« НазадПродовжити »
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:1 “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 aré 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 i North British Rericw, New Series, vol. vii., March 1867, p. 317.
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
Huxley, has been noticed in the third chapter of this work. The learned Professor has (as also has Professor Cope in America) shown that in very important and significant points the skeletons of the Iguanodon and of its allies approach very closely to that existing in the ostrich, emeu, rhea, &c. He has given weighty reasons for thinking that the line of affinity between birds and reptiles passes to the birds last named from the Dinosauria, rather than from the Pterodactyles (through Archeopteryxlike forms) to the ordinary birds. Finally, he has thrown out the suggestion that the celebrated footsteps left by some extinct three-toed creatures on the
ancient sandstone of Connecticut were made, not, as hitherto supposed, by true birds, but by more or less ornithic reptiles. But even supposing all that is asserted or inferred on this subject to be fully proved, it would not approach to a demonstration of specific origin by minute modification; for though the facts harmonize well with "Natural Selection," they are equally consistent with the rapid and sudden development of new specific forms of life. Indeed, Professor Huxley, with a laudable caution and moderation too little observed by some Teutonic Darwinians, guarded himself carefully from any imputation of asserting dogmatically the theory of “Natural Selection," while upholding fully the doctrine of evolution.
But, after all, it is by no means certain, though very probable, that the Connecticut footsteps were made by very ornithic reptiles, or extremely sauroid birds. And it must not be forgotten that a completely carinate bird (the Archeopteryx) existed at a time when, as yet, we have no evidence of some of the Dinosauria having come into being.
1 See also the Popular Science Revicw for July 1868.
Moreover, if the remarkable and minute similarity of the coracoid of a pterodactyle to that of a bird be merely the résult of function and no sign of genetic affinity, it is not inconceivable that the pelvic and leg resemblances of
Dinosauria to birds may be functional likewise, though such an explanation is, of course, by no means necessary to support the view maintained in this book.
But the number of forms represented by many individuals, yet by no transitional ones, is so great that only
SKELETON OF AN ICHTHYOSAURUS.
two or three can be selected as examples. Thus those remarkable fossil reptiles, the Ichthyosauria and Plesiosauria, extended, through the secondary period, probably