The leading feature in the development and separation of man from amongst other animals is undoubtedly the relatively enormous size of the brain in man, and the corresponding increase in its activities and capacity. It is a very striking fact that it was not in the ancestors of man alone that this increase in the size of the brain took place at this same period, viz. the Miocene. The great mammals such as the titanotherium, which represented the rhinoceros in early Tertiary times, had a brain which was in proportion to the bulk of the body, not more than one-eighth the volume of the brain of the modern rhinoceros (see Fig. 5). Other great mammals of the earlier Tertiary period were in the same case; and the ancestors of the horse, which are better known than those of any other modern animal, certainly had very much smaller brains in proportion to the size of their bodies than has their descendant. [Illustration: FIG. 5. Four casts of the brain-cavities of a series of large Ungulate Mammals in order to shew the relatively small size of the cerebral hemispheres of the extinct creature from which A is taken. A is that of _Dinoceras_, a huge extinct Eocene mammal which was as large as a Rhinoceros; B is that of Hippopotamus; C of Horse; and D of Rhinoceros.] We may well ask to what this sudden and marked increase in the size of the brain in several lines of the animal pedigree is due. It seems that the inborn hereditary nervous mechanism by which many simple and necessary movements of the body are controlled and brought into relation with the outer world acting upon the sense-organs, can be carried in a relatively small bulk of brain-substance. Fish, lizards, and crocodiles with their small brains carry on a complex and effective life of relation with their surroundings. It appears that the increased bulk of cerebral substance means increased ‘educability’--an increased power of storing up individual experience--which tends to take the place of the inherited mechanism with which it is often in antagonism. The power of profiting by individual experience, in fact educability, must in conditions of close competition be, when other conditions are equal, an immense advantage to its possessor. It seems that we have to imagine that the adaptation of mammalian form to the various conditions of life had in Miocene times reached a point when further alteration and elaboration of the various types, which we know then existed, could lead to no advantage. The variations presented for selection in the struggle for existence presented no advantage--the ‘fittest’ had practically been reached, and was destined to survive with little change. Assuming such a relative lull in the development of mere mechanical form, it is obvious that the opportunity for those individuals with the most ‘educable’ brains to defeat their competitors would arise. No marked improvement in the instrument being possible, the reward, the