The evidence thus afforded by comparative anatomy is so cogent that anyone who goes impartially and attentively through a collection of skeletons can convince himself at once of the morphological unity of the vertebrate stem. The evolutionary evidence of comparative ontogeny, or embryology, is less easy to grasp and less accessible, but not less important. It came to light at a much later date, and its extreme value was only made clear, by means of the biogenetic law, some forty years ago. It shows that every vertebrate, like every other animal, develops from a single cell, but that the course of its embryonic development is peculiar, and characterised by embryonic forms that are not found in the invertebrates. We find in them especially the _chordula_, or chorda-larva, a very simple worm-shaped embryonic form, without limbs, head, or higher sense-organs; the body consists merely of six very simple primitive organs. From these are developed steadily the hundreds of different bones, muscles, and other organs that we afterwards distinguish in the mature vertebrate. The remarkable and very complex course of this embryonic development is essentially the same in man and the ape, and in the amphibians and fishes. We see in it, in accordance with the biogenetic law, a new and important witness to the common descent of all vertebrates from a single primitive form, the _chordæa_. But, important as these arguments of comparative embryology are, one needs many years' study in the unfamiliar and difficult province of embryology before one can realise their evolutionary force. There are, in fact, not a few embryologists (especially of the modern school of experimental embryology) who do not succeed in doing so. It is otherwise with the palpable proofs that we take from a remote science, paleontology. The remarkable fossil remains and impressions of extinct animals and plants give us directly the historical evidence we need to understand the successive appearance and disappearance of the various species and groups. Geology has firmly established the chronological order of the sedimentary rocks, which have been successively formed of mud at the floor of the ocean, and has deduced their age from the thickness of the strata, and determined the relative date of their formation. The vast period during which organic life has been developing on the earth runs to many million years. The number is variously estimated at less than a hundred or at several hundred million years.[7] If we take the smaller number of 200 million years, we find them distributed amongst the five chief periods of the earth's organic development in such a way that the earlier or archeozoic period absorbs nearly one half. As the sedimentary rocks of this period, chiefly gneisses and crystalline schists, are in a metamorphosed condition, the fossil remains in them are unrecognisable. In the next succeeding strata of the paleozoic period we find the