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.