class in countries not extremely differing from that from which the species was brought, it will not be necessary to acclimatize, in the strict sense of the word. In the case of the latter class, however, acclimatization is a necessary preliminary to naturalization, and in many cases to useful domestication, and we have therefore to inquire whether it is possible.
Acclimatization by Individual Adaptation.—It is evident that acclimatization may occur (if it occurs at all) in two ways, either by modifying the constitution of the individual submitted to the new conditions, or by the production of offspring which may be better adapted to those conditions than their parents. The alteration of the constitution of individuals in this direction is not easy to detect, and its possibility has been denied by many writers. C. Darwin believed, however, that there were indications that it occasionally occurred in plants, where it can be best observed, owing to the circumstance that so many plants are propagated by cuttings or buds, which really continue the existence of the same individual almost indefinitely. He adduced the example of vines taken to the West Indies from Madeira, which have been found to succeed better than those taken directly from France. But in most cases habit, however prolonged, appears to have little effect on the constitution of the individual, and the fact has no doubt led to the opinion that acclimatization is impossible. There is indeed little or no evidence to show that any animal to which a new climate is at first prejudicial can be so acclimatized by habit that, after subjection to it for a few or many seasons, it may live as healthily and with as little care as in its native country; yet we may, on general principles, believe that under proper conditions such an acclimatization would take place.
Acclimatization by Variation.—A mass of evidence exists showing that variations of every conceivable kind occur among the offspring of all plants and animals, and that, in particular, constitutional variations are by no means uncommon. Among cultivated plants, for example, hardier and more tender varieties often arise. The following cases are given by C. Darwin:—Among the numerous fruit-trees raised in North America some are well adapted to the climate of the northern States and Canada, while others only succeed well in the southern States. Adaptation of this kind is sometimes very close, so that, for example, few English varieties of wheat will thrive in Scotland. Seed-wheat from India produced a miserable crop when planted by the Rev. M. J. Berkeley on land which would have produced a good crop of English wheat. Conversely, French wheat taken to the West Indies produced only barren spikes, while native wheat by its side yielded an enormous harvest. Tobacco in Sweden, raised from home-grown seed, ripens its seeds a month earlier than plants grown from foreign seed. In Italy, as long as orange trees were propagated by grafts, they were tender; but after many of the trees were destroyed by the severe frosts of 1709 and 1763, plants were raised from seed, and these were found to be hardier and more productive than the former kinds. Where plants are raised from seed in large quantities, varieties always occur differing in constitution, as well as others differing in form or colour; but the former cannot be perceived by us unless marked out by their behaviour under exceptional conditions, as in the following cases. After the severe winter of 1860–1861 it was observed that in a large bed of araucarias some plants stood quite unhurt among numbers killed around them. In C. Darwin’s garden two rows of scarlet runners were entirely killed by frost, except three plants, which had not even the tips of their leaves browned. A very excellent example is to be found in Chinese history, according to E. R. Huc, who, in his L’ Empire chinois (tom. ii. p. 359), gives the following extract from the Memoirs of the Emperor Khang:—“On the 1st day of the 6th moon I was walking in some fields where rice had been sown to be ready for the harvest in the 9th moon. I observed by chance a stalk of rice which was already in ear. It was higher than all the rest, and was ripe enough to be gathered. I ordered it to be brought to me. The grain was very fine and well grown, which gave me the idea to keep it for a trial, and see if the following year it would preserve its precocity. It did so. All the stalks which came from it showed ear before the usual time, and were ripe in the 6th moon. Each year has multiplied the produce of the preceding, and for thirty years it is this rice which has been served at my table. The grain is elongate and of a reddish colour, but it has a sweet smell and very pleasant taste. It is called Yu-mi, Imperial rice, because it was first cultivated in my gardens. It is the only sort which can ripen north of the great wall, where the winter ends late and begins very early; but in the southern provinces, where the climate is milder and the land more fertile, two harvests a year may be easily obtained, and it is for me a sweet reflection to have procured this advantage for my people.” Huc adds his testimony that this kind of rice flourishes in Manchuria, where no other will grow. We have here, therefore, a perfect example of acclimatization by means of a spontaneous constitutional variation.
That this kind of adaptation may be carried on step by step to more and more extreme climates is illustrated by the following examples. Sweet-peas raised in Calcutta from seed imported from England rarely blossom, and never yield seed; plants from French seed flower better, but are still sterile; but those raised from Darjeeling seed (originally imported from England) both flower and seed profusely. The peach is believed to have been tender, and to have ripened its fruit with difficulty, when first introduced into Greece; so that (as Darwin observes) in travelling northward during two thousand years it must have become much hardier. Sir J. Hooker ascertained the average vertical range of flowering plants in the Himalayas to be 4000 ft., while in some cases it extended to 8000 ft. The same species can thus endure a great difference of temperature; but the important fact is, that the individuals have become acclimatized to the altitude at which they grow, so that seeds gathered near the upper limit of the range of a species will be more hardy than those gathered near the lower limit. This was proved by Hooker to be the case with Himalayan conifers and rhododendrons, raised in Britain from seed gathered at different altitudes.
Among animals exactly analogous facts occur. When geese were first introduced into Bogota they laid few eggs at long intervals, and few of the young survived. By degrees the fecundity improved, and in about twenty years became equal to what it is in Europe. The same author tells us that, according to Garcilaso, when fowls were first introduced into Peru they were not fertile, whereas now they are as much so as in Europe. C. Darwin adduced the following examples. Merino sheep bred at the Cape of Good Hope have been found far better adapted for India than those imported from England; and while the Chinese variety of the Ailanthus silk-moth is quite hardy, the variety found in Bengal will only flourish in warm latitudes. C. Darwin also called attention to the circumstance that writers of agricultural works generally recommend that animals should be removed from one district to another as little as possible. This advice occurs even in classical and Chinese agricultural books as well as in those of our own day, and proves that the close adaptation of each variety or breed to the country in which it originated has always been recognized.
Constitutional Adaptation often accompanied by External Modification.—Although in some cases no perceptible alteration of form or structure occurs when constitutional adaptation to climate has taken place, in others it is very marked. C. Darwin collected a large number of cases in his Animals and Plants under Domestication.
In his Contributions to the Theory of Natural Selection (p. 167), A. R. Wallace has recorded cases of simultaneous variation among insects, apparently due to climate or other strictly local causes. He found that the butterflies of the family Papilionidae, and some others, became similarly modified in different islands and groups of islands. Thus, the species inhabiting Sumatra, Java and Borneo are almost always much smaller than the closely allied species of Celebes and the Moluccas; the species or varieties of the small island of Amboyna are larger than the same species or closely allied forms inhabiting the surrounding islands; the species found in Celebes possess a peculiar form of wing, quite
