A whale is a warm-blooded, air-breathing, milk-giving animal. Its fins are hands with finger-bones, having a large number of joints (see Fig. 84); its tail is a piece of cartilage or gristle, and not a fish’s fin with bones and rays; it has teeth in its gums even if it never cuts them; and it gives suck to its little one just as much as a cow does to her calf (see Fig. 85). Nay! the whalebone whales have even the traces of hind legs entirely buried under the skin (see Fig. 84), and in the Greenland whale the hip-joint and knee-joint can be distinguished with some of their muscles, though the bones are quite hidden and useless. [Illustration: Fig. 84. Skeleton of a Whalebone Whale (Mysticete), and Section of the Mouth with Whalebone. _b_, blowhole; _a_, upper arm; _fa_, fore arm; _h_, hand; _p_, _th_, _l_, small remains of pelvis or hip-bone, thigh, and leg; _r_, roof of the palate; _w_, _w_, plates of whalebone; _f_, whalebone fringe. ] We see then that the whale undoubtedly belongs to the same type as the four-footed land animals, although it branched off into the water so long ago that it may have come from some _very_ early milk-giver. But why then has it become so like a fish? For the same reason that the penguin’s wings have become so fin-like, and the seal’s arms and legs have become flippers, namely, that during the long time in which the whales have taken to a watery life, those which could swim best and float best in the water have been the most successful in the struggle for existence; and as a fish’s shape is by far the best for this purpose the warm-blooded milk-giver has gradually imitated it, though belonging to quite a different order of animals. [Illustration: Fig. 85. The Humpback Whale[187] suckling her young (_after Scammon_).] We saw this imitation already beginning in the seals, with their bodies sloping off towards the tail and their legs fastened back in a line with the body; but they have not gone so far in this direction as the whales have, since they still have hind legs and furry bodies. The sea cows, on their line, have gone a little farther, for they have lost their hind legs, and their skin is smooth, with very few hairs upon it. But it remained for the whales to take up the best fish-form, the old spindle-shape, thinning before and behind, with the strong fleshy tail ending in two tail lobes, which act like a screw in driving the body along.