Taking up the problems of psychogenesis where these were left in my previous work, I have in the present treatise to consider the whole scope of mental evolution in man. Clearly the topic thus presented is so large, that in one or other of its branches it might be taken to include the whole history of our species, together with our pre-historic development from lower forms of life, as already indicated in the Preface. However, it is not my intention to write a history of civilization, still less to develop any elaborate hypothesis of anthropogeny. My object is merely to carry into an investigation of human psychology a continuation of the principles which I have already applied to the attempted elucidation of animal psychology. I desire to show that in the one province, as in the other, the light which has been shed by the doctrine of evolution is of a magnitude which we are now only beginning to appreciate; and that by adopting the theory of continuous development from the one order of mind to the other, we are able scientifically to explain the whole mental constitution of man, even in those parts of it which, to former generations, have appeared inexplicable. In order to accomplish this purpose, it is not needful that I should seek to enter upon matters of detail in the application of those principles to the facts of history. On the contrary, I think that any such endeavour—even were I qualified to make it—would tend only to obscure my exposition of those principles themselves. It is enough that I should trace the operation of such principles, as it were, in outline, and leave to the professed historian the task of applying them in special cases.