"Biologists turn to the physical organization of man. They examine his whole structure, his bony frame, and all that clothes it. They resolve him into the finest particles into which the microscope will enable them to break him up. They consider the performance of his various functions and activities, and they look at the manner in which he occurs on the surface of the world. Then they turn to other animals, and taking the first handy domestic animal--say a dog,--they profess to be able to demonstrate that the analysis of the dog leads them, in gross, to precisely the same results as the analysis of the man; that they find almost identically the same bones, having the same relations; that they can name the muscles of the dog by the names of the muscles of the man, and the nerves of the dog by those of the nerves of the man, and that such structures and organs of sense as we find in the man, such also we find in the dog; they analyze the brain and spinal cord, and they find that the nomenclature which fits the one answers for the other. Moreover, they trace back the dog's and the man's development, and they find that at a certain stage of their existence, the two creatures are not distinguishable the one from the other; they find that the dog and his kind have a certain distribution over the surface of the world comparable in its way to the distribution of the human species.... Thus biologists have arrived at the conclusion that a fundamental uniformity of structure pervades the animal and vegetable worlds, and that plants and animals differ from one another simply as modifications of the same great plan. Again they tell us the same story in regard to the study of function. They admit the large and important interval which, at the present time, separates the manifestations of the mental faculties observable in the higher forms of mankind, and even in the lowest forms, such as we know them, mentally from those exhibited by other animals; but, at the same time, they tell us that the foundations or rudiments of almost all the faculties of man are to be met with in the lower animals; that there is a unity of mental faculty, as well as of bodily structure, and that here also, the difference is a difference of degree and not of kind."--Lecture on "The Study of Biology," by Professor Huxley, _Nature_, vol. xv. p. 219. Delivered at South Kensington Museum, London, December 16, 1876. On the grounds here admirably summarized, it is clear that the whole organism of our world has been constructed on a common plan. This being true, similarities will appear in process of development, and in the structure and functions of different orders. This similarity, however, does not help us to explain "the large and important interval" which appears when mental characteristics are considered. It makes the diversity of mental power more difficult to explain by reference to organism, in fact contributing to the strength