290 THE POPULAR SCIENCE MONTHLY. 



which fits into a theory strengthens it. The theory is not a thing com- 

 plete from the first, but a thing which grows, as it were asymptotically, 

 toward certainty. Darwin's theory, as pointed out nine or 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 bridged over, 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 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 as nearly as possible to the theory 

 during the phases of its growth 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 unattain- 

 able. I write thus with the theory of contagium vivum more especially 

 in my mind, and must regret the attitude of denial assumed by Prof. 

 Virchow toward that theory. " I must beg my friend Klebs to pardon 

 me," he says, " if, notwithstanding the late advances made by the doc- 

 trine 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." Prof. Virchow, that 

 is to say, will continue to deny the germ theory, however great the 

 probabilities on its side, however numerous the cases of which it renders 

 a just account, 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 fun- 

 gus 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 on the one side has begotten excess on the other. 



■^o^ 



In publishing the volume of " Fragments," to which the foregoing 

 article is introductory, I could not entirely ignore the criticisms which 

 one or two among them have evoked. Of such strictures, however, my 

 knowledge is incomplete, their authorship causing me to give some of 

 them a spacious berth. Nor as regards those with which I am acquainted 

 have I deemed it necessary to offer direct refutations. They fall spon- 

 taneously to pieces in presence of the facts here set forth. — Author''s 

 advance sheets. 



