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gill arches except the ventral ends of two or three of them which are transformed into the cartilages of the larynx.

Corresponding to the gill arches of fishes, there are several arterial arches which supply the gill filaments with blood. The utility of a series of such arches in the fishes is evident, since it allows the blood to be exposed to a larger amount of surface in the filaments of the several sets of gills. But why should we find essentially the same arrangement in the embryos of air-breathing vertebrates? It is a very remarkable fact that

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FIG. 200-Aortic arches in different classes of vertebrates: I, fish; II, a tailed amphibian; III, a lizard; IV, a bird. Vessels carrying impure blood shown in black; those carrying oxygenated blood shaded; those represented in mere outline are formed in the embryo but disappear in the adult. ad, dorsal aorta; ao, ao2, permanent arch of the aorta; ast, ventral aorta coming from the heart; a, b, first two temporary arches; dB, ductus Botalli; k, gill capillaries; pu, pulmonary artery. In I, representing one of the lung fishes, pu supplies the socalled lung. (After Hertwig.)

in the embryonic development of reptiles, birds, and mammals there are five complete aortic arches laid down very much as in the embryos of fishes, although no gills ever make their appearance. The arterial system, even of a human being, develops as if it were planned to supply blood to the gills of a fish. Afterward, the system is largely broken down, leaving parts only to form some of the permanent arterial vessels of the adult. In the mammals the left half of the fourth arch becomes the arch of the aorta. Parts of the anterior arches are utilized in the formation of the carotid arteries, but the rest of these arches disappear. The pulmonary arteries supplying the lungs are given off from the right half of the fifth pair, only the basal part of which persists. In the birds, which start out with much e same fishlike plan of aortic arches, it is the right half of the th arch which persists to form the permanent arch of the aorta.

The arterial systems of amphibians and reptiles present many transitional stages between the arterial system of fishes and that of birds and mammals. The lizard Lacerta, for instance, has two complete and symmetrical aortic arches, although it probably does not need them. Birds and mammals get along with half an arch, and the only reason why the lizard has so many is doubtless because it came from fishlike ancestors. The lizard Lacerta inherits two complete arches in the adult state just as we inherit five of them in our embryonic state.

It not infrequently happens that structures which, according to the theory of evolution, have long been lost, make a temporary appearance in the development of the embryo. Attention has already been called to the rudimentary teeth of foetal whales and calves. Whales are mainly devoid of hair, there being in some species only a few bristles remaining along the sides of the jaws, but there is sometimes a larger number of bristles that appear in early stages of development. The collar bone is absent in sheep, deer, and many other ruminants, but in the embryo sheep it is completely formed. The jaws of rodents show a space devoid of teeth between the incisors and the molars. Have the teeth in this region been lost? Here again, embryology throws light on the problem, since it has been found in some rodents that several of the missing teeth start to develop, but are resorbed before birth.

These and many other similar facts are quite unintelligible according to the theory of special creation. But they are just what we should expect according to the theory of evolution. Embryos are frequently very conservative in their habits. It takes them a very long time to get over doing things in the same old way after the need for so doing no longer exists.

F. THE EVIDENCE FROM PALEONTOLOGY

If the present inhabitants of the globe are the product of a long process of evolution we should expect to find some indications of the fact in the succession of fossil forms of life. The studies of the geologists have proven that the earth itself has

undergone a long evolution extending through many millions of years. At a relatively early period the surface of the earth became largely or wholly covered with water. As a result of upheavals of the surface, continents were raised out of the primitive shallow seas, and at various periods the crust was thrown into folds that gave rise to chains of mountains. With the elevation of land there began the processes of erosion and deposition of sediments in the bottom of the seas. In this manner stratified rocks were gradually built up whose total thickness has been estimated to be from fifty to one hundred miles. Upheavals occurring at different periods of the earth's history have exposed to view many of the rocky strata deposited in different geological eras. By studying the relations of strata in different regions geologists have been able to piece together a fairly well connected history of the earth's crust. They can determine what deposits are very old and what are relatively recent, and to what particular chapter in the world's history a given series of strata belongs.

It is in the rocky strata that we find the records of the history of life. As Agassiz has remarked, "The crust of our earth is a great cemetery where the rocks are the tombstones on which the buried dead have written their own epitaphs." Records of the existence of living beings have been left in various ways. Sometimes the bones or even other tissues may be preserved as such. Frequently fossil remains are known only from casts. An organism may become imbedded in mud or clay; after its tissues have decayed, the space it occupied becomes filled by the infiltration of foreign material, thereby forming a cast which takes the outline of the organism it replaces. Many forms simply leave imprints due to their settling down upon soft mud. Some of the rare traces of extinct jellyfish are preserved in this way in one of the very oldest fossil-bearing rocks. Sometimes the only records left by certain animals are the tracks which they made on the mud of some ancient shore. A great many organic remains consist of petrifactions. In these, the substances of the organic body are replaced by mineral matter which preserves a faithful

record not merely of external form, but often also of internal structure.

What do these various kinds of fossils teach us as to the evolution of life? The theory of evolution would naturally lead one to expect that the oldest rocks would contain the simplest organisms, and that there would be a gradual advance in the organization of fossil forms as we pass to more recent periods. Were we to find that the most highly developed creatures were in the oldest deposits, and that as new groups made their appearance they were represented by their most specialized members, the situation for the evolutionists would be, at least, awkward. It would indeed be surprising if the evidences from the sources considered in the preceding paragraphs should be contradicted by the testimony of the rocks. If the rocks contained a full record of all the species that have lived upon the earth their verdict would be crucial, but in considering the history of life as it is revealed to us by fossil forms we should bear in mind that the records are very incomplete. A great many organisms have soft bodies so that they cannot be preserved as fossils. Terrestrial plants and animals are preserved only under very exceptional conditions. Of the birds, mammals, and insects that are living around us, how many will be converted into fossils which may possibly be studied by some future paleontologist? Their bodies, if not eaten by some other animal, simply decay and disappear. In the whole Jurassic period, only two imperfect specimens of birds have been discovered. Doubtless numerous birds, and probably many kinds of birds existed during that period. These particular individuals probably happened to die upon mud flats at low tide, and became buried by a subsequent deposit of sediment as the tide returned. It is only through some peculiar combination of circumstances that such terrestrial forms are preserved in the first place, and it is only by other fortunate circumstances that the rocks containing the remains are exposed so that their contents may be studied. Large parts of the fossiliferous strata of the earth are hopelessly buried under the ocean. Over much of the land the

rocks are buried so deeply as to be practically inaccessible for study. Extensive areas of rocks which once bore fossils have subsequently been metamorphosed by heat so as completely to obliterate all traces of life. Again, the rocks belonging to older epochs have been largely eroded to form the materials for subsequent deposits. The paleontologist must gather his materials where he can and be thankful for what he finds; at best he can obtain but a very fragmentary picture of the vast procession of living forms which have peopled the globe at successive periods of its history.

The early critics of The Origin of Species laid great stress upon the absence of connecting links which, it was claimed, ought to be discovered among extinct species of plants and animals. This objection Darwin attempted to meet in advance in his chapter on "The Imperfection of the Geological Record." This record, as Darwin reminds us, is

a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of a slowly-changing language, more or less different in the successive chapters, may represent forms of life which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced.

We hear little today about the absence of connecting links as a difficulty for the evolutionist, the reason being that many of them have been found. Much additional knowledge of extinct forms has accumulated since Darwin wrote, and, while the history of life is exceedingly fragmentary, the new discoveries have served partly to fill in many of the gaps in the older records. The widest gap that remains is at the beginning. The Cambrian period, the first subdivision of the Paleozoic era, is the earliest in which fossils are abundantly preserved, but before this time. there were laid down deposits of enormous thickness which contain very few indications of organic remains. Most of the

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