316 DR. WILLIAM ROBERTS. 



rather expect to see a speck of protoplasm slowly formed, 

 Avithout definite shape or dimensions, and nourishing itself, 

 like the chlorophyll body, on a purely mineral diet. The 

 more one reflects on this subject, the more clearly does it 

 appear that the spontaneous origin of saprophytes is logically 

 impossible. Speaking as an evolutionist, I should rather 

 infer that saprophytes were a late development ; pro- 

 bably a degradation from some algal forms which had found 

 their profit in feeding on waste organic matter, and which 

 gradually lost their chlorophyll through want of use, and 

 with it their power of feeding on an exclusively mineral diet. 



We now approach the more practical side of our subject ; 

 that which concerns us as practitioners of medicine and 

 students of pathology. I have already directed your atten- 

 tion to the analogy between the action of an organised fer- 

 ment and a contagious fever. The analogy is probably real, 

 in so far at least that it leads us to the inference that con- 

 tagium, like a ferment, is something that is alive. We know 

 of nothing in all our experience that exhibits the pheno- 

 mena of growth and self-propagation except a thing possessed 

 of life. 



This living something can only be one of two things ; 

 either it is an independent organism (a parasite) multiplying 

 within the body or on its surface, or it is a morbid cell or 

 mass of protoplasm detached from the diseased body and 

 engrafted on the healthy body. Possibly, both these con- 

 ceptions may have their application in the explanation of 

 different types of infective disease. In regard to the latter 

 conception, however, the graft theory, which has been so 

 ably developed by my friend Dr. Hoss, I will only say that 

 it has not, as yet, emerged from the region of pure specula- 

 tion. It lacks an established instance or prototype; and it 

 fails to account for the long-enduring dormant vitality so 

 characteristic of many contagia, which conforms so exactly 

 with the persistent latent vitality of seeds or spores, but which 

 contrasts strongly with the fugitive vitality of detached pro- 

 toplasm. 



If, then, the doctrine of a contagium vivum be true, we are 

 almost forced to the conclusion that a contagium consists (at 

 least in the immense majority of cases) of an independent 

 organism or parasite, and it is in this sense alone that I shall 

 consider the doctrine. 



It is no part of iny purpose, even if I had the time, to 

 give an account of the present state of knowledge on this 

 question in regard to every contagious disease. My object 

 is to establish the doctrine as a true doctrine ; to produce 



