154 ORIGIN OF SPECIES 



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 adap- 

 tation 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 pecu- 

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

 natural selection of varieties having different innate consti- 

 tutions, and how much to both means combined, is an ob- 

 scure question. That habit or custom has some influence, I 

 must believe, both from analogy and from the incessant ad- 

 vice given in agricultural works, even in the ancient Ency- 

 clopaedias 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 selection would inevitably tend to 

 preserve those individuals which were born with consti- 

 tutions 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 pub- 

 lished in the United States, in which certain varieties are 

 habitually 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 

 propagated in England by seed, and of which consequently 

 new varieties have not been produced, has even been ad- 

 vanced, as proving that acclimatisation cannot be effected, 

 for it is now as tender as ever it was ! The case, also, of the 



