ACCLIMATIZATION. 127 



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 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 ani- 

 mals, 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 acclimatization of species to any 

 peculiar climate is due to mere habit, and how much to the 

 natural selection of varieties having different innate con- 

 stitutions, 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 

 animals 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 selection would inevitably tend 

 to preserve those individuals which were born with constitu- 

 tions best adapted to any country which they inhabited. In 

 treatises on many kinds of cultivated plants, certain varieties 

 are said to withstand certain climates better than others ; 

 this is strikingly shown in works on fruit-trees published in 

 the United States, in which certain varieties are habituallv 

 recommended for the Northern and others for the Southern 

 States ; and as most of these varieties are of recent origin, 

 they cannot owe their constitutional differences to habit. 

 The case of the Jerusalem artichoke, which is never prop- 

 agated in England by seed, and of which, consequently, new 



