146 THE STRUGGLE FOR LIFE. 
in order that there may be no useless mouths to fill. As 
civilization advances food becomes more plentiful, and is 
procured more easily by craft than by violence. As people 
live in closer association one with another, the proportions 
of the causes of mortality alter, since more weakly children 
reach maturity, for Nature selects, not the possessors of 
physical strength and great powers of digestion, but those 
who best resist epidemic and contagious disease. That 
such a selection is a reality and not a mere hypothesis is 
proved by numerous facts of which I will cite only two, one 
complementary to the other. In this country measles is looked 
upon as a comparatively mild disorder, and it has rather been 
the fashion for mothers to put children in the way of taking it, 
in order to get it over and done with. But measles when in- 
troduced accidentally into a country such as one of the Islands 
of the South Pacific, in which it has been before unknown, 
assumes the proportions of a veritable plague, men, women 
and children dying indiscriminately and wholesale. The com- 
plementary case is, that the European, who goes to the 
marshy districts of tropical Africa will almost certainly be 
attacked severely by malarious diseases which are compara- 
tively harmless to the native inhabitants. In both cases, 
natural selection acting through long periods of time has in 
these particulars altered the type of the race and produced 
a relative immunity. 
Another stage is reached when the introduction of a high 
pressure water supply renders possible the density of popu- 
lation now found in our own great cities. By this means a 
prime necessity of life is furnished to lofty buildings, and a 
means of removal of refuse provided, refuse which, if it could 
not be taken away by automatic means, would quickly poison 
the whole city. But, with the construction of sewers, new 
diseases such as typhoid fever and diphtheria appeared on 
the scene, for there is a true evolution of diseases or at 
least of those due to parasitic action. The germs of zymotic 
