adapted or is even dysteleological; while, on the other hand, natural and sexual selection has to account for most of what is adapted or what is present for ornament's sake only, and which is therefore unexplainable by the simple laws of structure. So truly is this the correct view of the case, that in fact the structure of organisms seems everywhere to be a compromise between the requirements of the laws of structure and the effects of natural selection, as we see—to employ an illustration familiar to the physiologist—in the crossing of the air-passage by the digestive passage in lung-breathing vertebrates, an arrangement full of peril to life. On a previous occasion I showed that, in accounting for this state of things, the Darwinian theory coincides, in its results, with the optimism of Leibnitz. Still I am very far from overlooking the difficulties which still remain on this point. One of the most serious of these, in my opinion, is the power of regeneration, as it is called by physiologists, and the nearly allied vis medicatrix naturæ, whether as seen in the healing of wounds, in the circumscription and compensation of internal morbid processes, or—at the outermost end of the series—in the reproduction of a complete fresh-water polyp out of each of the two halves into which one such polyp has been divided. This power could not have been acquired through natural selection; and here it seems inevitable for us to recognize laws of structure working toward an end. But have we not a like phenomenon in the restoration of mutilated crystals, a fact observed by Jordan, Lavalle, Pasteur, Sénarmont, Scharff, and others? So, too, the power which organisms possess of perfecting themselves by practice appears to me not to have yet been sufficiently studied with reference to natural selection.
As a third argument against the theory of natural selection—one which is supposed to negative all its claims to consideration—its opponents always urge in the last place that in no single instance has any one ever actually observed adaptive transformation of an organ by inheritance and selection of the fittest forms. What transformations have been thus effected in the past no man can tell, it is objected; and inasmuch as, even in the future, observations and experiments on this subject seem to be impracticable on many accounts, it is claimed that the doctrine of natural selection is not only an unproved hypothesis now, but that it is fated for ever to remain so. Taking their stand upon this ground, and contrasting themselves with the believers in Darwinism, its opponents boast not a little that they are upholding the standard of strict method, which requires us to accept as demonstrated only what is proved by experiment or by mathematical reasoning.
But this again is a mistake. If it is conceded that any one adapted structure can be explained by natural selection, and if therefore this theory is admitted to be legitimately deduced from legitimate premises, then, in order to suppose the operation of natural selection, wherever it is needed to explain phenomena, it is not necessary for us to actually demonstrate such operation in the individual instance. It might be an
