THE GEOGRAPHIC BASIS OF HISTORY 369
uninhabitable desert far west of the Nile indicate a great change of climate. Fraas goes further, and says : " An intellectual activity like that of the times of the Greeks, when Alexandria was the centre of all the arts and sciences, a true world's university, with the richest library on earth; or as that which existed from the times of the Platonists up to the first centuries of the Christian era, when the greatest thinkers of the church — such as Origen, the Gnostic — developed their philosophical-religious systems, — such a movement of thought demands, as an absolute necessity, a different climate, and a moister air, than that now prevailing in Egypt."
Still farther to the west along the Mediterranean coast of Africa and in the Sahara desert many writers, on grounds that appear to be reasonable, have inferred that desiccation has taken place during historic times. Their observations, however, have generally, possibly always, lacked the quantitative element which alone can make them conclusive. The ruin which has overtaken northern Africa since the days of Carthage is matter of common knowledge. Apparently, the famous march of the Third Legion far into the deserts of Sahara was rendered possible by more favorable climatic conditions, such as those which enabled Alexander to penetrate the deserts of Central Asia.
We all remember how cold and swampy Europe seemed when we read Cæsar. We have been in the habit of assuming that the climate of his day was like that of the present, and that we remember the snow and the swamps because the conqueror of Gaul emphasized the difficulties which he met. Possibly, however, Coesar was so great an artist that
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