Such ultimate causes of the differentiation of mental qualities must have been of two orders only, so far as I can see: (1) differences of physical environment; (2) spontaneous variations in different directions of the innate mental qualities of individuals, especially of the more gifted and energetic individuals of each people. In the mental evolution of animals these two factors are not distinguishable. We may say that the main and perhaps the sole condition of their evolution is the selection by the physical environment of spontaneous favourable variations and mutations of innate mental qualities; if we include under the term physical environment of the species all the other animal and vegetable species of its habitat. For it is only by its selective influence upon individual variations that physical environment can determine differentiation of races. But with man the case is different; spontaneous variation not only provides the new qualities which, by determining the survival of the individual in his struggle for existence with the physical environment, secure their own perpetuation by transmission to the after coming generations. The new qualities determine mental evolution in another manner, by a mode of operation which is almost completely absent in animal evolution; namely, the spontaneous variations create a social environment which profoundly modifies the influence of the physical environment, and itself becomes a principal factor in the determination of the trend of racial evolution.