ANIMAL AND VEGETABLE PARASITES. 533 



of aged trees ; the Oidium albicans vegetates upon the mucous sur- 

 faces of the mouth and pharynx ; the Botrytis Bassiana attacks the 

 body of the silkworm, and plants itself in its tissues ; while many 

 species of trematoid worms live attached to the gills of fish and of 

 water-lizards. 



These parasites are usually nourished by the fluids of the animal 

 whose body they inhabit. Each particular species of parasite is 

 found to inhabit the body of a particular species of animal, and is 

 not found elsewhere. They are met with, moreover, as a general 

 rule, only in particular organs, or even in particular parts of a 

 single organ. Thus the Tricocephalus dispar is found only in the 

 caecum ; the Strongylus gigas in the kidney ; the Distoma hepati- 

 cum in the biliary passages. The Distoma variegatum is found 

 only in the lungs of the green frog, the Distoma cylindraceum in 

 those of the brown. The Taenia solium is found in the intestine of 

 the human subject in certain parts of Europe, while the Taenia lata 

 occurs exclusively in others. It appears, therefore, as though some 

 local combination of conditions were necessary to the production 

 of these parasites; and they have been supposed, accordingly, to 

 originate by spontaneous generation in the localities where they 

 are exclusively known to exist. 



A little consideration will show, however, that the above condi- 

 tions are not, properly speaking, necessary or sufficient for the 

 production, but only for the development of these parasites. All the 

 parasites mentioned above 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, it is said, even 

 by millions. Now these eggs, in order that they may be hatched 

 and produce new individuals, require certain special conditions 

 which are favorable for their development; in the same manner 

 as the seeds of plants require, for their germination and growth, a 

 certain kind of soil and a certain supply of warmth and moisture. 

 It is accordingly no more surprising that the Ascaris vermicularis 

 should inhabit the rectum, and the Ascaris lumbricoides the ileum, 

 than that the Lobelia inflata should grow only in dry pastures, and 

 the Lobelia cardinalis by the side of running brooks. The lichens 

 flourish on the exposed surfaces of rocks and stone walls ; while 

 the fungi vegetate in darkness and moisture, on the decaying trunks 

 of dead trees. Yet no one imagines these vegetables to be spon- 

 taneously generated from the soil which they inhabit. The truth 



