Next, I may mention another bearing of biological 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 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 knowledge 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 explanation 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 studying, 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 perform 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 Vertebrata, 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, 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.