THE LESSON OF HEREDITY 



sity of descendants? Heredity, unaided, can give but 

 one answer to this question. It is because the elements 

 of this conglomerate ancestry have not been mixed 

 equally. In other words, because of marriages in 

 different degrees of consanguinity. 



The answer is not sufficient, yet it can account for 

 much. Let us examine it before seeking for other 

 causes. 



In the nature of the case, if men are all descended 

 from a common stock, all marriages must be in some 

 degree of consanguinity. But the degree may vary 

 from the incestuous union of brother and sister, which 

 was legalized among the ancients, or the marriage of 

 cousins, which is the limit fixed by most modern 

 civilizations, to the usual cases in which all trace of 

 relationship has long since been lost. 



At first sight, it is perhaps not apparent why marriages 

 in close degrees of consanguinity should be of especial 

 significance in their bearing on the problems of heredity. 

 But a moment's reflection will make this plain. In the 

 first place, a consanguineous union greatly restricts 

 the variety of tendencies of the descendants. A person 

 whose parents are cousins, for example, has only six 

 great grandparents, instead of the normal number of 

 eight; and thus, to carry the computation no farther 

 than that generation, his aggregate tendencies are 

 restricted in diversity by one-fourth in itself a serious 

 matter. And, in the second place, certain of these 

 restricted tendencies may be accentuated in a way that 

 may be yet more serious. These are the tendencies of 

 two great-grandparents in whom both lines of descent 



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