there must be a distinction of kind, because the psychological interval between the highest ape and the lowest man is so great; Mr. Wallace argues for the same conclusion on the ground that this interval is not so great as the theory of a natural evolution would lead us to expect: the brain of a savage, he says, is so much more efficient an instrument than the mind to which it ministers, that its presence can only be explained as a preparation for the higher efficiency of mental life as afterwards exhibited by civilized man. Lastly, Professor De Quatrefages contradicts both the English naturalists by vehemently insisting that, so far as the powers of intellect are concerned, there is a demonstrable identity of kind between animal intelligence and human, whether in the savage or civilized condition: he argues that the distinction only arises in the domain of morals and religion. So that, if our opinion on the issue before us were to be in any way influenced by the voice of authority, I might represent the judgments of these my most representative opponents as mutually cancelling one another—thus yielding a zero quantity as against the enormous and self-consistent weight of authority on the other side. But, quitting all considerations of authority, I proceeded to investigate the question _de novo_, or exclusively on its own merits. To do this it was necessary to begin with a somewhat tedious analysis of ideation. The general result was to yield the following as my classification of ideas. 1. Mere memories of perceptions, or the abiding mental images of past sensuous impressions. These are the ideas which, in the terminology of Locke, we may designate Simple, Particular, or Concrete. Nowadays no one questions that such ideas are common to animals and men.