lky 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 the