Scampering Types.—Small mammals in general have what may be called the scampering habit,—the ability to scurry away quickly in time of danger, without any pronounced specializations of the skeleton for running ; and it seems safe to assign this habit to the pre-Eocene insectivorous ancestors of the placentals. From such a scampering type, with small semi-plantigrade hind feet and semi digitigrade forefeet, adaptive radiation for different habits has brought about profound modifications of all parts of the skeleton. Cursorial Types.—In quadrupedal running or cursorial forms length and rapidity of stride, combined with strength and endur ance, are the great desiderata. The lower segments of the limbs become long, rod-like, angulated compound levers for striking the hard ground and catapulting the body forward ; the principal mus cles are bunched at the upper parts of the limbs and transmit their pulls through long cord-like tendons that pass over smooth pulley-like surfaces and are inserted into the lower segments of the leg. In extreme forms the foot is longer than the humerus, which gives great speed but requires great muscular strength.
The long narrow feet in running forms have parallel rather than spreading digits ; rising on its toes, the animal finally runs on its enlarged hoofs alone. The thumb and great toe are reduced in size and raised off the ground ; later they disappear, as do the fifth or outer digits of the fore- and hind-feet. In the perissodac tyls or odd-toed ungulates the middle or third digit in both feet becomes predominant, in the horses finally forming the sole func tional axis of the foot, the second and fourth digits being reduced to slender splints. In the artiodactyls, on the other hand, after the loss of the first digit the others become paired, the inner and outer digits, II. and V., being smaller, while III. and IV. are larger; in the final stages II. and V. become reduced and almost disappear, while III. and IV. become very long and fuse into the cannon-bone.
As the fore-and-aft movements of the limbs become empha sized, the elbows and knees are turned outward as little as pos sible ; twisting movements diminish. Consequently the shoulder girdle tends to lose the clavicle and the acromial process of the scapula disappears. In connection with the predominant f ore-and af t movements there are hinge-like joints between the humerus and the forearm, at the wrist and between the metacarpals and the digits.
The scapula is variously shaped : V-shaped in the swift-running artiodactyls, with vertically extended fossa for the infraspinatus muscle ; it is usually at least as long as the humerus, which is rela tively short with a projecting greater tuberosity for the attach ment of the powerful shoulder muscles. The olecranon process of the ulna becomes a broad thickened lever for the insertion of the massive and powerful triceps ; the strong tendon of the biceps passes through a broad channel in the humerus.
These and many other detailed adaptations for swift running have been worked out independently in many families and even in different orders of mammals, notably among the numerous plains-living ungulates, as in the horses of the northern world, the extinct pseudo-horses or smaller litopterns of Patagonia, the ante lopes and deer. The less advanced stages are seen among the car
nivorous hunters such as wolves and certain of the extinct creo donts, as well as in the marsupial wolf (Thylacinus) of Australia. Even some of the plains-living rodents, such as the chinchilla, show cursorial adaptations in the limbs. The ilium in cursorial types is a strongly braced T-shaped bone supporting the thick deep gluteal muscles which are inserted nearly at right angle to the long axis of the femur.
Graviportal or Striding Types.—Given an abundant food sup ply, many lines of evolving animals exploit the opportunity of becoming larger, living longer and leaving a larger progeny. These conditions have tended to transform slender speedy animals into great lumbering brutes. In these heavy-bodied forms the lateral or transverse growth components increase relatively faster than the linear ones ; slender bodies and narrow limbs and feet become broad and robust. This is clearly seen in such races as the extinct titanotheres, the rhinoceroses and other lines of ungulates, in which the earlier forms have slender narrow feet, the later ones broad short feet.
In such cases the extent to which the earlier cursorial adapta tions are disguised by the overlying graviportal changes depends inter alia upon how early the graviportal tendencies gain the ascendancy. When the graviportal tendency supervenes imme diately upon the short-footed scampering stage, so that the cur sorial stage is passed by, we have the extreme graviportal modifi cations illustrated in the Eocene Amblypoda and to a less extent in the elephants. The excessively short-toed feet, instead of cata pulting the body, roll forward on a great elastic cushion and the straightened legs, with long humeri and femora, swing forward like massive beams. The ilium widens transversely into a huge fan. Saltatorial or Leaping Types are usually developed from the scampering or cursorial types. In the jerboas and kangaroo rats, which have evolved from scampering rodents, the animals leap on their powerful hind-limbs, holding the body erect and using the fore-limbs chiefly in manipulating the food. The tail is used as a balancing organ in leaping. The kangaroos have doubtless been derived from arboreal phalangers, in which the fourth digit of the hind-foot was already enlarged, the first digit divergent and pre hensile, the second and third small and closely appressed or syn dactylous. The early hopping stage is illustrated by the tiny musk kangaroo (Hypsiprymnodon), in which the great toe is still present though reduced. In later stages the great toe disappears and the fourth digit becomes enormously enlarged, forming the lower joint of a catapult, the power for which is supplied by the massive muscles of the buttocks, thigh and shank. The tail acts as the third leg of a tripod. Bipedal leaping is convenient for hurdling obstacles in sudden alarms, but for heavy animals it is uneconom ical for long distances.