Some monogenists have simply enlarged Grecian theories, and explained everything by the prolonged duration of the same influences. Others have supposed that local changes in the atmospheric conditions of the world, anterior to the actual epoch, were the cause. This is a sort of progress beyond the preceding hypothesis, in the sense that at least we must recognise the insufficiency of actually existing causes, in order to explain the great differences observed at the present day between men. Isidore Geoffroy Saint-Hilaire has agreed with his father upon the great question of the influence of the surrounding medium; but death seized him before he could apply these theories to mankind. However, the high position that his _Histoire Naturelle Générale_ has taken in science obliges us to pause a moment on the subject of his opinions, which have, besides, easily triumphed over the ruins of Cuvier’s school. And if we do not agree with all the doctrines propounded by the second Geoffroy, we are all the more satisfied, since, differing from the son, we incline more to the theories of the father. Isidore Geoffroy believed in a decisive influence of the medium, but only under certain conditions. He believed that these influences are limited, as he himself calls it, every time that it relates to anything beyond the action of man, that is to say, on savage or free animals. In this case, the action of the _medium_, according to him, would be confined exclusively to the producing of varieties in form and in the colour of the skin,[210]--a form we never see varied in the same class of men; the colour of the skin,--which is sensibly the same among men, among whom the fair type is itself exceptional, and spread over a very small portion of the ancient north-west continent. In every case, Isidore Geoffroy acknowledged that these variations are sometimes very inconsiderable;[211] and although they have in no way approached those which separate human races, we may be allowed to believe that the differences observed among savage species were, in his eyes, much less important. He has endeavoured, on the contrary, to compare insignificant differences among free animals with varieties much more marked, and much clearer than those shown by domestic animals, and therefore, doubtless, he wished to make a step towards the fundamental question of anthropology, which was evidently at the bottom of his thoughts, and which he had for a long time resolved in a monogenist sense.