The Story of Evolution 107 



no risks, and receiving no spur to effort. Thus we see that evolu- 

 tion is not necessarily progressive; everything depends on the 

 conditions in reference to which the living creatures have been 

 evolved. When the conditions are too easygoing, the animal may 

 be thoroughly well adapted to them as a tapeworm certainly is 

 but it slips down the rungs of the ladder of evolution. 



This is an interesting minor chapter in the story of evolu- 

 tion the establishment of different kinds of parasites, casual 

 and constant, temporary and lifelong, external hangers-on and 

 internal unpaying boarders, those that live in the food-canal and 

 depend on the host's food and those that inhabit the blood or the 

 tissues and find their food there. It seems clear that ichneumon 

 grubs and the like which hatch inside a caterpillar and eat it alive 

 are not so much parasites as "beasts of prey" working from 

 within. 



But there are two sides to this minor chapter: there is the 

 evolution of the parasite, and there is also the evolution of coun- 

 teractive measures on the part of the host. Thus there is the 

 maintenance of a bodyguard of wandering amoeboid cells, which 

 tackle the microbes invading the body and often succeed in over- 

 powering and digesting them. Thus, again, there is the protec- 

 tive capacity the blood has of making antagonistic substances or 

 "anti-bodies" which counteract poisons, including the poisons 

 which the intruding parasites often make. 



THE EVIDENCES OF EVOLUTION HOW IT 

 CAME ABOUT 



1 



Progress in Evolution 



There has often been slipping back and degeneracy in the 

 course of evolution, but the big fact is that there has been pro- 

 gress. For millions of years Life has been slowly creeping 

 upwards, and if we compare the highest animals Birds and 

 Mammals with their predecessors, we must admit that they 



