1. The existence in the animal world of all grades of structures, from the humblest possible protozoan, whose body consists of a single simple speck, to the most powerful and complex of mammals. There are estimated to be something like a million species of animals living on the earth to-day. There may be several times this number. These species are linked together by millions of varieties, and are so related to each other that they may be all gathered together into various genera; these genera may be grouped into families, the families into orders, and the orders into seven or eight great primary phyla. By taking existing species and adding to them the extinct species of the rocks, and placing them all according to their structural affinities, it is possible to arrange them in the form of a tree with the various phyla, orders, families, genera, and species, branching and rebranching from the main trunk. The existence of structures, so graduated as to render such an arrangement possible, is in itself suggestive of a common relationship and origin. 2. Evolution is suggested by the similarities and homologies of structure found throughout the animal kingdom. Some of these similarities and homologies have already been mentioned. They are everywhere—remoter and more fundamental, some of them, others closer and more detailed. To the untrained mind, which sees surfaces only, and not even surfaces well, the animal world is an interminable miscellany of forms. But to the biologist, who looks deeper and with immense acumen over the whole field of animal life, there are only seven or eight different types of structure in the entire animal world. These seven or eight types correspond with the primary classes, or phyla, into which animals are divided, viz., protozoa, sponges, celenterates, echinoderms, worms, mollusks, arthropods, and vertebrates. However widely the members of each of these great groups may differ among themselves in colour, size, habits of life, and the like, the members of each group all resemble each other fundamentally. Moles differ from monkeys, bats from men, and birds from crocodiles and toads. They differ enormously. But they are all vertebrates with red blood, double body cavities, backbones, two pairs of limbs, and five fingers on each limb. When they are looked at superficially, there is not much similarity between a water-strider and a butterfly or between a stag-beetle and a gnat. But they are all, in reality, built according to the same plan. Like all other insects, they have six legs, a sheath-like skeleton, and bodies characteristically divided into head, thorax, and abdomen. It is the same with all other great classes of beings. All worms resemble each other; and so do all mollusks, although they may differ in particulars as widely as nautiluses and clams. Echinoderms have a radiate structure, celenterates and sponges are vase-like in shape, and protozoa are one-celled. The differences in