« НазадПродовжити »
tured, are often rendered sterile by captivity, so the influence of union with another species may produce a similar effect. It seems to the author an unwarrantable assumption that a cross with what, on the Darwinian theory, can only be a slightly-diverging descendant of a common parent, should produce an effect equal to that of captivity, and consequent change of habit, as well as considerable modification of food.
No clear case has been given by Mr. Darwin in which mongrel animals, descended from the same undoubted species, have been persistently infertile inter se, nor any clear case in which hybrids between animals, generally admitted to be distinct species, have been continually fertile inter se.
It is true that facts are brought forward tending to establish the probability of the doctrine of Pallas, that species may sometimes be rendered fertile by domestication. But even if this were true, it would be no approximation toward proving the converse, i. e., that races and varieties may become sterile when wild. And whatever may be the preference occasionally shown by certain breeds to mate with their own variety, no sterility is recorded as resulting from unions with other varieties. Indeed, Mr. Darwin remarks,” “With respect to sterility from the crossing of domestic races, I know of no well-ascertained case with animals. This fact (seeing the great difference in structure between some breeds of pigeons, fowls, pigs, dogs, etc.) is extraordinary when contrasted with the sterility of many closely-allied natural species when crossed.”
It has been alleged that the domestic and wild guineapig do not breed together, but the specific identity of these forms is very problematical. Mr. A. D. Bartlett, superintendent of the Zoological Gardens, whose experience is so great, and observation so quick, believes them to be decidedly distinct species.
* “Animals and Plants under Domestication,” vol. ii., p. 104.
Thus, then, it seems that a certain normal specific stability in species, accompanied by occasional sudden and considerable modifications, might be expected a prior? from what we know of crystalline inorganic forms and from what we may anticipate 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 also been shown that there are grounds for believing in a tendency to reversion generally, as it is distinctly present in certain instances. Also that specific stability is confirmed by the physiological obstacles which 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 considered 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 Minutelyintermediate Forms when such might be expected a priori.—Bats, Pterodactyls, Dinosauria, and Birds.-Ichthyosauria, Chelonia, and Anoura.-Horse Ancestry.—Labyrinthodonts and Trilobites.—Two Subdivisions of the Second Relation of Species to Time.—Sir William 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 prior; 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 paleontological 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 aérial form and 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 pterodactyls, 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
1 North British Review, New Series, vol. vii., March, 1867, p. 317.
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 pterodactyls, again, though a numerous group, are all true and perfect pterodactyls, 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 the interpretation given by Prof. Huxley" to the remains of Dinosaurian reptiles, and which were noticed in the third chapter of this work. The learned professor has (as also has Prof. 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, etc. He has given weighty reasons for thinking that the line of affinity between birds and
* See also the Popular Science Review for July, 1868.