270 THE EVOLUTION THEOKY 



another in this respect and yet are exposed to the same low tempera- 

 ture. 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 oi* life ; the whales remain always 

 in the water, the seals leave it often and therefore require the hair}' 

 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 laj^er 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 dis- 

 appeared. 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 Klikenthal 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 up(jn 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. 



Plant-galls aflbrd striking illustration of the extraordinary changes 

 that may be brought about in an organism or in its parts by external 

 influence in the course of the individual life. All possibility of 

 adaptation on tlie part of the plant is excluded in this case. The gall 

 can only depend upon the direct influence of a stimulus, which is 

 exercised by the young animal, the larva, upon the cells which sur- 

 round it ; and yet these cells vary to a considerable extent, become filled 

 with starch or form a woody layer, secrete special substances, such as 

 tannic acid, in large (quantities, or develop hairs, moss-like growths, 



