156 DARWINISM AND THE PROBLEMS OF LIFE 



are obviously better off than the frogs, in which the 

 oxidised blood that should go to the body is always 

 mixed with the bad. Why were the frogs placed at 

 this disadvantage at creation ? Have they less right 

 than the birds to a good constitution ? 



Natural selection explains the riddle. From the 

 heart of the frog has been developed that of the 

 amphibian, then that of the reptile, and from this 

 in turn that of the bird. When the amphibians 

 appeared, the heart was at the stage of development 

 in which we still find it in that class. This structure 

 sufficed for those descendants that remained amphibians, 

 but not for those that became reptiles and birds. 

 In the case of the latter, the heart had to be improved, 

 because the more energetic vitality of the two new 

 classes required a better supply of blood. Natural 

 selection only produces what is necessary ; the amphi- 

 bians had to retain the old heart, because they did 

 not need a more advanced one. 



But all the organs cannot be retained when a 

 species is transformed into a different one. 



When complete land-animals had been formed from 

 the fishes, the gills became useless. In fact, they 

 would be injurious to the new organisms, as they 

 perforate the sides of the gullet, and would allow 

 foreign bodies to pass too easily into it. Thus, for 

 instance, it often happens to the greedy perch that his 

 prey sticks in his gills from trying to swallow it too 

 quickly, and both animals perish. As soon, therefore, 

 as the gill-clefts ceased to be absolutely necessary for 

 breathing, they had to be got rid of by natural 



