672 NATURE OF REPRODUCTION. 



through several intermediate conditions ; so that the apparent difference 

 between them in configuration and structure no longer offered a serious 

 difficulty to the investigator. As a general rule, since that time, when- 

 ever a rare or comparatively unknown animal or plant has been suspected 

 to originate by spontaneous generation, it has only been necessary to 

 examine thoroughly its habits and functions, to discover its real methods 

 of propagation, and to show that they correspond, in all essential par- 

 ticulars, with the ordinary laws of reproduction. The limits within 

 which it is possible for the doctrine of spontaneous generation to be 

 applied have been successively narrowed, in the same degree that the 

 study of natural history has advanced ; the presumption of its existence 

 always hanging upon the outskirts of definite knowledge, and being 

 connected only with those animal or vegetable organisms which are for 

 the time imperfectly understood. The two groups from which it has 

 been most recently excluded by the progress of discovery are, 1. The 

 Entozoa, or internal parasites ; and 2. The Infusoria. 



I. Entozoa. These are organisms which live within the bodies of 

 other living animals, from whose organic juices they derive their nourish- 

 ment. 



There are many different kinds of entozoa, all of which are confined, 

 more or less strictly, to certain parts of the body which they inhabit. 

 Some of them are found in the intestines, others in the liver, the kidneys, 

 the lungs, or the heart and bloodvessels ; others on the surface of the 

 brain ; others even in the muscles or in the interior of the eyeball. 

 Each particular kind of parasite, as a rule, is peculiar to the species of 

 animal which it inhabits, and even to a particular part of the body, often 

 to a particular part of one organ. Thus, Ascaris lumbricoides is found 

 in the small intestine, Oxyuris vermicularis in the rectum, Trichoce- 

 phalus dispar in the cteciun. One kind of Distoma has its place in 

 the lungs of the green frog, another in those of the brown frog. Cysti- 

 cercus cellulosae is found in the connective tissue ; Trichina spiralis in 

 the substance of the muscles. 



With regard to many of these parasites the only difficulty in account- 

 ing for their existence, except on the presumption of their spontaneous 

 generation, lay in their being confined to such narrow limits and their 

 never being met with elsewhere. It seemed probable that some local 

 combination of conditions was necessary to the production of a para- 

 site, which was never to be found except in the biliary passages, the 

 kidneys, or the lungs of a living animal. A little consideration, how- 

 ever, makes it evident that these conditions are in reality neither neces- 

 sary nor sufficient for the production, but only for the development of 

 the parasites in question. Most of the internal parasites evidently 

 reproduce their species by generation. They have male and female 

 organs, and produce fertile eggs, often in great abundance. The eggs 

 contained in a single female Ascaris are to be counted by thousands ; 

 and in a tapeworm even by millions. These eggs, in order that they 

 may be hatched, and produce new individuals, require certain special 



