therefore the greatest number of possible adjustments. The struggle is between fish and fish, not between fishes and hard conditions of life. No form is excluded from the competition. Cold, darkness, and foul water do not shut out competitors, nor does any evil influence sap the strength. The heat of the tropics does not make the water hot. It is never sultry nor laden with malaria. The influence of tropical heat on land animals is often to destroy vitality and check activity. It is not so in the sea.

From conditions otherwise favourable in arctic regions the majority of competitors are excluded by their inability to bear the cold. River life is life in isolation. To aquatic animals river life has the same limitations that island life has to the animals of the land. The oceanic islands are far behind the continents in the process of evolution. In like manner the rivers are ages behind the seas.

Therefore the influences which serve as a whole to intensify fish life, and tend to rid the fish of every character or structure it can not "use in its business," are most effective along the shores of the tropics. One phase of this is the reduction in numbers of vertebræ, or, more accurately, the increase of stress on each individual bone.

Conversely, as the causes of these changes are still in operation, we should find that in cold waters, deep waters, dark waters, fresh waters, inclosed waters, and in the waters of past geological epochs, the process would be less complete, the numbers of vertebræ would be larger, while the individual vertebræ remain smaller, less complete, and less perfectly ossified.

This, in a general way, is precisely what we do find in examining the skeletons of a large variety of fishes.

Another phase is the process of cephalization, the process by which the head becomes emphasized and the shoulder bones and other structures become connected with it or subordinated to it. Still another is the reduction and change of the swim bladder and its utter loss of the function of lung or breathing organ which it occupied in the ganoid ancestors of modern fishes.

Cephalization through competition.

Analogy of the tropical waters to cities of men.

The life of the tropics, so far as fishes are concerned, offers many analogies to the life of cities, viewed from the standpoint of human development. In the cities in general, the conditions. of individual existence for the man are most easy, but there also competition of life is most severe. The struggle for existence is not a struggle with the forces and conditions of Nature. It is not a struggle with wild beasts, unbroken forests, or stubborn soil, but a competition between man and man for the opportunity of living.

It is in the city where the influences which tend to modernization and concentration of the characters of the species go on most rapidly. It is adaptation or death to each individual in the city: every quality not directly useful tends to become lost or atrophied.

Conversely, it is in the "backwoods," the region farthest from human conflicts, where primitive customs, antiquated peculiarities, and useless traits are longest and most persistently retained. The life of the "backwoods" may be not less active or vigorous, but it will lack specialization. It is from the unused possibilities of the "backwoods" that the progress of the future comes. The high specialization of favoured regions unfits its subjects for life under changed conditions. The loss of muscular power is often one of the results of skeletal specialization.

The coral reef is the metropolis of the fish. The deep sea, the arctic sea, and the isolated rivers-these are the ichthyological backwoods.

An exception to the general rule in regard to the numbers of vertebræ is found in the case of the eel. Eels inhabit nearly all seas, and everyOrigin of eels. where they have many vertebræ. The eels of the tropics are at once more specialized and more degraded. They are better eels than those of northern regions, but, as the eel is a degraded type, they have gone further in the loss of structures in which this degeneration consists.

It is not well to push this analogy too far, but perhaps we can find in the comparison of the tropics and the cities some suggestion as to the development of the eel.

In the city there is always a class which follows in no degree the general line of development. Its members are specialized in a wholly different way. By this means they take to themselves a field which others have neglected, making up in low cunning what they lack in humanity or intelligence.

The

Thus, among the fishes, we have in the regions of closest competition this degenerate and non-fishlike type, lurking in holes among the rocks, or creeping in the sand; thieves and scavengers among fishes. eels thus fill a place otherwise left unfilled. In their way they are perfectly adapted to the lives they lead. A multiplicity of vertebral joints is useless to the typical fish, but to the eel, strength and suppleness are everything. No armature of fin or scale or bone is so desirable as its power of escaping through the smallest opening.

IX.

EVOLUTION OF FOSSIL CEPHALOPODA.

General evidence of paleontology.

BY JAMES Perrin Smith.

Most of the paleontologic contributions to the evidence of evolution were gathered before the time of Darwin, and are therefore all the more trustworthy because the naturalists that gathered them were not evolutionists. It is indeed remarkable that their classifications have been so little changed by the introduction of the theory of evolution into the study of biology. Since this is the case the paleontologic record ought to show the order of appearance of genera in time, and their genetic relationship. It does do this in a general way. Thus in the echinoderms we have the cystoids, apparently the primitive stock, beginning in the Cambrian and disappearing in the Carboniferous; the blastoids, somewhat higher, began in the Upper Silurian and disappeared in the Carboniferous; the true crinoids, or sea lilies, began in the Lower Silurian and survived until the present day. Asteroids we know from the Cambrian on, and echinoids, or sea urchins, from the Lower Silurian until now.

Although their succession in time suggests genetic relationships, the crinoids, and especially the cystoids, being the most primitive type, the first known of the three great groups are apparently as widely separated from each other as they now are. Either they are par

allel developments from a common stock, or else the separation from the oldest stock of crinoids took place in the misty pre-Cambrian time.

The vertebrates, too, show evolution in a general way in the appearance of types, for we find fishes from the Lower Silurian on; at first only placoderms, bonyplated fishes, then upward by degrees to the teleosts (scale fish with bony skeleton), which began in the Upper Jura. Amphibians had branched off from the fish stock by the end of the Devonian, genuine reptiles from the amphibians by the end of Carboniferous time. Mammals appeared first in the Trias, probably as descendants from the amphibian stock. The first birds we find in the Upper Jura as transitions from reptiles.

The method used by most naturalists in the study of phylogeny has been a comparison of a series of adults from successive geologic horizons, together with a study of present and past distribution and migration of animals. The results of this are seen in all our classification and in all family trees. Such work as Marsh's origin of the horse, Cope's phylogeny of the reptiles, Baur's contributions to the origin of the mammals, and many others, abundantly justify this method of research.

But interesting and valuable as are the investigations in phylogeny made in this way, such genealogies can not, as a matter of course, be more than approximate, for we have nowhere a uniform succession of rocks, and nowhere an unbroken genetic series. Each region has often changed, belonging now to one faunal province, now to another, each great change in faunal geography showing some physiographic revolution here or elsewhere. Thus the local series is broken and filled out from other regions, species usually being classed together because of mere resemblance, while their real relationship is unknown.