84 DARWINISM AND HUMAN LIFE 



priate environment, or, what comes to the same 

 thing, to form a new habit. From the beginning, 

 necessity has been the mother of invention. For 

 animals, as for man, the exploration of new terri- 

 tory has been a constantly recurrent result of the 

 struggle for existence, and one of the most im- 

 portant. The open-air naturalist is familiar with 

 the way in which nearly related species fill shghtly . 

 different corners in the same crowded area. It is 

 interesting, also, to think of the gradual peopling of 

 strange habitats, such as the abysses of the ocean, 

 the dark caves, and under the ground ; or how 

 fishes come ashore, and mammals get into the air, 

 and crabs go up the mountains. 



(c) In the third place, there may be discriminate 

 ehmination of the less fit to the given conditions, 

 and it is this result that has most evolutionary 

 interest. The black rat's territory is invaded by 

 the brown rat, and soon there is only brown rat. 

 Probably Kropotkin is right in suggesting that 

 this case is less simple than Darwin supposed, for 

 the arrival of a second rat made man wake up, 

 and the weaker species yielded first. But there 

 is evidence enough to lead us to believe that the 

 struggle between brown rat and black rat leads 

 to the rapid extermination of the black. From 

 this extreme case we find every possible gradation, 

 till all we can say is that the less fit are slightly 

 handicapped in the race of life. But if the slight 

 handicapping teUs at all — and tells consistently — 

 as regards length and vigour of life, or number and 

 vigour of ofEspring, then it will serve as a selective 

 agent. 



It is very important to realise that the struggle 

 for existence may select without rapidly killing off 



