Зображення сторінки
PDF
ePub
[ocr errors]

growth and transformation of the most minute and the simplest organisms, which themselves, by all reason and analogy, owe their existence to immediate transformation from the inorganic world.

On the whole, then, we seem justified in asserting that there are grave difficulties in the way of the reception of the hypothesis of Pangenesis, which moreover, if established, would leave the evolution of individual organisms, when thoroughly analysed, little if at all less mysterious or really explicable than it is at present.

As was said at the beginning of this chapter, “ Pangenesis” and “ Natural Selection " are quite separable and distinct hypotheses. The fall of one of these by no means necessarily includes that of the other. Nevertheless, Mr. Darwin has associated them closely together, and, therefore, the refutation of Pangenesis may render it advisable for those who have hitherto accepted “Natural Selection" to reconsider their acceptance of that theory.

CHAPTER XI.

SPECIFIC GENESIS.

Review of the statements and arguments of the preceding chapters. —

Cumulative argument against predominant action of “Natural Selection.” – Whether anything 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 vibrating impulses. — Professor 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 endeavour to ascertain what, if anything, 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, limbs of vertebrates, the laryngeal structures of the new-born kangaroo, the pedicellariæ 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 colour, 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, have 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 favourable variations may hold their own in the struggle for life, against the overwhelming force and influence of mere number.

Again, we have considered, in Chapter III., 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

[ocr errors][merged small]

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,

[ocr errors]

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

« НазадПродовжити »