CHAP. V.] ACCLIMATISATION. 105 



acclimatised : thus the pines and rhododendrons, raised from seed 

 collected by Dr. Hooker from the same species growing at different 

 heights on the Himalaya, were found to possess in this country 

 different constitutional powers of resisting cold. Mr. Thwaites 

 informs me that he has observed similar facts in Ceylon ; analogous 

 observations have been made by Mr. H. C. Watson on European 

 species of plants brought from the Azores to England; and I 

 could give other cases. In regard to animals, several authentic 

 instances could be adduced of species having largely extended, 

 within historical times, their range from warmer to cooler latitudes, 

 and conversely ; but we do not positively know that these animals 

 were strictly adapted to their native climate, though in all ordinary 

 cases we assume such to be the case ; nor do we know that they 

 have subsequently become specially acclimatised to their new 

 homes, so as to be better fitted for them than they were at first. 



As we may infer that our domestic animals were originally 

 chosen by uncivilised man because they were useful and because 

 they bred readily under confinement, and not because they were 

 subsequently found capable of far-extended transportation, the 

 common and extraordinary capacity in our domestic animals of 

 not only withstanding the most different climates, but of being 

 perfectly fertile (a far severer test) under them, may be used as an 

 argument that a large proportion of other animals now in a state 

 of nature could easily be brought to bear widely different climates. 

 We must not, however, push the foregoing argument too far, on 

 account of the probable origin of some of our domestic animals 

 from several wild stocks; the blood, for instance, of a tropical 

 and arctic wolf may perhaps be mingled in our domestic breeds. 

 The rat and mouse cannot be considered as domestic animals, but 

 they have been transported by man to many parts of the world, 

 and now have a far wider range than any other rodent; for 

 they live under the cold climate of Faroe in the north and of the 

 Falklands in the south, and on many an island in the torrid zones. 

 Hence adaptation to any special climate may be looked at as a 

 quality readily grafted on an innate wide flexibility of constitution, 

 common to most animals. On this view, the capacity of enduring 

 the most different climates by man himself and by his domestic 

 animals, and the fact of the extinct elephant and rhinoceros having 

 formerly endured a glacial climate, whereas the living species are 

 now all tropical or sub-tropical in their habits, ought not to bo 

 looked at as anomalies, but as examples of a very common flexi- 

 bility of constitution, brought, under peculiar circumstances, into 

 action. 



How much of the acclimatisation of species to any peculiar 

 climate is due to mere habit, and how much to the natural selec- 

 tion 01 varieties having different innate constitutions, and how 



