It is during this Tertiary period that the mammals--the warm-blooded, hairy quadrupeds, which suckle their young--have developed (they had come into existence a good deal earlier), and we find the remains of ancestral forms of the living kinds of cattle, pigs, horses, rhinoceroses, tapirs, elephants, lions, wolves, bears, etc., embedded in the successive layers of Tertiary deposits. Naturally enough, those most like the present animals are found in late Pliocene, and those which are close to the common ancestors of many of the later kinds are found in the Eocene, whilst we also find, at various levels of the Tertiary deposit, remains of side-branches of the mammalian pedigree, which, though including very powerful and remarkable beasts, have left no line of descent to represent them at the present day. We have been able to trace the great modern one-toed horses, zebras, and asses, with their complicated pattern of grinding-teeth back by quite gradual steps (represented by the bones and teeth of fossil kinds of horses), to smaller three-toed animals with simpler tuberculated teeth, and even, without any marked break in the series, to a small Eocene animal (not bigger than a spaniel) with four equal-sized toes on its front foot, and three on its hind foot. We know, too, a less direct series of intermediate forms leading beyond this to an animal with five toes on each foot and "typical" teeth. In fact, no one doubts that (leaving aside a few difficult and doubtful cases) all such big existing mammals, as I mentioned above, as well as monkeys and man, are derived from small mammals--intermediate in most ways between a hedgehog and a pig--which flourished in very early Eocene times, and had five toes on each foot, and "a typical dentition." Even the elephants came from such a small ancestral form. The common notion that the extinct forerunners of existing animals were much bigger than recent kinds, and even gigantic, is not in accordance with fact. Some extinct animals were of very great size--especially the great reptiles of the period long before the Tertiaries, and before the chalk. But the recent horse, the recent elephant, the giraffe, the lions, bears, and others, are bigger--some much bigger--than the ancestral forms, to which we can trace them by the wonderfully preserved and wonderfully collected and worked-out fossilised bones discovered in the successive layers of the Pliocene, Miocene, and Eocene strata, leading us as we descend to more primitive, simplified, and smaller ancestors.