TRANSLATOR'S PREFACE. xi 



practical parts involve questions of the highest scientific interest, 

 and of a purely theoretical character. They afford, indeed, a 

 beautiful illustration of the fact, that there are no questions so 

 profound and theoretical in the physiology and natural history of 

 man, that are not intimately connected with the efficient per- 

 formance of the daily duties of the medical man. They teach, 

 that though scientific theories may be sometimes barren of im- 

 mediate practical results, they cannot fail to disencumber the 

 mind of those prejudices which lead to erroneous practice by the 

 medical man, and disastrous results to the patient. On this 

 ground I may, perhaps, be excused for referring to some of the 

 least apparently practical parts of this volume, not with a view 

 of adding any new matter to that already accumulated with so 

 much labour by the author, but with the hope of assisting the 

 reader to understand those generalisations which the subject of 

 the book involves, but which are necessarily not brought promi- 

 nently forward by the author. 



The history of the Entozoa has ever been supposed to involve 

 some of the most interesting questions relative to the generation 

 and reproduction of organic beings. Although it was easy to 

 account for the presence of worms in the stomach and intestines 

 by the ready explanation of the swallowing of the eggs, a difficulty 

 always presented itself in the case of those creatures called 

 hydatids, which evidently had an independent animal existence. 

 They exhibited no sexes, they produced no eggs, and the readiest 

 theory was that of spontaneous or equivocal generation. Even 

 as this theory was successively driven from every other part of 

 the animal and vegetable kingdom, it found a refuge amongst the 

 strange and paradoxical creatures imbedded in the tissues of man 

 and other animals, far removed from any external influences. 



The time has, however, at length arrived, when it can be de- 

 monstrated, that the cystic worm is no longer to be regarded as 

 the result of a " fortuitous concourse of atoms," but that it is the 

 offspring of the tape- worm, undergoing one stage of its growth, 

 through which it must pass before it can attain to the more dig- 



