Page:George Archdall Reid 1896 The present evolution of man.djvu/271

Rh maintain a purely saprophytic existence, and which are therefore diseases of crowded populations, were not evolved in this major half of the world, for here the necessary conditions did not exist. The inhabitants of the great continents, North and South America and Australia, were mostly scattered and nomadic. Such dense and settled communities as did arise, as in Mexico and Peru, were far inferior as regards numbers and antiquity to the settled populations of the Old World, such as those which for thousands of years inhabited Egypt, Asia Minor, India, and China. The conditions, therefore, in the great continents of the New World were unfavourable to the* evolution of non-malarial zymotic diseases. In the islands, particularly in the small isolated islands, the conditions were so unfavourable that it is scarcely possible that any such diseases could have been evolved. Such a disease as syphilis, which persists long in the infected individual, and is acquired by direct contact, might possibly have been evolved, as also might such a disease as tuberculosis, which not only persists long in the infected individual, but against which immunity cannot be acquired. But such diseases as small-pox, measles, scarlatina, influenza, &c., which do not persist long in the individual, and against which enduring immunity may be acquired, could not possibly have been evolved, for the simple reason that the supply of nutriment for the pathogenic organisms would not have been sufficient, either as regards quantity or duration, to permit of such an evolution from purely saprophytic to purely parasitic habits. A single epidemic of small-pox or measles, for example, would have so exhausted the nutritive supply of the microbes in a small island, no matter how crowded with human beings, that before a fresh and susceptible generation had arisen the microbes would have perished.