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trees growing at different heights on the Himalaya were found in this country 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 regard to animals； several authentic cases could be given of species within historical times having largely extended their range from warmer to cooler latitudes； and conversely； but we do not positively know that these animals were strictly adapted to their native climate； 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…extended transportation； I think the common and extraordinary capacity in our domestic animals of not only withstanding the most different climates but of being perfectly 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 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 living 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 constitution； 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 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 cautious in transposing animals from one district to another； for 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； I can see no reason to doubt that natural selection will continually tend to preserve those individuals which are born with constitutions best adapted to their native countries。 In treatises on many kinds of cultivated plants； certain varieties are said to withstand certain climates better than others： this is very 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 by seed； and of which consequently new varieties have not been produced； has even been advanced for it is now as tender as ever it was as proving that acclimatisation cannot be effected！ The case； also； of the kidney…bean has been often cited for a similar purpose； and with much greater weight； but until some one will sow； during a score of generations； his kidney…beans so early that a very large proportion are destroyed by frost； and then collect seed from the few survivors； with care to prevent accidental crosses； and then again get seed from these seedlings； with the same precautions； the experiment cannot be said to have been even tried。 Nor let it be supposed that no differences in the constitution of seedling kidney…beans ever appear； for an account has been published how much more hardy some seedlings appeared to be than others。
On the whole； I think we may conclude that habit； use； and disuse； have； in some cases； played a considerable part in the modification of the constitution； and of the structure of various organs； but that the effects of use and disuse have often been largely combined with； and sometimes overmastered by； the natural selection of innate differences。
Correlation of Growth
I mean by this expression that the whole organisation is so tied together during its growth and development； that when slight variations in any one part occur； and are accumulated through natural selection； other parts become modified。 This is a very important subject； most imperfectly understood。 The most obvious case is； that modifications accumulated solely for the good of the young or larva； will； it may safely be concluded； affect the structure of the adult； in the same manner as any malconformation affecting the early embryo； seriously affects the whole organisation of the adult。 The several parts of the body which are homologous； and which； at an early embryonic period； are alike； seem liable to vary in an allied manner： we see this in the right and left sides of the body varying in the same manner； in the front and hind legs； and even in the jaws and limbs； varying together； for the lower jaw is believed to be homologous with the limbs。 These tendencies； I do not doubt； may be mastered more or less completely by natural selection： thus a family of stags once existed with an antler only on one side； and if this had been of any great use to the breed it might probably have been rendered permanent by natural selection。
Homologous parts； as has been remarked by some authors； tend to cohere； this is often seen in monstrous plants； and nothing is more common than the union of homologous parts in normal structures； as the union of the petals of the corolla into a tube。 Hard parts seem to affect the form of adjoining soft parts； it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys。 Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child。 In snakes； according to Schlegel； the shape of the body and the manner of swallowing determine the position of several of the most important viscera。
The nature of the bond of correlation is very frequently quite obscure。 M。 Is。 Geoffroy St Hilaire has forcibly remarked； that certain mal