PEOFESSOK VIECHOW A^D EVOLUTION. 417 



experience and discovery. We shall probably never fill 

 all the gaps ; but this will not prevent a profound 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 possible a proportionate assent ; and, if it be a 

 theory which influences practice, our wisdom is to follow 

 its probable suggestions where more than probability is 

 for the moment unattainable. I write thus with the 

 theory of contagium vivum more especially in my 

 mind, and must regret the attitude of denial assumed 

 by Professor Virchow towards that theory. ' I must beg 

 my friend Klebs to pardon me,' he says, ' if, notwith- 

 standing the late advances made by the doctrine of 

 infectious fungi, I still persist in my reserve so far as 

 to admit only the fungus which is really proved, while I 

 deny all other fungi so long as they are not actually 

 brought before me.' Professor Virchow, that is to say, 

 will continue to deny the Grerm Theory, however great 

 the probabilities on its side, however numerous be the 

 cases of which it renders a just ceo unt, until it has 

 ceased to be a theory at all, and has become a congeries 

 of sensible facts. Had he said, ' As long as a single 

 fungus of disease remains to be discovered, it is your 

 bounden duty to search for it,' I should cordially agree 

 with him. But by his unreserved denial he quenches 

 the light of probability which ought to guide the 

 practice of the medical man. Both here and in relation 

 to the theory of evolution excess upon one side has 

 begotten excess upon the other. 



