no Lectures on Bacteria. [^ x. 



species which are able to develope as perfectly, or at least 

 nearly as perfectly, in the saprophytic as in the parasitic mode 

 of life. The ' or ' shows at once that there are gradations also 

 within this category, and these are, as might be expected, 

 of such a kind that some plants find the conditions more favour- 

 able in the parasitic, others in the saprophytic mode of life, 

 while others again show no difference in this respect. There 

 are many instances of these modifications of facultative para- 

 sitism among the Fungi, and we shall soon make acquaintance 

 with similar instances in the Bacteria. 



The mutual relations in each case between parasite and host, 

 the dependence of the one on the other, the benefit or injury 

 which the one receives from the other, are independent of these 

 strict requirements of parasitism which vary in each separate 

 case. In cases like that of the Trichinae, for example, we are in 

 the habit of speaking of this relation as one-sided, as one in 

 which the parasite derives its entire means of subsistence from 

 the living host, while the host receives nothing but harm from 

 the parasite through the necessary withdrawal of its substance 

 and other manifold chemical and mechanical disturbances which 

 it suffers. States of disturbance of the normal existence of a 

 living being, the normal requiring to be determined in each case 

 by experience, are known as diseases ; the parasites of which 

 we are speaking cause these states and are therefore injurious 

 to health, the exciting causes of disease. Further, the parasite 

 by means of its germs, spores, ova, or whatever other name is 

 given to its organs of propagation, may be transferred from the 

 host which has been made ill by it to others in which it will 

 also produce disease. The maladies caused by parasites are 

 therefore transferable from host to host, they are, to use the 

 common expression, infectious. 



But these cases, in which the parasite is injurious to health 

 and the injury is all on one side, are only one extreme among 

 those that are known. There are others in which the two 

 organisms live in common with equal advantage to both, and 



