and dolphins; still more it moulds similar and larger groups of animals into similar lines or radii of specialization. Thus we reach the grand idea of analogy as operating in the divergencies or adaptive radiations of groups, according to which great orders of animals tend in their families and suborders to minic other orders, and the faunæ or collective orders of continents to mimic the faunæ of other continents.
Amid this repetition on a grand scale of similar adaptations, which is altogether comparable to what we know as having occurred over and over again in human history, the paleontologist as a historian must keep constantly before him the second great idea of homogeny, of real ancestral kinship, of direct blood descent and hereditary relationship. The shark and the ichthyosaur superficially look alike, but their germ cells are radically different, their external resemblances are a mere veneer of adaptation so deceptive, however, that it may be a matter of half a century before we recognize the wolf beneath the clothing of the sheep, or the ass in the lion's skin.
These two great ideas of analogy or similarity of habit, and homogeny or similarity of descent, do not run on the same lines; they are the woof and the warp of animal history. Analogy corresponds to the woof or horizontal strands which tie animals together by their superficial resemblances in the present, homogenies are the warp, or the fundamental vertical strands which connect animals with their ancestors and their successors. The far reaching extent of analogous revolution was only dimly perceived by Huxley, and constituted his one great defect as a philosophical anatomist. Its power of transforming unlike and unrelated animals has accomplished miracles in the way of producing a likeness so exact that the inference of kinship is almost irresistible.
The paleontologist who would succeed as historian must first, therefore, render himself immune to the misguiding influences of analogy by taking certain further precautions which will now be explained by watching his procedure as historian.
Paleontology as the history of life takes its place in the background of recorded history and archeology, and simply from the standpoint of the human pedigree is of transcendent interest. Although it has progressed far beyond the dreams of Darwin and Huxley, the first general statement which must be made is that the actual points of contact between the grand divisions of the animal and plant kingdom, as well as between the lesser and even many of the minor divisions, have yet to be discovered. You recall that the older grand divisions of the Vertebrata, to which we must confine our attention, were suggested by the so-called Ages of Fishes, of Amphibians, of Reptiles and of Mammals. Even within these grand divisions we observe a succession of more or less closely analogous groups. Each of these groups has its
