But we should assuredly make a mistake if we were to regard the thick fur of these apes as a direct reaction of the organism to the cold. We see at once that this cannot be the case if we compare them with marine mammals, which differ just as much from one another in this respect and yet are exposed to the same low temperature. The whale and the dolphin are quite naked, absolutely hairless, but the seals possess a thick hairy coat. This striking difference is obviously connected with the mode of life; the whales remain always in the water, the seals leave it often and therefore require the hairy coat, especially in colder climates, since otherwise they would be too rapidly cooled by the evaporation of the water from their bodies. For the whales, on the other hand, even a very thick hairy coat would not have sufficed as a protection against cold, since water is a much better conductor of heat than air, and so it was necessary for them to become enveloped with the well-known thick layer of blubber, a deposit of fat lying under the skin, and this--after it was once developed--made the hairy coat superfluous, so that it disappeared. The seals certainly also possess a layer of fat under the skin, but it is only in the largest of them that it affords sufficient protection against the cooling effect of evaporation when they go upon land or on the ice, and it is therefore only in these larger ones that the hairy coat has markedly degenerated, as, for instance, in the walrus and the sea-lion; in all the smaller seals, in which the mass of the body is much less, the hairy coat is necessarily very thick and protected from soaking by being very oily, because the layer of fat under the skin would not be sufficient to prevent excessive cooling when on land. But the thick coat of hair is no more _produced_ by the cold than is the layer of fat. As Kükenthal has shown, all these characters are adaptations, and may depend here as elsewhere upon natural selection and upon the 'fluctuating' variations of the germ-plasm upon which that process is based. They are directed by personal selection because there is the need for them, and they are produced and augmented by germinal selection. In all these cases the _direct_ effect of external influences has nothing to do with the matter, but in other cases that alone brings about the whole change, which is then limited to the individual and does not affect the species as a whole at all.