448 POPULAR SCIENCE MONTHLY. 



THE DIFFEEENTIATION OF THE HUMAK SPECIES. 



By Professor LINDLEY M. KEASBEY, 

 bryn mawr college. 



ZOOLOGICALLY considered, the human race constitutes a single 

 ^-^ species which has in tlie course 'of time become subdivided into 

 a number of ethnic varieties. Scientifically speaking, this differentia- 

 tion of the human species into ethnic stocks is an instance of organic 

 variation, and, as such, subject to biological interpretation. Applying 

 the general principles of organic evolution to the particular case, ethnic 

 differentiation may, accordingly, be regarded as the result of environ- 

 mental agencies operating through the process of selection upon the 

 attribute of variability inherent in the anthropoid line. 



Eecent paleontological evidence renders it reasonably certain that 

 mankind was homogeneous before the ice-age, and that heterogeneity 

 entered in during the glacial period. It may be taken for granted, 

 therefore, that the first stages of ethnic differentiation were established 

 by the great geographic changes that occurred at this time. As the 

 ice-sheets advanced successively from the arctic and antarctic regions 

 and the thermal equator oscillated at long intervals about the geo- 

 graphical equator, the climate of the northern and southern hemi- 

 spheres alternated between equable and frigid conditions, and the 

 surface of the earth was modified accordingly. Thus so long as the 

 ice-age lasted, the human race was subjected to the influences of a 

 varying environment. Upon the final retreat of the glaciers, the con- 

 figuration of the earth gradually assumed its historic form and the 

 globe became divided as at present into temperature zones. After this, 

 time introduced no further changes in the general geographic condi- 

 tions, but place peculiarities became permanently established and 

 regions of the earth differed from each other in climate and topog- 

 raphy. So instead of the human race being affected as before by a 

 varying environment, varied environments henceforth influenced dif- 

 ferent portions of mankind. The effect of these varying and varied 

 environments was first to alter, and then to diversify, the conditions 

 of human survival. This necessitated adaptation on man's part, which 

 in turn led to the differentiation of the human species; so that by 

 the close of the glacial period heterogeneity was established where 

 homogeneity had previously prevailed. 



Owing to the numerous instances of migration and miscegenation 

 that have occurred since the ice-age, the ramifications of race are now- 



