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are almost identical on the two shores of the isthmus of Panama, and the great majority of the genera also, yet the species are almost wholly different.

Taking the enumeration of Gilbert and Starks, we find that out of 374 species, 43 are found apparently unchanged on both sides of the isthmus; 265 are represented on the Atlantic side by closely related species-in most cases the nearest known relative of the Pacific species-while 64 have no near analogue in the Atlantic. Of the latter group, some find their nearest relative to the northward or southward along the coast, and still others in the islands of Polynesia.

The almost unanimous opinion of recent students of the isthmus faunas finds expression in the following words of Gilbert and Starks ("Fishes of Panama Bay," p. 205):

"The ichthyological evidence is overwhelmingly in favor of a former open communication between the two oceans, which must have become closed at a period sufficiently remote from the present to have permitted the specific differentiation of a very large majority of the forms involved. That this differentiation progressed at widely varying rates in different instances, becomes at once apparent. A small minority (43) of the species (11 per cent of the species found on the Pacific side; about 2.5 of the combined fauna) remain wholly unchanged so far as we have been able to determine that point. larger number have become distinguished from their representatives of the opposite coast by minute, but not 'trivial' differences, which are wholly constant. From such representative forms we pass by imperceptible gradation to species much more widely separated, whose immediate relation in the past we cannot confidently affirm. . . .

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"It is obvious, however, that the striking resemblances between the two faunas are shown as well by slightly divergent as well as by identical species, and the evidence in favor of interoceanic connection is not weakened by an increase in the one list at the expense of the other. All evidence concurs in fixing the date of that connection at some time prior to the Pleistocene, probably in the early Miocene. When geological data shall be adequate definitely to determine that date, it will give us the best known measure of the rate of evolution in fishes."

From this discussion, it is probable that even in isolation some species change very slowly, that with similar conditions

the changes within isolated groups of a species may be parallel, and that the specific changes in different groups may progress with very different degrees of velocity.

The earliest known vertebrate remains are found in rocks of the Ordovician age, approximately of the epoch known as Trenton, at Cañon City, in Colorado. These remains consist of broken bits of bony shields of mailed fishes or fishlike forms known as Ostracophores. With these are fragments of scales, which seem to belong to more specialized forms. It is evident that these remains, as well as the remains of sharks which

[graphic]

FIG. 180.-An ostracoderm, Pterichyodes milleri, from the lower Devonian of Scotland. The jointed appendage on the head is not a limb. (After Traquair.)

appear later in the Upper Silurian, by no means reveal the actual first existence of vertebrates.

The sharks which appear in the Upper Silurian, although certainly primitive, even as compared with later sharks, are very far from the simplest even of known vertebrates. There seems to be good reason for the view that the vertebrate type of animal, with the nervous cord along the back and the alimentary canal marked by gill slits, was at first soft-bodied and wormlike, in fact, derived from a wormlike ancestry, and that, prior to the Ordovician and Silurian time, it was devoid of hard parts. The early sharks have teeth, and rough skin, fins, and sometimes fin spines, all susceptible of preservation in the rocks, even though the skeleton was soft and cartilaginous. The Ostracophores, some of which, at least, seem to be modified sharks, had no internal hard parts, but were protected by an external coat of mail, perhaps formed of coalescent prickles or scales.

From the sharks were doubtless descended the group of Fringe-fins or Crossopterygians, which were more distinctly fishlike. From these, on the one hand by continuous speciali

zation for aquatic life, the true fishes must have been derived. In the more primitive of these the air bladder retains the lunglike structure characteristic of the Fringe-fins. But in the more specialized forms this is reduced to a sac, at first with an open tube, then to a closed sac without tube in the adult, and finally in very many of the true fishes the air sac is altogether lost. On the other hand, in the Amphibia, which were probably also derived from the Crossopterygia, the air bladder is more highly specialized, fitting these animals for life outside

FIG. 181. The flying dragon (Draco). (After Seeley.)

the water, and the fins give place to fingers and toes as

befitting a terrestrial habit.

The amphibians deposit their eggs in damp places, and the young are hatched while the external gills are still functional.

Among the rep

tiles, which mark the next stage of adaptation for terrestrial life, the gills are absorbed before the animal leaves the egg. The reptile is therefore no longer confined to the neighborhood of the water for purposes of reproduction.

The bird, derived from the reptile, and at first distinguishable solely by the possession of feathers, loses later various reptilian traits and the group becomes one inhabiting the air.

From the reptiles again are derived the lowest mammals. The Monotremes of Australia lay eggs as reptiles do, these, like reptiles' eggs, being covered with a leathery skin. The higher mammals hatch the eggs within the body, nourish them with milk and, in general, care for them in a degree unknown within the class of reptiles. The traits of external hair, warm blood, double circulation of the blood from and to a two-chambered heart, and other characters of the mammals become fixed with time and the group diverges into a multitude of forms living and extinct, the last, and on the whole the most specialized of the series being Homo, the genus of man.

The first traces of man appear in the later geologic times after the end of the Tertiary. Human bones have been found in caves together with those of the cave-lion, cave-bear, and other extinct animals. In certain lakes in Switzerland and Austria have been found remains of peculiar dwellings, together with ancient fishing hooks and a variety of implements of stone and bronze. These houses were built on piles in the lakes, and connected with the shore by piers or bridges. The extinct race of men who lived in them is known as Lake-dwellers. Relics of man, especially rough stone tools and flint arrow and axe heads, and skulls and other bones,

[graphic]

FIG. 182.-Rough drawing of a mammoth on its own ivory, by a contemporary man. (After Le Conte.)

have been found under circumstances which indicate with certainty that man has existed long on the earth. But with these relics very few bones are found. This has been accounted for by supposing that man existed in a few wandering tribes scattered widely over Europe. In Java are found some ancient bones of manlike animals (Pithecanthropus), different, however, from any species or race of men living to-day, and showing traits which indicate a close relationship with the anthropoid apes.

The time of historic man-i. e., the period which has elapsed. since the history of man can be traced from carvings or buildings or writings made by himself-is short indeed compared with that of prehistoric man. Barbarous man writes no history and leaves no record save his tools and his bones. Iron and bronze rust, bones decay, wood disappears. Only stone im

plements remain to tell the tale of primitive humanity. These give no exact record of chronology.

So of the actual duration of man's prehistoric existence we can make no estimate. Speaking in terms of the earth's history, man is very recent, the latest of all the animals. In terms of the history of man, he is very ancient. The exact records of human history cover only the smallest fraction of the period of man's existence on earth.

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