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 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 unattainable. 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 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." 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 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 on the one side has begotten excess on the other.
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 spontaneously to pieces in presence of the facts here set forth.—Author's advance sheets.
