We have seen that certain conditions of life appear to be fatal to all races. In cases of this kind, we should distinguish how much of this insalubrity is due to the regions, and how much is the result of accidental circumstances, sometimes provoked by man himself. The plain of the Dombe in France was once as salubrious as the surrounding country. The exaggerated industry of the marshes transformed it into a pestilential region, where it was quite as fatal for foreign populations to live as it would have been in the swamps of the Senegal. Sanitary measures are now tending to restore it to its former condition. It is evident that we cannot reproach the Dombe with the deleterious influence which human intelligence seems to have undertaken to develop. Even when the latter does not step in to vitiate the conditions of life, we cannot charge a country with opposing unfavourable conditions to an indigenous or foreign race, when these conditions may be attributed to the negligence of the inhabitants, or to some special cause, which human intelligence might modify. Deprived of the care which rendered it healthy and luxuriant, the Campagna of Rome has become a branch of the Pontine Marshes. On the other hand, the environs of Rochefort have become healthy; Bouffarik, once one of the most dangerous spots in Algeria, has become the centre of a flourishing population. It was not, therefore, the general natural conditions which rendered these localities dangerous, especially to strangers, but simply _accident_. As soon as the cause is removed, acclimatisation becomes not only possible, but easy. Considered from this point of view, many countries, which now appear to repel all attempts at immigration, will, perhaps, at some future period, be particularly favourable to the development of colonizing races. It is clear that in all cases of this kind we must distinguish between _normal_ and _accidentally vitiated conditions of life_. I cannot enter into all the details which this distinction would allow, and shall confine myself to quoting a few facts. The very progress of civilization sometimes results in the vitiation of certain conditions of life. Such is the almost inevitable result of the crowding together of human beings in a relatively limited space. This is one of the points most clearly demonstrated in the statistical researches of M. Boudin upon the comparative mortality of the country and of barracks, for example. A comparison of our large towns and rural districts leads to the same result, and points to a special action upon the organs of reproduction. M. Boudin could not find a pure-blooded Parisian whose genealogy could be traced for more than three generations. At Besançon, town families become extinct in less than a century, and are replaced by others from the country. London, I have been assured, presents a similar phenomenon.