Exactly the same argument applies to man. If we consider man’s condition in his earlier stages, we can see that there must have been a period when he had neither fire nor clothing. At this time he must have lived in parts of the world where the temperature was that now found between the tropics of Cancer and Capricorn. As we know him now, he could survive the year around without fire and clothing only within this region. He might live a little north and south of these parallels of latitude, but not very far. Very few individuals of a community which attempted to spend a winter in the latitude of Washington, without either fire or clothing, would be alive when the warm weather of spring returned. It is pretty certain, therefore, that man lived exclusively in the tropics, up to the discovery of fire and clothing. There was a period, then, in man’s existence when the environment found in the tropics was better suited to his life than that found in the present temperate zones. Conditions gradually changed, until affairs were exactly reversed, and the temperate regions became better suited for man’s healthy life than the tropical regions. This was the condition of affairs when we first begin to get some knowledge of man, at the early dawn of history. When we first begin to learn anything about him historically, the most vigorous and healthy races, mentally and physically, were to be found in the temperate zones. The conditions that brought about this change of habitat in man were probably the spread of the various infectious diseases within the tropics. The hot tropical regions were much better fitted for the life of the germs which caused these infectious diseases than were the temperate regions, for exactly the same reasons that these tropical regions better suited the life and development of man. Due to his superior intelligence and superior powers of locomotion, man was enabled to spread through and occupy the tropical regions long before the germs of the various infectious diseases were able to do so. Yellow fever, for instance, was able to extend very little beyond the region where it originally developed, until man came along with his ships, and in this way enabled the yellow-fever germ to begin its travels about the world. As yellow-fever developed in America, the germ did not begin its travels until Columbus brought his ships into the Caribbean Sea. As the infections spread through the tropics, the environment in those regions became unfavorable to man, to such an extent that he ceased to be able to improve in his mental and physical characteristics. But as the environment of regions outside the tropics was still more unfavorable, actually deadly, he had no escape. At this period, then, man found himself inhabiting tropical regions where sanitary conditions, the infections, were very unfavorable to him, and he was unable to migrate to the temperate zones, because the sanitary conditions there, cold, were deadly to him.