104 EVOLUTION 



gruesome lists of pestiferous 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 parasitism, 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 Vertebrates, which 

 only fall into parasitism in the rarest cases, 

 that infestation is most frequent. Fishes 

 may even swarm externally with trematodes 

 and parasitic crustaceans, internally with 

 cysts and intestinal worms without number. 

 A vivid impression of the prevalence of 

 parasitism is afforded by the capture, not 

 infrequent at a channel zoological station or 

 by fisher folk anywhere, of the huge and 

 majestic sunfish, Orthagoriscus mola; and by 

 picking off with forceps into museum bottles 

 his crowds of uninvited passengers the tuft 

 of barnacles upon his back, the biting isopods 

 like enormous fleas upon his skin, the trema- 

 todes sucking like leeches upon his eyes; and 

 within to find not only his alimentary canal 

 crammed with worms more than with food, 



