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over the greater part of the globe. Yet no single transitional form has yet been met with in spite of the multitudinous individuals preserved. Again, with regard to their modern representatives the Cetacea, one or two aberrant forins alone have been found, but no series of transitional ones indicating minutely the line of descent. This group, the whales, is a very marked one; and it is curious, on Darwinian principles, that so few instances tending to indicate its mode of origin should have presented themselves. Here, as in the bats, we might surely expect that some relics of unquestionably incipient stages of its development would have been left.
SKELETON OF A PLESIOSAURUS.
The singular order Chelonia, including the tortoises, turtles, and terrapins (or fresh-water tortoises), is another instance of an extreme form without any, as yet known, transitional stages. Another group may be finally mentioned, viz. the frogs and toads (anourous Batrachians), of which we have at present no relic of any kind linking them on to the Eft group on the one hand, or to Reptiles on the other.
The only instance in which an approach towards a series of nearly related forms has been obtained is that of the existing horse, its predecessor Hipparion and other extinct allies. But even here we have no proof whatever of modi
fication by minute and infinitesimal steps; a fortiori no approach to a proof of modification by“ Natural Selection acting upon indefinite fortuitous variations. On the contrary, the series is an admirable example of successive modification in one special direction along one beneficial line, and the teleologist must here be allowed to consider that one motive of this modification (among probably an indefinite number of motives inconceivable to us) was the relationship in which the horse was to stand to the human inhabitants of this planet. These extinct forms, as Professor Owen remarks, " differ from each other in a greater degree than do the horse, zebra, and ass,” which are not only good zoological species as to form, but are species physiologically—i.e. they cannot produce a race of hybrids fertile inter se.
As to the mere action of surrounding conditions, the same Professor remarks: 2 “Any modification affecting the density of the soil might so far relate to the changes of limb-structure, as that a foot with a pair of small hoofs dangling by the sides of the large one, like those behind the cloven hoof of the ox, would cause the foot of Hipparion, e.g., and a fortiori the broader based three-hoofed foot of the Palæothere, to sink less deeply into swampy soil, and be more easily withdrawn than the more concentratively simplified and specialized foot of the horse. Rhinoceroses and zebras, however, tread together the arid plains of Africa in the present day; and the horse has multiplied in that half of America where two or more kinds of tapir still exist. That the continents of the Eocene or Miocene periods were less diversified in respect of swamp and sward, pampas or desert, than those of the
1 “Anatomy of Vertebrates," vol. iii. p. 792. 2 Ibid. p. 793.
Pliocene period, has no support from observation or analogy."
Not only, however, do we fail to find any traces of the incipient stages of numerous very peculiar groups of animals, but it is undeniable that there have been instances which appeared at first to indicate a gradual transition, and yet these instances have been shown by further investigation and discovery not truly to indicate anything of the kind. Thus at one time the remains of Labyrinthodonts which up till then had been discovered, seemed to justify the opinion that, as time went on, forms had successively appeared with more and more complete segmentation and
ossification of the backbone, which in the earliest forms was (as it is in the lowest fishes now) a soft continuous rod or notochord. Now, however, it is considered probable that the soft back-boned Labyrinthodont (Archegosaurus) was an immature or larval form, while Labyrinthodonts with completely developed vertebræ have been found to exist amongst the very earliest forms yet discovered. The same may be said regarding the eyes of the trilobites, some of
1 As a tadpole is the larval form of a frog.
the oldest forms having been found as well furnished with organs of sight as the very last of the group which has left its remains accessible to observation.
Such instances, however, as well as the abrupt way in which marked and special forms (as the Pterodactyles, &c., before referred to appear and disappear from the geological record, are of course explicable on the Darwinian theory, provided a sufficiently enormous amount of past time be allowed. The alleged extreme, and probably great, imperfection of that record may indeed be pleaded in excuse for the absence of transitional forms. But it is an excuse.1 Nor is it possible to deny the a priori probability of the preservation of at least a few minutely transitional forms in some instances, if every species without exception has arisen exclusively by such minute and gradual transitions.
It remains now to turn to the other considerations with regard to the relation of species to time; namely (1) the total amount of time which other sciences show to be allowable for organic evolution; and (2) the proportion existing, on Darwinian principles, between the time anterior to the earlier fossils, and the time since; as evidenced by the proportion between the amount of evolutionary change during the latter epoch and that which must have occurred anteriorly.
Sir William Thomson has lately? advanced arguments from three distinct lines of inquiry, agreeing in one approximate result. The three lines of inquiry are—1, The action of the tides upon the earth's rotation ; 2, The probable length of time during which the sun has illuminated this
As Professor Huxley, with his characteristic candour, fully admitted in his lecture on the Dinosauria before referred to.
2 “Transactions of the Geological Society of Glasgow,” vol. iii.
planet; and 3, The temperature of the interior of the earth. The result arrived at by these investigations is a conclusion that the existing state of things on the earth, life on the earth, all geological history showing continuity of life, must be limited within some such period of past time as one hundred million years. The first question which suggests itself, supposing Sir W. Thomson's views to be correct, is : Has this period been anything like enough for the evolution of all organic forms by "Natural Selection”? The second is : Has this period been anything like enough for the deposition of the strata which must have been deposited if all organic forms have been evolved by minute steps, according to the Darwinian theory?
As to Sir William Thomson's views the author of this book cannot presume to advance any opinion; but the fact that they have not been refuted pleads strongly in their favour, when we consider how much they tell against the theory of Mr. Darwin. The last-named author only remarks that “many of the elements in the calculation are more or less doubtful,”1 and Professor Huxley 2 does not attempt to refute Sir William Thomson's arguments, but only to show cause for suspense of judgment, inasmuch as the facts may be capable of other explanations.
Mr. Wallace, on the other hand, seems more disposed to accept them, and, after considering Sir William's objections and those of Mr. Croll, puts the probable date of the beginning of the Cambrian deposits 4 at only
Origin of Species,” 5th edition, p. 354. 2 See his address to the Geological Society, on February 19, 1869. 3 See Nature, vol. i. p. 399, February 17, 1870. 4 Ibid. vol. i. p. 454.