b ON PARASITES. 



of exactness. " The mathematician prescribes conditions for solution 

 and forms of result. He thus dictates to existence, — he determines 

 beforehand what means are wanted and what form the result shall 

 appear in." The natural philosopher on the other hand dictates 

 nothing, he only endeavors to distinguish between what is essential 

 and what is not, in the train of apparent causes to which any given 

 result may be attributed. Confined to study and observation only, 

 he creates nothing — changes nothing. The great field of the actual 

 is spread before him. It embraces facts only with which he is to 

 become acquainted. He reads natural phenomena right onward and 

 takes them in all their significancy as he finds them. Guided by the 

 light of experience the modern enquirer eschews all theories except 

 such as are based upon unmistakable facts. These he collects on 

 every side, and although they should not bear upon the particular 

 subject of investigation which he may have in hand he does not reject 

 them as worthless, but stores them up for future use, confident that 

 they occupy some position of importance in the economy of nature. 

 Thus whilst investigating a point in the physiology of respiration 

 Dalton discovered a rare species of Spiroptera in the right cavity of 

 a dog's heart. Donn6 discovered in a similar manner the Trichomonas 

 vaginalis, an infusorial animalcula in the morbid vaginal secretion of 

 a female laboring under gonorrhoea. Accident directed Claude 

 Bernard's attention to the glycogenic function of the liver. Num- 

 berless additional examples might be adduced, illustrative of the 

 importance of neglecting nothing in a physical examination. 



But a simple observation of great numbers of disconnected and 

 disjointed facts, although it may cause astonishment at the versatility 

 of nature, will afford small insight into the hidden laws which regulate 

 their occurrence. 



Facts to be of real value must be estimated comparatively and in 

 their proper connection. Such is especially the case in the subject of 

 inquiry which I have proposed to myself. Nowhere is the necessity 

 of carefully conducted and connected observations of more importance 

 than among parasites, and of these the Entozoa par excellence. In 

 one place a parasite is seen to reproduce by gemmation, in another 

 by fissation, and still a third by ovulation. Disconnected observation 

 would never establish a connection between all these three forms, and 

 yet nothing is more certain than that they occur in the same animal 

 at different stages of its existence. A microscopic ovule enclosing a 



