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 is often confounded with domestication or with naturalization; but these are both very different phenomena. A domesticated animal or a cultivated plant need not necessarily be acclimatized; that is, it need not be capable of enduring the severity of the seasons without protection. The canary bird is domesticated but not acclimatized, and many of our most extensively cultivated plants are in the same category. A naturalized animal or plant, on the other hand, must be able to withstand all the vicissitudes of the seasons in its new home, and it may therefore be thought that it must have become acclimatized. But in many, perhaps most cases of naturalization (see Appendix below) there is no evidence of a gradual adaptation to new conditions which were at first injurious, and this is essential to the idea of acclimatization. On the contrary, many species, in a new country and under somewhat different climatic conditions, seem to find a more congenial abode than in their native land, and at once flourish and increase in it to such an extent as often to exterminate the indigenous inhabitants. Thus L. Agassiz (in his work on Lake Superior) tells us that the roadside weeds of the north-eastern United States, to the number of 130 species, are all European, the native weeds having disappeared westwards; while in New Zealand there are, according to T. Kirk (Transactions of the New Zealand Institute, vol. ii. p. 131), no less than 250 species of naturalized plants, more than 100 of which spread widely over the country and often displace the native vegetation. Among animals, the European rat, goat and pig are naturalized in New Zealand, where they multiply to such an extent as to injure and probably exterminate many native productions. In none of these cases is there any indication that acclimatization was necessary or ever took place.

On the other hand, the fact that an animal or plant cannot be naturalized is no proof that it is not acclimatized. It has been shown by C. Darwin that, in the case of most animals and plants in a state of nature, the competition of other organisms is a far more efficient agency in limiting their distribution than the mere influence of climate. We have a proof of this in the fact that so few, comparatively, of our perfectly hardy garden plants ever run wild; and even the most persevering attempts to naturalize them usually fail. Alphonse de Candolle (Géographie botanique, p. 798) informs us that several botanists of Paris, Geneva, and especially of Montpellier, have sown the seeds of many hundreds of species of exotic hardy plants, in what appeared to be the most favourable situations, but that in hardly a single case has any one of them become naturalized. Attempts have also been made to naturalize continental insects in Britain, in places where the proper food-plants abound and the conditions seem generally favourable, but in no case do they seem to have succeeded. Even a plant like the potato, so largely cultivated and so perfectly hardy, has not established itself in a wild state in any part of Europe.

Different Degrees of Climatal Adaptation in Animals and Plants.—Plants differ greatly from animals in the closeness of their adaptation to meteorological conditions. Not only will most tropical plants refuse to live in a temperate climate, but many species are seriously injured by removal a few degrees of latitude beyond their natural limits. This is probably due to the fact, established by the experiments of A. C. Becquerel, that plants possess no proper temperature, but are wholly dependent on that of the surrounding medium.

Animals, especially the higher forms, are much less sensitive to change of temperature, as shown by the extensive range from north to south of many species. Thus, the tiger ranges from the equator to northern Asia as far as the river Amur, and to the isothermal of 32° Fahr. The mountain sparrow (Passer montana) is abundant in Java and Singapore in a uniform equatorial climate, and also inhabits Britain and a considerable portion of northern Europe. It is true that most terrestrial animals are restricted to countries not possessing a great range of temperature or very diversified climates, but there is reason to believe that this is due to quite a different set of causes, such as the presence of enemies or deficiency of appropriate food. When supplied with food and partially protected from enemies, they often show a wonderful capacity of enduring climates very different from that in which they originally flourished. Thus, the horse and the domestic fowl, both natives of very warm countries, flourish without special protection in almost every inhabited portion of the globe. The parrot tribe form one of the most pre-eminently tropical groups of birds, only a few species extending into the warmer temperate regions; yet even the most exclusively tropical genera are by no means delicate birds as regards climate. In the Annals and Magazine of Natural History for 1868 (p. 381) is a most interesting account, by Charles Buxton, of the naturalization of parrots at Northreps Hall, Norfolk. A considerable number of African and Amazonian parrots, Bengal parroquets, four species of white and rose crested cockatoos, and two species of crimson lories, remained at large for many years. Several of these birds bred, and they almost all lived in the woods the whole year through, refusing to take shelter in a house constructed for their use. Even when the thermometer fell 6° below zero, all appeared in good spirits and vigorous health. Some of these birds have lived thus exposed for many years, enduring the English cold easterly winds, rain, hail and snow, all through the winter—a marvellous contrast to the equable equatorial temperature (hardly ever less than 70°) to which many of them had been accustomed for the first year or years of their existence. Similarly the recent experience of zoological gardens, particularly in the case of parrots and monkeys, shows that, excluding draughts, exposure to changes of temperature without artificial heat is markedly beneficial as compared with the older method of strict protection from cold.

Hardly any group of Mammalia is more exclusively tropical than the Quadrumana, yet, if other conditions are favourable, some of them can withstand a considerable degree of cold. Semnopithecus schistaceus was found by Captain Hutton at an elevation of 11,000 feet in the Himalayas, leaping actively among fir-trees whose branches were laden with snow-wreaths. In Abyssinia a troop of dog-faced baboons was observed by W. T. Blanford at 9000 feet above the sea. We may therefore conclude that the restriction of the monkey tribe to warm latitudes is probably determined by other causes than temperature alone.

Similar indications are given by the fact of closely allied species inhabiting very extreme climates. The recently extinct Siberian mammoth and woolly rhinoceros were closely allied to species now inhabiting tropical regions exclusively. Wolves and foxes are found alike in the coldest and hottest parts of the earth, as are closely allied species of falcons, owls, sparrows and numerous genera of waders and aquatic birds.

A consideration of these and many analogous facts might induce us to suppose that, among the higher animals at least, there is little constitutional adaptation to climate, and that in their case acclimatization is not required. But there are numerous examples of domestic animals which show that such adaptation does exist in other cases. The yak of Thibet cannot long survive in the plains of India, or even on the hills below a certain altitude; and that this is due to climate, and not to the increased density of the atmosphere, is shown by the fact that the same animal appears to thrive well in Europe, and even breeds there readily. The Newfoundland dog will not live in India, and the Spanish breed of fowls in this country suffer more from frost than most others. When we get lower in the scale the adaptation is often more marked. Snakes, which are so abundant in warm countries, diminish rapidly as we go north, and wholly cease at lat. 62°. Most insects are also very susceptible to cold, and seem to be adapted to very narrow limits of temperature.

From the foregoing facts and observations we may conclude, firstly, that some plants and many animals are not constitutionally adapted to the climate of their native country only, but are capable of enduring and flourishing under a more or less extensive range of temperature and other climatic conditions; and, secondly, that most plants and some animals are, more or less closely, adapted to climates similar to those of their native habitats. In order to domesticate or naturalize the former