It has been stated above that palæontology is quite deficient. This is not only true of the record, but of the lack as yet of sufficient investigations. The greatest fields of investigation in this department have never been explored. The whole of the petrifactions accurately known do not probably amount to a hundredth part of those which, by more elaborate explorations, are yet to be discovered. The most ancient of all distinctly preserved petrifactions is the Eozoon Canadense, which was found in the lowest Laurentian strata in the Ottawa formation. Probably no discovery in palæontology ranks higher than the discovery of the descendants of the horse. The horse, for example, as far as his limbs and teeth go, differs far more from extant graminivora than man differs from the ape. Had not fossil ungulates been found, which demonstrate the common origin of the horse with didactyles and multidactyles, some would have deemed the horse a special miraculous creation. But now the links are complete, and the descent of the horse is found to follow exactly what the doctrine of evolution could have predicted. ONTOGENY. It has been stated that the palæontological record is quite incomplete, owing to many facts, some of which have been mentioned; fortunately, the history of the development of the organic individual, or ontogeny, comes in to fill up many deficiencies. Ontogeny is a repetition of the principal forms through which the respective individuals have passed from the beginning of their tribe, and its great advantage is that it reveals a field of information which it was impossible for the rocks to retain; for the petrification of the ancient ancestors of all the different animal and vegetable species, which were soft, tender bodies, was not possible. The annexed plate illustrates the dog, rabbit, and man in their first stages of development. Illustrations of a fish, an amphibious animal, a reptile, a bird, or any mammal, could also be given; for all vertebrate animals of the most different classes, in their early stages of development, cannot be distinguished, and the nearer the animal approaches man in the ascending scale, the longer does this similarity continue to exist--when reptiles and birds are distinctly different from mammals, the dog and the man are almost identical. The gill-arches of the fish exist in man, in dogs, in fowls, in reptiles, and in other vertebrate animals during the first stages of their development. Man also possesses, in his first stages, a real tail, as well as his nearest kindred--the tailless apes (orang-outang, chimpanzee, gorilla), and vertebrate animals in general. The tail, as has been stated, man still retains, though hidden as a rudiment. [Illustration: FIG. I.--Human Embryo.--_Ecker._] [Illustration: FIG. II.--Embryo of Dog.--_Bischoff._] [Illustration: FIG. III.--Dog Embryo.--_Huxley._] [Illustration: FIGS. IV, V, and VI.--Embryo of Rabbit in three stages of development.--_Haeckel._]