312 Heredity and Environment 



than half. The causes of this decline are chiefly voluntary being 

 assigned to health, expense and other causes. 



Death of Families. But the causes of sterility are not only 

 social and voluntary ones, which could be changed by custom and 

 public opinion; there are also involuntary and biological causes 

 of a deep-seated nature. Fahlenbeck has made a study of 433 

 noble families of Sweden which have become extinct in the male 

 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 contemporary 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- 



