102 EVOLUTION 



those ugly chapters of natural history which 

 follow the decline of so many forms of life, 

 even high and beautiful ones, into degenera- 

 tions well-nigh incredible, into parasitisms 

 even loathsome. For one thing, even the 

 most thoroughgoing creationist and Paleyan 

 of old must have had some qualms in ascrib- 

 ing the intricacies of parasitism to special 

 creation, or its pains and enfeeblement, so 

 varied and so widespread among all the 

 higher animals, to beneficent design; thus 

 the evolutionary parasitologist has had it 

 practically all his own way, yet has also 

 cured us, with his gruesome lists of pestif- 

 erous creatures and lurking dangers, of that 

 dream of this as the best of all possible worlds 

 to which a too facile evolutionism has been 

 wont to incline. Species of all manner of 

 groups, he shows us, may fall into parasi- 

 tism, the simplest bacteria and lower fungi, 

 the more active and long supposed innocent 

 Protozoa above all — so that for a generation 

 past the vast field of pathology has seemed 

 well-nigh divided between bacteriologist and 

 parasitologist proper. It is among the Ver- 

 tebrates, which only fall into parasitism in 

 the rarest cases, that infestation is most 

 frequent. Fishes may even swarm exter- 

 nally with trematodes and parasitic crus- 

 taceans, internally with cysts and intestinal 



