CHAPTER XX.

THE LAWS OF EVOLUTION EMPHASIZED BY THE STUDY OF THE GEOLOGICAL HISTORY OF ORGANISMS.

Testimony of Vertebrates.-The vertebrates might be used with great force to illustrate the general laws of evolution. No better example than the vertebrates could be selected to illustrate the fundamental law of the gradual increase, in differentiation and in rank, of the great classes of a branch in the order of their successive appearance and dominance in the geological formations.

In the lowest system of stratified rocks, the Cambrian, no trace of vertebrates has yet been found. In the Ordovician and Silurian only the lowest type of fishes, and they very rare, have been seen. Fishes were abundant in the Devonian. The Lower Carboniferous shows the first amphibians; and large-sized and extinct types of amphibians prevailed in the Carboniferous era. In this era also a few traces of true reptiles have been found. In the Triassic the great Dinosaurian reptiles were abundant on the land. In the Jurassic the shallower seas swarmed with the Enaliosaurs or sea-lizards, and in the lower Jurassic (Lias) the flying reptiles infested the air and culminated the reptilian domination of the Mesozoic time.

While reptiles were the masters of sea, land, and air, the lower types of mammals-the marsupials, and probably monotremes-began to appear in feeble representatives as early as the Triassic, and in the Cretaceous birds, too, make their appearance: though true birds in structure, they compete with the flying reptiles in their use of reptilian teeth for offence and defence.

Remarkable and Extreme Evolution of the Mammals in the Eocene. As we examine the earlier beds of the Tertiary rocks we observe for the first time the dominance of mammals; and

perhaps no more remarkable fact is established in the history of organisms than the sudden expansion of the placental mammals in the Eocene.

Over fifty genera, representing the chief ordinal types of the placental mammals, are already reported from the lowest Eocene, none having been discovered in the underlying Cretaceous. In Europe alone Zittel reports for the fauna of the Upper Eocene about 110 genera and about 200 species. To show the richness of this fauna, in spite of the imperfection of the records, he cites the facts that "Our present European land mammalian fauna contains 54 genera with about 150 species, and of these 60 per cent belong to the microfauna, consisting of the smaller forms of Rodents, Insectivora, Bats, and Carnivora, for which the conditions of preservation in earlier epochs were very unfavorable" ("The Geological Development, Descent, and Distribution of the Mammalia," by Karl A. von Zittel, Geol. Mag., Dec., III., vol. X., Sept., Oct., Nov., 1893).

If we glance at the whole group of mammals, we find the actually known forms included in three subclasses: the (I) Prototheria, with the order Monotremata; (II) Metatheria, represented by the order Marsupialia; and (III) Eutheria, or the Placentalia.

There can be no doubt as to the higher rank of the Placentalia over the marsupial and monotreme types. No certain traces of the Placentalia are known to occur below the Eocene. Stegadon, a genus of the Tillodontia, is thought to have appeared possibly in earlier beds.

Of these mammals, ten orders, fossil and recent, are recognized. Two of these are marine-Sirenia and Cetacea. The Edentata is a South American order, and has its representatives in the earliest known South American mammalian fauna (Vera Cruz fauna of Patagonia), which is probably equivalent to the northern Eocene.

If we omit the above three orders, of the remaining seven orders of land mammalia five are represented in the older Eocene of Europe-the Ungulates, with 5 suborders; the Rodents, the Insectivores, the (Carnivora) Creodonta, the Prosimiæ the forerunners of, if not true, Primates.

True Carnivores appeared in the newer Eocene, Cheiroptera in the middle Eocene, and true Primates in the older Miocene.

In these orders of placental mammals 56 genera appeared for the first time in the older Eocene, and there were successively added to them, in the middle Eocene 40 new genera, in the newer Eocene 105 new genera, Oligocene 5, older Miocene 49, newer Miocene 34, Pliocene 27; or previous to the opening of the Pleistocene 260 genera, distributed among the seven land orders of mammals, of which the first traces were obtained from the older Eocene beds of North America and Europe. The Australian, South American, and African types are not here included; and it must be remembered also that new discoveries are constantly adding to these statistics, and in general they augment the earlier more than the later totals.

Again, the fact that (? Prototheria and) Metatheria were already well developed in genera in the Mesozoic does not lessen the significance of the remarkable expansion of the mammals in the older Eocene period; nor does the imperfection of knowledge lessen the testimony to the relatively sudden expansion which the evidence now in hand indicates. The approach to recent time, and the increasingly better representation of the land faunas among the preserved remains, does not invalidate the truth of the general proposition, that all the grand features of structural modification, expressed in the subclass of placental mammals, made their appearance in distinct genera with great rapidity at the first stage of appearance of the Placentalia.

The prominent differences, expressed in the limbs, teeth, form, and habits, in the hoofed animals, the odd and even toes, the gnawing rodents, the flesh-eating Carnivores (Creodonts), the insect-eaters, the flying bats, and the climbing monkeys, were all seen among the members of the first fauna. of the new type of placental mammals, in the Eocene period.

Synthetic Types Illustrated by the Vertebrates of the Mesozoic. -No better illustration of the principle of the "synthetic " or "comprehensive" character of early types of organization is to be found than that presented by the Dinosaurian reptiles and the reptilian birds of the Mesozoic. Here we find

biped reptiles, three-toed and with avian pelvic structure; flying reptiles, with beaks instead of teeth; birds with teeth, and birds with long vertebrated tails.

So many points of combination of features have been seen in the Mesozoic fauna, which are now only found separated in the two great classes Aves and Reptilia, that zoologists have been forced to provide an intermediate group to include these ancient types, or to expand and combine the two classes into the one superclass Sauropsida of Huxley.

Specialization of Five Fingers in Reptiles and its Relation to Later Specializations. The principle of synthesis, or combination, in an early type, of the characteristics of two or more sepa

C

MA A III

FIG. 120.-Left forefoot of A, Phenacodus primavus Cope, Eocene; B, Hyracotherium venticolum Cope, Eocene; C, Paleotherium medium Cuv., Oligocene; D, Anchitherium aurelianense Blainv, Miocene; E, Hippotherium gracile Kaup., Pliocene; F, Equus caballus L., Recent. lunar; = magnum; cuneiform; = scaphoid; trapezoid; tz= trapezium; = unciform; I-V 1st to 5th finger or metacarpal bones; mc = metacarpal. (Steinmann and Döderlein.)

rate types of a later stage, is seen in the case of the Permian reptile Mesosaurus tumidus Cope, in which five tarsals are present, rather than four-the normal number of later reptiles. Such a fact shows, according to Cope, that five is the primitive number of tarsals, and that four is a specializationjust as we find in general in the evolution of paws, feet, and hands the full number of parts was provided before the specialized reduced number was evolved. The fewer number of fingers or of bones, entering into the mechanism of the foot or hand, is the result of selection and specialization of parts. rather than the direct production of any new function or part. The Eocene Phenacodus primævus Cope illustrates this princi

ple in the evolution of the forefoot of mammals, as shown in the figure on the opposite page.

Finger-bones and Teeth as Tests of Degree of Differentiation.-In tracing the history of mammals we find the principle of five fingers already developed before mammals began. Hence the wonderful modifications noted by Owen, Kowalevsky, Ryder, Marsh, Cope, and others, in the arrangement of the bones of the mammalian feet, their specialization in form, and relative size, shape, and position, have constituted the chief data for both classification and phylogenetic series.

The teeth, as highly specialized organs, and as terminal parts of the individual organization, coming into most immediate contact with the outside elements of resistance to the life of the individual, are particularly sensitive expressions of the stages of evolution.

Any device of offence or defence, particularly when hardness and resistance to attrition are characteristics of its structure, becomes at once a mark of the effects of environment in inducing modifications, and of the stage of progress attained by the individuals in their evolution. Their resistance to destruction makes such parts most valuable records in the rocks of the history of organisms.

Laws Derived from the Study of the Teeth of Mammals by Osborne. Professor H. F. Osborne, following the investigations of Rütemeyer and others, has recently written several instructive papers setting forth the laws to be observed in the history of the development of the teeth in mammals.

In a memoir (first read as the address of the vice-president of the section of Zoology, of the American Association for the Advancement of Science*) he narrates both concisely and admirably the laws expressed in the modification of the cusps or surface forms of the teeth of mammals.

Osborne shows how the tricuspid tooth is an evolution. from a simple monocuspid tooth, which is the primitive type of tooth in all earlier vertebrates. He shows further that the multiple succession of teeth characteristic of reptiles is the primitive method of arrangement, and this, as is also the in

"The Rise of the Mammalia in North America," Am. Jour. Sci., III., vol. XLVI., pp. 379–392 and 448-466.