280 ON THE STUDY OF BIOLOGY x 



Next, I may mention another bearing of biolo- 

 gical 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 

 which are certainly and unmistakably caused by 

 living organisms. This fact renders it possible, at 

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

 fections 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 

 sufficient knowledge of biological truths to be able 



ho a public document, inasmuch as it not only appeared in the 

 Journal of that learned body, but was re-published, in 1873, in 

 a volume of Critiques and Addresses, 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 higher Vertcbrata, the results of recent investiga- 

 tions, however we may sift and criticise them, seem to me to 

 leave a clear balance in favour of the evolution of living forms 

 one from another;" 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 opinion, except in the way of intensifying it, when 

 in consequence of the accumulation of similar evidence since 

 1870, I recently spoke of the denial of evolution as not worth 

 serious consideration. 



