THEORIES IN PRACTICE 357 



In the same way we conceive of the evolution- 

 ary changes through which new species were 

 evolved in the past as having been relatively sud- 

 den. I have already referred to the difficulty 

 with which the average mind can grasp the idea 

 that precisely the same sort of change in animal 

 and vegetable forms is taking place to-day that 

 has taken place in all other stages of evolution. 



It was one of the great merits of Darwin's 

 exposition of the "Origin of Species" that he 

 gave detailed illustrations of the struggle for 

 existence, and brought tangibly before the minds 

 of thoughtful people the conception that each 

 race of beings is more or less in competition with 

 every other race, and that the race that is adapt- 

 able enough to adjust itself to new conditions 

 is the only one that stands any prospect of 

 survival. 



The idea of the progression of the normal 

 increase of living creatures in geometrical ratio 

 and of the resulting overpopulation of any terri- 

 tory by the progeny even of a single pair, if 

 there were no counteracting factors, was of 

 course received by Darwin from Malthus. But 

 the application of that idea to all races of animals 

 and plants, and the logical deduction from its 

 application which first made possible anything 

 like a clear understanding of the reason why 



