Page:Popular Science Monthly Volume 60.djvu/456

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ZOOLOGICALLY considered, the human race constitutes a single species which has in the course 'of time become subdivided into a number of ethnic varieties. Scientifically speaking, this differentiation 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 environmental agencies operating through the process of selection upon the attribute of variability inherent in the anthropoid line.

Recent 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 geographical equator, the climate of the northern and southern hemispheres 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 configuration 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 conditions, but place peculiarities became permanently established and regions of the earth differed from each other in climate and topography. So instead of the human race being affected as before by a varying environment, varied environments henceforth influenced different 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-