Creation by Evolution/The Geographical Distribution of Animals
THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS
By William Berryman Scott
Blair Professor of Geology and Paleontology, Princeton University
Possible Explanations of Geographic Distribution
Different kinds of animals are found in different lands or in different seas. Even the animals of the same continent may show great differences, such as those between the animals of the Canadian forests and those of the coast of the Gulf of Mexico. A hasty examination of the facts might lead to the conclusion that animals were spread over the earth altogether in adaptation to climatic conditions, but this would be a mistake, for climate is only one factor in a very complicated problem, and similarity of climate in widely separated lands is insufficient in itself to bring about similarity of animals. The tropical parts of Australia, Africa, and South America have very similar climates, but their animals are altogether different. Climate, however, may be an effective barrier to the spread of animals and plants, but its action in this respect is entirely negative.
Two alternative views concerning the origin of new forms of life, animal or vegetable, have been presented. One, the older view, which generally prevailed until the publication of Darwin’s Origin of Species, in 1859, was that each kind of animal and plant had been separately created and was, within certain narrow limits, unchangeable and immutable. The other view is that offered by the theory of evolution—that all living things now in existence have arisen by natural descent, with modifications, from ancestors that might be traced back, step by step, to unknown simple forms of life.
By its very nature, the hypothesis or doctrine of special creation was helpless to explain the facts shown by the distribution of animals and plants on the earth. On the other hand, if the new theory failed to offer a satisfactory explanation of these facts it would thereby be shown to be inadequate and improbable.
Geographic Distribution in the Light of the Evolutionary Hypothesis
If the evolutionary hypothesis is true, then the present arrangement of living things on the surface of the earth, strange and inexplicable as some of its features may seem to be, must have been the necessary outcome of the whole vast series of changes—geographical, climatic, and biological—through which the earth has passed during unimaginably long periods of geological time. And it may be said, in anticipation, that the present distribution of animals is, in a broad and general sense, explained by what we know of the earth’s history as it is revealed by geology and palaeontology. Where explanation is lacking it is invariably due to our ignorance of parts of that history, and we may reasonably expect that the solution of any problem concerning a particular animal or group of animals will be found when all of that history shall have been deciphered. A great many new facts have been discovered since Darwin wrote, and these, as a whole, are strongly confirmatory of the hypothesis of evolution.
The mode of procedure followed in solving such a problem may be best illustrated by a single concrete example, which is furnished by the camel family. This very peculiar family consists of two sections. One section includes the true camels of the Old World, whose present native habitat appears to be Asia, for the only wild camels now in existence are found solely in central Asia. The other section includes the much smaller, lighter, and more graceful guanacos, llamas, and like animals of South America. No one who has studied the anatomy of the camels can doubt for a moment that these two sections of the family are closely related, despite the great differences in their external appearance. The geological history of the family, as revealed by fossils, shows that, for a long time during the Tertiary period, North America was its only home, for during that time the camels appear to have inhabited no other continent. Each successive group of rocky strata in our western plains has yielded remains of its own characteristic types of camels, whose development toward the modern type may be followed through many almost imperceptible gradations, all presumably arising by ordinary procreation. Later in the Tertiary period the fossils record the arrival of camels in Asia, on the one hand, and in South America on the other; and finally, at a very late geological date, they completely disappeared from North America. Their passage from North America to Asia was made possible by the existence of land connection where we now find the shallow Bering Sea. This connection was often made and broken in past ages.
According to the doctrine of special creation the relationship between the true camels and the llamas is not real but purely ideal. The fossil camels, which seem to record successive steps of development, all moving in the same direction, record only successive, disconnected acts of sudden creation in a way of which we have had no experience, and the present geographical arrangement of the members of the family bears no relation to its recorded geological history. Such an explanation seems to be altogether incredible, whereas the evolutionary explanation seems to be entirely adequate.
Extinction of Faunas
A great extinction of quadrupeds, which exterminated them over more than three-fifths of the land surface of the globe soon after the appearance of early Man in Europe, is an unexplained mystery. Nobody can yet say why there was such immense mortality among the huge and strange creatures that had roamed over all the continents. Whatever may have been the agent of destruction, it removed from North America a great variety of types, such as elephants and mastodons, many kinds of horses, bisons, tapirs, peccaries, camels and llamas, huge ground-sloths and giant armadillos (immigrants from South America) as well as giant wolves, great lion-like cats, and sabre-tooth tigers. It was this great extermination that put a gap of thousands of miles between the Asiatic and the South American section of the camel family by destroying them in the intervening areas.
Principal Areas of Geographic Distribution
It has long been customary to divide the lands of the earth into zoological regions in accordance with the animals that inhabit them. A map on which these regions are indicated by various tints appears to be a very irrational and arbitrary sort of thing. Few of the regions coincide with the continents. Australia and South America fall each into a single region, and Asia, Africa, and North America belong to two, or even three regions each. The northern part of North America, about to the 49th parallel of latitude, belongs to a vast region (what is called the Holarctic region), which includes Europe, northern Africa, and extratropical Asia. Most of the remainder of North America, including the Mexican plateau, makes up a separate region—the Sonoran. The hot lowlands of Mexico, Central America, and the southern tips of Florida and Lower California are included in the same region as South America (the Neotropical region), as are also the West Indian islands. The regions thus designated are determined primarily by the distribution of mammals, the warm-blooded quadrupeds, for these animals are better known than almost any other group except the birds, and their geological history has been much more fully and minutely deciphered than that of any other of the higher groups. These zoölogical regions are really an inevitable result of the many changes, climatic and geographic, through which the earth has passed while mammals were abundant and diversified.
It is practically certain that no group of mammals arose twice independently in unconnected areas, and it is this fact that enables us to trace, by the aid of fossils, the migration of mammals from continent to continent. If a group of mammals could arise independently and more than once the presence of a given group in North America and in Asia would be no indication that those continents were once connected, but if each group arose but once and spread as far as geographic and climatic conditions permitted, then its presence in two areas now disconnected indicates the former connection, direct or indirect, of those areas. The outlines of the zoölogical regions and their geographic relations afford a key to their history and to the manner in which they received their faunas. The complete zoölogical difference of Australia, for example, from any other continent is in itself sufficient to show that Australia has been geographically isolated for a long time, so that mammals of the higher, more advanced groups were unable to reach it.
The fact that zoölogical regions, as pictured on the map, when marked by differences of colour, seem to make a patchwork or a meaningless pattern on the map is due to the long, varied, and complicated changes in the geography of the regions and in their climate as recorded in their geological structure. Land areas that are now united were for ages separated by arms of the sea, and lands that are now separated were formerly united. Not very long ago, as geological time is reckoned, Great Britain and Ireland were joined to each other and to the continent of Europe, and the North Sea was a wide terrestrial plain, which stood not far above sea level. At about the same time the great islands of the East Indies were parts of the mainland of Asia, from which they became separated at different times. Borneo, Sumatra, Java, Celebes, the Philippines, the Japanese group, and the Kurile Islands were once joined to Asia. In the course of the ages Alaska and Siberia have been often connected and as often separated by uplift and depression of the land, and North and South America were brought together by the Isthmus at a relatively late period. Before that period the two continents had long been separated by a broad sea, which covered the site of the Isthmus and most of Central America.
We must also note the remarkable changes of climate to which the rocks and fossils bear unequivocal testimony. During the greater part of the Tertiary period the climate of the earth was mild and genial, a fact indicated by the fossil plants of the Arctic regions. This climate gradually gave way to one that was colder and that culminated in the glacial period, when so many lands were buried under sheets of moving ice, as Greenland is to-day.
This brief outline will show the great complexity of the conditions to which mammals were compelled to adapt themselves and will help to explain their present arrangement on the surface of the earth, but, it must be insisted, this explanation becomes meaningless if we deny the theory of evolution. If each individual species represents a separate act of creation, then there can be no relationship between species other than that of an ideal plan. On the other hand, if species are really related by community of descent, and new forms arise by the modification of older ones, then many of the facts of distribution are simply and naturally explained as due to changes in geography and climate, of which we have such clear and indisputable evidence.
If we study the mammals of South America of to-day we at once see that they fall naturally into two groups. Those of one group are very peculiar and are limited almost exclusively to the Neotropical region; those of the other are closely related to the mammals of North America. Confined to South America and Central America are a group of strange, bizarre creatures (the Edentata), such as the ant bear, the tree ant eaters, the sloths and armadillos, and, if we include the Pleistocene epoch in our purview, a bewildering variety of two extinct edentate groups, the immense groundsloths and the glyptodonts, or giant armadillos, the remains of which have been found buried in the Pampas of Argentina in astonishingly great number. In South America we find also the platyrrhine monkeys and marmosets and a host of peculiar rodents, cavies, agoutis, tree porcupines, the water hog (the largest living rodent), chinchillas, spiny rats, and many opossums, as well as another type of marsupial, which distantly resembles those of Australia. These constitute a most peculiar and characteristic assemblage of mammals, such as are found nowhere else in the world.
Mingled everywhere with these strange tropical creatures are mammals of northern type, which compose the second group mentioned above. This group includes the tapirs, the peccaries (or wild swine), the guanacos and llamas, many species of deer, cats great and small, wolves, skunks, weasels, otters, and raccoon-like animals, together with North American types of rodents, rabbits, squirrels, rats, mice, and the like. Though some of the animals of this group are obviously related to those of North America and Asia, nearly all are assigned to different species from their relatives that inhabit those regions, many even to peculiar genera. South American wolves, for example, are in many ways peculiar and must be placed in genera not found in other regions, but that they are related to the northern wolves is indisputable, whether we regard that relationship as ideal or real. If each of these species was created separately, their distribution is in no way explained by the history of the groups to which they belong. If, on the other hand, they arose by natural descent from a common ancestry, this history does explain the distribution, and in a most convincing manner.
Now let us go back to the time, in the middle of the Tertiary period, in the Miocene epoch, when North and South America were not connected, a fact demonstrated by the geological record of Central America and the Isthmus of Panama. We are now in a position to make a full and accurate comparison of the quadrupeds of the two Americas at that time because in Patagonia, on the one hand, and on our Great Plains, on the other, we have immense areas of soft rocks, accumulated at approximately the same time, which have in both the Americas yielded large numbers of well preserved fossil mammals. The separation of the two continents at that time is reflected in the complete difference of their mammals; in their mammalian life North and South America had literally nothing in common. Miocene Patagonia had no animal that can be regarded as an ancestor of any of those that are put in the second category—animals like those of North America and the Old World. All the mammals of that time and region were either such as gave rise to the peculiar South American forms of the Pleistocene and Recent epochs or such as died out without leaving any descendants. There was a great assemblage of hoofed animals, but they were peculiar, unknown from any other part of the world, and all are extinct and left no descendants in the modern world. There were predaceous creatures, beasts of prey, but no members of the order Carnivora, for these ancient flesheaters of Miocene Patagonia were marsupials, very like the so-called Tasmanian wolf (Thylacynus) and related to the opossums. Of the modern cats, wolves, skunks, and other northern carnivorous animals there was no trace; nor of the tapirs, peccaries, llamas, or deer of to-day. There were rodents in great variety, but they were all of the peculiar South American kinds; of the northern rats, mice, squirrels, and rabbits there was not a single representative.
In contemporaneous (middle Tertiary) North America there was an equally rich and varied mammalian fauna, but one totally different from that of the southern continent and very like that of the Old World. In addition to certain characteristically North American groups, not yet known from any other region, there were ancestral types of elephants, rhinoceroses, horses, tapirs, peccaries, deer, antelopes, camels, cats, sabre-tooth tigers, wolves, weasels, raccoons, rats and mice, squirrels, marmots, beavers, hares, and rabbits. The assemblage is essentially that of the Old World, though it shows certain local differences, and it is completely unlike that of South America.
About in the middle or perhaps in the later part of the Miocene epoch of the Tertiary period Central America and the Isthmus of Panama were raised above the sea, and North and South America were thus connected. This uplift made possible the migration of mammals in both directions, the northern forms going south, the southern forms going north. The earliest northern mammal yet discovered in South America is a raccoon-like carnivore, found in the upper Miocene of Catamarca, the Andean province of Argentine; and the first known Neotropical creature to arrive in North America was a ground sloth, found in the Miocene rocks of Oregon. From this beginning, the proportion of mammals common to both continents steadily increased, reaching a maximum in the Pleistocene, before the beginning of the great extinction already mentioned. In that extinction both continents lost a large proportion of their mammals. In particular, the southern invaders of North America, which had been so abundant in the Pleistocene, nearly all disappeared, leaving only the Canada porcupine as a remnant of that invasion.
In marked contrast to this failure of the Neotropical animals to establish themselves permanently in North America was the success of the northern immigrants in maintaining their foothold in South America. True, many of them, such as the mastodons, horses, short-faced bears, and sabre-tooth tigers, eventually died out, but they died out also in North America, having been among the many victims of the great Pleistocene extermination. However, large numbers are still there and constitute the second group of Neotropical mammals enumerated above. Some of these immigrants, notably the deer and the beasts of prey, have been so long in their southern home that they have undergone considerable modification and must be placed in genera different from those that include their relatives in North America. Among these modified immigrants are the little brocket of Chili and the Pampas deer of Patagonia, whereas the deer of the Guianas is a later arrival and differs but little from the deer of Florida. The wolves, bush dogs, skunks, coati mundis, etc., are obvious variants of northern types. Even the lack of certain animals in North America is reflected in South America, as, for instance, in the almost complete absence of bears, which reached North America from the Old World at a very late period.
Before the geology of the Isthmus of Panama was known, Messrs. Jordan and Evermann made a comparative study of the sea fishes on both sides of the Isthmus. The difference was so great that these authors concluded that the two seas had been separated by the upheaval of land in the middle of the Tertiary period (Miocene epoch), a result which was exactly confirmed by subsequent geological examination.
The geological and palaeontological history of North and South America in Tertiary time is known in greater fullness than that of most other continents, and it explains in a very satisfactory way the existing distribution of mammals in the Western Hemisphere, but only to one who believes the evolutionary theory. Otherwise, that history has no meaning or application, for the existing species are different from those which we find entombed in the rocks, and if they were not descended from the more ancient ones but created separately, then the history has no relation to the present arrangement of the animals. Can any one really believe that successive acts of creation were deliberately arranged so as to produce a false and illusory effect? Absurd as it may seem, such a belief is involved in the acceptance of the doctrine of special creation.
North America and Asia have been repeatedly connected and disconnected at the point where Bering Sea and Strait are now found. Here also many migrations back and forth between the two continents may be traced by the record of the rocks, and these migrations and counter migrations took place at widely separated geological dates. Many American mammals are the modified descendants of migrants from the Old World. Some came to America at a very ancient time; others came recently and are so little modified that naturalists disagree as to whether they should be assigned to European species or not. In short, our understanding of the distribution of existing mammals is dependent upon our knowledge of their history, and where that history is known the distribution is self-explanatory. If the doctrine of special creation is true, this knowledge is illusory, for according to that doctrine the modern species were separately created and therefore have no connection with the ancient species that look so deceptively like the ancestors of the living forms.
North America was separated from Asia at a very late date, as geological time is reckoned, and hence there is relatively little difference between the animals of the American and the Eurasiatic division of the Palaearctic region. The only large mammal that is found in the American and not in the Eurasiatic division is the musk-ox, but until a very recent period that peculiarly Arctic creature dwelt in Siberia and roamed as far westward as Great Britain, which was then a part of the mainland.
The distribution of mammals on the continents finds a satisfactory explanation in the theory of evolution and only in that theory, but the facts of island life furnish even more remarkable and more convincing testimony of the truth of that theory. Islands have been formed at different dates, some rising from the sea within the last few years, others having a history that goes back to the remotest geological antiquity. It is customary to group islands into two classes, suggested by their mode of formation—the continental and the oceanic. Continental islands are detached portions of continents, from which most of them are separated by shallow seas. They have the geological structure of the continents and are composed of the familiar stratified rocks, such as sandstone, slate, and limestone, and some contain granite-like igneous rocks and volcanic material. The faunas of these continental islands are determined by many factors, of which we may disregard several, such as area and climate, and consider only the distance of the island from the mainland and the geological date of its separation.
Great Britain, Ireland, and the islands of the East Indies or Malay Archipelago are continental islands that lie near the parent continents, are surrounded by comparatively shoal water, and although they were detached from the mainland at different times, they were yet, on the whole, of relatively recent origin as islands. Great Britain is, zoögically speaking, indistinguishable from an equal area of the European continent; the species are so generally identical that the islands must have been separated from the continent during the present geological epoch, the Recent. The great Asiatic islands, Borneo, Sumatra, Java, etc., were detached somewhat earlier and contain more peculiar and more characteristic species, but the difference from those of Asia is not great.
Oceanic islands lie far from any continent and rise abruptly, with steep submarine slopes, from profound depths of the sea. Most oceanic islands are composed of volcanic rocks and coral reefs. Nearly all of them seem to have been submarine volcanoes, which have built up their cones from the sea-floor; and the cones that lie in warm seas are generally capped with coral reefs, which may or may not bury the volcanic pedestal out of sight. The rocks that form the continents and continental islands—the sandstones, limestones, shales and slates, and the coarsely crystalline rocks, such as granite—are not found on the oceanic islands, whose history was radically different from that of the continents and their islands.
When we turn to the animals and plants that inhabit these remote islands we immediately note their marked difference from those of the continental islands. Oceanic islands have no land mammals other than bats, which are carried for great distances by storms, just as land birds are. On some of these islands mice are found, but these may have been introduced by men. Amphibians (that is, frogs, toads, newts, salamanders, and the like) are lacking, because not only are these animals unable to endure sea water, but sea water is fatal also to their eggs. Such islands have no true fresh-water fishes, crustaceans, or mussels. The fresh-water fishes and mussels that they do contain are those which readily enter streams from the ocean and so are much the same everywhere. The animals that are found in these remote islands are land birds, lizards, snails, and insects—such creatures, in short, as may reach them more or less accidentally, as it were, being carried by wind or floated on driftwood for long distances by ocean currents. Their plants also are of the kinds whose seeds are carried by wind and wave.
These oceanic islands may have received their plants and animals in one of two ways. The doctrine of special creation maintains that these plants and animals were directly created where we now find them. If we hold that they were specially created on each island we should expect to find on each island such forms as were particularly adapted to live on it, to which, in short, the environment was suitable and favourable. But this is not at all the case. Many of these oceanic islands are large and could support rather large animals, and the fact that the absence of such animals is due to their inability to reach the islands, except by human aid, is shown by the oft-repeated results of artificial introduction. Rats, mice, rabbits, swine, goats, cattle, and horses, when allowed to run wild on such islands, have thrived and multiplied exceedingly, showing that no unfavourable factor in the environment prevented their presence.
The fact that the oceanic islands really have been supplied with their plants and animals by the more or less accidental agency of the sea and the winds is indicated by a remarkable instance, of which we have the whole history, for it took place within recent years. In the Straits of Sunda, between Java and Sumatra, there is a volcanic island called Krakatau, which, in the summer of 1883 was the scene of a series of the most tremendous volcanic explosions that have occurred within historic time. In consequence of this terrific outburst, most of the island was destroyed and such parts of it as remained above water were so deeply buried by volcanic ash that all animal and vegetable life was completely destroyed; apparently not a living thing was left. Yet in twenty years, or less, the desolate island was restocked by the winds and ocean currents. The late Professor Selenka, of Munich, visited Krakatau some years after the catastrophe, and wrote of it:
Under the shade of a Casuarina, among cocoanut palms and thickets as high as a man's head, I found, to my astonishment, an active animal life of spiders, flies, bugs, beetles, and butterflies; even lizards half a yard in length animated the peaceful picture. All these plants and animals were brought hither by wind and water from Java and Sumatra, replacing the vanished world in the course of a few years.
If the living things on oceanic islands, animal and vegetable, were not created especially for them, they must have been carried to them by winds and currents. If this is true, the nature of the plants and animals of such an island must be determined by the antiquity of the island, its remoteness from land, and its position with reference to strong winds and marine currents. But even if these plants and animals reached the islands in the accidental manner described, might they not have remained unchanged and immutable? In that case there should be no particular relation between the remoteness of the islands and their geological date of formation, on the one hand, and the kind of creatures that inhabit them, on the other. But if species are mutable and subject to modification, then we ought to find that the islands had species that are peculiar to them, yet that would show more or less distinct relationship to those of the mainland from which the islands received the ancestors of their peculiar species. That this is the true solution of the problem is strongly indicated by the plants and animals of the Galapagos Islands, which first led Darwin to form his new views on the origin of species.
The Galapagos Archipelago is a group of five relatively large and ten small islands, all of volcanic origin, which rise steeply from great depths of the ocean. The one nearest to the coast of Ecuador is about 600 miles distant from it, and the islands lie almost on the Equator, in the zone of calms, in which strong winds seldom blow. The arrival of a new form from the mainland must be a very rare event; yet the islands contain many birds, reptiles, and insects, but no mammals. It is fortunate for our inquiry that no aborigines settled in the islands, which, when Darwin first visited them, were almost in a state of nature. Nearly all the animals and plants that inhabit the islands are peculiar to them; the species and many of the genera are found nowhere else in the world. This statement does not, of course, apply to the sea birds, which cross great stretches of the ocean with ease. Though the land birds belong to peculiar species, and most of them to peculiar genera, they are nevertheless of unmistakably South American types and belong to South American families. Large lizards, one a land species and the other marine, as well as the huge land tortoises that have given their name to the islands, are abundant. When the islands were first discovered there were fifteen species of these monstrous tortoises, each island and islet having its own species, but many of these have been extirpated. The species of land birds, like those of the tortoises, are peculiar to a single island each; the genera are mostly common to the group, as the islands are so far apart that communication between them is difficult.
On the theory of evolution these remarkable facts are easily explained. The ancestors of the existing birds and reptiles came, rarely and at long intervals, from the mainland of South America, and after settling in the islands became slowly modified, so that they were placed in genera nearly allied to those of the continent, yet different from them; and, in the isolation of the individual islands the species were free to develop into new forms peculiar to each.
In the Galapagos we witness the results of what may be called a great evolutionary experiment, under conditions unaffected by human interference, and such conditions are rare; but the Cape Verde islands, in the Atlantic, display very similar relations in their animals and plants. The species there are peculiar, but their affinity is as unmistakably African as that of the Galapagos species is South American.
Bermuda and Madeira are almost as far from the North American and African coasts, respectively, as the Galapagos Islands are from Ecuador, but they have hardly any peculiar birds, because they lie in the track of storms, and every year birds are arriving from the mainland in such numbers as to prevent the isolation of groups of them and the development of new types. This must not be understood to mean that new species do not arise on the continents; it merely means that they develop more rapidly under favourable isolation.
Islands like the Hawaiian group or St. Helena, which are very remote from any land and are of considerable geological antiquity, have birds and other land creatures so peculiar that their ancestry is obscure, or even unknown, so that it is difficult to decide what continent their ancestors really came from. This relation between the peculiarity of its species and the remoteness and antiquity of an island is just what we should expect according to the theory of evolution, but these factors should have no effect according to the theory of special creation.
An interesting illustration of this principle is afforded by the rails (Rallidae), a family of birds that is distributed all over the world, on continents and islands, except in the polar regions. The continental species can fly; many of the insular ones cannot. As the islands on which these flightless birds are found were never connected with any mainland, the advocate of the theory of special creation must hold that the birds were separately created on each island or reached it by crossing the sea; but they could not cross the sea by swimming, for no bird is able to swim across such breadths of sea. The birds must therefore have reached the islands by flying and have lost the power of flight after they settled in their insular homes. This loss of flight involves so great a modification of structure that the birds are assigned to new species and even to new genera, different from those to which their flying ancestors belonged. There is no doubt a close relation between the loss of the power of flight and a small, insular habitat. The absence of dangerous enemies, which prey upon the birds and upon their eggs and young, makes flight less essential to their existence. Very often, too, the prevalence of violent winds makes it advantageous for both birds and insects to remain on or near the ground, and not to attempt high or long flights, which involve the risk of being blown out to sea. Many birds do cross the sea for long distances when carried away by storms, but when land birds are so swept from the land they are likely to be destroyed. Most people who have taken a sea voyage have seen land birds, far out at sea, come aboard the ship, exhausted from their long flight, to rest in the rigging. Occasionally these waifs find new homes in remote lands, which have doubtless in this way received their bird inhabitants.
In the Galapagos there is a flightless cormorant, which lives by fishing in the sea. The penguins, those remarkable birds whose wings have been converted into swimming paddles or flippers, live on islands in the seas of the southern hemisphere, where they are safe from the attacks of enemies. The extinct dodo was a large, flightless pigeon, which lived on the island of Mauritius until it was exterminated by sailors and by introduced pigs. Another flightless pigeon, also extinct, was the solitaire of the Isle de Bourbon. New Zealand had many very large flightless birds, the moas, which were destroyed by the Maoris when they settled the islands; and the flightless, almost wingless, little kiwi (Apteryx) still lives in New Zealand. It is true that some flightless birds, such as the African ostrich and the South American rhea, live on the continents, but these are large and strong birds and very swift runners, and are therefore able to escape the large beasts of prey and to defend themselves against the smaller ones.
Much the same considerations apply to insects. In Madeira and in Kerguelen Land, a small group of volcanic islands in the Antarctic Ocean, there are large numbers of insects that are unable to fly. Madeira has 393 species of insects that are not found elsewhere, of which 178 species are incapable of flight. These species could not have reached the island in their present flightless state.
When all the facts here presented are taken together—the facts of the animal life of the continents and of the islands—it must appear to any unprejudiced mind that the evolutionary theory offers by far the most probable explanation of them. The alternative theory can offer no solution of problems of geographic distribution. If the theory of evolution were false it would surely be in conflict with the facts of the geographical distribution of plants and animals. When the distribution of a group is inexplicable by the theory of evolution we find that we have not yet deciphered the history of that group. After its history is made known its present distribution is manifestly the inevitable result of a natural sequence of events.
We have already deciphered much of what may be called the geographic history of the earth—the history of the many gradual changes that have taken place in the form and the extent of its lands and seas, as well as in its climate—and it is these changes that have determined in large part the distribution of its animals. All the facts discovered show reasonable natural succession; nowhere can we find evidences of sudden creation; and all are simply explained by the theory of evolution.
LIST OF BOOKS RECOMMENDED
- Brauer, August. Tiergeographie und Abstammungslehre, in Die Abstammungslehres, Jena, 1911.
- Darwin, Charles. The Origin of Species, Chaps. XII and XIII.
- Gill, Theodore. The Principles of Zoögeography. Proc. Biolog. Soc. Washington, Vol. II, pp. 1–39.
- Heilprin, Angelo. Geographical and Geological Distribution of Animals. International Scientific Series, Vol. LXVII.
- Lydekker, Richard. A Geographical History of Mammals, Cambridge, 1896.
- Merriam, C. Hart. The Geographic Distribution of Life in North America; Proc. Biolog. Soc. Washington, Vol. VII, pp. 1–39.
- Scott, W. B. The Theory of Evolution, Lecture V. New York, 1917.
- Wallace, A. Russel. The Geographical Distribution of Animals. 2 vols. New York, 1876.
“It is an interesting fact that on oceanic islands far removed from continents only those forms of life are found which could be borne to them by wind or wave. Only such birds as can be carried long distances by strong gales appear. The fauna of such islands contain no mammals except bats, and in every instance the life, both of plant and animal, is similar to that of the nearest mainland, yet differs from it in having distinct species. If special creation accounts for those forms, why are they not identical with those of the mainland? There is no answer. But evolution affords a simple and inevitable explanation. And if we admit that the original forms of life come from the mainland, and have since changed into new species, then the case of evolution is established.”—F. L. Darrow, Through Science to God.