io8 Lectures on Bacteria. [^ x. 



which take up their abode on or in other living creatures, and 

 feed on the substance of their bodies. The animal or plant 

 which supplies food and lodging to a parasite is termed its 

 host. Parasites are known in very diflferent divisions of the 

 animal and vegetable kingdoms, and many of them are well and 

 certainly understood. I need only mention intestinal worms on 

 the one hand, and on the other the long series of true Fungi 

 which are parasitic especially on plants. Our experience of 

 these forms, which are comparatively easy to examine, teaches 

 us that the adaptations to the parasitic mode of life are extra- 

 ordinarily complex and present an extreme variety of gradations 

 , between one case and another, that is between one species and 

 another, and that these are dependent on the one hand upon 

 the more or less strict requirements of the parasitic mode of 

 life, and on the other upon the mutual relations between parasite 

 and host. 



To attempt to go at all at length into the above relations, 

 would lead us here much too far into details. But we must call 

 attention for our present guidance to one or two of the most 

 important points. 



As regards the nature of the parasitism, we have first the case 

 which is farthest removed from the life of saprophytes, that of 

 obligate parasites, which by the provisions of their nature can 

 only complete the course of their development in the parasitic 

 and not in the saprophytic mode of life. To take an example 

 from amongst those with which we are best acquainted, this is 

 true strictly and excluding all deviation into saprophytism of the 

 Entozoa, such as tapeworms and Trichinae ; among Fungi, of 

 those which live inside plants and have been termed rusts 

 (Uredineae). These organisms as a matter of fact live only in- 

 side their living hosts and feed on them. It is quite conceivable 

 that the conditions necessary for their development may arise 

 or be artificially produced outside the living host, and it would 

 certainly be an instructive experiment to grow a tapeworm from 

 the ovum in a nutrient solution ; but this has never been actually 



