GENEUAL CONSIDEKATIONS. 115 



vaccination with the spores of Saprolegnia, &c.) The' fitness 

 of the soil is, moreover, increased by the humidity, which 

 increases daily and steadily in consequence of the germination of 

 the spores, and which is favorable to the growth of the plants. 

 It is always a sign favorable to their growth if this moisture 

 shows at first a slight acrid reaction ; this is, however, not so 

 very indispensable as has been commonly thought, since many 

 Fungi grow on neutral or alkaline soil ; as, for instance, in the 

 peritoneum of the Herbivora and on ulcers of the trachea. 



Fungi prosper the more the richer the soil is in organic nitroge- 

 nous substances, especially in such as are in a state of decom- 

 position. 



Here also we find exemplified the great law which must be 

 obeyed wherever plants are to prosper : " The, choice of the 

 locality depends upon the peculiar properties of the soil sought for 

 or avoided by the various species of plants." For certain species 

 prefer certain parts of their animal host. 



2. The nature of the gaseous medium seems quite indifferent 

 to the development of vegetable parasites, for they are found in 

 atmospheric air when on the skin, in an air rich in carbonic 

 acid when in the cavity of the mouth and lungs, in an air not 

 overcharged with carbonic acid gas when in the peritoneum* 

 Thus, for instance, the Algce of the process of fermentation 

 thrive best in an atmosphere rich in carbonic acid, whilst the 

 Fungi appear to absorb oxygen and give off carbonic acid. Hu-. 1 

 midity of the surrounding gaseous media (humidity of the atmo^ 

 sphere, humidity in the cavities of the body which contain air) 

 favours the development of these parasites. 



3. Most favorable for the development of vegetable para- 

 sites is the temperature of the body of the mammalia, especially 

 in their natural cavities. Their growth may be increased in 

 cold-blooded animals by raising the temperature of the surround-* 

 ing air. 



A knowledge of the circumstances which favour their growth 

 enables us to form a " general theory of the treatment of vegeta- 

 ble parasites." We have chiefly to look to a change in the me- 

 dium in which they grow, and to bring on a state in which they 

 cannot thrive so well. Of late Bazin has done much towards 

 advancing this department of science ; his method contemplates 

 more particularly the locality of the parasite, which it endeavours 

 to change, and makes direct war against the predisposing disease by 



