Ctiap. V.] ACCLIMATISATION. 113 



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 ex- 

 traordinary capacity in our domestic animals of not only withstand- 

 ing 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, how- 

 ever, push the foregoing argument too far, on account of the pro- 

 bable 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 trans- 

 ported by man to many parts of the world, and now have a fai 

 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 be looked at as anomalies, but 

 as examples of a very common flexibility 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 selection 

 of varieties having different innate constitutions, and how much to 

 both means combined, is an obscure question. That habit or custom 

 has some influence, I must believe, both from analogy and from the 

 incessant advice given in agricultural works, even in the ancient 

 Encyclopaedias of China, to be very cautious in transporting ani- 

 mals from one district to another. And as it is not likely that man 

 should have succeeded in selecting so many breeds and sub-breeds 

 with constitutions specially fitted for their own districts, the result 

 must, I think, be due to habit. On the other hand, natural selec- 



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