Part 7

For the purposes of palaeontology, the most essential data of geology are those which indicate the relative ages of the strata that make up the hard outer crust of the earth, for only through them can the order of animal succession be ascertained. It does not matter exactly how old the earth may be. While it is possible to determine the approximate length of time required for the construction of sedimentary rocks like those which natural agencies are producing to-day, there are few definite facts to guide speculation as to the mode or duration of the process by which the first hard crystalline surface of the earth was formed. But palaeontology does not care so much about the earliest geological happenings, for it is concerned with the manifold animal forms that arose and evolved after life appeared on the globe. Questions as to the way life arose, and as to the earliest transformations of the materials by which the earth was first formed are not within the scope of organic evolution, although they relate to intensely interesting problems for the student of the process of cosmic evolution.

According to the account now generally accepted, the original material of the earth seems to have been a semi-solid or semi-fluid mass formed by the condensation of the still more fluid or even gaseous nebula out of which all the planets of the solar system have been formed and of which the sun is the still fiery core. As soon as the earth had cooled sufficiently its substances crystallized and wrinkled to form the first mountains and ridges; between and among these were the basins which soon filled with the condensing waters to become the earliest lakes and oceans. The wear and tear of rains and snows and winds so worked upon the surfaces of the higher regions that sediments of a finer or coarser character like sand and mud and gravel were washed down into the lower levels. These sediments were afterwards converted into the first rocks of the so-called stratified or sedimentary series, as contrasted with the crystalline or plutonic rocks like the original mass of the earth and the kinds forced to the surface by volcanic eruptions. Later the earth wrinkled again in various ways and places so that new ridges and mountains were formed with new systems of lakes and oceans and rivers; and again the elements continued to erode and partially destroy the higher masses and to lay down new and later series of sedimentary rocks upon the old.

It seems scarcely credible that the apparently weak forces of nature like those we have mentioned are sufficiently powerful to work over the massive crust of the earth as geology says they have. Our attention is caught, as a rule, only by the greater things, like the earthquakes at San Francisco and Valparaiso, and the tidal waves and cyclones of the South Seas; but the results of these sporadic and local cataclysms are far less than the effects of the persistent everyday forces of erosion, each one of which seems so small and futile. When we look at the Rocky Mountains with their high and rugged peaks, it seems almost impossible that rain and frost and snow could ever break them up and wear them down so that they would become like the rounded hills of the Appalachian Mountain chain, yet this is what will happen unless nature's ways suddenly change to something which they are not now. A visitor to the Grand Canon of the Colorado sees a magnificent chasm over a mile in depth and two hundred miles long which has actually been carved through layer after layer of solid rock by the rushing torrents of the river. Perhaps it is easier to estimate the geological effects of a river in such a case as Niagara. Here we find a deep gorge below the famous falls, which runs for twenty miles or so to open out into Lake Ontario. The water passing over the brim of the falls wears away the edge at a rate which varies somewhat according to the harder or softer consistency of the rocks, but which, since 1843, has averaged about 104 inches a year. Knowing this rate, the length of the gorge, and the character of the rocky walls already carved out, the length of time necessary for its production can be safely estimated. It is about 30,000 to 40,000 years, not a long period when the whole history of the earth is taken into account. A similar length of time is indicated for the recession of the Falls of St. Anthony, of the Mississippi River, an agreement that is of much interest, for it proves that the two rivers began to make their respective cuttings when the great ice-sheet receded to the north at the end of the Glacial epoch.

What has become of the masses washed away during the formation of these gorges? As gravel and mud and silt the detritus has been carried to the still waters of the lower levels, to be laid down and later solidified into sandstone and slate and shale. All over the continents these things are going on, and indefatigable forces are at work that slowly but surely shear from the surface almost immeasurable quantities of earth and rock to be transported far away. In some instances it is possible to find out just how much effect is produced in a given period of time, especially in the case of the great river systems. For example, the mass of the fine particles of mud and silt carried in a given quantity of the water of the Mississippi as it passes New Orleans can be accurately measured, and a satisfactory determination can also be made of the total amount of water carried by in a year. From these figures the amount of materials in suspension discharged into the Gulf of Mexico becomes known. It is sufficient to cover one square mile to the depth of 269 feet; in twenty years it is one cubic mile, or five cubic miles in a century. Turning now to the other aspect of this process, and the antecedent causes which produce these effects, it appears that the area of the Mississippi River basin is 1,147,000 square miles--about one third of the total area of the United States. Knowing this, and the annual waste from its surface, it is easy to demonstrate that it will take 6000 years to plane off an average of one foot of soil and rock from the whole of this immense area. Of course only an inch or a few inches will be taken from some regions where the ground is harder or rockier, or where little rain falls, while many feet will be washed away from other places. The waters of the Hoang-ho come from about 700,000 square miles of country, from which one foot of soil is washed away in 1464 years. The Ganges River, draining about 143,000 square miles, carries off a similar depth of eroded materials from its basin in 823 years! Should we add to the above figures those that specify the bulk of the chemical substances in solution carried by these waters, the total would be even greater. We know that in the case of the Thames River, calcareous substances to the amount of 10,000 tons a year are carried past London, and all this mineral has been dissolved by rain-water from the chalky cliffs and uplands of England, so that the land has become less by this amount. Thus we learn that vast alterations are being made in the structure of great continents by rain and rivers, as well as by glaciers and other geological agencies. And at the same time that old strata are undergoing destruction new ones are in process of construction at other places, where animal remains can be embedded and preserved as fossils. The forces at work seem weak, but they continue their operations through ages that are beyond our comprehension and they accomplish results of world-building magnitude.

Thus the whole process of geological construction is such that older exposed strata continually undergo disintegration, but this involves the destruction of any fossils that they might contain. The very forces that preserve the relics of extinct animals at one time undo their work at a later period. There are many other influences besides that destroy the regularity of rock layers or change their mineralogical characters by metamorphosis. It is easier to see how volcanic outbursts alter their neighboring territory. The intense subterranean heat and imprisoned steam melt the deeper substances of the earth's crust, so that these materials boil out, as it were, where the pressure is greatest, and where lines of fracture and lesser resistance can be found. Because so much detritus is annually added to the ocean floors--enough to raise the levels of the oceans by inches in a century--it is natural that greater pressures should be exerted in these areas than in the slowly thinning continental regions. These are some of the reasons why volcanoes arise almost invariably along the shores or from the floors of great ocean beds. The chain that extends from Alaska to Chili within the eastern shore of the Pacific Ocean, and the many hundreds of volcanoes of the Pacific Islands bring to the surface vast quantities of eruptive rocks which break up and overlie the sedimentary strata formed regularly in other ways and at other times. The volcanoes of the Java region alone have thrown out at least 100 cubic miles of lava, cinders, and ashes during the last 100 years--twenty times the bulk of the materials discharged into the Gulf of Mexico by the Mississippi River in the same period of time.

From these and similar facts, the naturalist finds how agencies of the present construct new rocks and alter the old; and so in the light of this knowledge, he proceeds with his task of analyzing the remote past, confident that the same natural forces have done the work of constructing the lower geological levels because these earlier products are similar to those being formed to-day. After learning this much, he must immediately undertake to arrange the strata according to their ages. This might seem a difficult or even an impossible task, but the rocks themselves provide him with sure guidance.

Wherever a river has graven its deep way through an area of hard rocks, as in the case of Niagara, the walls display on their cut surfaces a series of lines and planes showing that they are superimposed layers formed serially by deposits that have differed some or much at different times according to the circumstances controlling the erosion of their constituent particles. A layer of several feet in thickness may be composed of compact shale, while above it will be a zone of limestone, and again above this another layer of shale. Successive strata like these, where they are parallel and obviously undisturbed, are evidently arranged in the order of their formation and age. But by far the most impressive demonstration of the basic principle of geology employed for the determination of the relative ages of rocks is the mighty Canon of the Colorado. As the traveler stands on the winding rim of this vast chasm, his eye ranges across 13 miles of space to the opposite walls, which stretch for scores of miles to the right and left; upon this serried face he will see zone after zone of yellow and red and gray rock arranged with mathematical precision and level in the same order as on the steep slopes beneath him. Plain common sense tells him that the great sheets of rock stretched continuously at one time between the now separate walls, and that the various strata of sandstone and limestone were deposited in successive ages from below upwards in the order of their exposure. When now he extends his explorations to another state like Utah or Wyoming, he may find some but not all of the series exhibited in the Grand Canon, overlaid or underlaid by other strata which in their turn can be assigned to definite places in the sequence. By the same method, the geologist correlates and arranges the rocks not only of different parts of the same state, or of neighboring states, but even those of widely separated parts of North America and of different continents. But he learns that he must refrain from over-hasty conclusions, for he soon finds that the sedimentary rocks have not been constructed at the same rate in different places during one and the same epoch, and that rocks formed even at one period are not always identical in nature. But his guiding principle is sensible and reasonable, and by employing it with due caution he provides the palaeontologist with the requisite knowledge for his special task, which is to arrange the extinct animals whose remains are found as fossils of various earth ages in the order of their succession in time.

CONDENSED TABLE OF PALAEONTOLOGICAL FACTS

_________________________________________________________________________ | | | | YEARS | NUMBER OF | | | ORDER OF NECESSARY FOR | FEET IN | GEOLOGICAL | GEOLOGICAL | APPEARANCE OF FORMATION | THICKNESS | AGE | EPOCH | CHARACTERISTIC | | | | GROUPS ____________|_________|____________|_____________|______________ | | | | | | | | M B R A F I | | | | a i e m i n b | | | | m r p p s v r | | Recent | | m d t h h e a | | or | | a s i i e r t | | Quaternary | | l l b s t e | | | | s e i e s | | | | s a - ____________|_________|____________|_____________|||||||__ | | | | | | | | | | | | | Pleistocene | | | | | | | | | Cenozoic | Pliocene | | | | | | | 5,000,000 | 25,000 | or | Miocene | | | | | | | | | Tertiary | Oligocene | | | | | | | | | | Eocene | | | | | | | ____________|_________|____________|_____________|||||||__ | | | | | | | | | | | | Mesozoic | Cretaceous | | | | | | | 4,000,000 | 23,000 | or | Jurassic | | | | | | | | | Secondary | Triassic | | | | | | ____________|_________|____________|_____________|___||||___ | | | | | | | | | | | Permian | | | | | | | Palaeozoic | Carboniferous | | | | 21,000,000 | 106,000 | or | Devonian | | | | | Primary | Silurian | | | | | | Cambrian | | | ____________|_________|____________|_____________|______________ | | | | 20,000,000 | 30,000 | Azoic | Archaen | ____________|_________|____________|_____________|_______________

After what seems an unduly long preparation, we now come to the actual biological evidence of evolution provided by the results of this division of zooelogical science. But all of the foregoing is fundamentally part of this department of knowledge and it is absolutely essential for any one who desires to understand what the fossils themselves demonstrate.

The oldest sedimentary rocks are devoid of fossil remains and so they are called the Azoic or Archaean. They comprise about 30,000 feet of strata which seem to have required at least 20,000,000 years for their formation. This period is roughly two-fifths of the whole time necessary for the formation of all the sedimentary rocks, and this proportion holds true even if the entire period of years should be taken as 100,000,000 instead of 50,000,000 or less. The earth during this early age was slowly organizing in chemical and physical respects so that living matter could be and indeed was formed out of antecedent substances--but this process does not concern us here. The important fact is that the second major period, called the Palaeozoic, or "age of ancient animals," saw the evolution of the lowest members of the series,--the invertebrates,--and the most primitive of the backboned animals, like fishes and amphibia. The rocks of this long age include about 106,000 feet of strata, demanding some 21,000,000 or 22,000,000 years for their deposition. Thus it is proved that the invertebrate animals were succeeded in time by the higher vertebrates, which is exactly what the evidences of the previous categories have shown. When we remember that the lower animals are devoid as a rule of skeletal structures that might be fossilized, and when we recall the fact that the strata of the palaeozoic provided the materials out of which the upper layers were formed afterwards, we can understand why the ancient members of the invertebrate groups are not known as well as the later and higher forms like vertebrates. Yet all the fossils of these relatively unfamiliar creatures clearly prove that no complex animal appears upon a geological horizon until after some simple type belonging to a class from which it may have taken its origin; in brief, there are no anachronisms in the record, which always corresponds with the record written by comparative anatomy, wherever the facts enable a comparison to be made.

But the extinct animals of the third and fourth ages are more interesting to us, because there are more of them and because they are more like the well-known organisms of our present era. These two ages are called the Mesozoic or Secondary, and the Cenozoic or Tertiary. The former is so named because it was a transitional age of animals that are intermediate in a general way between the primitive forms of the preceding age and those of the next period; the latter name means the "recent-animal" age, when evolution produced not only the larger groups of our present animal series, but also many of the smaller branches of the genealogical tree like orders and families to which the species of to-day belong.

Confining our attention to the large vertebrate classes, the testimony of the rocks proves, as we have said, that fishes appeared first in what are called the Silurian and Devonian epochs, where they developed into a rich and varied array of types unequaled in modern times. At that period, they were the highest existing animals--the "lords of creation," as it were. To change the figure, their branch constituted the top of the animal tree of the time, but as other branches grew upwards to bear their twigs and leaves, as the counterparts of species, the species of the branch of fishes decreased in number and variety, as do the leaves of a lower part of a tree when higher limbs grow to overshadow them.

Following the fishes, the amphibia arose during the coal age or Carboniferous, usurping the proud position of the lower vertebrate class. The reptiles then appeared and gained ascendancy over the amphibia, to become in the Mesozoic age the highest and most varied of the existing vertebrates. At that time there were the great land dinosaurs with a length of 80 feet, like Brontosaurus; aquatic forms like Ichthyosaurus and Plesiosaurus, whose mode of evolution from terrestrial to swimming habits was like that of seals and penguins of far later eras. Flying reptiles also evolved, to set an example for the bats of the mammalian class, for both kinds of flying organisms converted their anterior limbs into wings, although in different ways.

During the Triassic and Jurassic periods of the Mesozoic age, the first birds and mammals appeared to follow out their diverging and independent lines of descent. Palaeontology makes it possible to trace the origin and development of many of the different branches that grew out of the mammalian limb from different places and at different times during the Mesozoic and the following age, called the Cenozoic, or age of recent animals. It is unnecessary, however, for us to review more of the details: the main result is obvious; namely, that the appearance of the great classes of vertebrates is in the order of comparative anatomy and embryology. Not only, then, is the fact of evolution rendered trebly sure, but the general order of events is thrice and independently demonstrated to be one and the same. Surely we must see that no reasonable explanation other than evolution can be given for these basic facts and principles.

Turning now to the second division of palaeontological evidence, we come to those groups where abundant materials make it possible to arrange the animals of successive epochs in series that may be remarkably complete. For the reasons specified, the backboned animals provide the richest arrays of these series, and such histories as those of horses and elephants have taken their places in zooelogical science as classics. But even among the invertebrates significant cases may be found. For example, in one restricted locality in Germany the shells of snails belonging to the genus Paludina have been found in superimposed strata in the order of their geological sequence. The ample material shows how the several species altered from age to age by the addition of knobs and ridges to the surface of the shell, until the fossils in the latest rocks are far different from their ancestors in the lowermost levels. Yet the intervening shells fill in the gaps in such a way as to show almost perfectly how the animals worked out their evolutionary history. This example illustrates the nature of many other known series of mollusks and of brachiopods, extending over longer intervals and connecting more widely separated ages like the Secondary and the present period.

Since the doctrine of evolution and its evidences began to occupy the thoughts of the intellectual world at large, no fossil forms have received more attention than the ancient members of the horse tribe. As we have learned, a modern horse is described by comparative anatomy as a one-toed descendant of remote five-toed ancestors. When the hoofed animals of modern times were reviewed as subjects for comparative anatomical study, the odd-toed forms arranged themselves in a series beginning with an animal like an elephant with the full number of five digits on each foot and ending at the opposite extreme with the horse. A reasonable interpretation of these facts was that the animals with fewer toes had evolved from ancestors with five digits, of which the outer ones had progressively disappeared during successive geological periods, while the middle one enlarged correspondingly. The facts provided by palaeontology sustain this contention with absolutely independent testimony. Disregarding some problematical five-toed forms like Phenacodus, the first type of undoubted relationship to modern horses is Hyracotherium, a little animal about three feet long that lived during the Eocene period of the Cenozoic epoch. Its forefeet had four toes each, and its hinder limbs ended with three toes armed with small hoofs, but one of its relatives of the same time has a vestige of another digit on the hind foot. By the geological time mentioned, therefore, the earliest true horses had already lost some of the toes that their progenitors possessed. In the Miocene the extinct species, obviously descended from the Eocene forms, had lost more of their toes; still higher, that is, in the rocks formed during succeeding periods of time, the animals of this division are much larger and each of their feet has only three toes, of which the middle one is the largest while the ones on the sides are small and withdrawn from the ground so as to appear as useless vestiges. To produce modern horses and zebras from these nearer ancestors, few additional changes in the structure of the feet are necessary, for the lateral toes need only to become a little more reduced and the middle one to enlarge slightly to give the one-toed limb of modern types, with its splint-like vestiges still in evidence to show that the ancestor's foot comprised more of these terminal elements. Comparing the animals of successive periods, these and other skeletal structures demonstrate that the ancestry of each group of species is to be found in the animals of the preceding epoch, and that the whole history of horses is one of natural transformation,--in a word, of evolution.

No less interesting in their own way are the remains of other hoofed forms that lead down to the elephants of to-day and to the mammoth and mastodon of relatively recent geologic times. Common sense would lead to the conclusion that a form like a modern tapir was the prototype from which these creatures have arisen, and common sense would lead us to expect that if any fossils of the ancestors of the modern group of elephants occurred at all they would be like tapirs. Thus a fossil of much significance in this connection is Moeritherium, whose remains have been found in the rocks exposed in the Libyan desert, for this creature was practically a tapir, while at the same time its characters of muzzle and tusk mark it as very close to the ancestors of the larger woolly elephants of later geological times, when the trunk had grown considerably and the tusks had become greatly prolonged. Again the fossil sequence confirms the conclusions of comparative anatomy, regarding the mode by which certain modern animals have evolved.

The fossil deer of North America, as well as many other even-toed members of the group of mammalia possessing hoofs, provide the same kind of conclusive evidence. The feature of particular interest in the case of their horns, is a correspondence between the fossil sequence and the order of events in the life-history of existing species,--that is, between the results of palaeontology and of embryology. Horns of the earliest known fossil deer have only two prongs; in the rocks above are remains of deer with additional prongs, and point after point is added as the ancient history of deer is traced upwards through the rocks to modern species. We know that the life-history of a modern species of animals reviews the ancestral record of the species, and what happens during the development of deer can be directly compared with the fossil series. It is a matter of common knowledge that the year-old stag has simple spikes as horns, and that these are shed to be replaced the following year by larger forked horns. Every year the horns are lost and new ones grow out, and become more and more elaborately branched as time goes on, thus giving a series of developmental stages that faithfully repeats the general order of fossil horns. Even Agassiz, who was a believer in special creation and an opponent of evolution, was constrained to point out many other instances, mainly among the invertebrata, where there was a like correspondence between the ontogeny of existing species and their phylogenetic history as revealed by the fossil remains of their ancestors.

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In the last place, we must give more than a passing consideration to some of the extinct types of animals that occupy the position of "links" between groups now widely separated by their divergence in evolution from the same ancestors. Perhaps the most famous example is Archaeopteryx found in a series of slates in Germany. This animal is at once a feathered, flying reptile, and a primitive bird with countless reptilian structures. Its short head possesses lizard-like jaws, all of which bear teeth; its wings comprise five clawed digits; its tail is composed of a long series of joints or vertebrae, bearing large feathers in pairs; its breastbone is flat and like a plate, thus resembling that of reptiles and differing markedly from the great keeled breastbone of modern flying birds, whose large muscles have necessitated the development of the keel for purposes of firm attachment. In brief, this animal was close to the point where reptiles and birds parted company in evolution, and although it was a primitive bird, it is in a true sense a "missing link" between reptiles and the group of modern birds. Other fossil forms like Hesperornis and Ichthyornis, whose remains occur in the strata of a later date, fill in the gap between Archaeopteryx and the birds at the present time, for among other things they possess teeth which indicate their origin from forms like Archaeopteryx, while in other respects they are far nearer the birds of later epochs. That these links are not unique is proved by numerous other examples known to science, such as those which connect amphibia and reptiles, ancient reptiles and primitive mammals, as well as those which come between the different orders of certain vertebrate classes.

In summarizing the foregoing facts, and the larger bodies of evidence that they exemplify, we learn how surely the testimony of the rocks establishes evolution in its own way, how it confirms the law of recapitulation demonstrated by comparative embryology, and how it proves that the greater and smaller divisions of animals have followed the identical order in their evolution that the comparative study of the present day animals has independently described.

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The facts of geographical distribution constitute the fifth division of zooelogy, and an independent class of evidences proving the occurrence of evolution. This department of zooelogy assumed its rightful status only after the other divisions had attained considerable growth. Many naturalists before Darwin and Wallace and Wagner had noticed that animals and plants were by no means evenly distributed over the surface of the globe, but until the doctrine of evolution cleared their vision they did not see the meaning of these facts. As in the case of all the other departments of zooelogy the immediate data themselves are familiar, but because they are so obvious the mind does not look for their interpretation but accepts the facts at their face value. While the phenomena of distribution are no less fascinating to the naturalist, and no less effective in their demonstration of evolution, their comprehensive treatment would demand more space than the whole purpose of the present description of organic evolution would justify. Thus a brief outline only can be given of the salient principles of this subject in order that their bearing upon the problem of species may be indicated.

Even as children we learn many facts of animal distribution; every one knows that lions occur in Africa and not in America, that tigers live in Asia and Malaysia, that the jaguar is an inhabitant of the Brazilian forests, and that the American puma or mountain lion spreads from north to south and from east to west throughout the American continents. The occurrence of differing human races in widely separated localities is no less familiar and striking, for the red man in America, the Zulu in Africa, the Mongol and Malay in their own territories, display the same discontinuity in distribution that is characteristic of all other groups of animals and of plants as well. As our sphere of knowledge increases, we are impressed more and more forcibly by the diversity and unequal extent of the ranges occupied by the members of every one of the varied divisions of the organic world. Another fact which becomes significant only when science calls our attention to it is the absence from a land like Australia of higher mammals such as the rabbit of Europe. The hypothesis of special creation cannot explain this absence on the assumption that the rabbit is unsuited to the conditions obtaining in the country named, for when the species was introduced into Australia by man, it developed and spread with marvelous rapidity and destructive effect. It may seem impossible that facts like these could possess an evolutionary significance, but they are actual examples of the great mass of data brought together by the naturalists who have seen in them something to be interpreted, and who have sought and found an explanation in the formularies of science.

The general principles of distribution appear with greatest clearness when an examination is made of the animals and plants of isolated regions like islands. The Galapagos Islands constitute a group that has figured largely in the literature of the subject, partly because Darwin himself was so impressed by what he found there in the course of his famous voyage around the world in the "Beagle." They form a cluster on the Equator about six hundred miles west of the nearest point of the neighboring coast of South America. Although the lizards and birds that live in the group differ somewhat among themselves as one passes from island to island, on the whole they are most like the species of the corresponding classes inhabiting South America. Why should this be so? On the hypothesis of special creation there is no reason why they should not be more like the species of Africa or Australia than like those of the nearest body of the mainland. The explanation given by evolution is clear, simple, and reasonable. It is that the characteristic island forms are the descendants of immigrants which in greatest probability would be wanderers from the neighboring continent and not from far distant lands. Reaching the isolated area in question the natural factors of evolution would lead their offspring of later generations to vary from the original parental types, and so the peculiar Galapagos species would come into being. The fact that the organisms living on the various islands of this group differ somewhat in lesser details adds further justification for the evolutionary interpretation, because it is not probable that all the islands would be populated at the same time by similar stragglers from the mainland. The first settlers in one place would send out colonies to others, where independent evolution would result in the appearance of minor differences peculiar to the single island. In this manner science interprets the general agreement between the animals of the Azores Islands and the fauna of the northwestern part of Africa, the nearest body of land, from which it would be most natural for the ancestors of the island fauna to come.

The land-snails inhabiting the various groups of islands scattered throughout the vast extent of the Pacific Ocean provide the richest and most ideal material for the demonstration of the principles of geographical distribution. In the Hawaiian Islands snails of the family of Achatinellidae occur in great abundance, and like the lizards of the Galapagos Islands different species occur on the different members of the group. Within the confines of one and the same island, they vary from valley to valley, and the correlation between their isolation in geographical respects and specific differences on the other hand, first pointed out by Gulick, makes this tribe of animals classical material. In Polynesia and Melanesia are found close relatives of the Achatinellidae, namely, the Partulae, which are thus in relative proximity to the Achatinellidae and not on the other side of the world. Furthermore, the Partulae are not alike in all of the groups of Polynesia where they occur; the species of the Society Islands are absolutely distinct from those of the Marquesas, Tonga, Samoan, and Solomon Islands, although they agree closely in the basic characters that justify their reference to a single genus. The geological evidence tells us that these islands were once the peaks of mountain ranges rising from a Pacific continent which has since subsided to such an extent that the mountain tops have become separate islands. Thus the resemblances between Hawaiian and Polynesian snails, and the closer similarities exhibited by the species of the various groups of Polynesia, are intelligible as the marks of a common ancestry in a widespread continental stock, while the observed differences show the extent of subsequent evolution along independent lines followed out after the isolation of the now separated islands. The principle may be worked out in even greater detail, for it appears that within the limits of one group diverse forms occupy different islands, evolved in different ways in their own neighborhoods; while in one and the same island, the populations of the different valleys show marked effects of divergence in later evolution, precisely as in the case of the classic Achatinellidae of the Hawaiian Islands.

The broad and consistent principle underlying these and related facts is this: there is a general correspondence between the differences displayed by the organisms of two regions and the degree of isolation or proximity of these two areas. Thus the disconnected but neighboring areas of the Galapagos Islands and South America support species that resemble each other closely, for the reasons given before; long isolated areas like Australia and its surroundings possess peculiar creatures like the egg-laying mammals, and all of the pouched animals or marsupials with only one or two exceptions like our own American opossum,--a correlation between a geological and geographical discontinuity on the one hand and a peculiarity on the other that reinforces our confidence in the faunal evolutionary interpretation of the facts of distribution.