Acclimatisation is that process whereby animals or plants become adapted to, and so thrive in a climate different from that in which they are indigenous. The process, of course, varies widely, according to the amount of difference between the old and the new climate. In cases where the difference is extreme, important changes take place in the constitution, and are often attended with certain diseases described as 'diseases of acclimatisation.' Thus, Europeans settling in tropical parts are liable to disease of the liver; while natives of tropical lands, when resident in England, are exposed to pulmonary disease. The power of bearing changes of climate is greatest in the Anglo-German race, and usually bears a direct ratio to the intellectual development of a race. Some regions have, however, as yet baffled European colonisation. Civilised people display greater ingenuity and strength of will than savages in accommodating themselves to changes of climate, by making careful corresponding changes in their mode of life. Uloa and Humboldt assert that persons of and above middle age best stand transportation to tropical climates. Among animals, we find great powers of adaptation to various climates in the horse, dog, cat, and rat; and among plants, in the various cereals, in potatoes, and in several weeds common to almost all climates; but there seems to be a limit to the power, at least as seen in the individual. Acclimatisation beyond a certain point is the work of some generations. Almost all the domestic animals now commonly spread over Europe, and even in high northern latitudes, were originally natives of warm climates. The change produced by the acclimatising of animals may be either an improvement or a deterioration; of the latter, we have an instance in the Shetland pony; of the former, we see an example in the merino sheep of Spain. The reindeer may serve as an instance of the want of the faculty of becoming acclimatised; removed from the cold north to the fertile valleys of a temperate clime, it degenerates and dies. On the other hand, the horse, which is native in the East, arrives at its highest development in England; and the Syrian sheep, brought northward as far as Spain, becomes remarkable for its fine fleece. Spain has a climate much warmer than that of Silesia and Pomerania; and yet the merino sheep bred in these countries have become superior to their ancestors imported from Spain. This is a proof that art may do very much in modifying the influences of climate. Silkworms, brought from China first into Italy, have been acclimatised not only in the south of France, but even on the coast of the Baltic. Recently, attempts have been made to acclimatise in France the llama, the vicuña, and the alpaca of Peru, and with some success in the last instance, as alpacas have been found to thrive pretty well in the Pyrenees.
In America, some interesting experiments in naturalisation have been made. Many European birds have been set at liberty by local societies, and a few species promise to become Americanised. The camel breeds well in a half wild state in Nevada and Arizona; while alpacas, though repeatedly tried, have nowhere thriven. Ostrich-farming promises well in the Argentine Republic, but the Californian experiments with African ostriches have been reported to be failures. Various Australasian trees, notably the Eucalyptus, thrive wonderfully in California, and successful experiments have also been made with them in the cotton-growing states; the tea-plant also grows well in various parts of the United States. The camel does well in Australia, and has been found highly useful in the desert tracts. Several species of trout, salmon, and other fishes have been successfully naturalised in Australasia, notably in Tasmania and New Zealand. America and Australia alike have become the abodes of many noxious weeds from Europe. The English sparrow is a great nuisance in North America; the English rabbit is extremely destructive in Australia and New Zealand. In like manner, the Anacharis canadensis, a harmless water-plant in America, has proved an annoying obstruction in many British canals. On the other hand, in the island of Arran and elsewhere interesting and successful experiments in acclimatisation, especially of Australian plants, have been made. In the case of one species, Eucalyptus or gum-tree, the rate of growth has been even greater than in Australia. The introduction of coffee into the West Indies and of cinchona into India offer further examples of successful acclimatisation. It has been very generally believed that plants may become gradually inured to a climate so different from that to which they have been accustomed, that if they had been at once transferred to it they would have perished. On the other hand, it is maintained that each species of plant has certain limits of temperature within which it will succeed, and that alleged instances of acclimatising have been merely instances of plants formerly supposed to be more delicate than they really were. But as it is certain that different varieties of the same species are often more and less hardy, it would seem that in the production of new varieties by seed, there is still a prospect of the acclimatising, to a certain extent, of species of which the existing varieties are too delicate to grow well in the open air. Of Acclimatisation Societies, the best known is the Paris Société d'Acclimatation. The progress which has been made during the past few years in the science of medicine and in sanitation, renders it more probable that attempts made by Europeans to become acclimatised in tropical countries will be successful.
Biologically considered, acclimatisation is part of the general process of modification of organism by environment. When the conditions in the new home are approximately similar, no fresh changes will be imprinted on the organism, and the survival of the imported form is obviously natural. Such cases are instances simply of dispersion, generally by human selection, and hardly of acclimatisation in the strict sense. At the other extreme, the sum of the external forces, or 'natural selection,' may be predominantly adverse, the consequent changes pathological, the result non-survival. The term acclimatisation should thus be restricted to cases between these two extremes, where the plastic organism becomes actively and passively adapted to the new environment. That modifications do take place in consequence of a change of climate and other external conditions, has been recognised from the time of Hippocrates, but how soon these may become really hereditary is still matter of dispute. See CLIMATE, GEOGRAPHICAL DISTRIBUTION, DOMESTICATION, ENVIRONMENT, HEREDITY, PISCICULTURE; Darwin's Animals and Plants under Domestication; works on anthropology by Tylor and Waitz; H. Weber's Climatic Therapeutics.