Geological Evidences of Evolution.
In an article in Nature (vol. xiv. p. 275), Professor Judd calls attention to some recent discoveries in the Hungarian plains, of fossil lacustrine shells, and their careful study by Dr. Neumayr and M. Paul of the Austrian Geological Survey. The beds in which they occur have accumulated to the thickness of 2000 feet, containing throughout abundance of fossils, and divisible into eight zones, each of which exhibits a well-marked and characteristic fauna. Professor Judd then describes the bearing of these discoveries as follows—
"The group of shells which affords the most interesting evidence of the origin of new forms through descent with modification is that of the genus Vivipara or Paludina, which occurs in prodigious abundance throughout the whole series of freshwater strata. We shall not, of course, attempt in this place to enter into any details concerning the forty distinct forms of this genus (Dr. Neumayr very properly hesitates to call them all species), which are named and described in this monograph, and between which, as the authors show, so many connecting links, clearly illustrating the derivation of the newer from the older types, have been detected. On the minds of those who carefully examine the admirably engraved figures given in the plates accompanying this valuable memoir, or still better, the very large series of specimens from among which the subjects of these figures are selected, and which are now in the museum of the Reichsanstalt of Vienna, but little doubt will, we suspect, remain that the authors have fully made out their case, and have demonstrated that, beyond all controversy, the series with highly complicated ornamentation were variously derived by descent—the lines of which are in most cases perfectly clear and obvious—from the simple and unornamented Vivipara achatinoides of the Congerien-Schichten (the lower division of the series of strata). It is interesting to notice that a large portion of these unquestionably derived forms depart so widely from the type of the genus Vivipara, that they have been separated on so high an authority as that of Sandberger, as a new genus, under the name of Tulotoma. And hence we are led to the conclusion that a vast number of forms, certainly exhibiting specific distinctions, and according to some naturalists, differences even entitled to be regarded of generic value, have all a common ancestry."
It is, as Professor Judd remarks, owing to the exceptionally favourable circumstances of a long-continued and unbroken series of deposits being formed under physical conditions either identical or very slowly changing, that we owe so complete a record of the process of organic change. Usually, some disturbing elements, such as a sudden change of physical conditions, or the immigration of new sets of forms from other areas and the consequent retreat or partial extinction of the older fauna, interferes with the continuity of organic development, and produces those puzzling discordances so generally met with in geological formations of marine origin. While a case of the kind now described affords evidence of the origin of species complete and conclusive, though on a necessarily very limited scale, the very rarity of the conditions which are essential to such completeness serves to explain why it is that in most cases the direct evidence of evolution is not to be obtained.
Another illustration of the filling up of gaps between existing groups is afforded by Professor Huxley's researches on fossil crocodiles. The gap between the existing crocodiles and the lizards is very wide, but as we go back in geological time we meet with fossil forms which are to some extent intermediate and form a connected series. The three living genera—Crocodilus, Alligator, and Gavialis—are found in the Eocene formation, and allied forms of another genus, Holops, in the Chalk. From the Chalk backward to the Lias another group of genera occurs, having anatomical characteristics intermediate between the living crocodiles and the most ancient forms. These, forming two genera Belodon and Stagonolepis, are found in a still older formation, the Trias. They have characters resembling some lizards, especially the remarkable Hatteria of New Zealand, and have also some resemblances to the Dinosaurians—reptiles which in some respects approach birds. Considering how comparatively few are the remains of this group of animals, the evidence which it affords of progressive development is remarkably clear.[184]
Among the higher animals the rhinoceros, the horse, and the deer afford good evidence of advance in organisation and of the filling up of the gaps which separate the living forms from their nearest allies. The earliest ancestral forms of the rhinoceroses occur in the Middle Eocene of the United States, and were to some extent intermediate between the rhinoceros and tapir families, having like the latter four toes to the front feet, and three to those behind. These are followed in the Upper Eocene by the genus Amynodon, in which the skull assumes more distinctly the rhinocerotic type. Following this in the Lower Miocene we have the Aceratherium, like the last in its feet, but still more decidedly a rhinoceros in its general structure. From this there are two diverging lines—one in the Old World, the other in the New. In the former, to which the Aceratherium is supposed to have migrated in early Miocene times, when a mild climate and luxuriant vegetation prevailed far within the arctic circle, it gave rise to the Ceratorhinus and the various horned rhinoceroses of late Tertiary times and of those now living. In America a number of large hornless rhinoceroses were developed—they are found in the Upper Miocene, Pliocene, and Post-Pliocene formations—and then became extinct. The true rhinoceroses have three toes on all the feet.[185]
The Pedigree of the Horse Tribe.
Yet more remarkable is the evidence afforded by the ancestral forms of the horse tribe which have been discovered in the American tertiaries. The family Equidae, comprising the living horse, asses, and zebras, differ widely from all other mammals in the peculiar structure of the feet, all of which terminate in a single large toe forming the hoof. They have forty teeth, the molars being formed of hard and soft material in crescentic folds, so as to be a powerful agent in grinding up hard grasses and other vegetable food. The former peculiarities depend upon modifications of the skeleton, which have been thus described by Professor Huxley:—
"Let us turn in the first place to the fore-limb. In most quadrupeds, as in ourselves, the fore-arm contains distinct bones, called the radius and the ulna. The corresponding region in the horse seems at first to possess but one bone. Careful observation, however, enables us to distinguish in this bone a part which clearly answers to the upper end of the ulna. This is closely united with the chief mass of the bone which represents the radius, and runs out into a slender shaft, which may be traced for some distance downwards upon the back of the radius, and then in most cases thins out and vanishes. It takes still more trouble to make sure of what is nevertheless the fact, that a small part of the lower end of the bone of a horse's fore-arm, which is only distinct in a very young foal, is really the lower extremity of the ulna.
"What is commonly called the knee of a horse is its wrist. The 'cannon bone' answers to the middle bone of the five metacarpal bones which support the palm of the hand in ourselves. The pastern, coronary, and coffin bones of veterinarians answer to the joints of our middle fingers, while the hoof is simply a greatly enlarged and thickened nail. But if what lies below the horse's 'knee' thus corresponds to the middle finger in ourselves, what has become of the four other fingers or digits? We find in the places of the second and fourth digits only two slender splintlike bones, about two-thirds as long as the cannon bone, which gradually taper to their lower ends and bear no finger joints, or, as they are termed, phalanges. Sometimes, small bony or gristly nodules are to be found at the bases of these two metacarpal splints, and it is probable that these represent rudiments of the first and fifth toes. Thus, the part of the horse's skeleton which corresponds with that of the human hand, contains one overgrown middle digit, and at least two imperfect lateral digits; and these answer, respectively, to the third, the second, and the fourth fingers in man.