THE INFLUENCES OF ENVIRONMENT 269 



horses to become smaller, but that the climatic conditions as a whole 

 are concerned in the matter. Whether the total amount of variation 

 which has taken place in the horses which have lived wild there 

 for a hundred years would take place in the course of a single life, or 

 whether it is a cumulative phenomenon, has still to be decided. 



Similar statements, for the most part still more uncertain, are madf 

 in regard to changes in the hair of goats, sheep, cattle, cats, and sheep- 

 dogs, which are referred to climatic influence. The raw climate of 

 many highlands, like Tibet and Angora, is said to have directly 

 produced the long and fine-haired breeds. But there is a lack of 

 proof that adaptation or artificial selection did not also play a part, 

 and the fact that similar long-haired breeds have arisen among 

 rabbits and guinea-pigs in quite different places and under quite 

 different climatic conditions, but under the directing care of man, 

 speaks in favour of our supposition. But, on the other hand, it does 

 not seem impossible that the climate may have a variational influence 

 upon certain determinants of the germ-plasm, for we have already 

 seen that the influence of cultivation may incite plants and animals 

 to hereditary variations, and that slovvdy increasing disturbances 

 in the equilibrium of the determinant .system may thereby be pro- 

 duced, which ma}^ suddenly find marked expression as ' mutations.' 

 But there is little probability that adaptations, that is, transformations 

 corresponding to the altered climate, can arise in this way. The 

 thick fur of the Arctic mammals is assuredly not a direct effect of 

 the cold, although it has developed in all Arctic animals, not only 

 in the modern polar bears, foxes, and hares of the polar regions, Imi 

 also in the shaggy-haired mammoth of diluvial Siberia, whose tropical 

 relatives of to-day, the elephants, have an almost naked skin. 

 Another interesting case, recently brought to light, shows that 

 a group of animals which, in correspondence with their otherwise 

 exclusively tropical distribution, have only a moderately developed 

 coat of hair, may, on migrating to a cold country, grow as good a fur 

 as the members of other families. I refer to one of the higher apes, 

 RhinopithecuH roxellanoi, which live in companies in the forest on 

 the high mountains of Tibet, notwithstanding that the snow lies 

 there for six months ^. 



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 



' See Milne-Edwards, RechircJies pour seriir a I'hisfoire nat. d. mammifires, Paris, 

 1868-74. 



