2S0 ON THE STUDY OF BIOLOGY x 



Xext, I may mention another bearing of bio- 

 logical knowledge — a more practical one in the 

 ordinary sense of the word. Consider the theory 

 of infectious disease. Surely that is of interest to 

 all of us. Now the theory of infectious disease is 

 rapidly being elucidated by biological study. It is 

 possible to produce, from among the lower animals, 

 examples of devastating diseases which spread in 

 the same manner as our infectious disorders, and 

 wliich are certainly and unmistakably caused Ijy 

 living organisms. This fact renders it possible, at 

 any rate, that that doctrine of the causation of in- 

 fectious disease which is known under the name of 

 " the germ theory " may be well-founded; and, if 

 so, it must needs lead to the most important 

 practical measures in dealing with those terrible 

 visitations. It may be well that the general, as 

 well as the professional, public should have a 

 sullicient knowledge of biological truths to be able 



be said to be a public floonment. ina?murh as it not only 

 appparod in tho Journal of that learned body, but w;is re- 

 published, in 1873, in a volume of Critiques and Addrrssfs^ 

 to which my name is attached. Therein will be found a 

 pretty full statement of my reasons for enunciating two 

 propositions: (1) that "when we turn to the hi^d)er Verfe- 

 hratn, the results of recent investigations, however we may 

 sift and criticise them, seem to me to leave a clear balance 

 in favour of the evolution of livinir forms one from an- 

 other;" and (2) that the case of the horse is one which 

 "will stand rigorous criticism." 



Thus I do not see clearly in what way I can be said to 

 have changed my opiiiion, except in tho way of intensify- 

 inc: it. when in C(»nsef|uence of the accumulation of similar 

 evidence since 1870, I recently spoke of the denial of evolu- 

 tion as not worth serious consideration. 



