PROFESSOR VIRCHOW AND EVOLUTION. 659 



the more honor, I infer, is due to those who, three cen- 

 turies in advance, 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 

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

 covery of facts in the nineteenth century is indebted to 

 the stimulus derived from the theoretic discussions of 

 preceding 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 in- 

 dependent of the efforts of our "forefathers" to penetrate 

 this enigma. 



Finally, I do not know that I should agree with 

 Professor 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 

 discovery 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. 

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

 Helmholtz and Hooker, was then exactly in this condition 

 of growth; and had they to speak of the subject to-day 

 they would be able to announce an enormous strengthen- 

 ing of the theoretic fiber. Fissures in continuity which 

 then existed, and which left little hope of being ever 

 spanned, have been since filled in, so that the further the 

 theory is tested the more fully does it harmonize with 

 progressive experience and discovery. We shall probably 

 never fill all the gaps; but this will not prevent a pro- 

 found belief in the truth of the theory from taking root in 

 the general mind. * Much less will it justify a total denial 

 of the theory. The man of science who assumes in such 

 a case the position of a denier is sure to be stranded and 

 isolated. The proper attitude, in my opinion, is to give to 

 the theory during the phases of its growth as nearly as 



