I now pass to another line of argument. The theory of evolution by natural selection supposes that hereditary characters admit of being slowly modified wherever their modification will render an organism better suited to a change in its conditions of life. Let us, then, observe the evidence we have of such adaptive modifications of structure, in cases where the need of such modification is apparent. For the sake of clearness, I shall begin by again taking the case of the whales and porpoises. The theory of evolution infers, from the whole structure of these animals, that their progenitors must have been terrestrial quadrupeds of some kind, which became aquatic in their habits. Now the change in the conditions of their life thus brought about would render desirable great modifications of structure. These changes would, in the first instance, begin to affect the least typical--that is, the least strongly inherited structures--such as the skin, claws, and teeth, &c. But as time went on, the adaptation would begin to extend to the more typical structures, until the shape of the body began to be affected by the bones and muscles required for terrestrial locomotion becoming better adapted for aquatic locomotion, and the whole outline of the animal more fish-like in shape. This is the stage which we actually observe in the seals, where the hind legs, although retaining all their typical bones, have become shortened up almost to rudiments, and directed backwards, so as to be of no use for walking, but serving to complete the fish-like taper of the body. But in the whales the modification has gone even further than this, so that the hind legs have ceased to be apparent externally, and are only represented internally by remnants so rudimentary that it is impossible to make out with certainty the homologies of the bones; moreover, the head and the whole body have become completely fish-like in shape. But profound as these changes are, they only affect those parts of the organism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus the arm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all inclosed in a fin-shaped sack, so as to render them quite useless for any other purpose than swimming. Similarly, the head, although it so closely resembles the head of a fish in shape, still retains the bones of the mammalian skull in their proper anatomical relation to one another, but modified in form so as to offer the least possible amount of resistance to the water. In short it may be said that all the modifications have been effected with the least possible divergence from the typical mammalian type, which is compatible with securing so perfect an adaptation to a purely aquatic mode of life.