determinate form. Next comes the history of the processes by which system is induced. There were hollows and valleys in those early times, most probably vast and deep, but not irregular. The constant fall of rain must originate brooks and streams, coursing their downward way toward the lowest levels. Animated with this descending impulse, they remove barriers at the outlets of lakes and pools, excavate gorges through ridges of impediment, and wear off numerous fragments from every projecting point. This eroded material would be urged forward by the current till the lowest possible level was reached, probably the bottom of an arm of the sea or bay, and remain there while the basin was filling up. Thus we should have a formation, composed of layers of the sand, clay, and limestone, originally a chemical precipitate, but now altered into sedimentary deposits. When the first accessible hollows had been filled up, a great interval of time had elapsed, and the external envelope of the earth would shrink, on account of its refrigeration, and fall upon the collapsed nucleus. Hence new valleys would be formed, and the streams would carry the detritus into them, and another set of strata lying upon the edges of the first formation would be deposited. This process has been going on uninterruptedly from that day to the present, and the face of the earth has been changed a hundred times. How long this process went on before the introduction of life it is impossible to say, for the oldest strata known to exist contain the remains of the Eozoic reef-building colonies, in the formations known as the Laurentian.

As some of the older Laurentian beds are composed of pebbles, it is obvious that earlier formations exist, from which the sedimentary material has been derived. Possibly we may be able ultimately to separate from the various systems of the age under consideration those characterized by the presence of the first existing plants-since in the order of Nature there must have been plants before animals. If we follow the analogy of the duration of the earlier periods, we may believe that this Eophytic age exceeded the Eozoic in length; and, furthermore, that the time before the introduction of life was far greater than what has lapsed subsequently. If the law admits of universal application, that the simpler the organism the longer it has lived, then we may perhaps claim that the earlier the period the greater has been its duration. The extent of work performed in these early ages has certainly far exceeded any thing yet known of the operations in the Zoic periods.

The series of changes prior to the introduction of life may therefore be registered as distinct ages, as well marked by special features and a natural order of succession as the periods defined by Paleontology. The minute details of the history are wanting, but, with such substantial bases of probability as have been set forth, human thought will construct theoretical systems that will command universal acceptance.

As now understood, the following titles may express the characteristic features of all the great ages of the world, from the birth of matter to the advent of man:

MATTER CONVERTED INTO VAPORS.

NEBULA COMPOSED OF THE ENTIRE SOLAR SYSTEM.

THE EARTH-NEBULA.

PERIOD OF IGNEOUS FLUIDITY.

AGE OF CHEMICAL CHANGES.

BEGINNING OF THE SEDIMENTARY PERIOD.

INTRODUCTION OF VEGETATION; or, Eophytic Period.
INTRODUCTION OF ANIMAL LIFE; or, Eozoic Period.
PALEOZOIC ERA.

MESOZOIC ERA.

CENOZOIC ERA, COMPLETED BY THE ADVENT OF MAN.

THR

WALKING, SWIMMING, AND FLYING.

By E. LEWIS, JR.

HROUGHOUT the realms of Nature motion is indispensable to physical stability and organic existence. It is everywhere present, and equally among molecules and masses the mind searches in vain for evidence of absolute rest. It has been declared that organic life is a result of motion;" certain it is that motion is a condition of life. It appears in the endless manifestations of beauty and utility, in the world of living creatures of which ourselves are a part. The heavens are more beautiful when clouds are drifting, and the motions of animals give a charm to a landscape which disappears in the solitude of a desert. Stillness to the eye, like silence to the ear, becomes at last painfully oppressive. We scarcely realize, perhaps we seldom consider, how much of the joy and value of existence depends upon the movement of beings, and the marvelous perfection of the means by which it is effected. Walking, swimming, and flying, are the means by which we traverse the three great highways of Nature— the land, the water, and the air. If we change our position, it is in one or other of these. There is no more fascinating chapter of science than this. The mere fact of animal locomotion is felt to be an expression of beneficence, and of adaptation of means to ends which surpasses human ingenuity.

What laws of motion are revealed, what principles of mechanics are brought into action, when animals walk, swim, or fly, has been discussed by many writers, but by none in a more able or interesting manner than by Dr. Pettigrew, who, in a volume soon to appear in

the International Series, has given the results of a long course of observations and study upon the subject.'

The three modes of progression, apparently so unlike, are nevertheless essentially the same. The limbs of the quadruped, the wings of the bird, and the fins of the fish, are built upon the same general plan of structure, and are applied fundamentally to the same uses. They are traveling surfaces, and their wide range of modification is in direct relation to the media in which they are used. The one treads the solid ground, another the water, and another the yielding and elastic air. "But walking merges into swimming, aud swimming into flying, by insensible gradations; and these modifications result from the fact that the earth affords a greater amount of support than the water, and the water than the air."

Most terrestrial quadrupeds can swim as well as walk, and some can fly. Many marine animals both walk and swim, and birds and insects walk, fly, and swim, indiscriminately. It is not surprising, therefore, that, between the typical foot, wing, and fin, innumerable modifications in structure and form occur; indeed, so graduated are they that it may be difficult to determine where one form ends and another begins.

In Fig. 1 we have several illustrations of the traveling surfaces of

A-Extreme form of compressed foot, as seen in the deer, ox, etc., adapted specially for land transit.

B-Extreme form of expanded foot, as seen in the ornithorhynchus, etc., adapted more particularly for swimming.

C-Intermediate form of foot, as seen in the otter.

D-Foot of frog. Here the foot is equally serviceable in and out of the water.
E-Foot of the seal, which opens and closes in the act of natation.

animals. The small feet of the quadruped, the webbed feet of the ornithorhynchus, the otter, the walrus, and the triton, indicate with certainty the media to which they are adapted, and perhaps in nothing is modification of structure and form to habits more apparent than in the locomotive appendages of animals. The webbed structure between the toes of animals which live partially on the land, and of some terrestrial animals, as the water-dog, is wonderfully significant.

The wing of the penguin, Fig. 2, is scarcely more than a flipper, and the same is true of the auk.

Sir John Lubbock describes a species of insect whose wings are

1 "Animal Locomotion; or, Walking, Swimming, and Flying." By J. Bell Pettigrew, F. R. S. International Scientific Series. New York: D. Appleton & Co.

VOL. IV.-34

used as fins only. "Every variety of motion peculiar to land, air, and water-navigating animals, as such, is imitated by others which take to the elements in question, secondarily or at intervals." It is probably true, however, that no animal which lives indiscriminately in two media attains the highest development for traveling in or upon either. In such cases the maximum speed is not attained. Those animals, says the author, which swim the best, walk, as a rule, with difficulty, and vice versa, as the movements of the auk and the seal, in and out of water, amply testify. It is evident that all the supposed gaps between typical forms for locomotion are bridged by forms intermediate, and the author's position is fully sustained, that walking, swimming, and flying, are essentially the same.

THE LITTLE PENGUIN, adapted exclusively for swimming and diving. In this quaint bird the wing forms a perfect screw, and is employed as such in swimming and diving.

Before entering upon the question of the movement and functions of specialized organs for locomotion, attention is invited to the interesting statement that, however wonderful and beautiful, in its way, the bony skeleton may be, it is after all only an adjunct to locomotion, and of motion in general-that all the really essential movements of an animal occur in the soft parts. "The osseous system is therefore to be regarded as secondary in importance to the muscular, of which it may be considered a differentiation. Instead of regarding the muscles as adapted to the bones, the bones ought to be regarded as adapted to the muscles. Bones have no power either of originating or perpetuating motion. This begins and terminates in the muscles."

The bones are the passive organs of locomotion, in the movement of which muscular force is expended. In land animals, as a rule, the bones are harder and more elastic than in aquatic species. The cartilaginous and spongy bones of many fishes would be ill suited to bear the strains and shocks of terrestrial progression.

The velocity with which a limb may be moved will depend upon tho acuteness of the angles of its several bones. Hence the fleetness of many animals, in which the angles formed by the bones are acute. This is well shown in the skeleton of a deer, of which Fig. 3 is an ex

cellent illustration. Here we have not only the sharp angles, but leverlike adjustment of the several bones.

From these arises the power possessed by many animals to bound or leap enormous distances. The kangaroo has been known to leap twenty feet. The jerboa, when pursued, will pass over nine feet at a bound, and repeats so rapidly that a swift horse can scarcely overtake it. The greyhound and the hare will pass over sixteen feet at a stride. Animals of great weight and moderate speed have nearly straight limbs. Those of the deer are more angular than those of a horse, and those in the wing of a bird more angular than those of the fleetest quadruped.

FIG. 3.

SKELETON OF THE DEER (after Pander and d'Alton). The bones in the extremities of this, the fleetest of quadrupeds, are inclined very obliquely toward each other, and toward the scapular and iliac bones. This arrangement increases the leverage of the muscular system, and confers great rapidity on the moving parts. It augments elasticity, diminishes shock, and indirectly begets continuity of movement.

The forms of joints which predominate in the animal kingdom are the hinge and the ball and socket. The latter gives to the extremities their extraordinary range of motion, and a power of rotation so indispensable, as we will see, to the effectiveness of all the organs of locomotion.

It has been shown that a spiral configuration occurs in the bones and joints of the wing of the bat and the bird, and in the extremities of most quadrupeds. "The bones of animals are, as a rule, twisted levers, and act after the manner of screws." Thus it is that their traveling surfaces in progression may be turned at almost any angle, getting from the resisting media in which they move as much propelling power as possible, with a minimum of slip or waste.

It is because the traveling surfaces of animals are screws struct