Evolution of Intelligence 75 



There is no uncertainty about this matter. 

 Average age at reproduction and average longev- 

 ity have changed in races of men now living, 

 and changes in one have followed changes in the 

 other. We know that the direct or immediate 

 cause of change in average age at reproduction 

 has been usually some military necessity or tribal 

 custom based upon military necessity. We can- 

 not assume that this military necessity grew out 

 of some spontaneous variation in the germ-plasm 

 which increased natural longevity. The only 

 logical conclusion is that natural longevity is de- 

 termined by the average age at which ancestors 

 have reproduced during several preceding gen- 

 erations. 



To get a line on this matter I made a direct 

 test of the length of life of different brothers 

 and sisters in the same family. A person's 

 natural longevity is not determined by the length 

 of time he lives, because practically no one lives 

 the full term of his natural life. Each person is 

 either killed or kills himself before he reaches 

 the point of dying of old age. While age at 

 death is no criterion of a person's longevity, 

 still, by taking a large number of persons prop- 



