Do not ships, in which men live crowded together for months under very unsatisfactory sanitary conditions, develop deleterious principles, to which the crews become accustomed by degrees, but which are, nevertheless, capable of producing the most serious affections in the midst of surrounding populations, which, till then, had been in a flourishing condition. Have we here one of those phenomena to which, according to Darwin, we must attribute the terrible mortality and increasing sterility of the Polynesian races? Among the diseases introduced by European sailors, ought we not to reckon phthisis, which is said to have become epidemic, as well as hereditary, in those islands? The probabilities seem to me to be in favour of an answer in the affirmative. Neither land nor sky have changed in these archipelagoes since the time of their discovery, and yet the Polynesian Islanders disappear with a terrible rapidity, whilst their mixed races and even pure-blooded Europeans show a redoubled fertility—a double contradiction given by facts to autochthonic doctrines.
It is not always easy to determine, in judging of the more or less deleterious action of given conditions of life, what should be attributed to normal conditions, and what is the result of accidental vitiating elements. The soil, cold, heat, dryness, or humidity of a country are not all. The difference presented from the point of view of acclimatisation by the two hemispheres is a striking example.
The hot regions of the southern hemisphere are, as a rule, more accessible to white races than those in similar latitudes in the northern hemisphere. From 30 to 35 degrees of N. lat. we find Algeria, and especially the southern part of the United States, which present serious difficulties against our acclimatisation. In the same latitude of the southern hemisphere, lie the southern portion of the Cape and New South Wales, where all European races prosper almost immediately. M. Boudin’s calculations give the differences exactly. He has found that the mean mortality of French and English armies was about eleven times greater in our hemisphere than in the southern hemisphere.
Struck by this contrast, M. Boudin endeavoured to discover its cause, and found that it lay in the greater or less frequency and gravity of marsh fevers. North of the equator these fevers may be traced in Europe as far as the 59th deg. of latitude. In the south they rarely pass the tropics, and often cease at an even smaller distance. Tahiti, which is only 18 deg. from the geographical equator, and almost beneath the thermal equator, is free from them. In the southern hemisphere, the mean annual number of cases of fever in the united English and French colonies was 1·6 in 1000; in the northern hemisphere it was 224·9 in 1000.
Thus, marsh fevers are almost 200 times more frequent to the north than to the south of the equator, although in South America and Australia, for example, vast tracts are covered with standing water under a burning sun. They are, moreover, of a far less serious nature in the southern hemisphere. The immense lagoons of Corrientes only occasion slight fevers. We know how dangerous, on the contrary, are those of the Pontine Marshes, which are situated at a much greater distance from the equator. It would be much more difficult for a European to live in Italy upon the banks of the Carigliano, than in America upon those of the Parana.
In spite of some experiments and ingenious theories, these differences between localities, apparently presenting almost identical general physical conditions, have not yet been explained. The researches of M. Boudin, however, justify us in regarding these marsh miasmata as very probably the greatest and often the only obstacle to the acclimatisation of Europeans in the greater number of those places to which the spirit of enterprise has led them. There is something very encouraging and instructive in this fact. We know by what combination of circumstances these pestilential miasmata are engendered; we know how it is possible to resist them. Man can, then, wherever he may go, fight against nature, and at least somewhat ameliorate the conditions of acclimatisation. It has, until now, been impossible to make a whole country healthy in a short space of time. This was a work which time alone seemed to be able to accomplish, very often at a heavy cost of human life. It seems as if the introduction of the eucalyptus would, in a great measure at least, tend to diminish these sacrifices.
Should, however, the tree brought from Australia by M. Ramel justify all our hopes, we shall find that some care must still be taken in the choice of station. I shall presently show how, in countries which are apparently most dangerous, there are circumscribed spots where acclimatisation takes place almost immediately. It is clear that new comers ought to look carefully for these favoured localities, and pitch their tents there. The contrary has almost always been, and still is, the case. They allow themselves to be seduced by the beauty and fertility of the alluvial lands situated at the mouth of some river, or upon the shores of some bay calculated to facilitate commerce, without considering their unhealthiness. They settle down and build there, without being disturbed by the losses which overwhelm fresh arrivals; and thus it is that pestilential flats, like that of Batavia, have become inhabited.
V. I cannot here consider in any detail the action of conditions of life upon human races, without anticipating considerations which will be more appropriate in another chapter. I shall only point out a very general fact, and one of great interest in the problem of acclimatisation.
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.