294 Heredity and Environment 



line, and he shows that the last male died unmarried in 45 per 

 cent of these families, and before the age of 21 in 39 per cent, 

 while the line ended in infertile marriage in n per cent and in 

 daughters only in 5 per cent. 



The extinction of families, however, is often confused with the 

 extinction of family names, which means only that the family 

 has died out in the direct male line. Biological inheritance does 

 not necessarily follow family names. Owing to the elimination 

 of one-half of the chromosomes in the formation of the sex cells 

 and the replacing of these in fertilization by chromosomes from 

 another source it happens that many persons bear the name of 

 some progenitor but do not have a single one of his chromosomes 

 or inherited traits ; on the other hand, many persons who do not 

 bear his name may have some of his chromosomes and traits. As- 

 suming that there are 48 chromosomes in the human species and 

 that these never break up or lose their identity it is evident that 

 no person can inherit from more than 48 ancestors though he may 

 be descended from an innumerable number. 



Much confusion is caused also by the expression "hereditary 

 lines," as if each family were separate and distinct from all 

 others. But this is, of course, never true. The only hereditary 

 lines which exist are those of individual chromosomes or genes 

 and these divide and diverge like the branches of a tree. An 

 individual containing many chromosomes received from many 

 sources belongs to no single hereditary line, but rather to a net- 

 work of many lines. 



It has been said that if the birth rate of the "Mayflower" fam- 

 ilies continues to decrease at the present rate for the next 300 

 years, all the survivors at that time could be sent back in the 

 original "Mayflower." But there is no reason to suspect that 

 the decreasing birth rate will go on indefinitely at a constant ratio, 

 and to assume that it will do so is merely to look forward to the 

 extinction of all families, classes, races and nations in which the 

 birth rate has been decreasing ; this includes practically the entire 

 population of the United States and Western Europe and it is 



