often show striking indications of progressive development as we pass from the earlier to the later subdivisions of the Tertiary era. The most complete series of fossils is furnished by the remains of extinct horses to which we have already briefly referred. Back in the Eocene, skeletons have been found of a little animal, the Eohippus, not larger than a fox. This form represents the earliest known member of a most remarkable series of extinct species which lead to the horses of the present day. The evolutionist would expect, a priori, that horses descended from ancestral mammals having the typical number of five digits. The earliest Eocene type approaches this condition in having five digits on the forefoot, and four on the hind foot. In the forefeet the inner digit is represented by only a slender rudiment of the first metacarpal bone, while the outer, or fifth, digit is similarly reduced in the hind feet. The middle digit in both fore and hind feet is significantly larger than the others, a condition which is prophetic of further development in succeeding forms. The neck and limbs of the Eohippus were relatively short, and the teeth were of the primitive and unspecialized type, although the molars and premolars show the beginning of the complicated folds of enamel which characterize the grinding teeth of modern horses.

The small size of these ancient horses need not surprise us, for the mammals of the preceding Mesozoic era were small creatures whose diminutive size may have been useful in enabling them to keep discreetly out of the way of the great reptiles. The succeeding types of horses became larger and larger, and at the same time there was a gradual reduction in the size and number of the lateral digits of the feet, a relatively greater development of the middle digit, an elongation of the neck and legs, a gradual reduction of the fibula and ulna, and a more complicated development of the teeth. Such a large number of the intermediate stages have now been discovered between ancient and modern horses that the series affords an almost conclusive demonstration of the evolution of the horse from primitive, five-toed ancestral mammals. The scene of this evo

FIG. 212-A series of fossil horses from the Eocene to the present: a, forefoot; b, hind foot; c, radius and ulna; d, tibia and fibula; e, tooth seen from the side; seen from the grinding surface, showing the development of folds fter Romanes.)

f,

lution was mainly on the North American continent. Specimens of the old genus Eohippus are found in both Europe and North America, and it is probable that these animals migrated to North America during the Eocene, as they could well have done, by means of the land connection existing at that time across Bering Strait. Horses seem to have died out in the Old World, since we meet with them again only in the late Tertiary when some of the species may have migrated back from North America.

The western plains of North America have yielded remains of fossil horses throughout most of Tertiary time. The Eohippus was followed by Protorohippus, which was a little larger and had no rudimentary first metacarpal. Orohippus and Epihippus, somewhat later Eocene types, showed a further reduction in the size of the lateral digits and a considerable advance in the structural development of the teeth. In the Mesohippus of the Miocene (I am omitting several intermediate types) the digits of the forelegs are reduced to three with a rudiment of a fourth, while there are three digits on the hind feet. The second and fourth digits still retained small hoofs, but in the Pliohippus of the Pliocene these small hoofs had disappeared. In the Pleistocene era, which followed the Tertiary, horses became extinct in both North and South America. The reason for their disappearance is entirely unknown, since, so far as can be ascertained, conditions were not unfavorable for their life. After their introduction by the Spaniards, horses became wild in both the American continents, where they roamed in large herds over the grassy plains.

The series of fossils which speak so strongly in favor of the evolution of the horse does not stand alone. Other series of Tertiary fossils tell a similar story, although the intervening stages are not quite so complete. One remarkable series leads up to our modern species of elephants. These large and curiously organized creatures are apparently quite sharply separated from the other mammals. Although now confined to southern Asia and Africa, elephants formerly roamed over all of the continents

A

B

Ε ́

A

Α'

FIG. 213-Evolution of the elephant, showing increased complexities of tooth structure: A, A', recent elephant (Elephas) from the Pleistocene; B, Stegodon from the Pliocene; C, C', Mastodon from the Pleistocene; D, D', Trilophodon from the Miocene; E, E', Paleomastodon from the Oligocene; F, F', Moritherium from the Eocene. (After Lull.)

except Australia. The earliest member of the elephant series is a small tapirlike animal, the Moritherium, found in the upper Eocene strata of Egypt. The skull shows only the beginning of that curious elongation of the jaws and enlargement of the tusks which characterizes the later members of the group. The grinding teeth are still generalized in structure, although they possess a number of crests which become very highly developed and specialized in recent species. A somewhat later genus, Palæomastodon, which is also found in Egypt, shows a larger development of the tusks, a marked elongation of the lower jaw, and certain peculiarities of the skull that are indicative of the presence of a proboscis. In the genus Tetrabelodon there is a still greater development of the tusks and lower jaw, and the tooth structure has become more complicated. The genera Dibelodon and Mastodon are characterized by a shortening of the lower jaw and a loss of the lower tusks. Through the intermediate genus Stegodon, the Mastodons are connected with the genus Elephas which contains, in addition to the two living species of elephants, several extinct species including the well known mammoth.

Space forbids the consideration of the series of deer, camels, rhinoceroses, and carnivores which are continually being made more nearly complete by the discovery of new forms. Although the geological record may be very imperfect, nevertheless, so far as it goes, it not only is consistent with the doctrine of evolution, but it has supported this doctrine by a large amount of positive evidence. It has shown that from the period of the earliest organic remains there has been a gradual advance in the various forms of plant and animal life. It has revealed the existence of many types intermediate between existing groups. It has shown that when new groups are introduced they are represented by their more primitive members. It has shown that, as we trace groups back in time, they tend to draw more closely together; and it has afforded several remarkable series of connecting links which throw much light upon the derivation of existing forms. In a word, the whole progress of paleontology has served