58 
necessary in order to successfully inoculate guinea-pigs with 
the specific disease, and the progress of the disease in the 
animal becomes slower and slower, until at length the in- 
fluence is protective against the consequences of inoculation 
with an unsolarised or virulent culture. 
It has been pointed out by a French writer that these 
experiments again confirm the doctrine enunciated by M. 
Paul Bert, that any influence which arrests the develop- 
ment of a virus converts it into a vaccine. In the paper on 
Pasteur and the Germ Theory,” which I read before the 
Society last year, I argued tliat the difference between a 
harmless saprophyte and a deadly parasite was a difference of 
vigour. If I may formulate the idea again, I would say that 
a saprophyte is an organism which is able to utilize for its own 
] ife the residual forces in matter in wliich the specific or co- 
ordinating vital force has been extinguished; while the para- 
site is an organism which, in consequence of a given process of 
culture, is enabled to overcome the still existing specific 
vital force of the living organism on which it preys, and 
to divert that force to its own development. In some 
mysterious way light and oxygen are favourable to the 
vigour of the higher organism, and inimical to the vigour of 
the lower organism. These conditions determine which of 
the two is to be the subordinate. I have been led to think 
that the life-history of microbia is analogous to the earliest 
stages of the life-history of the higher organisms in this 
respect, and I am arranging some experiments in order to 
test this idea, the results of which I hope to communicate 
to the Societ}^ in due time. The higher plants and animals 
begin their lives in darkness, and as they grow attain to 
light and fresh air ; and then, if I may so express it, the 
higher life is progressively evolved. 
It is well to point out distinctly that, as yet, however 
much reason there may be for believing that pathogenic 
microbia are really evolved from originally harmless ferments 
