About this time, two great sanitary discoveries, the most important ever made by man, namely, fire and clothing, came to the knowledge of our tropical ancestors. The greatest sanitarian that the human race has ever produced was probably the individual who discovered fire, and next in importance, the individual who first wore some kind of clothing. These two discoveries enabled man to overcome the hitherto insurmountable sanitary obstacle of the temperate regions, namely, cold. With the application of these two sanitary discoveries, the human race was enabled to migrate from the tropics and continue healthy development in the temperate regions. At the present time, we have just reversed the process; we have just made sanitary discoveries that will enable man to return from the temperate regions to which he was forced to migrate long ages ago, and again live and develop in his natural home, the tropics. These sanitary discoveries are those that have enabled us to control yellow fever and malaria. The practical application of these great discoveries has just been demonstrated during the construction of the Panama Canal. This was not the first demonstration with regard to either disease. But the conditions were such at Panama that they have attracted the attention of the whole world, and probably the general knowledge that the white man can live and thrive in the tropics will date in future times from the construction of this great work. A given amount of labor applied to land will produce a very much larger amount of wealth than will the same labor applied in the same way produce in the temperate regions. The white man, of all the races of the human family, is the most eager in his pursuit of wealth. As it becomes generally known that he can live in the tropics and maintain his health, necessarily a large emigration will occur from the present civilized temperate regions to the tropics. The largest areas of land suitable for cultivation lie in the tropics, and much the largest bodies of rich alluvial lands, such as the valleys of the Amazon and the Congo. Not only are these lands more productive than the lands of the temperate zone, but climatic conditions enable the farmer to produce several crops a year. The tropics, when occupied and cultivated by the white man, will produce many times the amount of food now produced in the temperate regions. The great civilizations of man are now already established and developed in the temperate zones of Europe and America, and it is probable that, for centuries to come, these great empires will be located where they are at present, and that the tropics will be the agricultural, food-producing regions from which these centers of civilization will be supplied.