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Show 140 LAWS OF VARIATION. CIIAP. v. We may I• n1.!! er thi·s from our· frequent ina. bilityd to pre- di.C t wh et he r or n ot an imported plant will en ure· our cli.m ate, an d f rom the number of plants and ·a nima1ds brought from warmer countries which here eilJ?Y ~oo health. We have reason to beli~ve that species In a state of nature are limited i~ their :anges by the competition of other organic bmngs ~uite as ~uch as, or more than, by adaptation to particular climates. But whether or not the adaptation be generally very close, w.e h ave evi' dence, in the case of some few plants·, of tdh eu b ecomi·n g, to a certain extent, naturally. hab·i tuda te h to different temperatures, or becoming acclimatise : t us the pines and rhododendrons, raised fro~ seed coll~cted by Dr. Hooker from trees growing at different hmghts on the Himalaya, were found in this c~u~try to possess different constitutional powers of resisting cold. Mr. Thwaites informs me that he has observed similar facts in Ceylon and analogous observations have been made by Mr. H. C. Watson on European species of plants brought from the Azores to England. In re?ard to animals, several authentic cases could be given of species within historical times having l~rgely extended their range from warmer to cooler latitudes, and co~versely · but we do not positively know that these ani- mals we' re strictly adapted to thm· r nati·v e cl i rna t e, but in all ordinary cases we assume such to be the case ; nor do we know that they have subsequently become acclimatised to their new homes. . . As I believe that our domestic animals were originally chosen by uncivilised man because they were useful and bred readily under confinement, and not because they were subsequently found capable of far-exte_nded transportation, I think the common and extra?rdinar~ capacity in our domestic animals of not only withstant ing the most different climates but of being perfect Y CHAP. v. ACCLIMATISATION. 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 probable origin of some of our domestic animals from several 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." a~imals. On this view ' th~ c~paCity 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 living. species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but m~re~y as examples of a very common .flexibility of constitutiOn, brought, under peculiar circumstances, into play. Ho_w m~ch of. the acclimatisation of species to any peculiar chmate 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 ~very obscure question. That habit or custom has so~e 1nfiu~nce I must believe, both from analogy, and from ~he mces~nt advice given in agricultural works, even In the anCient Encyclopredias of China, to be very cau- |
