92 THE AMERICAN JOURNAL OF SOCIOLOGY
20 and o centrigrade. In Asia and America this zone of favorable social conditions is only one-half as large. While civilization moves northward from the semi-tropical zones above the equator, it also tends to develop in the semi-tropical zones north and south of the equator, and even in the south temperate zones.
South of the equator it is Africa which has the warmest average temperature (29 to 50), a condition almost as unfavor- able for civilization as excessive cold. The nature of its climate, together with the small development of its sea-coast relative to its continental area a development inferior to that of Asia, North America, South America, Australia, and especially of Europe explains the backwardness of the civilization of the greater part of the continent, save in the north and south. In order to draw it into the current of civilization, Europe must take it in tow, unfortunately in a manner generally too violent, and of such a nature that it lowers its appropriate moral value and civilization.
The geographical structure and limits of the earth, however, must be considered as only relatively constant. The whole sur- face of the globe was originally aqueous. It is, also, necessary to recall that, since the appearance of land, certain portions have sunk, making place for the seas, while others have risen. The Mediterranean was once an inland sea, and then in the place of the Straits of Gibraltar there was an isthmus joining Spain to Africa. Since the appearance of man, England was united to the continent, and Europe was joined to the northern part of America. The South Sea once formed a vast continent, of which the present islands were mountain peaks.
In the place of the Indian Ocean there was, perhaps, a conti- nent along southern Asia from the Sunda Islands to the shore of Africa. We have seen that this was Lemuria, where man originated.
On the other hand, seas were replaced by land, with inland lakes, such as certain deserts of Africa and Asia. Ancient Iran evolved about one of these large lakes, now transformed into a desert. The waves continually wear away certain shores and
