Page:Popular Science Monthly Volume 10.djvu/225

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213
PROFESSOR HUXLEY'S LECTURES.

Had that constitution been other than what it was, the bones would have been dissolved, the beds of sandstone would have fallen together, become one mass, and not the slightest indication that the animal had existed would have been discovered.

I know of no more striking evidence than this fact affords, of the caution which should be used in drawing the conclusion, from the absence of organic remains in a deposit, that animals did not exist at the time it was formed. I believe that, having the right understanding of the doctrine of evolution on the one hand, and having a just estimation of the importance of the imperfection of the geological record on the other, all difficulty from the kind of evidence to which I have adverted is removed; and we are justified in believing that all such cases are examples of what I have designated negative or indifferent evidence—that is to say, they in no way directly advance the theory of evolution, but they are no obstacle in the way of our belief in the doctrine.

I now pass on to the consideration of those cases which are not—for reasons which I will point out to you by-and-by—demonstrative of the truth of evolution, but which are such as must exist if evolution be true, and which therefore are, upon the whole, strongly in favor of the doctrine. If the doctrine of evolution be true, it follows that, however diverse the different groups of animals and of plants may be, they must have all, at one time or other, been connected by gradational forms; so that, from the highest animals, whatever they may be, down to the lowest speck of gelatinous matter in which life can be manifested, there must be a sure and progressive body of evidence—a series of gradations by which you could pass from one end of the series to the other. Undoubtedly that is a necessary postulate of the doctrine of evolution. But, when we look upon animated Nature as it at present exists, we find something totally different from this. We find that animals and plants fall into groups, the different members of which are pretty closely allied together, but which are separated by great breaks or intervals from other groups. And we cannot at present find any intermediate forms which bridge over these gaps or intervals. To illustrate what I mean: Let me call your attention to those vertebrate animals which are most familiar to you, such as mammals, and birds, and reptiles. At the present day these groups of animals are perfectly well defined from one another. We know of no animal now living which in any sense is intermediate between the mammal and the bird, or between the bird and reptile; but, on the contrary, there are actually some very distinct and anatomical peculiarities, well-defined marks, by which the mammal is separated from the bird, and the bird from the reptile. The distinctions are apparent and striking if you compare the definitions of these great groups as they now exist. At the present day there are numerous forms of what we may call broadly the pig tribe,