We will next consider what of late years has become the most important of the lines of evidence, not only in favour of the general fact of evolution, but also of its history: I mean the evidence which has been yielded by the newest of the sciences, the science of Embryology. But here, as in the analogous case of adult morphology, in order to do justice to the mass of evidence which has now been accumulated, a whole volume would be necessary. As in that previous case, therefore, I must restrict myself to giving an outline sketch of the main facts. First I will display what in the language of Paley we may call "the state of the argument." It is an observable fact that there is often a close correspondence between developmental changes as revealed by any chronological series of fossils which may happen to have been preserved, and developmental changes which may be observed during the life-history of now existing individuals belonging to the same group of animals. For instance, the successive development of prongs in the horns of deer-like animals, which is so clearly shown in the geological history of this tribe, is closely reproduced in the life-history of existing deer. Or, in other words, the antlers of an existing deer furnish in their development a kind of _résumé_, or recapitulation, of the successive phases whereby the primitive horn was gradually superseded by horns presenting a greater and greater number of prongs in successive species of extinct deer (Fig. 26). Now it must be obvious that such a recapitulation in the life-history of an existing animal of developmental changes successively distinctive of sundry allied, though now extinct species, speaks strongly in favour of evolution. For as it is of the essence of this theory that new forms arise from older forms by way of _hereditary_ descent, we should antecedently expect, if the theory is true, that the phases of development presented by the individual organism would follow, in their main outlines, those phases of development through which their long line of ancestors had passed. The only alternative view is that as species of deer, for instance, were separately created, additional prongs were successively added to their antlers; and yet that, in order to be so added to successive species every individual deer belonging to later species was required to repeat in his own lifetime the process of successive additions which had previously taken place in a remote series of extinct species. Now I do not deny that this view is a possible view; but I do deny that it is a probable one. According to the evolutionary interpretation of such facts, we can see a very good _reason_ why the life-history of the individual is thus a condensed _résumé_ of the life-history of its ancestral species. But according to the opposite view no reason can be assigned why such should be the case. In a previous chapter--the chapter on Classification--we have seen that if