Изображения страниц

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 pedicellariæ of Echinoderms, between Ichthyosauria and Cetacea, with very many other similar coincidences, have also been indicated as instances in point.

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 colour 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 organic forms seem to have an innate tendency to remarkable 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 favour 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 cases 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, &c. 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 not so; while the ordinary rule, that the progress of forms has been "from the more general to the more special," has been shown to have 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 presented by the inorganic world, and perhaps also by the lowest forms of the organic world. It has also been seen that with regard to minute spontaneous variations in races, there is a rapidly increasing 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 definite 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 proclivity 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 found 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. It has also been maintained 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 both 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 so very many 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 of any great weight, yet have a certain value when taken in conjunction with all the other objections.

The facts of homology, serial, bilateral and vertical, have also been passed in review. Such phenomena, 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. Besides this, "Natural Selection " has been shown to be impotent to explain these phenomena, while the exist ence 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 endeavour to show the utter inadequacy of Mr. Darwin's theory with regard to the higher psychical phenomena of man (especially the evolution of moral perceptions), and with regard to the evolution of individual organisms by the action of Pangenesis. And it has been implied that if Mr. Darwin's latter hypothesis could be shown to be untenable, an antecedent doubt would be thrown upon his other conception, namely, the theory of "Natural Selection."

A cumulative argument thus arises against the prevalent action of "Natural Selection," which, to the mind. of the author, is conclusive. As before observed, he was not originally disposed to reject Mr. Darwin's fascinating theory. Reiterated endeavours to solve its difficulties

have, however, had the effect of convincing him that that theory, as the one or as the leading explanation of the successive evolution and manifestation of specific forms, is untenable. At the same time he admits fully that "Natural Selection" acts, and must act, and that it plays in the organic world a certain though a secondary and subordinate part.

The one modus operandi yet suggested having been found insufficient, the question arises: Can another be substituted in its place? If not, can anything that is positive, and if anything, what, be said as to the question of specific origination?

Now in the first place, it is of course axiomatic that the laws which conditioned the evolution of extinct and of existing species are of as much efficacy at this moment as at any preceding period, that they tend to the manifestation of new forms as much now as ever before. It by no means necessarily follows, however, that this tendency is actually being carried into effect, and that new species of the higher animals and plants are now being produced. They may be so or they may not, according as existing circumstances favour, or conflict with, the action of those laws. It is possible that lowly organized creatures may be continually evolved at the present day, the requisite conditions being more or less easily supplied. There is, however, no such evidence at present as to higher forms; while, as we have seen in Chapter VII., there are a priori considerations which militate against their being similarly evolved.

The presence of wild varieties, and the difficulty which often exists in the determination of species, are sometimes adduced as arguments that high forms are now in process


« ПредыдущаяПродолжить »