406 PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 



come filled with a human population there is no such chance 

 for free development, for, then the average is maintained by 

 means of the prevailing uniformity both of physical structure 

 and of prevailing customs. In this respect as in many others, 

 the present is by no means a measure of the past. To this the 

 Glacial period itself bears indubitable testimony. During the 

 extraordinary changes in physical conditions throughout that 

 period, natural selection itself would act with abnormal rapidity 

 in the production of varieties and species, thus telling against 

 extravagant estimates of human anticjuity. 



