CHAPTER VIII.PROOFS FROM GEOGRAPHICAL DISTRIBUTION OF ORGANISMS.

Fig. 53.—Reptile-stage.

Now, it is a most remarkable fact that substantially these same stages, which are permanent conditions in the taxonomic series, are passed through as transient stages in the embryonic development of the human brain, and in the order given above. The very early condition of the human brain is represented inFig. 51. It is evidently nothing more than the intercranial continuation of the spinal cord, enlarged a little into three swellings or ganglia. These are the early representatives of the medulla, the optic lobes, and the thalamus; which last may be regarded as the basal and most fundamental part of the cerebrum. This stage may be regarded as lower than that of the ordinary fish. I have called it, therefore, thesub-fish stage. The cerebellum is a subsequent outgrowth from the medulla, as is the cerebrum and olfactivelobes from the thalamus.Fig. 52may be said, therefore, to represent fairly the fish-stage. Henceforward the principal growth is in the cerebrum and cerebellum, both of which are subsequent outgrowths of the original simple ganglia, the medulla, and the thalamus. The cerebrum especially increases steadily in relative size, first becoming larger than but not covering the optic lobes (Fig. 53). This represents the reptilian stage. Next, by further growth, it covers partly the optic lobes (Fig. 54). This may be called the bird-stage. Then itcovers wholly the optic lobes, and encroaches on the cerebellum behind and olfactive lobes in front (Fig. 55). This is the mammalian stage. Finally, it covers and overhangs all, and thus assumes the human stage (Fig. 56).

Fig. 54.—Bird-stage.of, olfactive lobe;cr, cerebrum;th, thalamus;ol, optic lobe;cb, cerebellum;m, medulla.

Fig. 55.—Mammalian stage.

Fig. 56.—Human stage.

We have spoken thus far only of relativesize; but progressive changes take place also in complexity of structure—i. e., in the depth and number of convolutions of the cerebrum and cerebellum. The cerebrums of fish, of reptile, bird, and lower mammals are smooth. About the middle of the mammalian series it begins to be convoluted. These convolutions become deeper and more numerous as we go upward in the scale, until they reach the highest degree in the human brain. The object of these inequalities is to increase the surface of gray matter—i. e., the extent of the force-generating as comparedwith the force-transmitting part of the brain, or battery as compared with conducting-wire. Now, in embryonic development the human brain passes also through these stages of increasing complexity of organization. Here also the ontogenic is similar to the taxonomic series.

Now, why should this peculiar order be observed in the building of the individual brain? We find the answer, the only conceivable scientific answer to this question, in the fact thatthis is the order of the building of the vertebrate brain by evolutionthroughout geological history. We have already seen that fishes were the only vertebrates living in the Devonian times. The first form of brain, therefore, was that characteristic of that class. Then reptiles were introduced; then birds and marsupials; then true mammals; and, lastly, man. The different styles of brains characteristic of these classes were, therefore, successively made by evolution from earlier and simpler forms. In phylogeny this order was observed because these successive forms were necessary for perfect adaptation to the environment at each step. In taxonomy we find the same order, because, as already explained (page 11), every stage of advance in phylogeny is still represented in existing forms. In ontogeny we have still the same order, because ancestral characteristics are inherited, and family history recapitulated in the individual history.

Fig. 57.—A, brain of extinct Ichthyornis;B, modern tern.

Fig. 58.—A, brain of Eocene dinoceras;B, Miocene brontothere;C, modern horse.

But not only is this order found in the evolution of the whole vertebrate department, but something ofthe same kind is found also in the evolution ofeach class. The earliest reptiles, the earliest birds, and the earliest mammals had smaller and less perfectly organized brains than their nearest congeners of the present day. This is shown in the accompanying figures (Figs. 57and58). To carry out one example more perfectly: In the history of the horse family, in connection with the changes of skeletal structure already described (page 108), we have also corresponding changes in the size and structure of the brain;pari passuwith the improvement of the mechanism we have also increased engine-power and increased muscular energy and therefore increased activity and grace. The brain of a modern horse, though not very large, is remarkable for the complexity of its convolutions. The great energy, activity, and nervous excitability of the horse are the result of this structure.

Cephalization.—Thus, in going up the phylogenic, the taxonomic, or the ontogenic series, we find a gradual process of development headward, brainward, cerebrumward; or, more generally, we might say that in all organic evolution we find an increasing dominance of the higher over the lower, and of the highest over all. For example, in the lowest plane of either series we find first the different systems imperfectly or not at all differentiated. Then, as differentiation of these progress, we find an increased dominance of the highest system—thenervous system; then in the nervous system, the increasing dominance of its highest part—thebrain; then in the brain the increasing dominance of its highest ganglion—thecerebrum; and, lastly, in the cerebrum the increasing dominance of its highest substance—the exterior gray matter—as shown by the increasing number and depth of the convolutions. This whole process may be calledcephalization.

Shall the process stop here? When evolution is transferred from the animal to the human plane, from the physiological to the psychical, from the involuntary and necessary to the voluntary and free, shall not the same law hold good? Yes! all social evolution, all culture, all education, whether of the race or the individual, must follow the same law. Allpsychical advance is a cephalization—i. e., an increasing dominance of the higher over the lower and of the highest over all; of the mind over the body, and in the mind of the higher faculties over the lower; and, finally, thesubordination of the whole to the highest moral purpose.

Fig. 59.—Homocercal tail-fin.A, form;B, structure.

Fig. 60.—Heterocercal or vertebrated tail-fin.A, form;B, structure.

4. Fish-Tails.—Still another and last example: It has long been noticed that there are among fishes two styles of tail-fins. These are the even-lobed, or homocercal (Fig. 59), and the uneven-lobed, or heterocercal (Fig. 60). The one is characteristic of ordinary fishes (teleosts), the other of sharks and some other orders. Instructurethe difference is even more fundamental than inform. In the former style the backbone stops abruptly in a series of short, enlarged joints, and thence sends off rays to form the tail-fin(Fig. 59,B); in the latter the backbone runs through the fin to its very point, growing slenderer by degrees, and giving off rays above and below from each joint, but the rays on the lower side are much longer (Fig. 60,B). This style of fin is, therefore,vertebrated, the othernon-vertebrated.Figs. 59and60show these two styles in form and structure. But there is still another style found only in the lowest and most generalized forms of fishes. In these the tail-fin is vertebrated and yet symmetrical. This style is shown inFig. 61,AandB.

Fig. 61.—Vertebrated but symmetrical fin.A, form;B, structure.

Now, in the development of a teleost fish (Fig. 58), as has been shown by AlexanderAgassiz,24the tail-fin is first likeFig. 61; then becomes heterocercal, likeFig. 60; and, finally, becomes homocercal likeFig. 59. Why so? Not because there is any special advantage in this succession of forms; for the changes take place either in the egg or else in very early embryonic states. The answer is found in the fact thatthis is the order of changein the phylogenic series. The earliest fish-tails were either likeFig. 61orFig. 60; never likeFig. 59. The earliest of all were almost certainly likeFig. 61; then they became likeFig. 60; and, finally, only much later in geological history (Jurassic or Cretaceous), they became likeFig. 59. This order of change is still retained in the embryonic development of the last introduced and most specialized order of existing fishes. The family history is repeated in the individual history.

Fig. 62.—Tail of the Archæopteryx.

Fig. 63.—Tail of a modern bird.

Similar changes have taken place in the form and structure of birds’ tails. The earliest bird known—the Jurassic Archæopteryx—had a long reptilian tail of twenty-one joints, each joint bearing a feather on each side, right and left (Fig. 62). In the typical modern bird, on the contrary, the tail-joints are diminished in number, shortened up, and enlarged, and give out long feathers, fan-like, to form the so-called tail (Fig. 63). The Archæopteryx’ tail isvertebrated, the typical bird’snon-vertebrated. This shortening up of the tail did not take place at once, but gradually. The Cretaceous birds, intermediate in time, had tails intermediate in structure. The Hesperornis of Marsh had twelve joints. Atfirst—in Jurassic—the tail is fully a half of the whole vertebral column. It then gradually shortens up until it becomes the aborted organ of typical modern birds. Now, in embryonic development, the tail of the modern typical birdpasses through all these stages. At first the tail is nearly one half the whole vertebral column; then, as development goes on, while the rest of the body grows, the growth of the tail stops, and thus finally becomes the aborted organ we now find. The ontogeny still passes through the stages of the phylogeny. The same is true of all tailless animals. The frog is tailed in the larval condition, because its ancestors were tailed amphibians. Even man himself is endowed with a much more considerable tail, viz., eight or nine joints, in his early embryoniccondition.25

We have taken all our examples from vertebrates, but quite as many and as good examples might be found among articulates. Insects, in the larval state, are worm-like in form. Hence it is probable that the earliest progenitors of this class were worm-like. Again, some insects have aquatic larvæ. The progenitors ofthese—in fact, of all insects—were probably aquatic. Crabs, in a larval condition, are long-tailed, and we know that the long-tailed crustaceans (Macrourans) preceded the short-tailed (Brachyourans). Water-breathing animals preceded air-breathers; the same is true in the ontogeny of the frog, of many insects, and, we might add, even of mammals. For the breathing of thefœtus in uterois essentially by exposure of fœtal blood to the oxygenated blood of the mother in a sort ofgill-fringes(placental tufts). But why should we multiply examples? The whole of embryology, in every department, is made up of examples of the same law.

Illustration of the Differentiation of the Whole Animal Kingdom.—Finally, the law of differentiation in the evolution of the whole animal kingdom may be well illustrated by means of the different directions taken in the development of the eggs of all the various kinds of animals. Suppose, then, we have one thousand eggs, representing all the different departments, classes, orders, families, etc., of animals. Many of these may doubtless be identified by form or size, or some other superficial character, as the eggs of this or that animal,but structurally they are all alike. At first, i. e., as germ-cells, they all represent theearliest conditionof life on the earth, and thelowest formsof lifenow. If we now watch their development, we find that some remain in this first condition without further change. These we set aside. They areProtozoa. The remainder continue to develop, but at first it would be impossibleto say to which of the several departments or primary groups they each belonged. Then, by cell-multiplication, the original single cell becomes a cell-aggregate. It may be compared now to a compound protozoan, such as Foraminifera. The cell-aggregate then differentiates into layers, and forms, in fact, a two-layered sac called a gastrula. This is the structure of some of the lowest cœlenterates, such as the hydra. Thus far all seem to go together. But now, for the first time, the primary groups are declared. If it be a vertebrate, for example, the most fundamental characters—the cerebro-spinal axis, the vertebral column, and the double cavity, neural and visceral, are outlined. Suppose, now, we set aside all other departments, and fix our attention on the vertebrates. At first we could not tell which were mammals, birds, reptiles, or fishes; but after a while the classes are declared. We now set aside all other classes and watch the mammals. After a while the order declares itself. We select the ungulates. Then the family is declared, say theEquidæ; then the genus,Equus; and, lastly, the species,Caballus.26

The same would be true if we followed any other line of development, whether in vertebrates or in any other department. Observe, then, that, in following any one line as we have done, there is an increasing specialization,and, if we followed all the lines, an increasing differentiation, like the branching and rebranching of a tree. Now, this is the type and illustration of what took place in the development of the animal kingdom. We conclude that the animal kingdom appeared first as Protozoa, then as living cell-aggregates or compound protozoans, then as gastrula or two-layered sacs with oral opening. Then the great primary departments, unless we except the vertebrates, commenced to separate. This took place before the primordial period; for in the primordial fauna we have all the departments, except vertebrates, already declared. This completely explains why it is that we are able to trace homology only within the limits of each primary group.

But the question has doubtless already occurred to the thoughtful reader, “Why should the steps of the phylogeny be repeated in the ontogeny?” The general answer is doubtless to be found in the law of heredity—that wonderful law, so characteristic of living things. We have compared it to a brief recapitulation from memory—the minor points, especially if they be also early, dropping out. But can we not explain it further? It is probable that we find a more special explanation in “the law of acceleration,” first brought forward by Prof. Cope. By the law of heredity each generation repeats the form and structure of the previous, and in the order in which they successively appeared. But there is a tendency for each successively-appearing character to appear a little earlier in each successive generation; andby this means time is left over for the introduction of still highernewcharacters. Thus, characters which were once adult are pushed back to the young, and then still back to the embryo, and thus place and time are made for each generation to push on still higher. The law of acceleration is a sort of young-Americanism in the animal kingdom. If our boys acquire knowledge and character similar to that of adults of a few generations back, they will have time while still young and plastic to press forward to still higher planes.

Proofs from Rudimentary and Useless Organs.—These have to a large extent been anticipated under previous heads. The tails of birds and the gill-arches of reptiles are rudimentary. The finger-bones of a whale’s paddle or a turtle’s flipper may be regarded as useless, at least so far as the exact number of constituent pieces is concerned; for an extended surface, without visible joints or separate fingers, is all that is seen, and apparently all that is required. The splint-bones of a horse’s foot or the dew-claws of a dog’s foot are certainly useless. We have already, in speaking of modifications of structure and of embryonic conditions, given many examples of this kind, but it may be well to add some striking examples with this special point in view.

If different orders of existing mammals were indeed made by gradual modification of some generalized primal form, then it is evident that these useless remnants of once useful parts would be most common in the most highly modified forms. Now, of all mammals, thewhales are perhaps the most modified or changed from the original mammalian form—so much modified, in fact, that the popular eye scarcely recognizes them as mammals at all. Here, then, we might expect, and do indeed find, many examples:

1. The baleen whales have no teeth, and no use for them. They have instead a wonderful armature of fringed whalebone plates (baleen), by means of which they gather theirfood.27Yet the embryo of the whale has a full set of rudimentary teeth deeply buried in the jawbone, and formed in the usual way characteristic of mammalian teeth—i. e., by an infolding of the epithelial surface of the gum—but the teeth are never cut; in fact, they reach their highest development in mid-embryonic life, and are again absorbed. Why, then, this waste of developmental energy? Why should teeth be formed only to be reabsorbed without being cut? The only conceivable answer is, because the ancestors of the whale, before the family of whales was fairly established, had teeth which were gradually, from generation to generation, aborted, because no longer used, the baleen plates having taken their place. If whales were made at once out of hand as we now see them, is it conceivable that these useless teeth would have been given them?

2. Again, many whales have rudimentary pelvic bones, but no hind-limbs. Why should there be pelvic bones,when the sole object of these bones is to act as a basis for hind-limbs? In some whales, for example the right whale, there are also rudiments of hind-legs, but these are buried beneath the skin and flesh, and therefore, of course, wholly useless. The only explanation of these facts is that the ancestors of all the whales before they had become whales were quadrupeds, which afterward took to the water, and little by little the hind-legs, for want of use, dwindled away to the useless remnants which we now find.

3. Again, whales seem to be hairless, yet rudimentary hairs are found in the skin. Their organs of smell are rudimentary, but made on the pattern of those of mammals, not of fishes—i. e., they are air-smelling, not water-smelling organs. From all these, as well as many other facts, it is evident that the whales descended in early Tertiary times from some marsh-loving, powerful-tailed, short-legged, scant-haired quadruped by modifications gradually induced by increasing aquatic habits.

Examples of such rudimentary organs might be multiplied without limit. As might be expected, some are found even in man. Such, for example, are the muscles for moving the ear, necessary in animals but useless in man, and therefore rudimentary. Similarly useless in man are the scalp-muscle, used by animals to erect the crest or bristles on the head, and the skin-muscle of the neck and chest, used by animals for shaking the skin of those parts. Most persons have lost the power of using these. For my part I can use them all—ear-muscles,scalp-muscle, skin-muscle—but they serve no useful purpose.

Again, and finally, in man and many mammals we find a slender, worm-like appendage about three inches long, attached to the cæcum of the large intestine. Anatomists and physiologists, under the influence of that philosophy which maintains that every part of the fearfully and wonderfully made human frame wasdirectlycontrived to subserve some useful purpose, have puzzled themselves to find the use of this. It probably has no use; on the contrary, it is a continual source of danger. If the human body had been made at once out of hand, it would not have been there. How came it, then? It is the rudimentary remnant of an organ—a greatly enlarged cæcum—which has served, and in some mammals still serves, a useful purpose. All these cases are survivals; they are organs which, like many customs in society, have outlived their usefulness, but still continue by heredity.

But why multiply examples? All along the track of evolution organs become useless by changes in the habits of their possessors. They are not, however, shed or dropped bodily at once. No; they areretained by heredity, butdwindle by disuse, more and more, until they pass away entirely. But even when they are entirely gone in the adult, they are often found still lingering in the embryo. They are among the most obvious and convincing proofs of the origin of organic forms by derivation.

It is well known that the kinds of organisms found in widely-separated countries differ more or less conspicuously. The traveler in Australia or in Africa finds all, the traveler in Europe nearly all, the animals and plants wholly different from those he has been accustomed to see at home. Even the visitor from the Atlantic to the Pacific coast, if he observes at all, will find nearly all organisms strange to him. The facts of geographical diversity of organisms are so numerous and complex that, at first sight, they seem utterly lawless. Only recently this subject has been redeemed from chaos and reduced to something like order and law by the light thrown upon it by the theory of evolution. We will give, in very brief outline, the most important facts, and then show how they may be explained.

Geographical Faunas and Floras.—The group of animals and plants inhabiting any locality, whether peculiar to that locality or not, is called, in popular language,its fauna and flora. But, in a true scientific sense, a fauna and flora is anaturalgroup of animals and plants in one place,differingmore or less conspicuously from other groups in other places, andseparated from them by physico-geographical boundaries, or by physical conditions of some kind.The members of such a group can only exist in certain harmonic relations with external conditions, and with one another. These relations with one another are often complex and nicely adjusted, so that change in one term is propagated through the whole series of terms, giving rise often to the most unexpected results, until finally a new equilibrium is established. Thus, the destruction of certain insectivorous birds, in mere wanton sport, may give rise to the multiplication of insect pests, and this to the destruction of certain kinds of plants, and this to the diminution of certain herbivores, and this in its turn to the disappearance of certain carnivores. It is well known that the introduction of rabbits into New Zealand and Australia has produced the most unexpectedly disastrous effect upon certain crops, on account of the absence of the fierce and active carnivores which keep in check their excessive multiplication in Europe.

Now, among the physical conditions which limit faunas and floras, and separate them from each other, the most important and universal is temperature.

Temperature-Regions.—If we travel from equator to pole, we pass through mean temperatures varying from80° to 0°. This gives rise to a very regular zonal arrangement of plant-forms: 1. We have first a region in which palms and palm-like forms are abundant and characteristic, and which therefore may be called the region of palms. It corresponds with the tropic zone. 2. We next have a region in which hard-wood foliferous trees are most abundant and characteristic; first mostly evergreens and then deciduous trees, and therefore may be called the region of hard-wood forests. This corresponds with the temperate-zone. 3. Then we find a region characterized predominantly by pines and pine-like trees and birches, and may be called the region of pines. This is the sub-Arctic region. 4. Then a region without trees, but only shrubs and summer plants. This is the Arctic region. 5. And, finally, an almost wholly plantless region of perpetual ice—the polar region.

These regions are determined wholly by temperature, and therefore, in going up a mountain-slope to snowy summits, we pass through similar regions in smaller space. For example, in going from sea-level to the summits of the Sierra, 14,000 to 15,000 feet high, we commence in a region of predominantly hard-wood trees; but at 3,000 feet the forests become almost wholly coniferous, at 11,000 to 12,000 feet the vegetation becomes shrubby, and at 13,000 feet we reach perpetual snow.

We have taken plants first, because these, being fixed to the soil and incapable of voluntary seasonalmigrations, are more strictly and simply limited by temperature—i. e., the arrangement of different kinds in zones is more simple and conspicuous. But the same rule holds also for animals. In passing from equator to pole, animal kinds also change frequently, so that there are many temperature-faunas in which the animals are all very different. In both animals and plants, species, genera, families, etc., are limited by temperature. These are familiar facts; we recall them to the reader in order that we may base thereon a clearer definition of these limits.

More Perfect Definition of Regions.—1. The area over which any form spreads is called itsrange. Now, the range of a species is more restricted than that of a genus, because, when a species is limited by temperature, another species of the same genus may carry on the genus. For the same reason the range of a family is usually greater than that of a genus, and so on for higher classification-groups. For example, pines range on the slopes of the Sierra from about 2,000 feet to 11,000 feet, but not the same species. In ascending, we meet first the nut-pine (Pinus Sabiniana), then the yellow-pine (P. ponderosa), then the sugar-pine (P. Lambertiana), then the tamarack-pine (P. contorta), and last, thePinus flexilis, etc.

Fig. 64.

2. Where two contiguous temperature-regions come in contact, there is no sharp line between; on the contrary, theyshade gradually, almost imperceptibly, into one another, the ranges of species overlapping and interpenetrating,and the two species coexisting on the borders of their ranges. This is represented by the diagram (Fig. 64), in which the horizontal lines represent the north and south ranges of species of two groups,AandB, separated by the dotted line.

3. Species also pass out gradually on the borders of these ranges and others come in gradually, sofar as number and vigor of individuals are concerned. Ifa a′andb b′(Fig. 65) represent the north and south range of two species, andb a′their overlap or area of coexistence, then the height of the curvesAandBwill represent the number and vigor of the individuals in different parts of the range.

Fig. 65.

4. While, therefore, there is a shading of contiguous groups into each other by overlap of species-ranges; while there is also a gradual passing out of species so far as number and vigor of individuals is concerned, yet, inspecific characterswe observe usually no such gradation. Species seem to come in on one border with all their specific characters perfect, remain substantially unchanged throughout their range, and pass out on the other border, still the same species. In other words, one species takesthe place of another, usually bysubstitution, not bytransmutation. It isas ifspecies had originated, no matter how, each in its own region, and had spread in all directions as far as physical conditions and struggle with other species would allow. This important subject will be more fully discussed later.

5. We have thus far spoken of species as limited by temperature alone, but they are limited also bybarriers. If, then, there be an east and west barrier, such as a high mountain-range, or a wide sea or desert, there will be no shading or gradation of any kind, because the barrier prevents overlapping, interpenetration, and struggle on the margins. For example: The species north and south of the Himalayas, or north and south of Sahara, are widely different. It is, again,as ifthey originated each where we find them and spread as far as they could, but the physical barrier prevented mingling and shading.

6. There are temperature-regions south as well as north of the equator. Now, although the climatic conditions are quite similar, the species of corresponding temperature-regions north and south are wholly different. It is, again, as if they originated where we find them, and were kept separate by the barrier of tropical heat between. If carried over, they often do perfectly well.

If the land-surfaces were continuous all around the globe, there is little doubt that each temperature region with its characteristic species would also be substantiallycontinuous. There would, it is true, be some local variations dependent upon soil and humidity, etc., but substantially the same species would exist all around. The distribution would be almost wholly zonal. But the intervening oceans are complete barriers to continental species. Hence we ought to expect, and do find, that the faunas and floras of different continents are almost totally different. Each apparently originated on its own continent, and did not spread to other continents, only because they could not get there. It is necessary to explain this in more detail.

Fig. 66.—Polar projection of the earth. 1, tropical; 2, temperate; 3, sub-arctic; 4, arctic; 5, polar regions.

Fig. 66represents a polar view of the earth, showing the eastern and western continents, and the five temperature zones already described. Now, if we examine the species in each region, commencing at the pole, we find that those of Nos. 5 and 4 are almost identical all around. The reason is obvious. The continents come close together there, with ice-connection if not land-connection all around.There is but one circumpolar region. But, as soon as we come down to No. 3 and No. 2, the species on the two continents are nearly all different, because there is an impassable barrier between, either in the form of ocean or of Arctic cold. For example, the animals and plants inhabiting the United States are almost wholly different from those in Europe, not only in species, but even largely in genera and to some extent in families. There are some exceptions to this rule, but these are of the kind which prove the rule, or rather the principle on which the rule is founded. These exceptions are mainly of three kinds: 1.Introduced species.—All our weeds, many garden-plants, and many animal pests are of this kind. They were not found here when America was discovered, only because they could not get here; for, when brought here, they do so well that they often overrun the country and dispossess the native species, as we ourselves have done the Indians. 2.Hardy or else wide-migrating species.—Hardy species have wide range; they may belong to No. 4 as well as No. 3. If so, they range down to No. 3 on both continents. Migrating birds, such as ducks and geese, etc., breed in summer in No. 4, and migrate southward in winter on both continents from the common circumpolar ground. 3.Alpine species.—It is a curious fact that species on tops of snowy mountains in temperate regions of the two continents are wonderfully similar, though so completely isolated. We are not yet prepared to discuss this point. We shall do so later. Suffice it to say now that it can be completely explained.

In region No. 1 the continental diversity is still greater. Not only species and genera, but whole families and even orders, are peculiar to each continent. The great pachyderms—elephant, rhinoceros, hippopotamus—are peculiar to the Eastern; the edentates—sloths and armadillos—to the Western. The humming-birds, those gems of the forests, of which there are over four hundred species, and the whole cactus family, are peculiar to America, while the tailless monkeys are equally characteristic of the Eastern Continent.

The continents do not come together again toward the south, and, therefore, as might be expected, the great difference between the two persists to the southern points. The faunas of the southern points of South America, Africa, and Australia are very different.

Subdivisions of Continental Faunas and Floras.—Besides the subdivisions of continental faunas, north and south, determined by temperature as already explained, if there be in any continent an impassable barrier running north and south, there will be a corresponding difference in the species on the two sides, east and west. We give but one example: The North American Cordilleras or Rocky Mountains, with their high ranges and desert plains, constitute a very great barrier between the eastern and western portions of the United States. Hence, we find an extraordinary difference between the species inhabiting California and those found in the eastern portion of the country. Speaking generally, all the species and many of the genera are peculiar. The exceptions,too, are significant. Leaving out introduced species, of which there are many, they are mostly strong-winged or widely-migrating birds, such as the turtle-dove, the turkey-buzzard, the bald eagle, and, of course, many water-birds.

Special Cases.—If any body of land is widely separated from all other lands by deep seas, we invariably find a corresponding peculiarity of its species. Thus, the species inhabiting Australia and Madagascar are perhaps the most peculiar in the world. We do not dwell further on these, because we will discuss them hereafter. There is a little group of very small islands—the Galapagos—about six hundred miles off the western coast of South America, and surrounded on all sides by deep sea. These islands are stocked with a collection of curious animals not found elsewhere on the surface of the earth; but among them are no mammals at all. We might multiply examples without limit. Even the rivers emptying in the same sea sometimes have each its peculiar species of mussels. In the Altamaha River there are several species of unios—such, for instance, as the wonderful spinous unio—not found elsewhere. How came they there? Howsoever they may have come there, they are now kept isolated there by barriers of land and of salt water.

Many other curious details will come up in our discussion of the origin of diversity.

Marine Species.—Precisely the same principles apply here; but diversity in the case of marine species isperhaps less marked, and certainly less general, because of the universal oceanic connection. Open-sea species are therefore almost universal. But many marine species are confined to shallow water, and therefore to shore-lines. The species on the two shores of the same ocean, or the two coasts of the same continent, are different, being isolated east and west by barriers of deep sea or of land, and north and south by temperature. Also about isolated lands, like Australia and Madagascar, the species are peculiar.

Thus, then, species, genera, etc., are limited in every direction; north and south by temperature, and in all directions by barriers, in the form of oceans, deserts, and mountain-chains. Add to these, peculiar climates and soils, and we see that, from this point of view, the whole surface of the earth may be divided and sub-divided into regions, sub-regions, provinces, etc. It would carry us too far to explain the primary and secondary divisions adopted by Mr. Wallace, and the somewhat different ones suggested by Mr. Allen. Our main object is to discuss thecauseof this diversity, and especially to show the light shed upon it by the theory of evolution. We have only given a sketch of the facts sufficient for this purpose.

It will be observed that all along we have assumed a sort of provisional theory. We have said in every case, it isas iforganic forms originated where we findthem, and have gone thence wherever they could—as far in every direction as physical conditions and struggle with competing species would allow. This view has been formulated as the “theory of specific centers of origin.” There would be less objection to this as a first provisional theory did it not assume a supernaturalmodeof origin. But, in the minds of those who hold it, it has usually assumed expressly or tacitly the form of “specific centers of creation,” thus implying the immutability of specific types and the supernaturalism of specific origin (page 68). In this latter or usual form it completely fails to account for the facts given above. For, if this were the mode of origin, each species ought in every case to be perfectly adapted to its own environment, and to no other. But, on the contrary, introduced species often flourish better than in their own country, and better than the natives of their new homes. In the less objectionable form of “specific centers of origin,” without defining the mode of origin, it accounts well for many of the more obvious facts of geographical diversity, as itnowexists, but not all. According to this view, the amount of diversity ought to be in strict proportion to the completeness of isolation, or impassableness of the separating barriers; but this is notexactlytrue. There is another element, not yet mentioned, which is just as important as impassableness, but which until recently has been left entirely out of account. This is the element oftime—the amount of time since the barrier was set up,or during which it has continued to exist. These two elements, it is true, are closely connected with each other; for, since all changes in physical geography have taken place very slowly—since barriers in the form of mountain-ranges and seas have increased by slow process of growth—it is evident that impassableness is, to some extent, a measure of time. But they are by no means in strict proportion. The one or the other may predominate.

Now, this time-element connects geographical distribution with changes of physical geography and climate ingeologicaltimes, and especially with thelatestof these changes, viz., those occurring during theGlacial epoch. During that remarkable epoch extraordinary changes of climate, from extreme Arctic rigor to great mildness, enforced wide migrations of species southward and northward; while concomitant changes of physical geography, by elevation of the earth’s crust over wide areas, opened highways between previously-isolated continents, permitting migrations in various directions, and by subsequent depression again isolating the migrated species in their new homes. It is evident, then, that the recognition of the element of almost unlimited time at once introduces into the question of geographical distribution theidea of evolution. If the study of geographical distribution, asit now exists, and as a part of science of physical geography, gave rise naturally to the theory of “specific centers of origin,” the study of the same, in connection with geological time,and as a part of geological science, now demands its explanation by the theory of evolution.

It must be borne in mind, then, that geographical diversity of organisms is not a question of the present epoch only. There has been geographical diversity in every previous geological epoch; it is, therefore, a question of geology as well as of biology. It is probable, however, that diversity has increased with the course of geological times, and is greater now than ever before. In other words, in the evolution of the organic kingdom, the law of differentiation has prevailed here, as in other departments of biology. A clear statement of the causes of thepresentdistribution of organisms must embrace also the causes of geographical diversitygenerally. We give, therefore, at once a brief statement of what seems to us the most probable view, and shall then proceed to show how it explains the present distribution.

Most Probable View of the General Process.—Bearing in mind, then, this time-element, the phenomena of geographical diversity are best explained by the following suppositions: 1. A gradual progressive movement (evolution) of the organic kingdom, marching, as it were, abreast, at equal rate along the whole line—i. e., in all parts of the earth, and throughout all geological times, under the action of all the forces or factors, and following all the laws, of evolution already explained (pages 19 and 73). If this were all, there would be nogeographicaldiversity, althoughorganic diversitymight be as great as it is now. There would be differentiation of forms and structure everywhere, but no differentiation of groups in different localities. 2. Under the influence of different conditions in different places, more or less isolated from one another by climatic or physical barriers, the onward movement (evolution) of organic forms takes different directions and different rates, and gives rise to local groups, which become more and more differentiated, without limit as time goes on. This element, acting by itself throughout all geological times, would ere this have produced an extreme geographical diversity, such as does not anywhere exist. 3. From time to time, at long intervals, extensive changes of physical geography and climate, produced by crust elevations, partly enforce by change of temperature, and partly permit by opening of gateways, extensive migrations and dispersals of species, by which mingling and struggle for life and final readjustment takes place, and extreme diversity is prevented. Such mingling of different faunas and floras on the same ground, and the severe struggle for life that thus ensues, and the survival of the fittest in many directions, are, as already shown, among the most powerful factors of evolution. They tend toincrease organicdiversity, but todiminish geographicaldiversity. 4. At the close of such great periods of change as indicated in the last, by contrary movement of the earth-crust—i. e., subsidence—new barriers are set up and new isolations are produced, and the process of divergence again commences andincreases steadily so long as the barriers continue to exist.

Now, the last of these periods of great changes and extensive migrations, and subsequent isolations, was the Glacial epoch. It was this epoch, therefore, which mainly determined the present geographical distribution of species. Thus, the present distribution is a key to the directions of the last great migrations, and therefore to the nature of the changes in physical geography and climate which then occurred; and, conversely, the character of these changes, determined in other ways,furnishes the only key to the present distribution of species.

Before applying the foregoing principles in the explanation of special cases, it may be well to give a very brief outline of the condition of things during the Glacial epoch.

In America, during this epoch, by increasing cold the southern margin of the great northern ice-sheet crept slowly southward, until it reached the latitude of about 38° to 40°. Arctic species were thus driven southward slowly, from generation to generation, until they occupied the whole of the United States, as far as the shores of the Gulf, while temperate species were forced still farther south, into Central and South America. This period of extreme rigor and southward migration was followed by a period of great mildness, during which the ice and its accompanying Arctic conditions retreated northward, followed by Arctic species.More than one advance and retreat, apparently, occurred during this time. Again, during the same time, brought about by northern elevation, there was broader connection than now exists between North and South America, and free migrations between, in both directions, enforced by extreme changes in temperature. Also, during this or previous time, there were broad connections between North America and Asia, in the region of Behring Strait, and between America and Europe, in high-latitude regions, and extensive migrations of faunas and floras between were thus permitted. The necessary result of all these migrations of species, partly enforced by changes of climate, partly permitted by opening of gateways since closed, was exceptionally rapid changes in organic forms. This was the result of two causes: First, the severer pressure of a changing physical environment; and, second, a severer struggle for life between the natives and the invaders.

In Europe, during the same time and from similar causes, there were at least three or four different faunas struggling together for mastery on the same soil. First, there were the Pliocene indigenes, who had, if any, pre-emption right to the soil; second, invaders from Arctic regions, driven southward by increasing cold; third, invaders from Asia, permitted by the removal of the old sea-barrier which once extended from the Black Sea to the Arctic, and of which the Caspian and Aral are existing remnants, and thus opening a gateway for migration which has remained open ever since;fourth, invaders from Europe and Asia into Africa, and sometimes back again into Europe, by opening of gateways through the Mediterranean, which have been since closed. One of these highways was through Gibraltar, and one from Italy to Africa through Sicily. As in America, so here, in even greater degree, the severe pressure of changing environment and the severe struggle for life produced rapid changes of organic forms. Many species were destroyed; others saved themselves by modifications adapted more perfectly to the changed conditions. There is little doubt that man came into Europe with the Asiatic invasion, and was one of the principal agents of change, especially in the way of destruction of many old forms.

Such is a very brief outline of the last great geological change and its general results. Being the last, this one has left the strongest and most universal impress on thepresentgeographical distribution. But similar changes by crust oscillations, if not also by extreme changes of climate, have repeatedly occurred in geological times, and some of the most remarkable geographical faunas and floras are the result of these earlier geological changes. We will now give a few examples illustrating these principles:

1.Australiais undoubtedly more peculiar in its fauna and flora than any other known country. Not only are all its species peculiar, not found elsewhere on the face of the earth, but its genera, its families, and even many of its orders of animals and plants, arealso peculiar. These facts are so familiar that it is unnecessary to dwell on them. I need only mention, among plants, the whole of the simple-leaved acacias, already mentioned onpage 86, of which there are so many species, and the whole family of the eucalyptids, of which there are several hundred species. Among animals I need mention only the order of monotremes, or egg-laying mammals, and nearly the whole order of marsupials, or pouched animals, of which there are over two hundred species. On the other hand, the true typical mammals are entirely absent, with the exception of a few bats and a few rats, which have evidently been accidentally introduced from abroad.

Another very noteworthy fact, which must be taken in connection with the last, is that Australian forms are far less advanced in the race of evolution than those of any other country—i. e., that many old forms which have long ago become extinct elsewhere are still retained there. A few examples will suffice. The marsupials just mentioned are an old form once universally distributed, but now nearly extinct everywhere, except in Australia; the cestracion, or Port Jackson shark, and the ceratodus, are Palæozoic and Mesozoic forms retained only in Australia.

What is the explanation of these remarkable facts? We find the sufficient answer in the fact that Australia has been long isolated from all other countries. While geographical changes in geological times have mingled more or less the organic forms of other countries, andthe sharp struggle for life has produced more rapid advance and the production of many new and higher forms better armed for the battle of life, Australia has remained isolated from competition, and therefore comparatively unprogressive.

Can we tell when Australia was finally isolated? Approximately we can. The class of mammals is divided into two groups, which differ widely from each other; so widely, that they are called sub-classes. These are placental mammals, or true typical mammals, and non-placental or reptilian mammals. The non-placentals include only the marsupials and the monotremes (ornithorhyncus and echidna). The monotremes actually lay eggs and incubate them. In the marsupials the embryo has no placental connection with the mother, and is born in a very imperfect condition, utterly unfit for independent life, and placed in the pouch (marsupium), andpermanentlyattached there to the teat until it is capable of independent life; after which only it voluntarily nurses like other new-borns. In other words, the gestation commenced in the womb is completed in the pouch. The uterine gestation in the opossum is only seventeen days, while the marsupial gestation is about two and a half months. In a kangaroo seven feet high in sitting position the embryo at birth is only one inch long—a pink, hairless, almost amorphous mass. The monotremes are pure oviparous animals, like birds and reptiles. The marsupials might well be calledsemi-oviparous. In pure egg-layers the whole embryonic developmentis outside of the body; in pure young-bearers the whole is within the body; in marsupials it is partly within and partly without. Now—1. The monotremes are found nowhere but in Australia and the neighboring New Guinea. 2. The marsupials are also all confined to the Australian region, except a few oppossums in America. 3. There are some two hundred and thirty species of non-placentals in the Australian region. 4. As already said, there are no true mammals at all in Australia, except a few bats and rats which have come accidentally from abroad. 5. But non-placentals existed abundantly inMesozoic times everywhere, both in Europ-Asia and in America, while true mammals did not appear at all on the surface of the earth until theTertiary, when they almost immediately became very abundant everywhere, except in Australia.Evidently, therefore, Australia was isolated before the Tertiary.The enormous difference between its fauna and flora and those of other countries is due to at least three things: 1. So long an isolation necessarily produced great divergence of forms. This alone, however, would not affect thegrade of organization. 2. Saved from wide migrations, and especially invasions from Eurasia, the great field of competitive struggle, it was left far behind in the race of evolution. Hence many of its forms are archaic; its mammalian fauna, for instance, is still in the Mesozoic stage. 3. Its distance from other large continents is so great that accidental colonization has been very slight, only extending to a few bats and a few rats.

I stop a moment to insist on the effect of competitive struggle in developing organic forms strong for the battle of life. Of all the continents, Eurasia has been the scene of most frequent geological changes, and therefore the arena of fiercest competitive struggle through wide and frequent migrations. Eurasian species, therefore, are the strongest of all. They have conquered wherever they have gone. Species in isolated regions are usually the weakest. The great moas and the dodo could not have continued to exist unless protected in a sort of bomb-proof. Kangaroos would now be quickly exterminated by the introduction of fierce Eurasian carnivores.

2.Africa.—The fauna of that part of Africa north of Sahara is essentially Mediterranean—i. e., a sub-group of the Eurasian. Sahara, rather than the Mediterranean Sea, is the true intercontinental barrier. The true African region, therefore, is south of Sahara. Now, according to Mr. Wallace, whom I mainly follow here, the true African mammalian fauna consists of two very different groups of animals. The one is a group of very small, curious animals, mostly low forms of insectivores and lemurs, very peculiar to this region, though more resembling those of Madagascar than of any other region; the other is a group of large and powerful animals which dominate the region. These latter are similar to, though not identical with, those which inhabited Eurasia in Pliocene times. The great carnivores, pachyderms, and ruminants of the region are examples of this group. Now, the explanation of these facts is as follows: The indigenesof Africa are the animals of the first group. Africa, in Tertiary times, was isolated from the great field of combat, Eurasia, and therefore its animals were small, of low grade, and peculiar. During later Tertiary (Pliocene) times, then, Africa was inhabited by animals of the first group, while Eurasia was dominated by animals of the second group. These two groups were then separated by the Desert of Sahara, or else by a sea in that region. Some time during the Glacial epoch geographical changes removed this barrier, and climatic changes drove the Eurasian animals southward into Africa, where, finding congenial climate, they took possession of the continent, dominating the feebler natives. Subsequently they were isolated there by the formation of the desert, and the process of divergence commenced, and has gone on to the formation of many new forms. Meanwhile the change, partly by extinction and partly by modification, has gone on still more rapidly in Eurasia, but in a different direction. Hence, Africa is regarded as one of the primary faunal regions.

3.Madagascar.—This, next to the Australian, is probably the most peculiar faunal region known. There is probably not a single mammalian species found there which is known to occur anywhere else. It is remarkable also as the principal home of that strange, generalized, ancient form of monkeys—the lemurs. And yet its animals, though very different, have a distant resemblance to those of Africa; not, however, to the present dominant type, but to those we have called the indigenes. Notone of the northern invaders is found there. The obvious conclusion from these facts is, that Madagascar was formerly united with Africa, and both were occupied by the same mammalian fauna (which may be called African indigenes, although they were considerably different from their descendants of the present day), but became separated before the northern invasion. The effect of this invasion was to hasten the steps of change in the indigenous fauna of Africa, partly by extermination, partly by modification, while the isolated portion in Madagascar went on at the usual slow rate of change in isolated regions. The time since the separation (which was certainly during the Tertiary period) has been sufficiently long to produce very great divergence in both, butespecially in the African indigenes. In the fauna of Madagascar, therefore, we have a nearer approach to the original fauna of both. On account of this long isolation, we have here many ancient types which are extinct elsewhere. The lemurs are such an ancient type. These are a wonderfully-generalized type of monkeys—a connecting link between monkeys and other mammals, especially insectivores. As might be supposed, from the law of differentiation, already explained (page 11), they are the earliest form, the progenitors, of monkeys. In fact, in early Tertiary times, they were found not only in Africa and Madagascar, but all over the earth, as the only representatives of the monkey family. The true monkeys were not introduced until the mid-Tertiary. In Eurasia and in America (which at that time was probablyconnected with Eurasia) wide migrations and frequent conflicts of faunas produced comparatively rapid evolution of new and higher forms, while in isolated Africa old types continued until the invasion. Madagascar was spared this invasion, and therefore old types are still preserved there. At present, at least three quarters of all lemurs are confined to Madagascar, although a few species are still found in Africa and in the great East Indian islands.

4.Island-Life.—Mr. Wallace has divided islands into two kinds, continental and oceanic islands. The division is undoubtedly a good one, although we may not always be able to refer an example with certainty to the one or the other class.Continentalislands are those on the borders of continents, and separated from the latter only byshallow water.Oceanicislands are those, usually very small, found in the midst of the ocean, with abyssal depth all about. Continental islands may be regarded as appendages to the neighboring continent—as outliers of continents separated by submergence, and have, in fact, been thus formed. Oceanic islands have been formed geologically recently by volcanic action building up from the sea-bottom. Continental islands have a continental structure—i. e., they are composed of stratified as well as of igneous rocks. Their structure is a record of geological history, like that of the neighboring continent. Oceanic islands are composed wholly of volcanic rocks; or, if there be any stratified rocks, these are only of the most recent date. As examples of continental islands wehave New Zealand as an appendage of Australia, the great East Indian (Borneo, Java, Sumatra, etc.) and the Japanese Islands, etc., as appendages of Asia; the British Islands, appendages of Europe; the West Indian Islands, appendages of America; Madagascar, an appendage of Africa, etc., etc. As examples of oceanic islands we have the Azores and Bermudas in the Atlantic, and the Polynesian islands in mid-Pacific.

a. Continental Islands.—Now, the fauna of continental islands, as might be expected from the mode of origin of these islands, is similar to, though not identical with, that of the neighboring continent; the amount of difference being in proportion to thelength of time sincethey were separated and thewidth of the separation.Madagascar, for example, has been long separated from its parent continent, and by a wide and deep channel. Its fauna, therefore, differs greatly from that of Africa, although resembling it more than that of any other country. The separation ofNew Zealandfrom Australia has been not quite so long, and the divergence, therefore, is not so great. These two will be sufficient illustrative examples of long separation, and therefore of great differentiation of forms.

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