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Review of the Statements and Arguments of Preceding Chapters.--Cumulative Argument against Predominant Action of “Natural Selection.”—Whether any thing positive as well as negative can be enunciated.—Constancy of Laws of Nature does not necessarily imply Constancy of Specific Evolution.—Possible Exceptional Stability of Existing Epoch-Probability that an Internal Cause of Change exists.Innate Powers must be conceived as existing somewhere or other.—Symbolism of Molecular Action under Wibrating Impulses.—Prof. Owen's Statement.—Statement of the Author's View.—It avoids the Difficulties which oppose “Natural Selection.”—It harmonizes Apparently Conflicting Conceptions,—Summary and Conclusion.
HAVING now severally reviewed the principal biological facts which bear upon specific manifestation, it remains to sum up the results, and to endeavor to ascertain what, if any thing, can be said positively, as well as negatively, on this deeply interesting question.
In the preceding chapters it has been contended, in the first place, that no mere survival of the fittest accidental and minute variations can account for the incipient stages of useful structures, such as, e.g., the heads of flat-fishes, the baleen of whales, vertebrate limbs, the laryngeal structures of the new-born kangaroo, the pedicellariae of Echinoderms, or for many of the facts of mimicry, and especially those last touches of mimetic perfection, where an insect not only mimics a leaf, but one worm-eaten and attacked by fungi.
Also, that structures like the hood of the cobra and the rattle of the rattlesnake seem to require another explanation.
Again, it has been contended that instances of color, as in some apes; of beauty, as in some shell-fish; and of utility, as in many orchids, are examples of conditions which are quite beyond the power of Natural Selection to originate and develop. Next, the peculiar mode of origin of the eye (by the simultaneous and concurrent modification of distinct parts), with the wonderful refinement of the human ear and voice, has been insisted on ; as also, that the importance of all these facts is intensified through the necessity (admitted by Mr. Darwin) that many individuals should be similarly and simultaneously modified in order that slightly favorable variations may hold their own in the struggle for life, against the overwhelming force and influence of mere number. Again, we have considered, in the third chapter, the great improbability that from minute variations in all directions alone and unaided, save by the survival of the fittest, closely-similar structures should independently arise; though, on a non-Darwinian evolutionary hypothesis, their development might be expected a priori. We have seen, however, that there are many instances of wonderfully close similarity which are not due to genetic affinity; the most notable instance, perhaps, being that brought forward by Mr. Murphy, namely, the appearance of the same eye-structure in the vertebrate and molluscous sub-kingdoms. A curious resemblance, though less in degree, has also been seen to exist between the auditory organs of fishes and of Cephalopods. Remarkable similarities between certain placental and implacental mammals, between the bird's-head processes of Polyzoa and the pedicellariae ef Echinoderms, between Ichthyosauria and Cetacea, with very many other similar coincidences, have also been pointed out. Evidence has also been brought forward to show that similarity is sometimes directly induced by very obscure conditions, at present quite inexplicable, e. g., by causes immediately connected with geographical distribution; as in the loss of the tail in certain forms of Lepidoptera and in simultaneous modifications of color in others, and in the direct modification of young English oysters, when transported to the shore of the Mediterranean. Again, it has been asserted that certain groups of or. ganic forms seem to have an innate tendency to remark. able developments of some particular kind, as beauty and singularity of plumage in the group of birds of paradise. It has also been contended that there is something to be said in favor of sudden, as opposed to exceedingly minute and gradual modifications, even if the latter are not fortuitous. Cases were brought forward in Chapter IV., such as the bivalve just mentioned, twenty-seven kinds of American trees simultaneously and similarly modified, also the independent production of pony breeds, and the case of the English greyhounds in Mexico, the offspring of which produced directly acclimated progeny. Besides these, the case of the Normandy pigs, of Datura tatula, and also of the black-shouldered peacock, have been spoken of. The teeth of the labyrinthodon, the hand of the potto, the whalebone of whales, the wings of birds, the climbing tendrils of some plants, etc., have also been adduced as instances of structures, the origin and production of which are probably due rather to considerable modifications than to minute increments. It has also been shown that certain forms which were Once supposed to be especially transitional and intermediate (as, e.g., the aye-aye) are really by no means so ; while the general rule, that the progress of forms has been “from the more general to the more special,” has been shown to present remarkable exceptions, as, e.g., Macrauchenia, the Glyptodon, and the sabre-toothed tiger (Machairodus).
Next, as to specific stability, it has been seen that there may be a certain limit to normal variability, and that if changes take place they may be expected a priori to be marked and considerable ones, from the facts of the inorganic world, and perhaps also of the lowest forms of the organic world. It has also been seen that with regard to minute spontaneous variations in races, there is a rapidlyincreasing difficulty in intensifying them, in any one direction, by ever such careful breeding. Moreover, it has appeared that different species show a tendency to variability in special directions, and probably in different degrees, and that at any rate Mr. Darwin himself concedes the existence of an internal barrier to change when he credits the goose with “a singularly inflexible organization; ” also, that he admits the presence of an internal proclivity to change when he speaks of “a whole organization seeming to have become plastic, and tending to depart from the parental type.” We have seen also that a marked tendency to reversion does exist, inasmuch as it sometimes takes place in a striking manner, as exemplified in the white silk fowl in England, in spite of careful selection in breeding. Again, we have seen that a tendency exists in nature to eliminate hybrid races, by whatever means that elimination is effected, while no similar tendency bars the way to an indefinite blending of varieties. This has also been enforced by statements as to the prepotency of certain pollen of identical species, but of distinct races. To all the preceding considerations have been added others derived from the relations of species to past time. It has been contended that we have as yet no evidence of minutely intermediate forms connecting uninterruptedly together undoubtedly distinct species. That while even “horse ancestry’ fails to supply such a desideratum, in very strongly-marked and exceptional kinds (such as the Ichthyosauria, Chelonia, and Anoura), the absence of links is very important and significant. For if every species, without exception, has arisen by minute modifications, it seems incredible that a small percentage of such transitional forms should not have been preserved. This, of course, is especially the case as regards the marine Ichthyosauria and Plesiosauria, of which such numbers of remains have been discovered. Sir William Thomson’s great authority has been seen to oppose itself to “Natural Selection,” by limiting, on astronomical and physical grounds, the duration of life on this planet to about one hundred million years. This period, it has been contended, is not nearly enough, on the one hand, for the evolution of all organic forms by the exclusive action of mere minute, fortuitous variations; on the other hand, for the deposition of all the strata which must have been deposited, if minute fortuitous variation was the manner of successive specific manifestation. Again, the geographical distribution of existing animals has been seen to present difficulties which, though not themselves insurmountable, yet have a certain weight when taken in conjunction with all the other objections. The facts of homology, serial, bilateral, and vertical, have also been passed in review. Such facts, it has been contended, are not explicable without admitting the action of what may most conveniently be spoken of as an internal power, the existence of which is supported by facts not only of comparative anatomy but of teratology and pathology also. “Natural Selection” also has been shown to be impotent to explain these phenomena, while the existence of such an internal power of homologous evolution diminishes the a priori improbability of an analogous law of specific origination. All these various considerations have been supplemented by an endeavor to show the utter inadequacy of Mr. Dar