During the Triassic and Jurassic periods of the Mesozoic age, the first birds and mammals appeared to follow out their diverging and independent lines of descent. Palaeontology makes it possible to trace the origin and development of many of the different branches that grew out of the mammalian limb from different places and at different times during the Mesozoic and the following age, called the Cenozoic, or age of recent animals. It is unnecessary, however, for us to review more of the details: the main result is obvious; namely, that the appearance of the great classes of vertebrates is in the order of comparative anatomy and embryology. Not only, then, is the fact of evolution rendered trebly sure, but the general order of events is thrice and independently demonstrated to be one and the same. Surely we must see that no reasonable explanation other than evolution can be given for these basic facts and principles. Turning now to the second division of palaeontological evidence, we come to those groups where abundant materials make it possible to arrange the animals of successive epochs in series that may be remarkably complete. For the reasons specified, the backboned animals provide the richest arrays of these series, and such histories as those of horses and elephants have taken their places in zooelogical science as classics. But even among the invertebrates significant cases may be found. For example, in one restricted locality in Germany the shells of snails belonging to the genus _Paludina_ have been found in superimposed strata in the order of their geological sequence. The ample material shows how the several species altered from age to age by the addition of knobs and ridges to the surface of the shell, until the fossils in the latest rocks are far different from their ancestors in the lowermost levels. Yet the intervening shells fill in the gaps in such a way as to show almost perfectly how the animals worked out their evolutionary history. This example illustrates the nature of many other known series of mollusks and of brachiopods, extending over longer intervals and connecting more widely separated ages like the Secondary and the present period.