The essential features of the evolution of intelligence seem, then, to be, first, the development of controlling nerve-centres, by which the responsive action of reflex automatic or instinctive centres may be checked, augmented, or modified; secondly, the increased differentiation and integration of these control centres with extension of the range and complexity of experience in close touch with practical needs; thirdly, the condensation and concentration of experience by the formation of generic products through the reiterated emphasis begotten of recurrent situations having certain salient features in common, though differing in details; and fourthly, an increased plasticity of behaviour, especially in early life, enabling an animal to deal effectually with an environment far less simple than that to which the more stereotyped instinctive behaviour is fitted by inheritance to respond. And this evolution of intelligent behaviour is working its way up to, though as such it cannot reach, the succeeding phase of mental evolution in which the data, supplied by intelligence, are treated with a new purpose for higher ends in the rational thought which seeks to explain the phenomena, and frame an ideal scheme of their relations and interconnections. Two further points may be noticed. First, that it is during the early and plastic days or months of life that intelligence is setting its seal on animal behaviour, and stamping it with its distinctive character. Adult life is very much what youth has made it; and old age is stereotyped through habit. In times of progress, the character of the race is determined by plastic possibilities of the young. Among them it is that the incidence of elimination makes itself felt, resulting in the survival of those whose intelligence can mould behaviour in accordance with the new circumstances of a wider life.