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 made 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 slowly increasing disturbances in the equilibrium of the determinant system may thereby be produced, which may 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, but 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, _Rhinopithecus roxellanæ_, which live in companies in the forest on the high mountains of Tibet, notwithstanding that the snow lies there for six months[26]. [26] See Milne-Edwards, _Recherches pour servir à l'histoire nat. d. mammifères_, Paris, 1868-74.