176 ACCLIMATISATION. [Chap. V. 



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 cir- 

 cumstances, 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, T 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 con- 

 stitutions 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 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 



