Chap. V. 
ACCLIMATISATIOlSr. 
141 
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 pro¬ 
bable origin of some of our domestic animals from seve¬ 
ral wild stocks: the blood, for instance, of a tropical 
and arctic wolf or wild dog 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, 
living free under the cold climate of ^Faroe in the 
north and of the Falklands in the south, and on many 
islands in the torrid zones. Hence I am inclined to look 
at adaptation to any special climate as a quality readily 
grafted on an innate wide flexibility of constitution, 
which is common to most animals. On this view, the 
capacity of enduring the most different climates by man 
himself and by his domestic animals, and such facts as 
that former species of the elephant and rhinoceros were 
capable of enduring a glacial climate, whereas the liv¬ 
ing species are now all tropical or sub-tropical in their 
habits, ought not to be looked at as anomalies, but 
merely as examples of a very common flexibility of con¬ 
stitution, brought, under peculiar circumstances, into 
play. 
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 
a very 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 can- 
