The caterpillars of the Bombyx hesperus feed in a state of nature on the leaves of the Café diable, but, after having been reared on the Ailanthus, they would not touch the Café diable, and actually died of hunger.[[753]]
It has been found possible to accustom marine fish to live in fresh water; but as such changes in fish, and other marine animals, have been chiefly observed in a state of nature, they do not properly belong to our present subject. The period of gestation and of maturity, as shown in the earlier chapters,—the season and the frequency of the act of breeding,—have all been greatly modified under domestication. With the Egyptian goose the rate of change in the season has been recorded.[[754]] The wild drake pairs with one female, the domestic drake is polygamous. Certain breeds of fowls have lost the habit of incubation. The paces of the horse, and the manner of flight in certain breeds of the pigeon, have been modified, and are inherited. The voice differs much in certain fowls and pigeons. Some breeds are clamorous and others silent, as in the Call and common duck, or in the Spitz and pointer dog. Every one knows how dogs differ from each other in their manner of hunting, and in their ardour after different kinds of game or vermin.
With plants the period of vegetation is easily changed and is inherited, as in the case of summer and winter wheat, barley,
and vetches; but to this subject we shall immediately return under acclimatisation. Annual plants sometimes become perennial under a new climate, as I hear from Dr. Hooker is the case with the stock and mignonette in Tasmania. On the other hand, perennials sometimes become annuals, as with the Ricinus in England, and as, according to Captain Mangles, with many varieties of the heartsease. Von Berg[[755]] raised from seed of Verbascum phœnicium, which is usually a biennial, both annual and perennial varieties. Some deciduous bushes become evergreen in hot countries.[[756]] Rice requires much water, but there is one variety in India which can be grown without irrigation.[[757]] Certain varieties of the oat and of our other cereals are best fitted for certain soils.[[758]] Endless similar facts could be given in the animal and vegetable kingdoms. They are noticed here because they illustrate analogous differences in closely allied natural species, and because such changed habits of life, whether due to use and disuse, or to the direct action of external conditions, or to so-called spontaneous variation, would be apt to lead to modifications of structure.
Acclimatisation.—From the previous remarks we are naturally led to the much disputed subject of acclimatisation. There are two distinct questions: Do varieties descended from the same species differ in their power of living under different climates? And secondly, if they so differ, how have they become thus adapted? We have seen that European dogs do not succeed well in India, and it is asserted,[[759]] that no one has succeeded in there keeping the Newfoundland long alive; but then it may be argued, probably with truth, that these northern breeds are specifically distinct from the native dogs which flourish in India. The same remark may be made with respect to different breeds of sheep, of which, according to Youatt,[[760]] not one brought "from a torrid climate lasts out the second year," in the Zoological Gardens. But sheep are capable of some degree of acclimatisation, for Merino sheep bred at the Cape of Good Hope have been found
far better adapted for India than those imported from England.[[761]] It is almost certain that the breeds of the fowl are descended from the same species; but the Spanish breed, which there is good reason to believe originated near the Mediterranean,[[762]] though so fine and vigorous in England, suffers more from frost than any other breed. The Arrindy silk-moth introduced from Bengal, and the Ailanthus moth from the temperate province of Shan Tung, in China, belong to the same species, as we may infer from their identity in the caterpillar, cocoon, and mature states;[[763]] yet they differ much in constitution: the Indian form "will flourish only in warm latitudes," the other is quite hardy and withstands cold and rain.
Plants are more strictly adapted to climate than are animals. The latter when domesticated withstand such great diversities of climate, that we find nearly the same species in tropical and temperate countries; whilst the cultivated plants are widely dissimilar. Hence a larger field is open for inquiry in regard to the acclimatisation of plants than of animals. It is no exaggeration to say that with almost every plant which has long been cultivated varieties exist, which are endowed with constitutions fitted for very different climates; I will select only a few of the more striking cases, as it would be tedious to give all. In North America numerous fruit-trees have been raised, and in horticultural publications,—for instance, in Downing,—lists are given of the varieties which are best able to withstand the severe climate of the northern States and Canada. Many American varieties of the pear, plum, and peach are excellent in their own country, but until recently hardly one was known that succeeded in England; and with apples,[[764]] not one succeeds. Though the American varieties can withstand a severer winter than ours, the summer here is not hot enough. Fruit-trees have originated in Europe as in America with different constitutions, but they are not here much noticed, as the same nurserymen do not supply a wide area. The Forelle pear flowers early, and when the flowers have just set, and this is the critical period, they have been observed, both in France and England, to withstand with complete impunity a frost of 18° and even 14° Fahr., which killed the flowers, whether fully expanded or in bud, of all other kinds of pears.[[765]] This power in the flower of resisting cold and afterwards producing fruit does not invariably depend, as we know on good authority,[[766]] on general constitutional vigour.
In proceeding northward, the number of varieties which are enabled to resist the climate rapidly decreases, as may be seen in the list of the varieties of the cherry, apple, and pear, which can be cultivated in the neighbourhood of Stockholm.[[767]] Near Moscow, Prince Troubetzkoy planted for experiment in the open ground several varieties of the pear, but one alone, the Poire sans Pepins, withstood the cold of winter.[[768]] We thus see that our fruit-trees, like distinct species of the same genus, certainly differ from each other in their constitutional adaptation to different climates.
With the varieties of many plants, the adaptation to climate is often very close. Thus it has been proved by repeated trials "that few if any of the English varieties of wheat are adapted for cultivation in Scotland;"[[769]] but the failure in this case is at first only in the quantity, though ultimately in the quality, of the grain produced. The Rev. J. M. Berkeley sowed wheat-seed from India, and got "the most meagre ears," on land which would certainly have yielded a good crop from English wheat.[[770]] In these cases varieties have been carried from a warmer to a cooler climate; in the reverse case, as "when wheat was imported directly from France into the West Indian Islands, it produced either wholly barren spikes or furnished with only two or three miserable seeds, while West Indian seed by its side yielded an enormous harvest."[[771]] Here is another case of close adaptation to a slightly cooler climate; a kind of wheat which in England may be used indifferently either as a winter or summer variety, when sown under the warmer climate of Grignan, in France, behaved exactly as if it had been a true winter wheat.[[772]]
Botanists believe that all the varieties of maize belong to the same species; and we have seen that in North America, in proceeding northward, the varieties cultivated in each zone produce their flowers and ripen their seed within shorter and shorter periods. So that the tall, slowly maturing southern varieties do not succeed in New England, and the New English varieties do not succeed in Canada. I have not met with any statement that the southern varieties are actually injured or killed by a degree of cold which the northern varieties withstand with impunity, though this is probable; but the production of early flowering and early seeding varieties deserves to be considered as one form of acclimatisation. Hence it has been found possible, according to Kalm, to cultivate maize further and further northwards in America. In Europe, also, as we learn from the evidence given by Alph. De Candolle, the culture of maize has extended since the end of the last century thirty leagues north of its former boundary.[[773]] On the authority of the great Linnæus,[[774]] I may quote an analogous case, namely, that in Sweden tobacco raised from home-grown seed ripens its seed a month sooner and is less liable to miscarry than plants raised from foreign seed.
With the Vine, differently from the maize, the line of practical culture has retreated a little southward since the middle ages;[[775]] but this seems due to commerce, including that of wine, being now freer or more easy. Nevertheless the fact of the vine not having spread northward shows that acclimatisation has made no progress during several centuries. There is, however, a marked difference in the constitution of the several varieties,—some being hardy, whilst others, like the muscat of Alexandria, require a very high temperature to come to perfection. According to Labat,[[776]] vines taken from France to the West Indies succeed with extreme difficulty, whilst those imported from Madeira, or the Canary Islands, thrive admirably.
Gallesio gives a curious account of the naturalisation of the Orange in Italy. Daring many centuries the sweet orange was propagated exclusively by grafts, and so often suffered from frosts that it required protection. After the severe frost of 1709, and more especially after that of 1763, so many trees were destroyed that seedlings from the sweet orange were raised, and, to the surprise of the inhabitants, their fruit was found to be sweet. The trees thus raised were larger, more productive, and hardier than the former kinds; and seedlings are now continually raised. Hence Gallesio concludes that much more was effected for the naturalisation of the orange in Italy by the accidental production of new kinds during a period of about sixty years, than had been effected by grafting old varieties during many ages.[[777]] I may add that Risso[[778]] describes some Portuguese varieties of the orange as extremely sensitive to cold, and as much tenderer than certain other varieties.
The peach was known to Theophrastus, 322 B.C.[[779]] According to the authorities quoted by Dr. F. Rolle,[[780]] it was tender when first introduced into Greece, and even in the island of Rhodes only occasionally bore fruit. If this be correct, the peach, in spreading during the last two thousand years over the middle parts of Europe, must have become much hardier. At the present day different varieties differ much in hardiness: some French varieties will not succeed in England; and near Paris, the Pavie de Bonneuil does not ripen its fruit till very late, even when grown on a wall; "it is, therefore, only fit for a very hot southern climate."[[781]]
I will briefly give a few other cases. A variety of Magnolia grandiflora, raised by M. Roy, withstands cold several degrees lower than that which any other variety can resist. With camellias there is much difference in hardiness. One particular variety of Noisette rose withstood the severe frost of 1860 "untouched and hale amidst a universal destruction of other Noisettes." In New York the "Irish yew is quite hardy, but the common yew is liable to be cut down." I may add that there are varieties of the sweet potato (Convolvulus batatas) which are suited for warmer, as well as for colder, climates.[[782]]
The plants as yet mentioned have been found capable of resisting an unusual degree of cold or heat, when fully grown. The following cases refer to plants whilst young. In a large bed of young Araucarias of the same age, growing close together and equally exposed, it was observed,[[783]] after the unusually severe winter of 1860-61, that, "in the midst of the dying, numerous individuals remained on which the frost had absolutely made no kind of impression." Dr. Lindley, after alluding to this and other similar cases, remarks, "Among the lessons which the late formidable winter has taught us, is that, even in their power of resisting cold, individuals of the same species of plants are remarkably different." Near Salisbury, there was a sharp frost on the night of May 24th, 1836, and all the French beans (Phaseolus vulgaris) in a bed were killed except about one in thirty, which completely escaped.[[784]] On the same day of the month, but in the year 1864, there was a severe frost in Kent, and two rows of scarlet-runners (P. multiflorus) in my garden, containing 390 plants of the same age and equally exposed, were all blackened and killed except about a dozen plants. In an adjoining row of "Fulmer's dwarf bean" (P. vulgaris), one single plant escaped. A still more severe frost occurred four days afterwards, and of the dozen plants which had previously escaped only three survived; these were not taller or more vigorous than the other young plants, but they escaped completely, with not even the tips of their leaves browned. It was impossible to behold these three plants, with their blackened, withered, and dead brethren all round them, and not see at a glance that they differed widely in constitutional power of resisting frost.
This work is not the proper place to show that wild plants