to the existing conditions. This adaptation finds expression in the survival of the fittest in the struggle for life; that is to say, those individuals continue to live and reproduce their kind whose structure enables them to undergo changed conditions without succumbing, while others, because they cannot adapt themselves, perish, leaving no posterity, no trace of their having ever existed, save, perhaps, in the geological strata of their epoch. The special advantage which has once insured the survival of an organism, while its congeners which possessed no such advantage perished, is fixed by heredity; it grows under the influence of that same law of survival which insures the upper-hand in the struggle for life to the organisms possessing the advantage in the highest degree; in virtue of the law of the coordination and subordination of parts and functions, it brings about in the whole organism very extensive modifications which insure its fixity; and the sum total of the new characters becomes sufficiently stable to convey to the mind which observes it the impression of the persistence of forms and the existence of types, whereas in fact there exist only changes amid which there remain, in virtue of the law of heredity, traits of resemblance to a common ancestor or stock.
Such are, in brief, the principal laws of biological phenomena, and the chief theories which have been devised for the purpose of assigning to them causes. When, in order to establish or to impugn laws and theories so far-reaching as these, we can have recourse to direct experiment and observation, the mind is satisfied and its certitude reposes on an immovable basis. But when a theory has to do with origins in the remote past, or even in the present, but inaccessible to experiment, our certitude rests on no solid foundation. In the absence of experiment, we have to be content with opinions formed according to the rules of induction and of analogy, and possessing more or less probability. Among views of this sort, those appear to have greatest weight which, in their contexture and in the method of their formation, are most in harmony with those beliefs of which we are most certain; which rest on the same general principles; which, so to speak, are incorporated with our beliefs, so that, were they to succumb to criticism, their fall would compromise the entire system. In other words, they must occupy their own place in a general philosophy, there appearing as so many links in a chain attached, on the one hand, to laws and theories which account for them, and, on the other, to laws and theories which without them cannot be explained.
Could we look for this result from the only general system of philosophy which has existed down to the present day? Having been written at a time when the science of life had for its generalizations only conclusions from Bichat's researches, the hypotheses of Gall, and the results of classification, that portion of the positive philosophy which treats of biology is too far behind the actual state of science to
