416 FRAGMENTS OF SCIENCE. 



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 discovery 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 phi- 

 losopher 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 

 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, to- 

 wards 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 strengthening of the theoretic 

 fibre. 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 ia 

 tested the more fully does it harmonise with progressive 



