while the embryological development of the higher ones recalls in the younger condition of the germ the same peculiar character. I might multiply examples, and draw equally striking illustrations from the other classes; and though these correspondences cannot be fully established while our knowledge of the embryological growth of animals is so scanty, and information about their geological succession, yet wherever we have been able to trace the connected history of any group of animals in time, and to compare it with the history of their embryological development and their structural relations as they exist to-day, the correspondence is found to be so complete that we are justified in believing that it will not fail in other instances. I may add that a gradation of exactly the same character controls the geographical distribution of animals over the surface of the globe. Here again I must beg my readers to take much of the evidence, which, if expanded, would fill a volume, for granted, since it would be entirely inappropriate here. But I may briefly state that animals are not scattered over the surface of our globe at random, but that they are associated together in what are called _faunæ_, and that these faunæ have their homes within certain districts--called by naturalists _zoölogical provinces_. The limits of these provinces are absolutely fixed, in the ocean as well as on the land, by certain physical conditions connected with climate, with altitude, with the pressure of the atmosphere, the weight of the water, etc.; and this is true even for animals of migratory habits, for all such migrations are periodical, and have boundaries as definite and impassable as those that limit the permanent homes of animals. There is a certain series established by the relations between different kinds of animals, as thus distributed over the globe, which agrees with the gradation in their rank, their growth, and their succession in time;--the law which distributes animals in successive faunæ, and in accordance both with their relative superiority or inferiority, and with the physical conditions essential to their existence, being the same as that which controls their structural relations, their embryological development, and their succession in time.