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Chap. III. HEREDITY AND HUMAN PROBLEMS

Whereas the fruit fly breeds rapidly, has large families, and is capable of being brought up in large numbers in small bottles, human beings breed slowly, have very small families, and though they can be brought up on bottles they cannot be brought up in them. Whereas, too, the fruit fly has only four pairs of chromo­ somes, man has 24 pairs. Again, whereas with the fruit fly it is possible to bring up large numbers with the same or with different genetic constitutions in the same or in different environments, it is only on very rare occasions-i.e. identical twins-that human beings have the same genetic constitution, and the environments in which they are brought up can certainly only very rarely, and perhaps never, be made identical. It therefore follows that the genetic study of human beings presents an extremely difficult task. W hat certain knowledge there is derives from medical practice. On the psychological side (apart from certain mental abnormalities which are due to organic causes) very little certain knowledge exists about the mechanism of heredity.