{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