The chrysanthemum (_Pyretrum sinense_), which adorns our gardens, came, as we know, originally from China. Introduced into France in 1790, it flourished there and produced fruit which it was unable to ripen, so that commerce alone supplied our flower gardens with the necessary seed for more than sixty years. The attempt to rear it in hot-houses and frames met with very small success. In 1852 a few plants were observed to flower and to fruit sooner than the others; the seeds ripened, and France now produces all the seed which she requires. A small number of accidentally precocious plants have, therefore, acclimatised this beautiful flower. The history of the Egyptian goose (_Anser egyptiacus_) is still more striking. Brought to France in 1801, by Geoffroy Saint-Hilaire, this species at first laid in December, as in its native country. It reared its brood in the depth of winter, and consequently under very unfavourable circumstances. Several generations were, nevertheless, reared at the Museum. Now in 1844 the birds laid in February, the following year in March, and in 1846 in April, the time at which our common goose lays. Is it not clear that the organisation of the Egyptian goose had accommodated itself to the conditions imposed by our climate? This marvellous faculty of living beings is sometimes even inconvenient. French vines when removed to the island of Bourbon yield grapes continually, so that the mixture of clusters in every stage of development and maturity has been a serious obstacle in the manufacture of the wine. Silkworms have acted in a similar manner; they have laid their eggs and spun their cocoons with perfect indifference as to the season of the year, and in such an irregular manner as to force breeders to give up rearing them. Acclimatisation, that is to say, _physiological adaptation_ to new conditions of life, is an incontestable fact. All our domestic races which have been imported into America are prospering there. When the conditions of existence have been almost the same as those of their native country, they have changed but little. When the new conditions have differed too widely from the old ones, local races have been formed; and thus, though perhaps assisted by human industry, _pigs with fleece_ are to be found on the cold plateaux of the Cordilleras, _sheep with hair_ in the warm valleys of the Madeleine, and _hairless cattle_ in the burning plains of Mariquita. Is it not clear that these pigs, sheep, and oxen, these descendants of our races in temperate climates, have established a harmony between themselves and the conditions of life? II. But, as I said before, this harmony is scarcely ever obtained without struggles and sacrifices. In this respect again man resembles plants and animals. Let us see, in the first place, what may be learnt on this subject from these beings of inferior organisation.