70 EVOLUTION OF LIVING ORGANISMS. 



disease is " natural," that is to say depends on characters 

 developed in the ordinary environment, then natural 

 innate immunity is soon established by selection, and the 

 disease is eventually stamped out, only immune indivi- 

 duals surviving. But if resistance depends partly or 

 entirely on "acquired immunity," developed only under 

 the stimulus of the disease itself, then the disease will 

 persist, but become less dangerous as the capacity to 

 resist it is increased by selection of the individuals who 

 recover most easily. How effectually selection acts is 

 realised on observing the rapid and fatal spread of in- 

 fectious diseases imported by one race into a region in- 

 habited by another race where they were not previously 

 prevalent. Unless the invaded race happens by chance 

 to be preadapted to resist the new disease, it falls an easy 

 victim. Naturally each race is only adapted by selection 

 to resist the diseases of its own habitat ; town life is as 

 fatal to the prairie Indian as winter frost to the tropical 

 plant. 



Disease has played a most important part in the evolu- 

 tion of the human races. It is not so much by acts of 

 violence that the Spanish and other invaders of America 

 or the European colonists of Australasia have conquered 

 and almost exterminated the native inhabitants of those 

 countries, as by the introduction of diseases the natives 

 were unable to withstand. Small-pox, measles, and 

 tuberculosis have almost cleared the fine indigenous 

 races from off the North American continent, severely 

 handicapping them in the struggle against their Euro- 

 pean rivals. This is merely another instance of the con- 

 stant struggle between species or varieties which we have 

 discussed above. 



There can be no doubt, then, that the death-rate is 

 ^elective. The next point we have to deal with is the 

 part played by selection in the process of evolution. What 

 will be the effect of selection on succeeding generations ; 

 or, in other words, what will be the combined effect of 



