I was thinking as we went through the first three sessions of this symposium of a difficulty faced by human geneticists which is not faced by those who are concerned with the genetics of plants and animals, and which I do not think has been mentioned—though, certainly, it was implied in Dr Harrison's Galton Lecture.

The geneticist of plants and animals, if he is concerned with wild plants and wild animals, is considering their viability in a particular environment. The geneticist who is concerned with domestic strains is concerned with something which he knows much more precisely. He knows what he wants; he knows that he wants a cow that will give more milk, or a hen that will give more eggs.