Huxley,1 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 very 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 Review for July 1868.

Moreover, if the remarkable and minute similarity of the coracoid of a pterodactyle to that of a bird be merely the result 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

two or three can be selected as examples. Thus those remarkable fossil reptiles, the Ichthyosauria and Plesiosauria, extended, through the secondary period, probably

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,1 "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.