ABSTRACT
Fifty years of research into human genetics has made it clear that human beings abide by the same laws of heredity as other animals do. There are thousands of human pedigrees that illustrate our conformity to the Mendelian laws. Polymorphism seems to arise from two main causes. The first is when, for any reason whatsoever, it is an advantage for the population to be subdivided into two or more distinct types which depend upon and therefore sustain each other. The other main cause of polymorphism is when, for any reason whatsoever, a so-called ‘heterozygote’ is fitter than a homozygote. Human beings are to some extent committed to a genetic system which attaches a certain weight, perhaps great weight, to there being many different kinds of men. The superior fitness conferred by heterozygous constitutions is sometimes referred to as ‘heterosis’ or ‘hybrid vigour’, but agreed meanings have not yet taken firm shape.
