superior member with reference to locomotion that the functions of prehension and of manipulation of the hand have been able to acquire adaptations the most varied. The perfecting of the tactile sense must have been an immediate result of this emancipation. This result must have involved the acquisition of a multitude of new notions suggesting new movements, new actions. From that, the multiplication of the movements of the lingers and of their combinations, the increase in manual skill and all the psychologic consequences, reacting the one upon the other, which must have been produced necessarily, by increase, in variety and complexity, of newly acquired motive and sensorial representations. On this subject, I could not do better than to refer the reader to the beautiful pages devoted by Herbert Spencer to the parallelism of the sensorial and motor improvement in the animal series together with the intellectual improvement.[1]
It is impossible to say, even approximately, to what augmentation of cerebral weight the transformation in question may correspond, but there are grounds for believing that this augmentation must have been considerable, all the more so since the intellectual growth in question must have influenced simultaneously the sensorial and motive manifestations, and the order of sensations the psychologic importance of which is extreme, and the order of movements (the movements of the fingers) very numerous and which we know to be of great help in the function of expression. This function is perhaps the most important to be considered here, because its progress reacts in a capital manner upon intellectual and social development. It may have been noticed, among divers savage peoples, how much the language by gesture makes up for the imperfections of the spoken language; it is then allowable to suppose that the movements of the hands and of the fingers figured largely among the primitive means of expression of Pliocene man.
I do not believe it is possible to cite any ulterior cause of psychologic progress and of increase in brain weight comparable to the emancipation of the superior members with which . we have just been occupied. The perfecting of articulate language must have been consequently the principal factor supervening in the psychologic and cerebral progress, to which would be due the superiority of the lowest existing races over the Pithecanthropus.
The quantitative cerebral progression has been accompanied by an improvement in the general form of the brain. This improvement is already perceptible in the Pithecanthropus according to the general form of the skull; it seems, however,
- ↑ H. Spencer, Principles of Psychology, vol. i.
