This theory, though it would be regarded by many as a fair statement of his views, is one which Mr. Darwin would entirely repudiate. Whether fish and insect, reptile, bird and beast, are derived from one original stock or not, they are certainly not links in one sequence. I do not, however, propose to discuss the question of Natural Selection, but may observe that it is one thing to acknowledge that in Natural Selection, or the survival of the fittest, Mr. Darwin has called attention to a _vera causa_, has pointed out the true explanation of certain phenomena; but it is quite another thing to maintain that all animals are descended from some primordial source. For my own part, I am satisfied that Natural Selection is a true cause, and, whatever may be the final result of our present inquiries—whether animated nature be derived from one ancestral source, or from many—the publication of the Origin of Species will none the less have constituted an epoch in the History of Biology. But, how far the present condition of living beings is due to that cause; how far, on the other hand, the action of Natural Selection has been modified and checked by other natural laws—by the unalterability of types, by atavism, &c.; how many types of life originally came into being; and whether they arose simultaneously or successively,—these and many other similar questions remain unsolved, even admitting the theory of Natural Selection. All this has indeed been clearly pointed out by Mr. Darwin himself, and would not need repetition but for the careless criticism by which in too many cases the true question has been obscured. Without, however, discussing the argument for and against Mr. Darwin’s conclusions, so often do we meet with travesties of it like that which I have just quoted, that it is well worth while to consider the stages through which some group, say for instance that of insects, have probably come to be what they are, assuming them to have developed under natural laws from simpler organisms. The question is one of great difficulty. It is hardly necessary to say that insects cannot have passed through all the lower forms of animal life, and naturalists do not at present agree as to the actual line of their development. In the case of insects, the gradual course of evolution through which the present condition of the group has probably been reached, has been discussed by Mr. Darwin, by Fritz Müller, Haeckel, Brauer, myself and others. In other instances Palæontology throws much light on this question. Leidy has shown that the milk-teeth of the genus _Equus_ resemble the permanent teeth of the ancient _Anchitherium_, while the milk-teeth of _Anchitherium_ again approximate to the dental system of the still earlier _Merychippus_. Rütimeyer, while calling attention to this interesting observation, adds that the milk-teeth of _Equus caballus_ in the same way, and still more those of _E. fossilis_, resemble the permanent teeth of _Hipparion_.