INTRODUCTION TO SOCIOLOGY 563
The result of selection and adaptation in all cases is the cre- ation, and later the fixation, in the species of social or organic conditions which are advantageous to the individual.
The formation of new races of the human species is a necessary condition to the extension of the limits of the human species. Like the formation of new species in general throughout organic nature, it implies two indispensable, though apparently contra- dictory, factors. Indeed, these factors admirably complement each other and indissolubly conspire to create a new variety of the human species capable of adjusting itself to an environment dif- ferent from that which primarily existed. Darwin 1 indicates very admirably the progress which brings about, in connection with the formation of species in general, the balancing of its two antagonistic but concurrrent forces. Darwin says :
The conclusion to which I have arrived, is that the regions where the species are most numerous are those which have been most often isolated and separated from the regions, then united anew, and again separated. This requires a long period of time and some changes in the external con- ditions. The most general conclusion suggested to me by the geographic distribution of organic beings is that isolation is the chief or concomitant
factor in the appearance of new species I know that there are some
striking exceptions.
The glacial period, which rendered uninhabitable the very center of the European and Asiatic continent, produced a vast isolation of this kind, succeeding a previous reunion favored by a more homogeneous climate, which itself, in rescattering the species over greater diversified areas, had already isolated them. The species, including the human species, were driven back toward the north and south, and were separated for a long time. Organic peculiarities were formed and fixed by heredity, thanks to adaptation and to selection. Several human races became fixed and isolated, which later entered into communication with each other, scattering themselves anew, and in rescattering isolated themselves in regions naturally separated, where they formed new varieties. These extensions and successive isola- tions are especially remarkable in mountainous countries, such as the Hellenic peninsula, Germany, Spain, and Switzerland, where the periodic tides of invasions permit races to live side by
1 Vie ft correspondence, Vol. I, p. 504.
