THEORIES IN PRACTICE 359 



vegetable and animal races have evolved, was 

 due to Darwin. 



Alfred Russell Wallace conceived the same 

 idea independently, and must always be credited 

 with a share in the discovery. 



But of course it was Darwin's exposition that 

 gave the subject general vogue, and the scheme 

 of heredity that it connotes is with full propriety 

 spoken of as Darwinian evolution. 



The essentials of this scheme of heredity may 

 be stated in a few words, as follows: Animals 

 and plants tend to increase in geometrical ratio. 

 If unopposed, the progeny of a single pair of 

 animals or an individual plant would soon popu- 

 late and overpopulate the entire earth. Oppo- 

 sition to such overpopulation comes from the 

 rivalry of other animals and plants. The strug- 

 gle for existence thus induced puts a premium 

 on the individual animal or plant that is better 

 able than its fellows to seek means of sustenance. 

 Such an individual will, on the average, live 

 longer and produce more offspring than an indi- 

 vidual less well adapted to its surroundings. 



The preservation of these favored individuals 

 and their progeny may be described in a phrase 

 as "the survival of the fittest." 



The natural processes that determine such' 

 survival on one hand, and the destruction of 



