AUTHOR'S PEEFACE. 



When my permission was asked to publish a Translation of my Work 

 upon Parasites, which was just then appearing in a Second German 

 Edition, I was the more ready to grant the request, since the branch 

 of Science of which it treats is one which has been cultivated more 

 especially upon German soil and by German investigators, but has 

 by no means found in other countries such wide-spread attention as 

 its great scientiiic and practical importance render desirable. It is 

 true that English Literature possesses in the Translation of Kiichen- 

 meister's Work on Human Parasites, and in the Treatises of Cobbold 

 (Entozoa and Parasites of Man), writings which cover the same ground 

 as my own ; but Kiichennieister's work is entirely out of date, while 

 Cobbold aims at giving a general sketch rather than a complete 

 delineation of the group. 



This, however, is the aim which I have kept in view in the 

 compilation of my book. 



I have endeavoured to serve the interests both of the Physician 

 and the Hygienist, as well as of the Zoologist — the interests of practice 

 and of theory, which are by no means so diverse as at first sight 

 might appear. 



The relations which obtain between Parasites and their hosts are 

 in all respects conditioned by their natural history ; and without a 

 detailed knowledge of the organization, the development, and the mode 

 of life of the different species, it is impossible to determine the nature 

 and extent of the Pathological conditions to which they give rise, and 

 to find means of protection against these unwelcome guests. 



Even small and apparently isolated facts become often of great 

 significance in this connection, and hence the Physician cannot afford 

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