74 Evolution of Intelligence 



then we know the circumstances which produce 

 'the original changes, and can control them. If 

 changes in longevity is the leader in this matter, 

 then we must assume some mysterious agency 

 as being the cause. To say that an increase of 

 longevity over what existed at some earlier date 

 is a "mutation," is such an assumption of the 

 mysterious. 



We have a little direct evidence on this mat- 

 ter in the history of races of men at present liv- 

 ing. Some are much longer lived than other 

 ones. They are also the ones which reproduce 

 at the latest average age. When we go back into 

 the marriage customs of the past we find that 

 those races in which longevity is greatest today 

 are those which are descended from people who 

 long ago introduced exogamy, wife capture, wife 

 purchase, and other tribal customs which raised 

 the average age at reproduction. Those races 

 which are lowest and in which longevity is least, 

 are those races which have not artificially de- 

 layed the average age of reproduction. They are 

 the races in which boys and girls are free to 

 reproduce as soon as they are so disposed. 



