We must remember, however, that the rocks of the Silurian system, overlaid by ten miles' thickness of sediment entombing a hundred successive faunæ, each as rich and varied as the fauna of the present day, themselves teem with fossils fully representing all the existing classes of animals, except perhaps the highest.
If it be possible to imagine that this marvellous manifestation of Eternal Power and Wisdom involved in living nature can have been worked out through the law of 'descent with modification' alone, we shall certainly require from the Physicists the longest row of cyphers which they can afford.
Now, although the admission of a doctrine of evolution must affect greatly our conception of the origin and rationale of so-called specific centres, it does not practically affect the question of their existence, or of the laws regulating the distribution of species from their centres by migration, by transport, by ocean currents, by elevations or depressions of the land, or by any other causes at work under existing circumstances. So far as practical naturalists are concerned, species are permanent within their narrow limits of variation, and it would introduce an element of infinite confusion and error if we were to regard them in any other light. The origin of species by descent with modification is as yet only a hypothesis. During the whole period of recorded human observation not one single instance of the change of one species into another has been detected; and, singular to say, in successive geological formations, although new species are constantly appearing and there is abundant evidence of progressive change, no single case
