R. C. PUNNETT
Reading edition
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.
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 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 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.
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
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
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 F2 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
10. Small and Large-eyed Primulas 55
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 F1 (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 × F1 Cock 111
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. F2 Generation ex Silky Hen × Brown Leghorn Cock 127
29. Pedigree of Eurasian Family 131
30. Curve illustrating Influence of Selection 159
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
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.