We know that the animal and vegetable races of one species, although in reality subject to the same influences, have, nevertheless, their special aptitudes; and, more especially, some affection which is very general in one will be very rare in another. The case is precisely similar with human races. Marsh fevers act in the same manner upon all men. The Negro suffers and dies from fever on the banks of the Niger, but in a much less degree than the White. Moreover, the two races, when transposed to India, preserve, in this respect, almost the same relations. Compared with local races, the Negro still retains the ascendancy; he is everywhere the last attacked by malarious emanations. Born in a country where he is obliged almost incessantly and universally to breathe them, descended from ancestors, who from prehistoric times have lived in this poisoned air, he has become acclimatised to it more than any other race; on this account alone, he is able to prosper in places where the White would suffer for a long time. On the other hand, the Negro has a delicate chest, and no race is so subject to consumption, whilst this malady is much more rarely fatal to the White or to the Malay. From the extreme differences presented by the White and the Negro it follows that the general conditions of acclimatisation are reversed in the two races. A moderately warm air which is impregnated with malarious emanations is dangerous to the European. A moderate degree even of damp cold will be fatal to the Negro. These few facts are sufficient to show that the conditions of acclimatisation vary with the race; that the same climate cannot exercise the same kind of action upon different races, and that complete acclimatisation, that is to say, _naturalisation_, can only follow upon the harmony of these two terms—race and conditions of life. CHAPTER XX. CONDITIONS OF ACCLIMATISATION. I. The possibility of establishing the harmony, of which I have spoken in the preceding chapter, has been denied. It has been argued that it must exist beforehand, and that instead of becoming acclimatised, people merely become _accustomed_ to a given place. It will be easy to show from what takes place in animals and plants, that there is, in their case, something more than this, and that the organisation is sometimes modified in its most intimate relations so as to conform to the exigencies of conditions of life, which are by nature inflexible.