44 DARWINISM AND THE PROBLEMS OF LIFE 



animals in which neither the old nor the new features 

 were quite distinct ; that is to say, we have now to deal 

 with two species. The differences between them may 

 become so great in the course of time that crossing the 

 two may give sterile offspring, or be altogether 

 impossible. Thus we see that varieties are species in 

 the making ; species are varieties that have become 

 fixed. 



We know now what natural selection is. The best 

 equipped animals escape the longest from destruction by 

 enemies or from other injury. They are thus enabled 

 to leave most offspring, and the next generation shows 

 an average improvement. 



Now let us return to the bioccenoses. 



We examined life in a particular region, and 

 concluded that the number of animals in it varies 

 somewhat, but remains about the same on the average. 

 In this we assumed that the locality retains its natural 

 condition, and that, especially, man does not intervene 

 with his artificial culture. It will be quite otherwise if 

 civilisation comes on the scene ; then the impenetrable 

 forests will give place to fruitful fields, and boats will 

 shoot up and down the river that flows by. This 

 transformation of Nature must naturally lead to a change 

 in the fauna ; and the question arises to what extent this 

 will be modified, and whether man, for instance, can 

 extinguish, with his improved methods of hunting and 

 his instruments of destruction, the animals that are 

 harmful to him and increase those that are useful, in the 

 forests which he leaves standing. 



We have to inquire first, therefore, which animals 



