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Thus, then, it seems that a certain normal specific stability in species, accompanied by occasional sudden and considerable modifications, might be expected a priori from what we know of crystalline inorganic forms and from what we may suspect with regard to the lowest organic ones. This presumption is strengthened by the knowledge of the increasing difficulties which beset any attempt to indefinitely intensify any race characteristics. The obstacles to this indefinite intensification, as well as to certain lines of variation in certain cases, appear to be not only external, but to depend on internal causes or an internal cause. We have seen that Mr. Darwin himself implicitly admits the principle of specific stability in asserting the singular inflexibility of the organization of the goose. We have also seen that it is not fair to conclude that all wild races can vary as much as the most variable domestic ones. It has been shown besides that there are grounds for believing in a tendency to reversion generally, as it is distinctly present in certain instances; further, that the doctrine of specific stability is confirmed by the fact that physiological obstacles oppose themselves to any considerable or continued intermixture of species, while no such barriers oppose themselves to the blending of varieties. All these considerations taken together may fairly be looked upon as strengthening the belief that specific manifestations are relatively stable. At the same time the view advocated in this book does not depend upon, and is not identified with, any such stability. All that the author contends for is that specific manifestation takes place along certain lines, and according to law; and not in an exceedingly minute, indefinite, and fortuitous
manner. Finally, he cannot but feel justified, from all that has been brought forward, in reiterating the opening assertion of this chapter, that something is still to be said for the view which maintains that species are stable, at least in the intervals of their comparatively rapid successive manifestations.
SPECIES AND TIME.
Two relations of species to time.—No evidence of past existence of minutely graduated intermediate forms when such might be expected a priori. — Bats, Pterodactyles, Dinosauria, and Birds.—Ichthyosauria, Chelonia, and Anoura. -Horse ancestry.-Labyrinthodonts and Trilobites.-Two subdivisions of the second relation of species to time.-Sir Wm. Thomson's views. Probable period required for ultimate specific evolution from primitive ancestral forms.—Geometrical increase of time required for rapidly multiplying increase of structural differences.-Proboscis monkey.-Time required for deposition of strata necessary for Darwinian evolution.--High organization of Silurian forms of life. Absence of fossils in oldest rocks.—Summary and conclusion.
Two considerations present themselves with regard to the necessary relation of species to time, if the theory of "Natural Selection" is valid and sufficient.
The first is with regard to the evidences of the past existence of intermediate forms, their duration and succession.
The second is with regard to the total amount of time required for the evolution of all organic forms from a few original ones, and the bearing of other sciences on this question of time.
As to the first consideration, evidence is as yet against the modification of species by "Natural Selection" alone, because not only are minutely transitional forms generally
absent, but they are absent in cases where we might certainly a priori have expected them to be present.
Now it has been said: "If Mr. Darwin's theory be true, the number of varieties differing one from another a very little must have been indefinitely great, so great indeed as probably far to exceed the number of individuals which have existed of any one variety. If this be true, it would be more probable that no two specimens preserved as fossils should be of one variety than that we should find a great many specimens collected from a very few varieties, provided, of course, the chances of preservation are equal for all individuals." "It is really strange that vast numbers of perfectly similar specimens should be found, the chances against their perpetuation as fossils are so great; but it is also very strange that the specimens should be so exactly alike as they are, if, in fact, they came and vanished by a gradual change."
Mr. Darwin attempts to show cause why we should believe a priori that intermediate varieties would exist in lesser numbers than the more extreme forms; but though they would doubtless do so sometimes, it seems too much to assert that they would do so generally, still less universally. Now little less than universal and very marked inferiority in numbers would account for the absence of certain series of minutely intermediate fossil specimens. The mass of palæontological evidence is indeed overwhelmingly against minute and gradual modification. It is true that when once an animal has obtained powers of flight its means of diffusion are indefinitely increased, and we might expect to find many relics of an aërial form and
1 North British Review, New Series, vol. vii., March 1867, p. 317. 2 "Origin of Species," 5th edition, 1869, p. 212.
few of its antecedent state-with nascent wings just commencing their suspensory power. Yet had such a slow mode of origin, as Darwinians contend for, operated exclusively in all cases, it is absolutely incredible that birds, bats, and pterodactyles should have left the remains they have, and yet not a single relic be preserved in any one instance of any of these different forms of wing in their incipient and relatively imperfect functional condition!
Whenever the remains of bats have been found they have presented the exact type of existing forms, and there is as yet no indication of the conditions of an incipient elevation from the ground.
The pterodactyles, again, though a numerous group, are all true and perfect pterodactyles, though surely some of the many incipient forms, which on the Darwinian theory have existed, must have had a good chance of preservation.
As to birds, the only notable instance in which discoveries recently made appear to fill up an important hiatus, is that concerning the remains of some Dinosaurian reptiles, the interpretation of which, as given by Professor