So far the examples have been taken from one order of the highest class of backboned animals, called mammalia. When our survey is extended to other divisions of this class, additional laws of organic relationship are discovered. If in a series of evolving generations the line of modification proceeding from a terrestrial animal like a cat to semi-aquatic and marine types substantially like an otter and a seal should be carried further, it will inevitably lead to forms possessing characters such as those displayed by whales and the related porpoises, dolphins, and narwhals of the order cetacea. In their make-up all of these animals clearly possess the general characteristics of mammals, and they constitute collectively another limb which has sprung from the same stock as the carnivora, although at an earlier time. This we believe because of their plan of body and because their peculiar organization fits them even more perfectly than the seals for aquatic existence that is their only possible mode of life. In the case of the whales the bony framework of the fore limb is again like that of the cat's leg, although the whole structure is a flexible finlike paddle. The hind limb has disappeared as an efficient organ, but the significant fact is that small rudiments of hind limbs are present just where corresponding structures are placed in the seal. These vestiges cannot be reasonably accounted for, unless they are the degenerate hinder limbs of a remote four-footed ancestor. Furthermore the unborn whale possesses a complete coat of hair, which is afterwards replaced by blubber; but hair is a thatchlike coat to shed rain, as the way the hairs lie on a terrestrial mammal indicates. We are therefore forced to conclude that whales have originated from four-footed animals walking about on land, because no opposed explanation gives so reasonable an interpretation of the observed facts.