Variations in the aspect of men are best seen when an examination is made of nations arranged in a northerly and southerly direction; the result is such as would ensue to an emigrant passing slowly along a meridional track; but the case would be quite different if the movement were along a parallel of latitude. In this latter direction the variations of climate are far less marked, and depend much more on geographical than on astronomical causes. In emigrations of this kind there is never that rapid change of aspect, complexion, and intellectual power which must occur in the other. Thus, though the mean temperature of Europe increases from Poland to France, chiefly through the influence of the great Atlantic current transferring heat from the Gulf of Mexico and tropical ocean, that rise is far less than would be encountered on passing through the same distance to the south. By the arts of civilization man can much more easily avoid the difficulties arising from variations along a parallel of latitude than those upon a meridian, for the simple reason that in that case those variations are less. [Sidenote: Their political result.] But it is not only complexion, development of the brain, and, therefore, intellectual power, which are thus affected. With difference of climate there must be differences of manners and customs, that is, differences in the modes of civilization. These are facts which deserve our most serious attention, since such differences are inevitably connected with political results. If homogeneousness be an element of strength, an empire that lies east and west must be more powerful than one that lies north and south. I cannot but think that this was no inconsiderable cause of the greatness and permanence of Rome and that it lightened the task of the emperors, often hard enough, in government. There is a natural tendency to homogeneousness in the east and west direction, a tendency to diversity and antagonism in the north and south, and hence it is that government under the latter circumstances will always demand the highest grade of statesmanship. [Sidenote: Nature of transitional forms.] The transitional forms which an animal type is capable of producing on a passage north and south are much more numerous than those it can produce on a passage east and west. These, though they are truly transitional as respects the type from which they have proceeded, are permanent as regards the locality in which they occur, being, in fact, the incarnation of its physical influences. As long, therefore, as those influences remain without change the form that has been produced will last without any alteration. For such a permanent form in the case of man we may adopt the designation of an ethnical element. [Sidenote: Conditions of change in an ethnical element.]