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 Virchovv 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 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 rnind 
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 
