416 FRAGMENTS OP SCIENCE. 



advance, so put together the facts and analogies of con- 

 tagious disease as to divine its root and character. Pro- 

 fessor 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 stim- 

 ulus 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 chem- 

 istry from the efforts of the alchemists, who would de- 

 tach modern atomic doctrines from the speculations of 

 Lucretius and his predecessors, or who would claim 

 for our present knowledge of contagia an origin alto- 

 gether 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, towards 

 certainty. Darwin's theory, as pointed out nine and 

 ten years ago by Helmholtz and Hooker, was then ex- 

 actly in this condition of growth; and had they to 

 speak of the subject to-day they would be able to an- 

 nounce 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 is 

 tested the more fully does it harmonise with progressive 



