PROFESSOR VIRCHOW AND EVOLUTION 437 



pie, like the undulatory theory, has been a motive power 

 and not an anodyne. "At last," continues Professor 

 Virchow, "in the nineteenth century we have begun lit- 

 tle by little r;,Ily . to find contagia animata. ' ' So much 

 the more honor, 1 infer, is due to those who, three cent- 

 uries in a ^vance, so put together the facts and analogies 

 of contagious disease as to divine its root and character. 

 Professor Virchow seems to deprecate the "obstinacy" 

 with which this notion of a contagium vivum emerged. 

 Here I should not be inclined to follow him; because 1 

 do not know, nor does he tell me, how much the discov- 

 ery of facts in the nineteenth century is indebted to the 

 stimulus derived from the theoretic discussions of preced- 

 ing centuries. The genesis of scientific ideas is a subject 

 of profound interest and importance. He would be but a 

 poor philosopher who would sever modern chemistry from 

 the efforts of the alchemists, who would detach modern 

 atomic doctrines from the speculations of Lucretius and 

 his predecessors, or who would claim for our present 

 knowledge of contagia an origin altogether independent 

 of the efforts of our "forefathers" to penetrate this enigma. 



Finally, I do not know that I should agree with Pro- 

 fessor Virchow as to what a theory is or ought to be. I 

 call a theory a principle or conception of the mind which 

 accounts for observed facts, and which helps us to look 

 for and predict facts not yet observed. Every new dis- 

 covery which fits into a theory strengthens it. The theory 

 is not a thing complete from the first, but a thing which 

 grows, as it were asymptotically, toward certainty. Dar- 

 win's theory, as pointed out nine and ten years ago by 

 Helmholtz and Hooker, was then exactly in this condition 



