On casual
+  observation they might have <!-- Page 106 --><span class="pagenum"><a
+  name="page106"></a>{106}</span>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<sub>2</sub> male birds, however, showed the full
+  deep pigmentation of the Silky.</p>
+
+  <div class="figright" style="width:33%;">
+      <a href="images/122b.png"><img style="width:100%" src="images/122b.png"
+      alt="Fig. 20. Results of crossing Brown Leghorn hen × Silky cock." title="Fig. 20. Results of crossing Brown Leghorn hen × Silky cock." /></a>
+    <span class="sc">Fig. 20.</span>
+
+    <p class="cenhead">Scheme illustrating the result of crossing a Brown
+    Leghorn hen with a Silky cock (cf. Fig. 19).</p>
+  </div>
+
+  <p>When, however, the cross was made the other way, viz. Brown Leghorn
+  hen × Silky cock, the result was different. While the F<sub>1</sub> male
+  birds were almost destitute of pigment as in the previous cross, the
+  F<sub>1</sub> hens, on the other hand, were nearly as deeply pigmented as
+  the pure Silky <!-- Page 107 --><span class="pagenum"><a
+  name="page107"></a>{107}</span>(Pl. V., 2). The male Silky transmitted
+  the pigmentation, but only to his daughters. Such birds bred together
+  gave an F<sub>2</sub> 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.</p>
+
+  <div class="figleft" style="width:48%;">
+      <a href="images/123.png"><img style="width:100%" src="images/123.png"
+      alt="Fig. 21. Result of crossing F_1 birds with Brown Leghorn." title="Fig. 21. Result of crossing F_1 birds with Brown Leghorn." /></a>
+    <span class="sc">Fig. 21.</span>
+
+    <p class="cenhead">Scheme to illustrate the result of crossing
+    F<sub>1</sub> birds (<i>e.g.</i> Brown Leghorn × Silky) with the pure
+    Brown Leghorn.</p>
+  </div>
+
+  <p>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<sub>1</sub> birds and the pure Brown
+  Leghorn. The cross between the F<sub>1</sub> 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<sub>1</sub> hen. But when the F<sub>1</sub>
+  cock was mated to a Brown Leghorn hen, a definite proportion of the
+  chicks, one in eight, was deeply pigmented, and <i>these deeply pigmented
+  birds were always females</i> (cf. Fig. 21). And in this respect all the
+  F<sub>1</sub> males behaved alike, whether they were from the Silky hen
+  or from the Silky cock. We have, therefore, the paradox that the
+  F<sub>1</sub> 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<sub>1</sub> cock can
+  transmit this pigmented condition to a quarter of his female offspring
+  though he himself is almost devoid of pigment.</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/124.jpg"><img style="width:100%" src="images/124t.jpg"
+      alt="Plate V." title="Plate V." /></a>
+    <span class="sc">Plate V.</span>
+
+    <p class="cenhead">1, 2, F<sub>1</sub> Cock and Hen, ex Brown Leghorn
+    Hen × Silky Cock; 3, Silky Cock; 4, Hen ex Silky Hen × Brown Leghorn
+    Cock.</p>
+  </div>
+
+<p><!-- Page 108 --><span class="pagenum"><a name="page108"></a>{108}</span></p>
+
+  <div class="figright" style="width:28%;">
+      <a href="images/126.png"><img style="width:100%" src="images/126.png"
+      alt="Fig. 22. Scheme of inheritance for Silky hen × Brown Leghorn cock." title="Fig. 22. Scheme of inheritance for Silky hen × Brown Leghorn cock." /></a>
+    <span class="sc">Fig. 22.</span>
+
+    <p class="poem">Scheme to illustrate the nature of the F<sub>1</sub>
+    generation from the Silky hen and Brown Leghorn cock (cf. Fig. 23).</p>
+  </div>
+
+  <p>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 (<i>P</i>); (2) a factor which inhibits the
+  production of pigment (<i>I</i>); and (3) a factor for femaleness
+  (<i>F</i>), for which the female birds are heterozygous, but which is not
+  present in the males. Further, we make the assumptions (<i>a</i>) that
+  there is repulsion between <i>F</i> and <i>I</i> in the female zygote
+  (<i>FfIi</i>), and (<i>b</i>) that the male Brown Leghorn is homozygous
+  for the inhibitor factor (<i>I</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 <i>grossulariata</i>
+  factor. We may now proceed to show how this explanation fits the
+  experimental facts which we have given.</p>
+
+  <p>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 <!-- Page 109 --><span class="pagenum"><a
+  name="page109"></a>{109}</span>pigmentation factor. In crossing a Silky
+  hen with a Brown Leghorn cock we are mating two birds of the constitution
+  <i>FfPPii</i> and <i>ffppII</i>, and all the F<sub>1</sub> birds are
+  consequently heterozygous for both <i>P</i> and <i>I</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.</p>
+
+  <div class="figleft" style="width:27%;">
+      <a href="images/127.png"><img style="width:100%" src="images/127.png"
+      alt="Fig. 23. Scheme of inheritance for Brown Leghorn hen × Silky cock." title="Fig. 23. Scheme of inheritance for Brown Leghorn hen × Silky cock." /></a>
+    <span class="sc">Fig. 23.</span>
+
+    <p class="poem">Scheme to illustrate the nature of the F<sub>1</sub>
+    generation from the Brown Leghorn hen and Silky cock (cf. Fig. 22).</p>
+  </div>
+
+  <p>In the reciprocal cross, on the other hand, we are mating a Silky male
+  (<i>ffPPii</i>) with a Brown Leghorn hen which on our assumption is
+  heterozygous for the inhibitor factor (<i>I</i>), and in constitution
+  therefore is <i>FfppIi</i>. Owing to the repulsion between <i>F</i> and
+  <i>I</i> the gametes produced by such a bird are <i>Fpi</i> and
+  <i>fpI</i> in equal numbers. All the gametes produced by the Silky cock
+  are <i>fPi</i>. Hence the constitution of the F<sub>1</sub> male birds
+  produced by this cross is <i>ffPpIi</i> as before, but the female birds
+  must be all of the constitution <i>FfPpii</i>. 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 <!-- Page 110 --><span class="pagenum"><a
+  name="page110"></a>{110}</span>inhibitory factor owing to the repulsion
+  between these factors. The nature of the F<sub>2</sub> 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<sub>1</sub> males and females when crossed with the
+  Brown Leghorn. And, first, the cross of Brown Leghorn female by
+  F<sub>1</sub> male. The Brown Leghorn hen is on our hypothesis
+  <i>FfppIi</i>, and produces gametes <i>Fpi</i> and <i>fpI</i>. The
+  F<sub>1</sub> cock is on our hypothesis <i>ffPpIi</i>, and produces in
+  equal numbers the four kinds of gametes <i>fPI</i>, <i>fPi</i>,
+  <i>fpI</i>, <i>fpi</i>. 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 <i>P</i> in the absence of <i>I</i>, and this is
+  a female. The result, as we have already seen, is in accordance with the
+  experimental facts.</p>
+
+  <div class="figright" style="width:30%;">
+      <a href="images/128.png"><img style="width:100%" src="images/128.png"
+      alt="Fig. 24. Scheme of inheritance for Brown Leghorn hen × F_1 cock." title="Fig. 24. Scheme of inheritance for Brown Leghorn hen × F_1 cock." /></a>
+    <span class="sc">Fig.</span> 24.
+
+    <p class="poem">Diagram showing the nature of the offspring from a
+    Brown Leghorn hen and an F<sub>1</sub> cock bred from Silky hen × Brown
+    Leghorn cock, or <i>vice versa</i>.</p>
+  </div>
+
+  <p>On the other hand, the Brown Leghorn cock is on our hypothesis
+  <i>ffppII</i>. All his gametes consequently contain the inhibitor factor,
+  and when he is mated with an F<sub>1</sub> <!-- Page 111 --><span
+  class="pagenum"><a name="page111"></a>{111}</span>hen all the zygotes
+  produced must contain <i>I</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 <i>P</i>.</p>
+
+  <div class="figleft" style="width:50%;">
+      <a href="images/129.png"><img style="width:100%" src="images/129.png"
+      alt="Fig. 25. Scheme showing the heterozygous nature of the pure Brown Leghorn hen." title="Fig. 25. Scheme showing the heterozygous nature of the pure Brown Leghorn hen." /></a>
+    <span class="sc">Fig. 25.</span>
+
+    <p class="cenhead">Scheme to illustrate the heterozygous nature of the
+    pure Brown Leghorn hen. For explanation see text.</p>
+  </div>
+
+  <p>The interpretation of this case turns upon the constitution of the
+  Brown Leghorn hen, upon her heterozygous condition with regard to the two
+  factors <i>F</i> and <i>I</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 <i>ppii</i>. Such birds when crossed with the Silky give
+  dark pigmented birds of both sexes in F<sub>1</sub>, and the
+  F<sub>2</sub> generation consists of pigmented and unpigmented in the
+  ratio 3&nbsp;:&nbsp;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 <i>Fpi</i> and
+  <i>fpI</i>, the male birds produced by such a cross should be
+  heterozygous for <i>I</i>, <!-- Page 112 --><span class="pagenum"><a
+  name="page112"></a>{112}</span><i>i.e.</i> in constitution <i>ffppIi</i>,
+  while the hen birds, though identical in appearance so far as absence of
+  pigmentation goes, should not contain this factor but should be
+  constitutionally <i>Ffppii</i>. Crossed with the pure Silky, the
+  F<sub>1</sub> 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 <!-- Page 113 --><span class="pagenum"><a
+  name="page113"></a>{113}</span>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.</p>
+
+  <p>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.</p>
+
+  <p>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, <!-- Page 114 --><span
+  class="pagenum"><a name="page114"></a>{114}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 115 --><span class="pagenum"><a name="page115"></a>{115}</span></p>
+
+<h3>CHAPTER XI</h3>
+
+<p class="cenhead">SEX (<i>continued</i>)</p>
+
+  <p>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 <i>FfMM</i>, and
+  for a male <i>ffMM</i>. Both sexes are homozygous for the male element,
+  and the difference between them is due to the presence or absence of the
+  female element <i>F</i>.</p>
+
+  <p>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
+  (<i>Drosophila ampelophila</i>). 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 <!-- Page 116 --><span class="pagenum"><a
+  name="page116"></a>{116}</span>are inherited on the same lines as the
+  <i>grossulariata</i> and <i>lacticolor</i> 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.</p>
+
+  <div class="figright" style="width:23%;">
+      <a href="images/133.png"><img style="width:100%" src="images/133.png"
+      alt="Fertilisations in Drosophila." title="Fertilisations in Drosophila." /></a>
+  </div>
+  <p>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, <i>Mmff</i> and the female <i>Ffmm</i>. Each sex produces
+  two sorts of gametes, <i>Mf</i> and <i>mf</i> in the case of the male,
+  and <i>Fm</i>, <i>fm</i> 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 <i>MmFf</i> and <i>mmff</i>, as well as
+  the normal males and females, <i>Mmff</i> and <i>Ffmm</i>. 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 <i>M</i> and the ova without <i>F</i>, or between
+  the spermatozoa <!-- Page 117 --><span class="pagenum"><a
+  name="page117"></a>{117}</span>without <i>M</i> and the ova containing
+  <i>F</i>. 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
+  <i>grossulariata</i> factor and <i>F</i>, or else between the pigment
+  inhibitor factor and <i>F</i>, while in the latter there is repulsion
+  between the factor for red eye and <i>M</i>.</p>
+
+  <div class="figright" style="width:20%;">
+      <a href="images/134.png"><img style="width:100%" src="images/134.png"
+      alt="Fig. 26. Scheme of probable mode of inheritance of colour-blindness." title="Fig. 26. Scheme of probable mode of inheritance of colour-blindness." /></a>
+    <span class="sc">Fig. 26.</span>
+
+    <p class="poem">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.</p>
+  </div>
+
+  <p>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 <!--
+  Page 118 --><span class="pagenum"><a
+  name="page118"></a>{118}</span>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 <i>all</i> 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.</p>
+
+  <p>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. <a href="#page71">71</a>). 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 <i>X</i>, the gametes produced by the
+  colour-blind male are of two sorts only, viz. <i>Mfx</i> and <i>mfX</i>.
+  If he marries a normal woman (<i>Ffmmxx</i>), the spermatozoa <i>Mfx</i>
+  unite with ova <i>fmx</i> to give normal males, while the spermatozoa
+  <i>mfX</i> unite with ova <i>Fmx</i> 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. <!-- Page
+  119 --><span class="pagenum"><a name="page119"></a>{119}</span></p>
+
+  <p>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&nbsp;:&nbsp;1. If it were an ordinary
+  Mendelian case the ratio should be 3&nbsp;:&nbsp;1, and one out of every three
+  yellows so bred should be homozygous and give only yellows when crossed
+  with agouti. But Cuénot and others have shown that <i>all</i> 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 <!--
+  Page 120 --><span class="pagenum"><a
+  name="page120"></a>{120}</span>grounds for assuming that anything in the
+  nature of unproductive fertilisation takes place.<a name="footnotetag8"
+  href="#footnote8"><sup>[8]</sup></a></p>
+
+  <p>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&mdash;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 (<i>Bryonia dioica</i>) the
+  sexes are separate, some plants having only male and others only female
+  flowers. In another European species, <i>B. alba</i>, both male and
+  female flowers occur on the same plant. Correns crossed these two species
+  reciprocally, and also fertilised <i>B. dioica</i> by its own male with
+  the following results:&mdash;</p>
+
+<p><!-- Page 121 --><span class="pagenum"><a name="page121"></a>{121}</span></p>
+
+<table class="nobctr" summary="Bryonia dioica × B. alba." title="Bryonia dioica × B. alba.">
+<tr><td class="qspcsingle"> dioica </td><td class="qspcsingle"> &#x2640; </td><td class="qspcsingle"> × dioica </td><td class="qspcsingle"> &#x2642; gave </td><td class="qspcsingle"> &#x2640; &#x2640; and &#x2642; &#x2642;</td></tr>
+
+<tr><td class="qspcsingle"> &nbsp; &nbsp; " </td><td class="qspcsingle"> </td><td class="qspcsingle"> × alba </td><td class="qspcsingle"> &#x2642; &nbsp; &nbsp;" </td><td class="qspcsingle"> &#x2640; &#x2640; only</td></tr>
+
+<tr><td class="qspcsingle"> alba </td><td class="qspcsingle"> &#x2640; </td><td class="qspcsingle"> × dioica </td><td class="qspcsingle"> &#x2642; &nbsp; &nbsp;" </td><td class="qspcsingle"> &#x2640; &#x2640; and &#x2642; &#x2642;.</td></tr>
+</table>
+
+  <p>The point of chief interest lies in the striking difference shown by
+  the reciprocal crosses between <i>dioica</i> and <i>alba</i>. Males
+  appear when <i>alba</i> is used as the female parent but not when the
+  female <i>dioica</i> is crossed by male <i>alba</i>. 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 <i>dioica</i> 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. <i>Alba</i> &#x2640; × <i>dioica</i>
+  &#x2642; gives the same result as <i>dioica</i> &#x2640; × <i>dioica</i>
+  &#x2642;, and we must therefore suppose that alba produces male and
+  female ovules in equal numbers. <i>Alba</i> &#x2642; x <i>dioica</i>
+  &#x2640;, 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&mdash;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<sub>1</sub> plants. <!-- Page 122 --><span
+  class="pagenum"><a name="page122"></a>{122}</span></p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/140.jpg"><img style="width:100%" src="images/140t.jpg"
+      alt="Fig. 27. Single and double stocks." title="Fig. 27. Single and double stocks." /></a>
+    <span class="sc">Fig. 27.</span>
+
+    <p class="cenhead">Single and double stocks raised from the same single
+    parent.</p>
+  </div>
+
+  <p>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,<a name="footnotetag9"
+  href="#footnote9"><sup>[9]</sup></a> "that those plants that beare double
+  flowers, doe beare <span class="figright" style="width:44%;"><a
+  href="images/141.png"><img style="width:100%" src="images/141.png"
+  alt="Crosses of single and double stocks." title="Crosses of single and double stocks."
+  /></a></span>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<sub>1</sub> plants were
+  single. A distinction, however, appeared when a further generation was
+  raised from the F<sub>1</sub> plants. All the F<sub>1</sub> plants from
+  the pollen of the double-throwing single behaved like double-throwing
+  singles, but of the F<sub>1</sub> 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 <!-- Page
+  123 --><span class="pagenum"><a name="page123"></a>{123}</span>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 <!-- Page 124 --><span class="pagenum"><a
+  name="page124"></a>{124}</span>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.</p>
+
+  <p>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 125 --><span class="pagenum"><a name="page125"></a>{125}</span></p>
+
+<h3>CHAPTER XII</h3>
+
+<p class="cenhead">INTERMEDIATES</p>
+
+  <p>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 <!-- Page 126 --><span
+  class="pagenum"><a name="page126"></a>{126}</span>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<sub>2</sub> 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 <!-- Page 127 --><span class="pagenum"><a
+  name="page127"></a>{127}</span><span class="figright"
+  style="width:30%;"><a href="images/144.png"><img style="width:100%"
+  src="images/144.png" alt="Fig. 28. Scheme of inheritance for Silky hen ×
+  Brown Leghorn cock." title="Fig. 28. Scheme of inheritance for Silky hen × Brown Leghorn cock."
+  /></a> <span class="sc">Fig. 28.</span><br />Diagram to illustrate the
+  nature and composition of the F<sub>2</sub> generations arising from the
+  cross of Silky hen with Brown Leghorn cock.</span>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<sub>1</sub> &#x2642; is a <i>ffPpIi</i>, and such a bird
+  produces in equal numbers the four sorts of gametes <i>fPI</i>,
+  <i>fPi</i>, <i>fpI</i>, <i>fpi</i>. The constitution of the F<sub>1</sub>
+  &#x2640; in this case is <i>FfPpIi</i>. Owing to the repulsion between
+  <i>F</i> and <i>I</i> she produces the four kinds of gametes <i>FPi</i>,
+  <i>Fpi</i>, <i>fPI</i>, <i>fpi</i>, 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 (<i>FfPPii</i>) 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 <i>P</i> in the absence of
+  <i>I</i>. These are nearly as dark as the pure Silky. Four zygotes are
+  destitute of <i>P</i>, though they may or may not contain <i>I</i>. These
+  birds are completely devoid of pigment like the Brown Leghorn. The
+  remaining nine zygotes show <!-- Page 128 --><span class="pagenum"><a
+  name="page128"></a>{128}</span>various combinations of the two factors
+  <i>P</i> and <i>I</i>, being either <i>PPIi</i>, <i>PPII</i>,
+  <i>PpII</i>, or <i>PpIi</i>, 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 <i>PPIi</i> approaches in pigmentation a bird of
+  the constitution <i>Ppii</i>, while a bird of the constitution
+  <i>PpII</i> 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<sub>2</sub> generation depends
+  upon the fact that these two factors interact upon one another, and to
+  different degrees according as the zygote is for one <!-- Page 129
+  --><span class="pagenum"><a name="page129"></a>{129}</span>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 <i>ffPPII</i> when bred with
+  females of the constitution <i>FfPPIi</i> 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.</p>
+
+  <p>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 <!-- Page 130 --><span class="pagenum"><a
+  name="page130"></a>{130}</span>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.</p>
+
+  <div class="figright" style="width:42%;">
+      <a href="images/147.png"><img style="width:100%" src="images/147.png"
+      alt="Fig. 29. Pedigree of a family which originated from a cross between a Hindu and a European." title="Fig. 29. Pedigree of a family which originated from a cross between a Hindu and a European." /></a>
+    <span class="sc">Fig. 29.</span>
+
+    <p class="poem">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.</p>
+  </div>
+
+<p><!-- Page 131 --><span class="pagenum"><a name="page131"></a>{131}</span></p>
+
+  <p>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 <!-- Page 132 --><span class="pagenum"><a
+  name="page132"></a>{132}</span>hope to come to any conclusion until we
+  have evidence collected by critical and competent observers.</p>
+
+  <p>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<sub>1</sub>
+  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 (<i>Pararge egeria</i>) 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.
+  <i>egeriades</i>. 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.</p>
+
+  <p>And here it is impossible not to recall Mendel's own experiences with
+  the Hawkweeds (<i>Hieracium</i>). This <!-- Page 133 --><span
+  class="pagenum"><a name="page133"></a>{133}</span>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
+  <i>Hieracium</i>. 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 <!-- Page 134 --><span class="pagenum"><a
+  name="page134"></a>{134}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 135 --><span class="pagenum"><a name="page135"></a>{135}</span></p>
+
+<h3>CHAPTER XIII</h3>
+
+<p class="cenhead">VARIATION AND EVOLUTION</p>
+
+  <p>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 <!-- Page 136 --><span
+  class="pagenum"><a name="page136"></a>{136}</span>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?</p>
+
+  <p>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<sup>1</sup> = 2 possible forms,
+  where ten factors are concerned there are 2<sup>10</sup> = 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<sup>10</sup> = 59,049; for
+  twenty such factors the possible number of different individuals would be
+  3<sup>20</sup> = 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 <!--
+  Page 137 --><span class="pagenum"><a name="page137"></a>{137}</span>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.</p>
+
+  <p>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 <!-- Page 138 --><span class="pagenum"><a
+  name="page138"></a>{138}</span>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 <i>mutations</i>, and are inherited
+  according to the Mendelian scheme; the latter have been termed
+  <i>fluctuations</i>, 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 <!-- Page 139 --><span class="pagenum"><a
+  name="page139"></a>{139}</span>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.</p>
+
+  <p>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 <!--
+  Page 140 --><span class="pagenum"><a name="page140"></a>{140}</span>the
+  light of our present knowledge we must regard the mutation as the basis
+  of evolution&mdash;as the material upon which natural selection works.
+  For it is the only form of variation of whose heredity we have any
+  certain knowledge.</p>
+
+  <p>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 <!-- Page 141 --><span class="pagenum"><a
+  name="page141"></a>{141}</span>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.</p>
+
+  <p>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
+  <i>The Mutation Theory</i>. 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 <!-- Page 142 --><span class="pagenum"><a
+  name="page142"></a>{142}</span>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&mdash;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.</p>
+
+  <p>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 <!-- Page 143 --><span class="pagenum"><a
+  name="page143"></a>{143}</span>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.</p>
+
+  <p>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 <!-- Page 144 --><span class="pagenum"><a
+  name="page144"></a>{144}</span>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.</p>
+
+  <p>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 <i>Amauris</i>, 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 <i>Euralia</i> belonging to the entirely <!-- Page 145
+  --><span class="pagenum"><a name="page145"></a>{145}</span>different
+  family of the Nymphalidae, to which there is no evidence for assigning
+  the disagreeable properties of the Danaines. Now the different species of
+  <i>Euralia</i> show remarkably close resemblances to the species of
+  <i>Amauris</i>, 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.</p>
+
+  <p>One of the species of <i>Euralia</i> occurs in two very distinct forms
+  (Pl. VI.), which were previously regarded as separate species under the
+  names <i>E. wahlbergi</i> and <i>E. mima</i>. These two forms
+  respectively resemble <i>Amauris dominicanus</i> and <i>A. echeria</i>.
+  For purposes of argument we will assume <i>A. echeria</i> to be the more
+  recent form of the two. On the modern Darwinian view certain individuals
+  of <i>A. dominicanus</i> gradually diverged from the <i>dominicanus</i>
+  type and eventually reached the <i>echeria</i> 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 <i>A. echeria</i> in
+  places where this species was more abundant than <i>A. dominicanus</i>
+  were encouraged by natural selection, and under its guiding hand the form
+  <i>mima</i> eventually arose from <i>wahlbergi</i>.</p>
+
+  <p>According to Mendelian views, on the other hand, <!-- Page 146
+  --><span class="pagenum"><a name="page146"></a>{146}</span><i>A.
+  echeria</i> arose suddenly from <i>A. dominicanus</i> (or <i>vice
+  versa</i>), and similarly <i>mima</i> arose suddenly from
+  <i>wahlbergi</i>. If <i>mima</i> occurred where <i>A. echeria</i> was
+  common and <i>A. dominicanus</i> was rare, its resemblance to the more
+  plentiful distasteful form would give it the advantage over
+  <i>wahlbergi</i> and allow it to establish itself in place of the latter.
+  On the modern Darwinian view natural selection gradually shapes
+  <i>wahlbergi</i> into the <i>mima</i> form owing to the presence of <i>A.
+  echeria</i>; on the Mendelian view natural selection merely conserves the
+  <i>mima</i> 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 <i>mima</i> and <i>wahlbergi</i> 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 <i>Amauris</i> and <i>Euralia</i>
+  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
+  <i>Euralia</i> produced in any region that one has the best chance of
+  survival, through the operation of natural selection, which resembles the
+  most plentiful <i>Amauris</i> form. Mimetic resemblance is a true
+  phenomenon, but natural selection plays the part of a conservative, not
+  of a formative agent.</p>
+
+  <div class="figcenter" style="width:75%;">
+      <a href="images/164.jpg"><img style="width:100%" src="images/164t.jpg"
+      alt="Plate VI. Mimicry by Euralia sp." title="Plate VI. Mimicry by Euralia sp." /></a>
+    <span class="sc">Plate VI.</span>
+  </div>
+
+<p><!-- Page 147 --><span class="pagenum"><a name="page147"></a>{147}</span></p>
+
+  <p>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.</p>
+
+  <p>A suggestive contribution to this subject was recently made by G.&nbsp;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 <!--
+  Page 148 --><span class="pagenum"><a
+  name="page148"></a>{148}</span>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 <i>p</i> homozygotes of one kind, <i>r</i>
+  homozygotes of the other kind, and 2 <i>q</i> 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
+  <i>q</i><sup>2</sup> = <i>pr</i> 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 <i>q</i><sup>2</sup> = <i>pr</i> 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:&mdash;</p>
+
+<table class="nobctr" summary="Stable populations." title="Stable populations.">
+<tr><td class="spacsingle" align="center"> <i>p.</i></td><td class="spacsingle" align="center"> <i>2q.</i> </td><td class="spacsingle" align="center"> <i>r.</i></td></tr>
+<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 2 </td><td class="spacsingle" align="center"> 1</td></tr>
+<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 4 </td><td class="spacsingle" align="center"> 4</td></tr>
+<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 6 </td><td class="spacsingle" align="center"> 9</td></tr>
+<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 8 </td><td class="spacsingle" align="center"> 16</td></tr>
+<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 20,000 </td><td class="spacsingle" align="center"> 100,000,000</td></tr>
+<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 2<i>n</i> </td><td class="spacsingle" align="center"> <i>n</i><sup>2</sup></td></tr>
+</table>
+
+  <p>This, of course, assumes that all three classes are equally fertile,
+  and that no form of selection is taking place to the <!-- Page 149
+  --><span class="pagenum"><a name="page149"></a>{149}</span>benefit of one
+  class more than of another. Moreover, it makes no difference whether
+  <i>p</i> 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 <!-- Page 150 --><span class="pagenum"><a
+  name="page150"></a>{150}</span>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.</p>
+
+  <p>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 <!-- Page 151 --><span
+  class="pagenum"><a name="page151"></a>{151}</span>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 <i>Lathyrus odoratus</i>. 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 <i>Euralia
+  wahlbergi</i> and <i>E. mima</i> 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&mdash;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 <i>Mirabilis jalapa</i> cannot fertilise
+  <i>M.</i> <!-- Page 152 --><span class="pagenum"><a
+  name="page152"></a>{152}</span><i>longiflora</i>, 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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 153 --><span class="pagenum"><a name="page153"></a>{153}</span></p>
+
+<h3>CHAPTER XIV</h3>
+
+<p class="cenhead">ECONOMICAL</p>
+
+  <p>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<sub>2</sub> 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 <!--
+  Page 154 --><span class="pagenum"><a name="page154"></a>{154}</span>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.
+  <a href="#page44">44</a>), 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 <i>individually</i>, 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<sub>2</sub> generation if only a sufficient <!-- Page 155 --><span
+  class="pagenum"><a name="page155"></a>{155}</span>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.</p>
+
+  <p>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<sub>2</sub> 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<sub>2</sub> 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. <!-- Page 156
+  --><span class="pagenum"><a name="page156"></a>{156}</span></p>
+
+  <p>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<sub>2</sub> generation. Analysis shows that the difference between the
+  walnut <!-- Page 157 --><span class="pagenum"><a
+  name="page157"></a>{157}</span>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.</p>
+
+  <p>An excellent example of the practical application of Mendelian
+  principles is afforded by the experiments which Professor Biffen has
+  recently carried out in Cambridge. <!-- Page 158 --><span
+  class="pagenum"><a name="page158"></a>{158}</span>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 <!-- Page 159
+  --><span class="pagenum"><a name="page159"></a>{159}</span>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.</p>
+
+  <div class="figright" style="width:45%;">
+      <a href="images/177.png"><img style="width:100%" src="images/177.png"
+      alt="Fig. 30. Curves to illustrate the influence of selection." title="Fig. 30. Curves to illustrate the influence of selection." /></a>
+    <span class="sc">Fig.</span> 30.
+
+    <p class="cenhead">Curves to illustrate the influence of selection.</p>
+  </div>
+
+  <p>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 <!-- Page 160 --><span
+  class="pagenum"><a name="page160"></a>{160}</span>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 <!-- Page 161 --><span class="pagenum"><a
+  name="page161"></a>{161}</span>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>i.e.</i> 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>i.e.</i> 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 <!-- Page 162 --><span class="pagenum"><a
+  name="page162"></a>{162}</span>which the varying conditions of the
+  environment cause it to fluctuate. Each of these strains is termed a
+  <b>pure line</b>. 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</p>
+
+<table class="nobctr" summary="Stable populations." title="Stable populations.">
+<tr><td class="qspcsingle" align="center"> A </td><td class="qspcsingle" align="center"> </td><td class="qspcsingle" align="center"> fluctuating </td><td class="qspcsingle" align="center"> between </td><td class="qspcsingle" align="center"> 4 </td><td class="qspcsingle" align="center"> and </td><td class="qspcsingle" align="center"> 16 </td><td class="qspcsingle" align="center"> with </td><td class="qspcsingle" align="center"> a mean of </td><td class="qspcsingle" align="center"> 10</td></tr>
+<tr><td class="qspcsingle" align="center"> B </td><td class="qspcsingle" align="center"> </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 6 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 18 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 12</td></tr>
+<tr><td class="qspcsingle" align="center"> C </td><td class="qspcsingle" align="center"> </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 8 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 20 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 14</td></tr>
+</table>
+
+  <div class="figright" style="width:45%;">
+      <a href="images/179.png"><img style="width:100%" src="images/179.png"
+      alt="Fig. 31. Curves to illustrate the conception of pure lines in a population." title="Fig. 31. Curves to illustrate the conception of pure lines in a population." /></a>
+    <span class="sc">Fig.</span> 31.
+
+    <p class="cenhead">Curves to illustrate the conception of pure lines in
+    a population.</p>
+  </div>
+
+  <p>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.</p>
+
+  <p>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 <!-- Page
+  163 --><span class="pagenum"><a name="page163"></a>{163}</span>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.</p>
+
+  <p>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 <!-- Page 164
+  --><span class="pagenum"><a name="page164"></a>{164}</span>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.</p>
+
+  <p>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 <!-- Page 165 --><span
+  class="pagenum"><a name="page165"></a>{165}</span>running the risk of
+  losing it altogether, as is now so often the case.</p>
+
+  <p>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<sub>1</sub> 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 <!-- Page 166 --><span class="pagenum"><a
+  name="page166"></a>{166}</span>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. <!-- Page 167 --><span class="pagenum"><a
+  name="page167"></a>{167}</span></p>
+
+  <p>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.</p>
+
+  <p>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 <!-- Page 168
+  --><span class="pagenum"><a name="page168"></a>{168}</span>repetition, we
+  may safely ignore telegony as a factor in heredity.</p>
+
+  <p>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.</p>
+
+  <p>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 <!-- Page
+  169 --><span class="pagenum"><a name="page169"></a>{169}</span>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.</p>
+
+<hr class="full" />
+
+<p><!-- Page 170 --><span class="pagenum"><a name="page170"></a>{170}</span></p>
+
+<h3>CHAPTER XV</h3>
+
+<p class="cenhead">MAN</p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/189.jpg"><img style="width:100%" src="images/189t.jpg"
+      alt="Fig. 32. Normal and brachydactylous hands." title="Fig. 32. Normal and brachydactylous hands." /></a>
+    <span class="sc">Fig.</span> 32.
+
+    <p class="cenhead">Normal and brachydactylous hands placed together for
+    comparison. (From Drinkwater.)</p>
+  </div>
+
+  <div class="figright" style="width:40%;">
+      <a href="images/190.jpg"><img style="width:100%" src="images/190t.jpg"
+      alt="Fig. 33. Radiograph of a brachydactylous hand." title="Fig. 33. Radiograph of a brachydactylous hand." /></a>
+    <span class="sc">Fig.</span> 33.
+
+    <p class="cenhead">Radiograph of a brachydactylous hand.</p>
+  </div>
+
+  <p>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 <!-- Page 171 --><span class="pagenum"><a
+  name="page171"></a>{171}</span>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 <!-- Page 172 --><span class="pagenum"><a
+  name="page172"></a>{172}</span>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. <a href="#page173">173</a>. 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
+  <!-- Page 173 --><span class="pagenum"><a
+  name="page173"></a>{173}</span>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>i.e.</i> 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.</p>
+
+  <div class="figcenter" style="width:76%;">
+      <a href="images/192.png"><img style="width:100%" src="images/192.png"
+      alt="Fig. 34. Pedigree of brachydactylous family." title="Fig. 34. Pedigree of brachydactylous family." /></a>
+    <span class="sc">Fig.</span> 34.
+
+    <p class="cenhead">Pedigree of Drinkwater's brachydactylous family. The
+    affected members are indicated by black and the normals by light
+    circles.</p>
+  </div>
+
+<p><!-- Page 174 --><span class="pagenum"><a name="page174"></a>{174}</span></p>
+
+  <p>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.</p>
+
+  <p>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. <!-- Page 175 --><span class="pagenum"><a
+  name="page175"></a>{175}</span></p>
+
+  <div class="figcenter" style="width:45%;">
+      <a href="images/194.png"><img style="width:100%" src="images/194.png"
+      alt="Fig. 35. Pedigree of a hæmophilic family." title="Fig. 35. Pedigree of a hæmophilic family." /></a>
+    <span class="sc">Fig.</span> 35.
+
+    <p class="cenhead">Pedigree of a hæmophilic family. Affected (all
+    males) represented by black, and normals of both sexes by light
+    circles. (From Stahel.)</p>
+  </div>
+
+  <p>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.</p>
+
+  <p>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. <a
+  href="#page117">117</a>) we have already discussed an instance in which
+  the defect is rare, though not <!-- Page 176 --><span class="pagenum"><a
+  name="page176"></a>{176}</span>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 hæmophilia 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 hæmophilia 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
+  hæmophilic 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
+  hæmophilia must remain undecided.</p>
+
+  <p>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 <!-- Page 177
+  --><span class="pagenum"><a name="page177"></a>{177}</span>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&mdash;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
+  <!-- Page 178 --><span class="pagenum"><a
+  name="page178"></a>{178}</span>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.</p>
+
+  <p>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.</p>
+
+  <p>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. <!-- Page 179 --><span class="pagenum"><a
+  name="page179"></a>{179}</span>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.</p>
+
+  <p>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 <!-- Page
+  180 --><span class="pagenum"><a name="page180"></a>{180}</span>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.</p>
+
+  <p>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.,&mdash;qualities which we are accustomed to regard as
+  convenient units in classifying the different minds with which we are
+  daily brought into contact,&mdash;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
+  <!-- Page 181 --><span class="pagenum"><a
+  name="page181"></a>{181}</span>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.</p>
+
+  <p>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 <!-- Page 182 --><span
+  class="pagenum"><a name="page182"></a>{182}</span>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
+  <!-- Page 183 --><span class="pagenum"><a
+  name="page183"></a>{183}</span>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.</p>
+
+  <p>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.</p>
+
+  <p>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 <!-- Page 184 --><span class="pagenum"><a
+  name="page184"></a>{184}</span>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 <span class="correction" title="Original reads `simples'."
+  >simplest</span> cases, we must learn far more about them. At present we
+  are woefully ignorant <!-- Page 185 --><span class="pagenum"><a
+  name="page185"></a>{185}</span>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&mdash;the fateful race of gametes so irrevocably
+  bound to us by that closest of all ties, heredity.</p>
+
+<hr class="full" />
+
+<p><!-- Page 187 --><span class="pagenum"><a name="page187"></a>{187}</span></p>
+
+<h3>APPENDIX</h3>
+
+  <p>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
+  <b>selfed</b>.</p>
+
+  <p>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 <!-- Page
+  188 --><span class="pagenum"><a name="page188"></a>{188}</span>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.</p>
+
+  <p>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, <i>Megachile</i>, 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 <!--
+  Page 189 --><span class="pagenum"><a
+  name="page189"></a>{189}</span>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 <i>Megachile</i>.
+  Lastly, it not infrequently happens that the little beetle
+  <i>Meligethes</i> is found inside the keel. Such flowers should be
+  rejected for crossing purposes.</p>
+
+<hr class="full" />
+
+<p><!-- Page 191 --><span class="pagenum"><a name="page191"></a>{191}</span></p>
+
+<h3>INDEX</h3>
+
+  <div class="contents">
+    <div class="stanza">
+      <p><i>Abraxas grossulariata</i>, <a href="#page99">99</a></p>
+      <p>"Acquired" characters, <a href="#page14">14</a></p>
+      <p>Adaptation, <a href="#page143">143</a></p>
+      <p>Agouti mice, <a href="#page50">50</a></p>
+      <p>Albino mice, <a href="#page50">50</a></p>
+      <p>Albinos, nature of, <a href="#page53">53</a></p>
+      <p><i>Amauris</i>, <a href="#page144">144</a></p>
+      <p>Analysis of types, <a href="#page156">156</a></p>
+      <p>Ancestral Heredity, Law of, <a href="#page13">13</a></p>
+      <p>Andalusian fowls, <a href="#page70">70</a></p>
+      <p>Axil colour in sweet peas, <a href="#page93">93</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Bateson, W., <a href="#page14">14</a>, <a href="#page29">29</a>, <a href="#page55">55</a>, <a href="#page116">116</a>, <a href="#page132">132</a>, <a href="#page141">141</a></p>
+      <p>Biffen, R. H., <a href="#page157">157</a></p>
+      <p>Blue Andalusian fowls, <a href="#page71">71</a></p>
+      <p>Brachydactyly, <a href="#page171">171</a></p>
+      <p>Bryony, <a href="#page120">120</a></p>
+      <p>Bush sweet peas, <a href="#page63">63</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Castle, <a href="#page132">132</a></p>
+      <p>Cattle, horns in, <a href="#page86">86</a>, <a href="#page166">166</a></p>
+      <p>Colour, nature of, in flowers, <a href="#page48">48</a></p>
+      <p>Colour-blindness, <a href="#page117">117</a></p>
+      <p>Combs of fowls, <a href="#page33">33</a>, <a href="#page43">43</a></p>
+      <p>Correns, C., <a href="#page29">29</a>, <a href="#page120">120</a></p>
+      <p>Coupling of characters in gametes, <a href="#page93">93</a></p>
+      <p>Cuénot, <a href="#page50">50</a>, <a href="#page119">119</a></p>
+      <p>"Cupid" sweet peas, <a href="#page62">62</a></p>
+      <p>Currant moth, <a href="#page99">99</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Darwin, C., <a href="#page10">10</a>, <a href="#page65">65</a>, <a href="#page147">147</a>, <a href="#page163">163</a></p>
+      <p>De Vries, H., <a href="#page15">15</a>, <a href="#page29">29</a>, <a href="#page141">141</a></p>
+      <p>Discontinuity in variation, <a href="#page14">14</a></p>
+      <p>Dominant characters, <a href="#page18">18</a></p>
+      <p>Doncaster, L., <a href="#page99">99</a></p>
+      <p>Drinkwater, H., <a href="#page172">172</a></p>
+      <p>Dutch rabbits, <a href="#page60">60</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Eggs, <a href="#page2">2</a></p>
+      <p>Environment, influence of, <a href="#page137">137</a></p>
+      <p><i>Euralia</i>, <a href="#page144">144</a></p>
+      <p>Evolution, <a href="#page10">10</a>, <a href="#page85">85</a>, <a href="#page139">139</a></p>
+      <p>Eye, in primulas, <a href="#page55">55</a></p>
+      <p>Eye-colour, in man, <a href="#page176">176</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Factor, definition of, <a href="#page31">31</a></p>
+      <p>Factors, interaction of, <a href="#page42">42</a></p>
+      <p>Fertilisation, <a href="#page3">3</a></p>
+      <p>Fertilisation, self- and cross-, <a href="#page163">163</a></p>
+      <p>Fixation of varieties, <a href="#page153">153</a></p>
+      <p>Fluctuations, <a href="#page138">138</a></p>
+      <p>Fowls, coloured from whites, <a href="#page49">49</a>, <a href="#page73">73</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Galton, <a href="#page13">13</a>, <a href="#page179">179</a></p>
+      <p>Gametes, nature of, <a href="#page6">6</a></p>
+      <p>Gregory, R. P., <a href="#page55">55</a>, <a href="#page93">93</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Hæmophilia, <a href="#page176">176</a></p>
+      <p>Hardy, G. H., <a href="#page147">147</a></p>
+      <p>Heterozygote, definition of, <a href="#page28">28</a></p>
+      <p>Heterozygote, of intermediate form, <a href="#page68">68</a></p>
+      <p><i>Hieracium</i>, <a href="#page27">27</a>, <a href="#page132">132</a></p>
+      <p>Himalayan rabbits, <a href="#page60">60</a></p>
+      <p>Homostyle primulas, <a href="#page56">56</a></p>
+      <p>Homozygote, definition of, <a href="#page28">28</a></p>
+      <p>Hooded sweet peas, <a href="#page89">89</a></p>
+      <p>Horses, bay and chestnut in, <a href="#page167">167</a></p>
+      <p>Hurst, C. C., <a href="#page62">62</a>, <a href="#page176">176</a>, <a href="#page180">180</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Immunity in wheat, <a href="#page158">158</a></p>
+      <p>Individuality, <a href="#page135">135</a></p>
+      <p>Inhibition, factors for, <a href="#page74">74</a>, <a href="#page108">108</a></p>
+      <p>Intermediates, <a href="#page125">125</a></p>
+<!-- Page 192 --><span class="pagenum"><a name="page192"></a>{192}</span>
+    </div>
+
+    <div class="stanza">
+      <p>Johannsen, W., <a href="#page160">160</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Lop-eared rabbits, <a href="#page132">132</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Mendel, <a href="#page8">8</a>, <a href="#page17">17</a>, <a href="#page26">26</a>, <a href="#page132">132</a></p>
+      <p>Mental characters, <a href="#page180">180</a></p>
+      <p>Mice, inheritance of coat colour in, <a href="#page50">50</a></p>
+      <p>Mimicry, <a href="#page143">143</a></p>
+      <p><i>Mirabilis</i>, <a href="#page151">151</a></p>
+      <p>Morgan, T. H., <a href="#page116">116</a></p>
+      <p>Mulattos, <a href="#page129">129</a></p>
+      <p>Mutation, <a href="#page83">83</a>, <a href="#page138">138</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Nägeli, C., <a href="#page26">26</a></p>
+      <p>Natural selection, <a href="#page11">11</a>, <a href="#page140">140</a>, <a href="#page142">142</a>, <a href="#page149">149</a></p>
+      <p>Nettleship, E., <a href="#page175">175</a></p>
+      <p>Night-blindness, <a href="#page175">175</a></p>
+    </div>
+
+    <div class="stanza">
+      <p><i>Pararge egeria</i>, <a href="#page132">132</a></p>
+      <p>Parkinson, J., <a href="#page122">122</a></p>
+      <p>Pea comb, <a href="#page33">33</a></p>
+      <p>Peas, coloured flowers in, <a href="#page24">24</a></p>
+      <p>Peas, tall and dwarf, <a href="#page18">18</a></p>
+      <p>Pigeons, <a href="#page86">86</a></p>
+      <p>Pin-eye in primulas, <a href="#page55">55</a></p>
+      <p><i>Pisum</i>, <a href="#page17">17</a></p>
+      <p>Primulas, <a href="#page31">31</a>, <a href="#page55">55</a>, <a href="#page68">68</a>, <a href="#page93">93</a></p>
+      <p>Pollen, <a href="#page3">3</a></p>
+      <p>Pollen of sweet peas, <a href="#page92">92</a></p>
+      <p>Pomace fly, <a href="#page115">115</a></p>
+      <p>Population, inheritance of characters in a, <a href="#page147">147</a></p>
+      <p>Presence and Absence theory, <a href="#page35">35</a></p>
+      <p>Pure lines, <a href="#page162">162</a></p>
+      <p>Purity of gametes, <a href="#page24">24</a></p>
+      <p>Purity of type, <a href="#page155">155</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Rabbits, <a href="#page53">53</a>, <a href="#page60">60</a></p>
+      <p>Ratios, Mendelian&mdash;</p>
+      <p class="i2">3&nbsp;:&nbsp;1, <a href="#page20">20</a></p>
+      <p class="i2">9&nbsp;:&nbsp;3&nbsp;:&nbsp;3&nbsp;:&nbsp;1, <a href="#page25">25</a>, <a href="#page34">34</a></p>
+      <p class="i2">9&nbsp;:&nbsp;3&nbsp;:&nbsp;4, <a href="#page51">51</a></p>
+      <p class="i2">9&nbsp;:&nbsp;7, <a href="#page49">49</a></p>
+      <p>Ray, John, <a href="#page143">143</a></p>
+      <p>Recessive characters, <a href="#page19">19</a></p>
+      <p>Repulsion between factors, <a href="#page90">90</a></p>
+      <p>Reversion, <a href="#page59">59</a>, <a href="#page165">165</a></p>
+      <p class="i2">in rabbits, <a href="#page59">59</a></p>
+      <p class="i2">in sweet peas, <a href="#page62">62</a></p>
+      <p class="i2">in fowls, <a href="#page65">65</a></p>
+      <p class="i2">in pigeons, <a href="#page65">65</a></p>
+      <p>Rose comb, <a href="#page33">33</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Saunders, E. R., <a href="#page54">54</a>, <a href="#page122">122</a></p>
+      <p>Seeds, nature of, <a href="#page4">4</a></p>
+      <p>Segregation, <a href="#page22">22</a></p>
+      <p>Selection, <a href="#page162">162</a></p>
+      <p>Sheep, horns in, <a href="#page76">76</a></p>
+      <p>Silky fowls, <a href="#page30">30</a>, <a href="#page105">105</a></p>
+      <p>Single comb, <a href="#page32">32</a></p>
+      <p>Species, nature of, <a href="#page150">150</a></p>
+      <p>Species, origin of, <a href="#page11">11</a></p>
+      <p>Speckled wood butterfly, <a href="#page132">132</a></p>
+      <p>Spermatozoa, <a href="#page3">3</a></p>
+      <p>Sports, <a href="#page147">147</a></p>
+      <p>Staples-Browne, R., <a href="#page66">66</a></p>
+      <p>Sterility, <a href="#page151">151</a></p>
+      <p>Sterility in sweet peas, <a href="#page93">93</a></p>
+      <p>Stocks, double, <a href="#page122">122</a></p>
+      <p>Stocks, hoariness in, <a href="#page54">54</a></p>
+      <p>Sweet pea, colour in, <a href="#page44">44</a>, <a href="#page79">79</a></p>
+      <p class="i2">history of, <a href="#page82">82</a></p>
+      <p class="i2">inheritance of hood in, <a href="#page89">89</a></p>
+      <p class="i2">inheritance of size in, <a href="#page62">62</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Telegony, <a href="#page167">167</a></p>
+      <p>Thrum-eye in primulas, <a href="#page55">55</a></p>
+      <p>Toe, extra toe in poultry, <a href="#page76">76</a></p>
+      <p>Tschermak, E., <a href="#page29">29</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Unit-character, definition of, <a href="#page31">31</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Variation, <a href="#page14">14</a>, <a href="#page137">137</a>, <a href="#page139">139</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Walnut comb, <a href="#page33">33</a></p>
+      <p>Weismann, A., <a href="#page13">13</a></p>
+      <p>Wheat, beard in, <a href="#page74">74</a></p>
+      <p class="i2">experiments with, <a href="#page157">157</a></p>
+      <p>White, dominant in poultry, <a href="#page72">72</a></p>
+      <p>Wilson, J., <a href="#page168">168</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Yellow mice, <a href="#page119">119</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Zygotes, nature of, <a href="#page5">5</a></p>
+    </div>
+  </div>
+
+<hr class="full" />
+
+<h2>Notes</h2>
+
+<hr class="short" />
+
+<div class="note">
+  <p><a name="footnote1" href="#footnotetag1">[1]</a> Cf. note on p. <a
+  href="#page171">171</a>.</p>
+
+  <p><a name="footnote2" href="#footnotetag2">[2]</a> It has been found
+  convenient to denote the various generations resulting from a cross by
+  the signs F<sub>1</sub>, F<sub>2</sub>, F<sub>3</sub>, etc. F<sub>1</sub>
+  on this system denotes the first filial generation, F<sub>2</sub> the
+  second filial generation produced by two parents belonging to the
+  F<sub>1</sub> generation, and so on.</p>
+
+  <p><a name="footnote3" href="#footnotetag3">[3]</a> Hurst's original
+  cross was between a Belgian hare and an albina Angora, which <span
+  class="correction" title="Original reads `turned to out be'.">turned out
+  to be</span> a masked Dutch.</p>
+
+  <p><a name="footnote4" href="#footnotetag4">[4]</a> The Spot is an almost
+  white bird, the colour being confined to the tail and the characteristic
+  spot on the head.</p>
+
+  <p><a name="footnote5" href="#footnotetag5">[5]</a> 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.</p>
+
+  <p><a name="footnote6" href="#footnotetag6">[6]</a> 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.</p>
+
+  <p><a name="footnote7" href="#footnotetag7">[7]</a> 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.</p>
+
+  <p><a name="footnote8" href="#footnotetag8">[8]</a> For the most recent
+  discussion of this peculiar case the reader is referred to Professor
+  Castle's paper in <i>Science</i>, December 16, 1910.</p>
+
+  <p><a name="footnote9" href="#footnotetag9">[9]</a> <i>Paradisus
+  Terrestris</i>, London, 1629, p. 261.</p>
+
+</div>
+
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<hr class="pg" />
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+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.
+
+
+
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