It seems probable that man was evolved from his prehuman ancestry as a single stock, probably a stock somewhat widely distributed in the heart of the Eurasian continent, or possibly in Africa according to the recent view of some authors, or in the area which is now the Indian Ocean. If this be true, it follows that the differentiation of the mental and physical qualities of the principal human races, the differentiation of the white and black and yellow and brown races, as well as of the chief subraces, such as the Semitic, the races of Europe—the _Homo Europaeus_, _Alpinus_ and _Mediterraneus_—was the work of the immensely prolonged prehistoric period. For these races and subraces, as we now know them, seem to have been in existence and to have had recognisably and substantially the same leading qualities, both mental and physical, that they now have, before the beginning of the historic period. The racial differentiation during the prehistoric period must have been much greater than during the historic period; and this was not only because the former period was immensely longer, but also because, in all probability, the rate of racial change has been on the whole slower in the historic period. The differentiation of racial types in the prehistoric period must have been in the main the work of differences of physical environment, operating directly by way of selection, by way of the adaptation of each race to its environment through the extermination of the strains least suited to exist under those physical conditions. But this process, this direct moulding of racial types by physical environment, must have been well nigh arrested as soon as nations began to form. For the formation of nations implies the beginning of civilisation; and civilisation very largely consists in the capacity of a people to subdue their physical environment, or at least to adapt the physical environment to men’s needs to a degree that renders them far less the sport of it than was primitive man; it consists, in short, in replacing man’s natural environment by an artificial environment largely of his own choice and creation.