The form of the present work needs a preliminary word of explanation. Its subject is "Mind in Evolution," but no one will expect that such a subject should be treated with any pretence of adequacy within a single volume or by a single writer. The contribution offered in the following pages is of a double character. There is, first, an attempt to sketch in outline what seem to the writer to be the main phases of mental development. There is, secondly, an attempt to fill in this outline so far as the lower phases are concerned. To put the same distinction in different words, a hypothesis is propounded as to the general trend of mental evolution, and an attempt is made to test this hypothesis so far as animal intelligence and the generic distinction between animal and human intelligence are concerned. For the rest, that is to say in all that relates to the higher development of the human mind in society, the outline is left to be filled in upon a future occasion. The whole subject naturally falls into the two main divisions of animal and human evolution, and the mass of matter to be dealt with is so great that it is convenient to keep the two parts separate.