144 THE FOUNDATIONS OF ZOOLOGY 



Each person has two parents, and four grandparents; but 

 even in a country like ours, which draws its people from all 

 quarters of the earth, each of the eight great-grandparents is not 

 always a distinct person ; for when the parents are cousins, this 

 number is six, or five, or even four, instead of eight. Among 

 more primitive folks, who stay at home generation after genera- 

 tion, and marry neighbors, a person whose ancestors have trans- 

 gressed none of our social laws may have a minimum ancestry 

 of only four in each generation. The maximum and the mini- 

 mum fixed by our customs are given, for ten generations, in the 

 two lines below: 



2 4816 32 64 128 256 512 1024 . . . 2046 



2-4-4-4-4-4- 4 - 4 - 4- 4 3$ 



Few persons who can trace their ancestry for ten generations 

 with completeness are descended from 1024 distinct persons in 

 the tenth generation; and in all old stable communities of simple 

 folks the number is very much smaller. In the long run, fhe 

 number of ancestors in each generation is determined by the 

 average sexual environment, and it must be a small and pretty 

 constant number. 



All genealogical study gives indirect evidence of this familiar 

 fact, which has not been adequately recognized by students of in- 

 heritance. I have made a computation from the genealogical his- 

 tory of the people of a small island on our coast. These people 

 lead a simple life, or at least they have done so in the past; but 

 most of the men have been sailors, and have ranged much farther 

 in search of mates than agricultural people. I have selected three 

 persons whose ancestry is recorded in detail for some seven or 

 eight generations. These three persons would not be popularly 

 regarded as near relations, for they have no parents or grand- 

 parents with like names, although two of the grandparents were 

 cousins. The generations are not quite parallel, for the period 

 covered by eight in one line is covered by seven in the two others, 

 and the average is about seven and a half. 



In seven and a half generations the maximum ancestry for one 

 person is 382, or, for three persons, 1 146. The names of 452 of 

 them, or nearly half, are recorded, and these 452 named ancestors 



