Popular Science Monthly/Volume 37/July 1890/Evolution and the Distribution of Animals I
|EVOLUTION AND THE DISTRIBUTION OF ANIMALS.|||
NO one with good eyes and brains behind them has ever looked forth on the varied life of the world—on forest or field or brook or sea—without at least once asking himself this question: "What is the cause of nature's endless variety?" We see many kinds of beasts and birds and trees and flowers and insects and blades of grass, yet when we look closely we find not one grass-blade in the meadow quite like another blade. Not one worm is like its fellow-worm, and not one organism in body or soul is the measure of its neighbor. You may search all day to match one clover-leaf, and, should you succeed, even then you have failed; for, if the two leaves agree in all physical respects, they may still be unlike in that which we can not see, their ancestries, their potentialities. Again, with each change of conditions, of temperature, of moisture, of space, of time, with each shifting of environment, the range in variety increases. "Dauer in Wechsel" (persistence in change); "this phrase of Goethe," says Amiel, "is a summing up of nature." And the naturalist will tell you that the real variety is far greater than that which appears. He will tell you that, where commonness seems to prevail, it is the cover of variety. The green cloak which covers the brown earth is the shelter under which millions of organisms, brown or green, carry on their life-work.
Each recognizable kind of animal or plant is known in biology as a species. The number of forms now considered as distinct species is far beyond the usual conception of those who have not made a special study of such matters. I have an old book in my library, the tenth edition of the Systema Naturæ, published by Linnæus in 1758. This book treats of all the species of animals known a little more than a century ago. In its eight hundred and twenty-three pages some four thousand different kinds of animals are named and briefly described. But for every one of these enumerated by Linnæus, more than one hundred kinds are known to the modern naturalist, and the number of species still unknown doubtless exceeds the number of those already recorded. Every year for the last quarter of a century there has been published in London a plump octavo volume known as the Zoölogical Record. Each of these volumes, larger than the whole Systema Naturæ, contains the names of the species new to science added to our lists during the year of which it treats; and in the record of each year we find the names of two or three times as many as are mentioned in the whole Systema Naturæ. Yet the field shows no signs of exhaustion. As these volumes stand on the shelf together, it is easy to see that the later volumes are the thickest, and that the record for the present year is the largest of all. The additional species named and described in 1889 are more than ten thousand. Moreover, what is true of the increase of knowledge in systematic zoölogy, is even more marked in the case of botany. Such, then, is the variety of life on the globe—a variety of which Linnæus and his successors had never dared to dream.
And yet, great as this variety is, there are, after all, only a few types of structure among all animals and plants—some three or four or eight or ten general modes of development—all the rest being minor variations from these few types.
It is even true that all life is but a series of modifications of a single plan; for all organisms are composed of cells, the essential element of which is always a single substance—protoplasm. All are governed by the same laws of development, reproduction, and susceptibility to outside influences. Unity in life is therefore not less a fact than is life's great diversity. In whatever way we account for the diversity, the essential unity must not be forgotten. The bonds of unity among organisms constitute what the naturalist calls homology.
That these resemblances have some deep significance, no thoughtful student of nature has ever doubted. What this significance may be is the underlying question in that branch of philosophy which has come to be known as evolution.
In the present discussion I shall take for-granted that answer to these questions which is associated with the name of Darwin; and, as a student of the relations and distribution of animals, I firmly believe that no answer to these questions fundamentally different from his will ever be possible.
The essence of the Darwinian theory is this, that the various species of the present day are all derived from pre-existing forms, more or less unlike them; that this derivation takes place through the operation of natural laws—the law of heredity, the law of response to external stimulus or environment, and the law less clearly understood by which variations from ancestral types are constantly produced; the "divine initiative" in the individual which struggles against sameness and monotony. The constant tendency of organisms to multiplication by geometric progression in a world of limited extent, already apparently full, brings about a constant struggle for existence among these organisms, and by this struggle, we have the progressive adjustment of individuals to their environment—an adjustment which is made more and more complete by the ceaseless destruction of the unadjusted. According to this theory, the * same causes which have produced difference of species in the past must be still at work, and must continue to produce similar differences in the future. To the theory of derivation is opposed the old idea of "special creation." But this theory of special creation has never had in science other than a provisional existence. It was a mere name for a process not understood. If each of the millions of species of animals and plants living and extinct came about by a "special creation," then special creation can not be an operation outside the limits of law. It is simply the name given in ignorance to the law by which species are produced. What has been done so many times must be done in some uniform way. What this way is, the theory of evolution professes in some degree to define.
The fact is, the theory of development gives the only clew by which the naturalist can be guided in his work. If the mutual affinities of species do not depend on the law of heredity, they are unintelligible. They are impossible. If the variation of species is really immutability in disguise, we can not trust our senses. We are left to choose between some form of the development theory and a hopeless unscientific agnosticism, content with the surface facts, and ignorant of the laws of which these facts are the expression.
I do not wish to-night to discuss either the general question of evolution nor that special theory of the method of evolution which is associated with the name of the master of modern zoölogy. I shall take evolution and Darwinism for granted, and confine myself to a statement of certain facts and principles in the science of zoögeography and to their bearing on the question of the origin of species. There are many difficulties in bringing the facts of this science down to the needs of concrete illustration. A science so broad as to include all human history at once with the history of every group of animated organisms can not well be compressed into a discussion of a single hour. And with this I may recall the additional difficulty, present in all discussions of the subject of evolution, of distinguishing single illustrations from arguments. Isolated cases of geographical variations in species would not have great value as arguments for the development theory were the cases really isolated. The force lies in this fact, that these cases are typical; that what may be said of one is true of a thousand.
In like manner the full force of the laws of homology and heredity can only be felt when their effect is cumulative, as in the mind of the anatomist who has followed each organ through its protean disguises in a wide range of forms.
Still, again, the force of the argument drawn from embryology does not come from a knowledge of the changes in a single egg. All these studies need the second premise, obtained by years of comparison in different fields of investigation, that no case is isolated. Without this premise, the argument would be incomplete. The few cases of development or change which can be brought to popular notice are simply illustrations and not proofs.
As Prof. Bergen has well said, "it is important that we should understand that none of the kinds of evidence in favor of evolution loses so much by being represented only by scattered instances as the argument from distribution." And, conversely, no argument is so strong when all the known facts are brought into consideration together. The universal fact of the mutability of species can be really understood or appreciated only by him by whose eyes multitudes of species have been seen to change. To the ordinary observer the species seems constant, just as the face of a cliff seems constant. To the student of nature mutability is everywhere. Just as the wind and rain and frost quietly but surely change the face of a cliff, so do other forces of nature as quietly but as surely change the face of a species.
And now we may notice that it was precisely this phase of the subject, the relation of species to geography, which first attracted the attention of both Mr. Darwin and Mr. Wallace.
Both these observers noticed that island life is neither strictly like nor unlike the life of the nearest land, and that the degree of difference varies with the degree of isolation. Both were led from this fact to the theory of derivation, and to lay the greatest stress on the progressive modification resulting from the struggle for existence.
In the voyage of the Beagle, you remember, Mr. Darwin was brought in contact with the singular fauna of the Galapagos Islands, that cluster of volcanic rocks which lies in the open sea some six hundred miles west of the coast of Equador and Peru. The sea birds of these islands are essentially the same as those of the coast of Peru. So with most of the fishes. We can see how this might well be, for both sea birds and fishes can readily pass from the one region to the other. But the land birds, as well as the reptiles, insects, and plants, are mostly peculiar to the islands. The same species are found nowhere else. But other species very much like them in all respects are found, and these all live along the coast of Peru. In the Galapagos Islands, according to Darwin's notes, "there are twenty-six land birds; of these, twenty-one or perhaps twenty-three are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with, the other animals and with a large proportion of the plants. . .. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, feels that he is standing on American land."
The question, then, is this: If these species have been created as we find them on the Galapagos, why is it that they should all be very similar in type to other animals, living under wholly different conditions, but on a coast not so very far away? And again, why are the animals and plants of another cluster of volcanic islands—the Cape Verde Islands—similarly related to those of the neighboring coast of Africa, and wholly unlike those of the Galapagos? If the animals were created to match their conditions of life, then those of the Galapagos should be like those of Cape Verde, the two archipelagoes being extremely alike in respect to soil, climate, and physical surroundings. If the species on the islands are products of separate acts. of creation, what is there in the nearness of the coasts of Africa or Peru to influence the act of creation so as to cause the island species to be, as it were, echoes of those on shore?
If, on the other hand, we should adopt the obvious suggestion that both these clusters of islands have been colonized by immigrants from the mainland, the fact of uniformity of type is accounted for, but what of the difference of species? If the change of conditions from continent to island may on the island cause such great and permanent changes as to form new species from the old, why may not like changes take place on the mainlands as well as on the islands? And if possible on the mainland of South America, what evidence have we that species are permanent anywhere? May they not be constantly changing? May not what we now consider as distinct species be only the present phase in the changing history of the series of forms which constitutes the species?
The study of these and many similar facts can lead to but one conclusion:
These volcanic islands rose from the sea destitute of land life. They were settled by the waifs of wind and of storm, birds and insects blown from the shore by trade winds, lizards carried on drift-logs and floating vegetation. Of these waifs few came perhaps in any one year, and few perhaps of those who came made the islands a home; yet, as the centuries passed on, suitable inhabitants were found. That this is not fancy we know, for we have the knowledge of many similar transfers. Every one who has approached our eastern shores by sea in the face of a storm will realize this. Hosts of land birds—sparrows, warblers, chickadees, and even woodpeckers—are carried out by the wind, a few falling exhausted on the decks of ships, a few others falling on off-shore islands, like the Bermudas, the remainder drowned in the sea.
Of the immigrants to the Galapagos the majority doubtless die and leave no sign. A few will remain, multiply, and take possession, and their descendants are thus native to the islands. But, isolated from the great mass of their species and bred under new surroundings, these island birds come to differ from their parents and still more from the great mass of the land species of which their ancestors were members. Separated from these, their individuality would assert itself. They would assume with new environment new friends, new foes, new conditions. They would develop qualities peculiar to themselves—qualities intensified by isolation. "Migration" says Dr. Coues, "holds species true; localization lets them slip/' This would be more exactly the truth should we say that localization holds peculiarities true; migration lets them slip. Local peculiarities disappear by wide association and are intensified when individuals of similar peculiarities are kept together. Should later migrations of the original land species come to the islands, the individuals surviving would in time form distinct species, or more likely, mixing with the mass of those already arrived, their special characters would be lost in those of the majority.
The Galapagos, first studied by Mr. Darwin, serve to us only as an illustration. The same problems come up in one guise or another in all questions of geographical distribution, whether of continent or island.
The relations of the fauna of different regions are intimate in direct relation to the ease by which barriers may be crossed. Distinctness is in direct proportion to isolation. What is true in this regard of the fauna of any region as a whole is likewise true of any of its individual species. The degree of resemblance among individuals is in direct proportion to the freedom of their movement, and variation within what we call specific limits is again proportionate to the barriers which prevent equal and perfect diffusion.
The various divisions or realms into which the surface of the earth may be divided on the basis of the differences in animal life each has its boundary in the obstacles offered to the spread of the average animal. Each species broadens its range as far as it can. It struggles knowingly or not to overcome the barriers of ocean or river, of mountain or plain, of woodland or desert, of moisture or drought, of cold or heat, of lack of food or abundance of enemies, whatever these barriers may be. Were it not for these barriers, every species would become what only man now is, practically cosmopolitan. Man is pre-eminently the barrier-crossing animal. The degree of hindrance offered by any barrier to the extension of species is only relative. That which constitutes an impassable barrier to some groups is a high-road to others. The river which opposes the passage of the monkey or the cat would be the king's highway to the frog or the turtle. The waterfall which checks the ascent of the fish would be the chosen home of the ouzel.
In spite of the great variety among the barriers existing on the earth, we may divide the globe roughly into five realms or areas of distribution, having their boundaries in the sea or in differences of climate. One or two of these realms are sharply defined; the others are surrounded by a broad fringe of debatable ground, which forms a region of transition to some other zone.
The largest of these realms is the holarctic realm, which comprises nearly all of Asia, Europe, and North America, the arctic and north temperate zones. The north temperate zone has practically a continuous climate, the chief variations being in elevation and rainfall. The close union of Alaska to Siberia forms an almost unbroken land area from the eastern coast of America around to western Europe. To the south the species increase in number and variety; the arctic regions are remarkable for what they lack, yet the general character of the life is almost unbroken over this vast district. Alfred Russel Wallace refers to this unity of northern life in these words:
"When an Englishman travels by the nearest sea route from Great Britain to northern Japan, he passes by countries very unlike his own both in aspect and in natural productions. The sunny isles of the Mediterranean, the sands and date-palms of Egypt, the arid rocks of Aden, the cocoa-groves of Ceylon, the tiger-haunted jungles of Malacca and Singapore, the fertile plains and volcanic peaks of Luzon, the forest-clad mountains of Formosa, and the bare hills of China pass successively in review, until after a circuitous journey of thirteen thousand miles he finds himself at Hakodadi in Japan. He is now separated from his starting-point by an almost endless succession of plains and mountains, arid deserts or icy plateaus; yet, when he visits the interior of the country, he sees so many familiar natural objects that he can hardly help fancying he is close to his home. He finds the woods and fields tenanted by tits, hedge-sparrows, wrens, wagtails, larks, redbreasts, thrushes, buntings, and house-sparrows, some absolutely identical with our own feathered friends, others so closely resembling them that it requires a practiced ornithologist to tell the difference. . .. There are also, of course, many birds and insects which are quite new and peculiar, but these are by no means so numerous or conspicuous as to remove the general impression of a wonderful resemblance between the productions of such remote islands as Britain and Yesso" (Island Life).
A journey to the southward from Britain or Japan or Illinois, or any point within the holarctic realm, would show the successive changes in the character of life, though gradual, to be more rapid. The barrier of frost which keeps the fauna of the tropics from encroaching on the northern regions once crossed, we come on the multitude of animals whose life depends on sunshine, the characteristic forms of the neotropical realm.
The neotropical realm includes South America, the West Indies, and the hot coast-lands of Mexico and Central America. To the northward, this realm overlaps the holarctic in the transition regions of Sonora, Arizona, Texas, and Florida; but to the southward the barrier of the broad ocean keeps it practically distinct from all others. The richness of this fauna in forms and species makes the great forests of the Amazon the dream of the naturalist. Joaquin Miller gives a vivid picture of the life of tropical America:
Birds hung and swung, green-robed and red,
Or drooped in curved lines dreamily,
Rainbows reversed from tree to tree,
Or sang—low hanging overhead,
Sang soft as if they sang and slept,
Sang low like some far waterfall,
And took no note of us at all.
Corresponding to the neotropical realm in position, but with a less rich and varied fauna, is the Ethiopian realm. This includes the greater part of Africa, merging gradually on the north into the holarctic realm, through the transition regions of Barbary, Italy, and Spain. In monkeys, herbivorous mammals, and reptiles, this region is wonderfully rich. In variety of birds and fishes the neotropical region far surpasses it.
The Indian realm comprises southern Asia and the neighboring islands. Its rich fauna has much in common with that of Africa, and it is, moreover, surrounded by transition districts which lead on the north to the holarctic, and on the west to the Ethiopian. On the east the Indian realm is lost in the islands of Polynesia, which represent each one its own degree of transition and isolation.
The Australian realm of Australia and its islands is more isolated than any of the others. It shows a singular development of low types of life, as though in the progress of evolution this continent had been left a whole geological age behind the others. It is certain that, could the closely competing fauna of the holarctic or Indian realms have been able to invade Australia, the dominant mammals and birds of that region would not have been marsupials and parrots. In the words of Prof. Bergen, "the antiquated forms of life are found in abundance only in regions where they have been long shut off from communication with the great land masses." The rapid multiplication which certain holarctic animals and plants have shown when transported to the Australian realm, demonstrates what might have taken place if impassable barriers had not previously shut them out.
Each of these great realms may be indefinitely subdivided into provinces and sections, for there is no end to the possibility of analysis. No township or school district has exactly the same animals or plants as any other; and, finally, in ultimate analysis no two animals or plants are alike. Modification comes with the growth of each new individual, and steadily increases with the individual's separation in time or space from the parent stock. Moreover, we observe apparent anomalies of distribution in every realm: here appears an animal, there a plant, which seems to have a character or a place which it ought not to hold. To the result of unexpected or chance crossing of barriers these apparent anomalies in geographical distribution are due. Anomalies in distribution, like anomalies in evolution, would cease to be such if we knew all the facts and circumstances of their previous history. The present range of the tapir in Farther India and in the northern part of South America, two widely separated regions, is at first sight an anomaly of distribution. This anomaly disappears when we know that formerly the tapir ranged over the holarctic realm and became gradually extinct with the changing climate. The bones of a tapir, much like one of the South American species, are found in recent clays in Indiana (Ellettsville), and similar remains exist in France, in China, and in Burmah. The isolated, unexterminated colonies are now left at the extremes of the animal's former range, and these colonies at present constitute what we call distinct species.
The more extended are our studies the fewer are the anomalies which arrest our attention, and the fewer are the distinctive or characteristic forms. There is little foundation for the current belief that each species of animal has originated in the area it now occupies, for in many cases our knowledge of paleontology shows the reverse of this to be true. Even more incorrect is the belief that each species occupies the district or the surroundings best fitted for its habitation. This is manifest in the fact of the extraordinary fertility and persistence shown by many kinds of animals and plants in taking possession of new lands, which have become, through the voluntary or involuntary interference of man, open to their invasion. Facts of this sort are the "enormous increase of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the sparrow in North America, though in none of these cases are the animals natives of the countries in which they thrive so well" (Wallace). The persistent spreading of European weeds to the exclusion of our native plants is a fact too well known to every farmer in America, The constant movement westward of the white-weed and the Canada thistle marks the steady deterioration of our grass-fields. Especially noteworthy has been this change in Australia and New Zealand. In New Zealand the weeds of Europe, toughened by centuries of struggle, have won an easy victory over the native plants. Edward Wakefield, in his history of New Zealand, says that "many animals and birds acquire peculiarities in the new country which would indeed astonish those accustomed to them in the old. They usually run to a much larger size and breed oftener. They also take to strange kinds of food. Birds deemed granivorous at home become insectivorous here, and vice versa. Some learn the habits of the native species. Skylarks imitate the native wagtail, and may often be seen perching on fences and telegraph wires. They sing in the night-time, too, a thing unheard of in the old country, and doubtless acquired from the nocturnal habits of New Zealand birds."
The European house-fly in New Zealand has completely extirpated the large blue-bottle fly which was formerly a source of great annoyance to the settlers. An account is given of a farmer who filled a bottle with house-flies and carried them eighty miles into the country, liberating them one by one, in the vicinity of his sheep-folds, in order to let them take the place of the native flies.
It is said that red clover would not grow in New Zealand until bumble-bees were introduced to fertilize its flowers. Wakefield estimates that the introduction of these large wild bees has been worth five million dollars to the farmers in New Zealand.
Dr. Hooker states that, in New Zealand, "the cow-grass has taken possession of the road-sides; dock-and water-cress choke the rivers, the sow-thistle is spread all over the country, growing luxuriantly up to six thousand feet; white clover in the mountain districts displaces the native grasses," and the native (Maori) saying is, "As the white man's rat has driven away the native rat, as the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself" (E. L. Youmans).
- An address delivered before the Chicago Institute, in a course on the Testimony of Science in regard to Evolution.