ON PARASITES. 5 



with their numbers and importance, and this with decided advantage 

 to the progress of natural science generally. 



No more important or welcome contributions have been made 

 within the last few years to the common stock of scientific infor- 

 mation than those concerning parasites, whether regarded for their 

 brilliant illustration of the theory of types, the general principles of 

 classification, and the doctrines of physiology, or their practical 

 bearing upon important industrial pursuits, and the science and the 

 art of medicine and veterinary surgery. Although all departments 

 of this extensive subject are full of interest, yet inasmuch as the 

 entozoa imd. especially those infesting the human subject have been 

 investigated with the most satisfactory results, and present points 

 of singular novelty in their history. I am especially attracted to- 

 wards them in collecting materials for this article. 



Accordingly after a few gener:ai observations which the subject 

 naturally suggests, and some necessary brief allusions to individual 

 species of vegetable parasites, which by their singularity or important 

 relations to man especially engage attention, I propose to take into 

 consideration the human entoEoa and exhibit as nearly as possible the 

 present condition of scientific knowledge concerning them. 



Some allusion will necessarily be made during the course of the 

 investigation to entozoa found only in the lower animals, in order to 

 illustrate more clearly the history of those infesting the human 

 subjects Some important facts I have myself been privileged to 

 verify ; the most of them are given upon the authority of Von 

 Siebold Kuchenmeister, and other distinguished and accurate ob- 

 servers. Whatever theories may be broached must be taken for 

 what they are worth. 



The rightful study of natural phenomena induces speculation but 

 does not permit the imagination to run away with the judgment, for 

 it continually induces the mind to trace effects back to their causes, 

 ajidfelix qui pohdi rerum cognoscere causas. It continually submits 

 the results of previous observation to the trial of new facts. Hence, 

 as the field of positive knowledge widens opinions and theories which 

 once seemed correct are found to be erroneous. The ideas of the 

 previous year are continually revolutionized by the discoveries of the 

 present one, until a primitive fact or law is discovered, which then 

 becomes a foundation for the particular branch of science in which it 

 is found, and as far as it extends gives to that branch the character 



