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It is found in communities where there is a tendency to inbreed. Thus it is rife among peasant families in the south of Sweden and among Polish Jews, both of which peoples intermarry extensively. In such communities the probabilities are that both parents, even if their relationship is not so close as that of first cousins, will be heterozygous for the gene. That is to say, these people carry the factor inherited from one and not both of their parents for amaurotic idiocy in their gene make up, but they themselves do not suffer from the disease. The trouble arises, however, when two such people of the same constitution marry. For in the combination the amaurotic idiocy is brought out in about one quarter of the offspring.
Favorable as well as unfavorable genes may be transmitted in the same way. A study of this comes from the mountainous districts of Cape colony, South Africa. Settlers in one valley are notably healthy, both much interbreeding because of the difficulties of communicating with neighbors. Settlers in another valley, where the original stock was not so sound, have degenerated in a marked degree, cases of cretinism and dwarfism being frequent. Again the explanation is that unfavorable genes masked in the ancestor who had received from one of his parents only and who therefore suffered no ill effects himself, were passed on and combining with their own kind time after time, produced the degeneracy.
Brother and sister unions, which take place even today in some parts of the world, do not necessarily result in degenerate offspring. An excellent illustration of this is afforded by the Pharaohs of Egyptr, who maintained a high level of physical and mental fitness through many generations. This is true, too, of the Incas, the king priest class of Peru, who also were brother and sister bred. The marriage of first cousins is not necessarily objectionable, it depends very much whether the parents came from a sound stock or not.
Before medicine and surgery had reached their present pitch of efficiency, weaklings tended to die, thus leaving society to be perpetuated for the most part by its more virile members. Today, medical care does not every thing possible to see that the weak survive. They marry and have children, to whom in some measure they hand down their own disabilities.
Galton, when he started to work out the trend of population, became alarmed at this state of affairs, and initiated a movement whose object was the betterment of the human race. This developed into a science which we now know as eugenics. Eugenics seeks to apply our knowledge of heredity and environment in order to raise the standard of human material. Galton and his followers strongly advocated the sterilization of all who are carriers of gross abnormalities such as insanity, or gross physical defects such as lobster claw. This is a condition in which the fingers and the toes are so united that either there are only two fingers or toes instead of five, or all five are joined together. It is obvious that such a condition is a grave handicap in industrial life.
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