The two words which I have chosen as the title of this discourse are technical terms. Round them whole regions of modern biological discovery are grouped. I am warned that terms unfamiliar to the laity may repel an audience; but since the common speech as yet contains no good equivalents by which the concepts gamete and zygote may be rendered, and my purpose this afternoon is to set forth the leading ideas which Science has gained from those concepts, I therefore make so bold as to begin by naming them. The ideas we derive from a knowledge of the inter-relations of gamete and zygote are of so wide an application that they cannot be ignored by any person who aspires to form just views of the nature of man and of life. No one having learnt these ideas can look on the world with quite the same eyes as before. They have something of that fundamental quality which we associate with chemistry, and just as it is impossible to talk easily or freely of any natural phenomena with those who are wholly ignorant of chemistry, so are we conscious of difficulty when we discuss not merely formal problems of evolution or of heredity, but any broad question of social organisation with those wholly ignorant of genetic fact.