VI PREFACE 



many questions relating to epidemics, endemics, and epizootics 

 due to parasites. The medical man who only looks at the 

 phenomena of parasitism as displayed within the human terri- 

 tory must of necessity acquire a cramped, narrow, and distorted 

 conception of the role played by parasites in the production 

 of disease. Let it be freely granted that to the practising 

 physician, as such, it matters little how many beasts, birds, 

 reptiles, or fishes perish annually from parasitic affections ; yet, 

 when it is demonstrable that a large proportion of the strictly 

 human entozoa require a change of hosts or, in other words, 

 need to pass through the bodies of the lower animals then it 

 is evident that some acquaintance on his part with the entozoa 

 infesting animals becomes a practical necessity. Knowledge of 

 the kind here offered will often materially aid him in recom- 

 mending prophylactic measures. Moreover, the study of 

 comparative pathology, almost ignored in England, conveys 

 with it other lessons of high value in relation to the healing 

 art. The great mind of John Hunter comprehended all this 

 long ago, as any student of the beautiful preparations contained 

 in the museum of the Royal College of Surgeons may readily 

 convince himself ; and this is all the more noteworthy, since the 

 subject concerns the physician rather than the surgeon. 



To the naturalist the second half of this book addresses 

 itself in a very direct manner. When engaged in his dissec- 

 tions, an appeal to its pages will often enable him to decide at 

 once as to the species of parasite accidentally encountered, and if 

 a full diagnosis be demanded it will guide him to better sources 

 of information. Many hundreds of correspondents, not having 

 ready access to the systematic writings of Rudolphi, Diesing, 

 and Dujardin, have requested me to identify their te finds." I 

 have rarely or never failed to comply with their requests ; but 

 it is hoped that the present work may prove of ready service to 

 subsequent inquirers, and thus place a reasonable limit upon the 

 number of future applicants. Since the manuscript of this work 

 was completed I have received Dr von Linstow's Compendium 



