THE PRESENT EVOLUTION OF MAN — PHYSICAL 259 



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 tuber- 

 culosis, wliich not only persists long in the infected 

 individual, but against which immunity cannot be 

 acquired. But such diseases as sraall-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. 



