188 Summary 



or dies to itself ; each is a retainer to some other part 

 of Nature. Organisms are linked together into a sys- 

 tem. This is a central fact in Natural History. It also 

 helps us to understand the fineness of Nature's sift- 

 ing, for the meshes of the sieve are between the 

 threads of the web of life. The interlinking of lives 

 in a progressive inter-relationship, like that between 

 flowers and their insect visitors, tends to make retro- 

 gression more difficult, gives additional security to 

 integration. The idea of the web of life may be illus- 

 trated all through Organic Nature, as in the linkages 

 between various insects and their partner yeasts, the 

 association of ants and acacias, the utilization of a 

 caterpillar by an ant. 



13. The second great idea in Darwinism is the 

 Struggle for Existence. This is a formula for all the 

 manifold efforts and answers-back that living crea- 

 tures make when they find themselves up against en- 

 vironing difficulties and limitations. It includes all 

 the reactions which secure the welfare not only of the 

 self but of the offspring, and even at times the kin. 

 It rises from internecine competition to an endeavor 

 after well-being; it includes the improvement of the 

 nest and the elaboration of parental care as well as 

 sharpening teeth and claws. Darwin realized the 

 subtlety and many-sidedness of "struggle" more 

 clearly than most of his followers have done, and 

 modern ecology justifies his breadth and depth of 

 view. Love is a factor in evolution just as much as 

 hunger. 



14. The central problem in evolution is the origin 



