That these facts are indeed most remarkable and deserving of all attention, cannot be questioned. But before we can agree that they are conclusive and demonstrative in Professor Huxley's sense a good many considerations require to be carefully weighed. (i.) It is obvious, in the first place, that here as in all other instances which we have seen, the one thing is lacking which is really wanted in order to prove Evolution, namely evidence of one species gradually shading off into another. The creatures of which we have heard, are each isolated from the rest, and indeed very much isolated, for each belongs to a different _genus_,[279] which shows that the differences between them are substantial. They are, in fact, farther apart from one another, than the zebra or the donkey from the horse, for both of these are classed in the genus _equus_,--or than the Bengal tiger is from the domestic pussy-cat, both belonging to the genus _felis_. These various ungulate forms thus stand a long way from one another, and if they were once connected together by a bridge, or rather a causeway, we ought certainly to find some traces of it, and not always of those particular types which require to be united. If we suppose the very distinct species actually known to have been the piers of such a bridge, yet what has become of the arches? Till some vestiges of these be found, or, at least, some positive evidence that arches there actually were, can it be said that the story of the fossil _equidae_ furnishes convincing testimony on behalf of the supposed evolution? Affinities these various forms undoubtedly exhibit: it has yet to be shown that affinities necessarily imply descent. There is, however, something even more remarkable. We have seen that Professor Huxley prognosticated beforehand the discovery of _Eohippus_, and specified pretty nearly the features it would be found to present. In the same way, Professor Marsh[280] anticipates and describes a still more remote ancestral form, for which, though it has not yet been found, he has provided an appellation, _Hippops_. But if either Professor really believes in Evolution, why does he take for granted that we shall chance upon one particular form, standing like a solitary outpost by itself, and not upon any other trace of the stream of life whereof it was but one transient phase? Such predictions may be evidence that the occurrence of these progressive forms is regulated by something analogous to Bode's Law of interplanetary distances, and that their discovery may be looked for at certain intervals. But the very fact that their actual position can be so accurately specified serves to show that it is very definitely fixed.