53 6 THE POPULAR SCIENCE MONTHLY. 



Next, I may mention another bearing of biological knowledge a 

 more practical ne 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 cases of devastating diseases which have all the appearance 

 of our infectious diseases, 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 infectious 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 most 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 to take a rational interest 

 in the discussion of such problems, and to see, what I think they may 

 hope to see, that to those who possess a sufficient elementary knowl- 

 edge of biology they are not all quite open questions. 



Let me mention another important practical illustration of the 

 value of biological study. Within the last forty years the theory of 

 agriculture has been revolutionized. The researches of Liebig, and 

 those of our own Lawes and Gilbert, have had a bearing upon that 

 branch of industry, the importance of which cannot be over-estimated ; 

 but the whole of these new views have grown out of the better expla- 

 nation of certain processes which go on in plants, and which, of course, 

 form a part of the subject-matter of biology. 



I might go on multiplying these examples, but I see that the clock 

 won't wait for me, and I must, therefore, pass to the third question to 

 which I referred: Granted that biology is something worth stiidying, 

 what is the best way of studying it? Here I must point out that, 

 since biology is a physical science, the method of studying it must 

 needs be analogous to that which is followed in the other physical 

 sciences. It has now long been recognized that if a man wishes to be 

 a chemist it is not only necessary that he should read chemical books 

 and attend chemical lectures, but that he should actually himself per- 

 form the fundamental experiments in the laboratory, and know exactly 

 what the words which he finds in his books and hears from his teach- 



be said to be a public document, inasmuch as it not only appeared in the journal of that 

 learned body, but was republished 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 Verlebrata, the 

 results of recent investigations, however we may sift and criticise them, seem to me to 

 leave a clear balance in favor 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, ex- 

 cept in the way of intensifying it, when in consequence of the accumulation of similar 

 evidence since 1810 I recently spoke of the denial of evolution as not worth serious con- 

 sideration. 



