Nevertheless, if the evolution of the entire animal and plant kingdoms had taken place within the period between the first deposits of stratified rocks and the present time, we might still have expected to find, despite the imperfections of the record, sufficient evidence to show how the present groups have arisen, and how they are related to one another. But, unfortunately, at the period when the history of the rocks begins, nearly all the large groups of animals were in existence, and some of them, indeed, as the trilobites and the brachiopods, appear to have reached the zenith of their development.
On the other hand, the subdivisions of the group of vertebrates have evolved during the period known to us. It is true that the group was already formed when our knowledge of it begins, but, from the fishes onwards, the history of the vertebrates is recorded in the rocks. The highest group of all, the mammals, has arisen within relatively modern times. The correctness of the transmutation theory could be as well established by a single group of geological remains as by the entire animal kingdom. Let us, therefore, examine how far the theory is substantiated by the paleontological record of the vertebrates. We find that the earliest vertebrates were fishes, and these were followed successively by the amphibians, reptiles, birds, and mammals, one of the last species of all to appear being man himself. There can be little doubt that this series, with certain limitations to be spoken of in a moment, represents a progressive series beginning with the simpler forms and ending with the more complicated. Even did we not know this geological sequence we would conclude, from the anatomical evidence alone, that the progression had been in some such order as the geological record shows. The limitation referred to above is this: that while the mammals arose later than the birds, we need not suppose that the mammals arose from the birds, and not even perhaps from the reptiles, or at least not from reptiles like those living at the present day. The mammals may in fact, as some anatomists believe, have come direct from amphibian-like forms. If this is the case, we find the amphibians giving rise on one hand to reptiles and these to birds, and on the other hand to mammals.