I pointed out, in fact, that, as we go back in time, the great intervals which at present separate some of the larger divisions of animals become more or less completely obliterated by the appearance of intermediate forms, so that if we take the particular case of reptiles and birds, upon which I dwelt at length, we find in the mesozoic rocks animals which, if ranged in series, would so completely bridge over the interval between the reptile and the bird that it would be very hard to say where the reptile ends and where the bird begins. Evidence so distinctly favorable to evolution as this is far weightier than that upon which men undertake to say that they believe many important propositions; but it is not the highest kind of evidence attainable for this reason, that, as it happens, the intermediate forms to which I have referred do not occur in the exact order in which they ought to occur, if they really had formed steps in the progression from the reptile to the bird; that is to say, we find these forms in contemporaneous deposits, whereas the requirements of the demonstrative evidence of evolution demand that we should find the series of gradations between one group of animals and another in such order as they must have followed if they had constituted a succession of stages, in time, of the development of the form at which they ultimately arrive. In other words, the complete evidence of the evolution of the bird from the reptile—what I call the demonstrative evidence, because it is the highest form of this class of evidence; that evidence should be of this character, that in some ancient formation reptiles alone should be found; in some later formations birds should first be met with; and in the intermediate strata we should discover in regular succession those forms which I pointed out to you which are intermediate between reptiles and birds.
The proof of evolution cannot be complete until we have obtained evidence of this character, and that evidence has of late years been forthcoming in considerable and continually increasing quantity. Indeed, it is somewhat surprising how large is the quantity of that evidence, and how satisfactory is its nature, if we consider that our obtaining such evidence depends upon the occurrence in a particular locality of an undisturbed series deposited through a long period of time, which requires the further condition* that each of these deposits should be such that the animal remains imbedded in them are not much disturbed, and are imbedded in a state of great preservation. Evidence of this kind, as I have said, has of late years been accumulating largely, and in respect to many divisions of the animal kingdom. But I will select for my present purpose only one particular case, which is more adapted to the object I have in view, as it relates to the origin, to what we may call the pedigree, of one of our most familiar domestic animals—the horse. But I may say that in speaking of the origin of the horse I shall use that term in a general sense as equivalent to the technical term Equus, and meaning not what you
