Popular Science Monthly/Volume 37/August 1890/Evolution and the Distribution of Animals II
|EVOLUTION AND THE DISTRIBUTION OF ANIMALS.|||
I was lately called to examine a specially interesting problem in geographical distribution, that of the dispersion of fishes in the Yellowstone Park. This region is a high volcanic plateau, formed by the filling of a mountain basin with a vast deposit of lava. The streams of the park are for the most part among the coldest and clearest of the Rocky Mountains, and apparently in every way suitable for the growth of trout. All the hot springs of the great Geyser basin are not sufficient to warm the waters of the Fire-hole River. Yet, with the exception of the Yellowstone itself, all these streams are destitute of fish-life. A reason for this is apparent in the fact that the plateau is fringed with cataracts which no fish can ascend. Each stream has a canon and waterfall near the point where it exchanges the hard bed of lava for the softer rock below. So the best of trout-streams, for an area of fifteen hundred square miles, are left without trout, because their natural inhabitants can not get to them.
On the theory that each species occupies those places best suited to its life, this fact would represent a great oversight on the part of Mother Nature. But with this is the curious fact that the Yellowstone itself, both above and below its falls, is well stocked with trout and with no other fish. This is an anomaly of distribution, but this anomaly disappears when we examine the continental divide as it appears at the head of the Yellowstone. At one point, the Two-Ocean Pass, only about an eighth of a mile of wet meadow and marsh separates the drainage of the Yellowstone from that of the Columbia. From the Columbia the Yellowstone has therefore received its trout. No doubt every anomaly of distribution would become perfectly simple could we only know all the facts which bear on the case.
In my studies of the fishes of America I have had occasion to especially investigate the barriers to their distribution, and the relative value of these as limiting the range of the different forms.
In general we may say that, with rare exceptions, in all waters not absolutely uninhabitable, there are fishes. The processes of natural selection have given to each kind of river or lake species of fishes adapted to the conditions of life which obtain there. There is no state of water, of bottom, of depth, of speed of current, but finds some species with characters adjusted to it. Each of these species has an ascertainable range of distribution, and within this range we may be reasonably certain to find it in any suitable waters.
But every species has beyond question some sort of limit to its distribution, some barrier which it has never passed in all the years of its existence. That this is true becomes evident when we compare the fauna of widely separated rivers. Thus the Sacramento, Hudson, St. Johns, and Rio Grande have not a single species common to any two of them. None' of them has any species peculiar to itself, and each one shares the greater part of its fauna with the water-basins nearest to it.
With the shore fishes, as with other water animals, the barriers are primarily the heights of the land and the depths of the sea—physical obstacles not to be crossed. Next in importance is the barrier of climate. With some forms of life this is absolute, for the palm and the banana are the index of the torrid zone as the dwarf birch and reindeer moss are the index of the frigid. "Plants" says Dr. Gray, "are the thermometers of the ages by which climatic extremes and climates in general are best measured." In many groups anatomical characters are not more profound or of longer standing than are the adaptations to heat and cold. Heat-loving animals are far more numerous in species than animals of cold climates, though the latter often make up by greater abundance of individuals. Barriers less important than those of climate arise from external surroundings—from absence of means of defense, from character of food, of air, of water, and the presence of various enemies. These conditions vary in their importance with each group of animals, yet apparently the least of them may be able to limit the range of species. To limit the range is the first step toward extinction, for to cease to advance is to retreat. Adverse conditions may invade even the heart of its distribution, causing reduction of numbers, which, if long continued, must mean rarity and final extermination. Extinction comes to those species we call rare, and its advent must be unnoticed. Circumstances become unfavorable to the growth or reproduction of some animal. Its numbers are reduced—it is rare—it is gone.
The air in Indiana but a few years since was dark with the hordes of passenger pigeons at the time of their fall migrations. The advance of a tree-destroying, pigeon-shooting civilization has gone steadily on, and now who has seen a passenger pigeon? I have seen them, and I have a skin or two in my collection, but the bird I knew as filling the trees in my boyhood is now in the same region an ornithological curiosity.
A very slight change in the environment of any species may be a matter of the greatest moment as regards its increase or permanence. The dependence of the clover on the number of cats in a certain neighborhood is an illustration given us by Mr. Darwin. The clover depends on the bumble-bee for the fertilization of its pods. The nests of the bumble-bee are destroyed by the field mouse, which is thus an enemy of the clover. The balance is restored by the work of the cat, who captures the mouse and prevents its ravages on the nests of the bee. The old nursery jingle of the cow that tossed the dog that worried the cat that killed the rat is repeated throughout nature. With any change in any of the elements in this series the whole equilibrium of nature is interrupted. For this equilibrium is apparent only—a sort of armed neutrality, an established order of things which the superficial observer mistakes for real peace and permanence.
In some groups we find evidence of a progressive adaptation of individuals to circumstances—for example, to climate, ending in the formation of new species to accord with changed conditions of temperature. We may illustrate this by means of the arctic birches. In Norway, as in most northern regions with a moist climate, there are large forests of birches. In the valleys, where the summers are warm and reasonably long, the birches of different species grow to be considerable trees. Farther to the north, or higher up the mountains, the summer is too short for the growth of birch-trees, and their place is taken by birches which never pass beyond the size of small bushes. Still higher up there are birches even where snow falls during every month of the year, and the distant sun gives only a glimpse of summer in July. Competition with other plants is, of course, not severe in such regions, but the birches must struggle against the weather. They can live and multiply, if only they can adjust themselves to the conditions of life. They must keep down their size, they must carry as little foliage as possible, and their stems must be tough enough to resist snow, and hardy enough to withstand almost perpetual frost. Their year's growth must be finished in a very short time, and leaves, flowers, and seeds must follow in the most rapid succession. In short, there is room for birch-trees here, if only the trees can be reduced to their lowest terms. And so birch-trees have crept up the mountain-sides even to the very edges of the perpetual snow. But such trees! All trees requiring sunshine, or long time for their summer's growth, are rigidly kept away by "natural selection." The cold climate dwarfs the individual, and the hard conditions exclude every individual not dwarfed. I have before me three birch-trees from a Norwegian mountain called the Suletind—the little trees known to the Norwegian peasants as "Hundsöire," or "dogs'-ears." The trunk of each tree is barely an inch in height. There are no branches, and but three leaves. Half inclosed by the uppermost leaf is the single little catkin of flowers. Leaves in June, blossoms in July, fruit in August, and then the little tree is ready for its nine months' sleep. These little trees are the Lapps of forest vegetation.
All natural history is full of similar cases of modifications. Everywhere there is the most perfect adaptation of life to its conditions. But this adaptation must come about through the survival of those organisms fittest to live under the conditions, while the unfit die out and leave no progeny. But fitness is a relative term; for in many cases, as with the Norwegian dwarf birches, the deformed or stunted may be the only ones fitted to survive. An advantage ever so slight must in the long run conquer. The gambler recognizes that final victory must always go with the percentage of the dealer.
The restlessness of individuals is the key to all these problems. Each species of animal or plant is first the product of heredity, and then of the various influences, reactions, and extinctions to which we give the name of natural selection. Each species may be conceived as making every year inroads on territory occupied by other species. If these colonies are able to hold their own in the struggle for possession, they will multiply in the new conditions, and the range of the species becomes widened. If the surroundings are different, new species or varieties may be formed with time; and these new forms may invade the territory of the parent species. Again, colony after colony of species after species may be destroyed by other species or by uncongenial surroundings.
Only in the most general way can the history of any species be traced; but, could we know it all, it would be as long and as eventful a story as the history of the colonization and settlement of North America by immigrants from Europe. Each region where animals or plants can live has been thousands of times discovered, its colonization a thousand times attempted. In these efforts there is no co-operation. Every individual is for himself, every struggle a struggle of life and death; to each species each member of every other species is an alien and ah enemy.
The arctic birches serve as one illustration only of the spread and change of organisms in the face of a barrier apparently insurmountable. I can not enter into detail as to the many ways in which individuals manage to cross the barriers which usually limit the species. These ways are as varied as the creatures themselves, and infinitely more varied than the barriers. It is enough to say that organisms have extended their range in regions where their existence is possible. Here, by the long-continued process of adjustment to circumstances, with the incessant destruction of the unadapted, these organisms have become so well fitted to their surroundings as to give rise to the popular impression that each species now inhabits that part of the world best fitted for its occupation. Yet the very reverse of this must be true, for in the growth of any species it is these features of adaptation which are the last to appear. If, as anatomists now teach, the history of the individual is an epitome of the history of the group to which the individual belongs, then adaptive characters appearing late in the growth of the individual must have appeared late in the history of the group. They are the last changes made in the organism—mere after-thoughts in the work of creation.
For example, the long pectoral fins of the flying-fish enable it to make great leaps through the air, after the manner of the grasshopper. Yet we can not say that the flying-fish was meant to be the bird among fishes, for its nearest relatives are without wings, and the wing-development is one of the last acquisitions of the individual. Its flight is simply an exaggeration of the leaping or skimming which related forms with shorter fins accomplish. The growth of the fins goes on with the increase of this power, and greater power comes with the growth of the fins.
To my mind the strongest arguments for the theory of development are those drawn from the changing character of the species themselves.
No phase in the history of systematic science is more instructive than the varying attitudes of the naturalist toward those local modifications of species called geographical variations.
It was early noticed that, while individuals of any one species in any limited region are substantially alike, this perfect identity disappears with the examination of wider extent of territory. These differences were often too small to justify the formation or recognition of a new species, but too evident to be wholly neglected. These subordinate species were termed by Linnæus varieties, and their geographical basis was often recognized. Thus, of his Homo sapiens, or aboriginal man, Linnæus recognized four varieties—asiaticus, americanus, afer, and europæus. As with the varieties of man, so with those of other animals and plants. The individuals of England were not quite those of the same species in Italy, and those in America showed their own peculiarities.
Sometimes these qualities could be exactly measured, in which case a new species was described. Sometimes they proved elusive, and the supposed new species were added to the great dust-heap of synonymy. The work of the systematic zoölogists of the last generation was chiefly in museum cataloguing and labeling. To them these half-tangible varieties were the object of special opprobrium. On the museum shelves they were simply a nuisance, obscuring the characters of the real species and throwing closetformed ideas of nature into utter confusion. Prof. Cope tells us how variant shells have been crushed under the heel of the indignant conchologist, because they would go neither into species A "nor species" B. "Specimens were often preserved from typical localities," so that no confusion might be introduced among the cherished specific characters. That Nature went on producing these varying and intermediate forms was no concern of the zoologist. That such forms were any part of Nature's plan apparently never occurred to the followers of Linnæus.
Says the botanist De Candolle: "They are mistaken who suppose that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. This seemed to be true so long as a genus was imperfectly known, and its species were founded on few specimens—that is to say, were provisional only; just as we come to know them better, intermediate forms flow in, and doubts as to the limits of the species become more numerous."
The ease with which slight variations have deceived and confused naturalists is one of the most discouraging features in the history of science. Such variations have formed the basis of thousands of useless and distracting names.
When Darwin was at work upon his monograph of the barnacles, he wrote to a friend:
"Systematic work would be easy were it not for this confounded variation, which, however, is pleasant to me as a specialist, though odious as a systematist. . . . How painfully true is your remark that no one has hardly a right to examine the question of species who has not minutely described many!... Certainly I have felt it humiliating, discussing and doubting and examining, over and over again, when in my mind the only doubt has been whether the form varied to-day or yesterday. . . . After describing a set of forms as distinct species, tearing up my manuscripts and making them one species, tearing that up and making them separate, and then making them one again (which has happened to me), I have gnashed my teeth, cursed species, and asked what sin I had committed to be so treated."
An epoch in systematic zoölogy began with the study of the collections made by the United States Pacific Railway Survey some thirty years ago. This was the first opening out to naturalists of the details of the fauna of a vast district under the same parallels of latitude, but showing every variation in rainfall, elevation, and physical surroundings. The most valuable results of these collections were seen in the study of birds. It was found in general that each bird of the Atlantic States had its counterpart in the prairies, the sage-plains, the mountains, and the Pacific slope. Differences were carefully sought for and found, for the school of Prof. Baird allowed nothing to escape their analysis. There were differences in size, in form and color, slight in degree, but nevertheless really existing, and these were made the basis of as many distinct species. Still further studies increased the number of these species, until at last a large proportion of our birds were represented by Eastern, Western, sage-brush, and prairie species. Sometimes these closely connected forms were distinguishable at first sight, as in the case of the yellowhammer, and its double, the red-shafted flicker; in other cases baffling the most skillful, as with the two species of the crow-blackbird.
An illustration of these forms and their relations may be taken from the common shore lark and its varieties, although it is fair to say that some of these variations have never been regarded as species.
The shore lark, or horned lark (Otocoris alpestris), ranges widely over the colder and open parts of Europe, Asia, and America. The common form, called alpestris, is familiar to most of us. In the Northwestern region, as far south as Utah, is another form, equally large, but paler in color (eucolæma). In the prairie region the lark is of the ordinary color, but smaller (praticola). In the sage-plains, it is a similarly small but pale lark, with brighter yellow in its throat; this is arenicola. In Texas the bird is still smaller and grayer (giraudi); while the small form found in New Mexico and Arizona has its plumage strongly washed with red; this is chrysolæma. In the interior of California the shore larks are still smaller and redder (variety rubea), while northward and coastwise appears a small lark with more streaked plumage; this is strigata. All these can be generally recognized by an expert ornithologist, and doubtless a closer analysis would reveal the basis for still finer subdivisions.
In 1871 Dr. Joel A. Allen published his masterly paper on the Mammals and Winter Birds of Florida. This memoir has had the practical effect of making all our ornithologists, for the most part against their will, believers in the theory of derivation of species.
Dr. Allen took up, as a matter of serious study, the variations in individual birds. He showed that the variation of individuals of the same species was far greater than had been supposed, and that the characters relied on to distinguish species were often due to slight increase in these variations. For example, in Northern birds the bodies would be larger, the bills smaller than in birds of the same species from the South, and the coloration of birds was often directly related to the degree of rainfall. He showed, in brief, that each one of these many variations must be held to define a distinct species, or else that the number of species of American birds would have to be greatly reduced, and the range of variation inside the species would need to be correspondingly extended.
This claim for attention on the part of the despised variety produced much consternation among students of birds. But facts must be recognized; and the final result has been, that we have now extended our idea of each species until it is large enough to include all that we know of intermediate and varying forms. When a hiatus appears, whether existing either in fact or in our material for study, there we put our line of definition. "We can only predicate and define species at all," says Dr. Coues, "from the mere circumstance of missing links. Species are the twigs of a tree separated from the parent stem. We name and arrange them arbitrarily, in default of a means of reconstructing the whole tree in accordance with Nature's ramifications."
What is true of birds is equally the case with other groups of animals. Continued explorations bring to light each year new species of American fishes, but the number of new forms discovered each year is usually less than 'the number of old supposed species which are found to intergrade with each other, and have so become untenable.
I have myself published three complete lists of the fresh-water fishes of North America. The one published in 1876 enumerated 670 species; that of 1878, 665 species; while the list of 1885 contained 587 species, although upward of 75 new species had been found in the nine years which elapsed between the first and the last of these three lists.
The old idea of a species as a separate entity, a special creation, has passed away forever. We can no more return to it than astronomers can return to the Ptolemaic notion of the solar system. The same lesson comes up from every hand. It is the common experience of all students of species. We have learned it from Gray and Engelmann and Coulter, and from each of the many students of American botany. We have learned it from Baird and Allen and Coues and Ridgway and Stejneger, and from all who have made life-studies of American birds. We have learned it from Cope and Marsh and Leidy, and from all who have searched the rocks for the bones of our ancestors.
I do not know of a single naturalist in the world, who has made a thoughtful study of the relations of species in any group, who entertains the old notion as to their distinct origin. There is not one who could hold this view, and look an animal in the face! The study of the problems of geographical distribution is possible only on the theory of the derivation of species. If we view all animals and plants as the results of special creations in the regions assigned to them, we have, instead of laws, only a jumble of arbitrary and meaningless facts. We have been too fully accustomed to the recognition of law to believe that any facts are arbitrary and meaningless. We know no facts which lie beyond the realm of law. I may close with the language of Asa Gray:
"When we gather into one line the several threads of evidence of this sort to which we have here barely alluded we find that they lead in the same direction with the clews furnished by [other lines of investigation]. Slender indeed each thread may be, but they are manifold, and together they bind us firmly to the doctrine of the derivation of species."
- An address delivered before the Chicago Institute, in a course on the Testimony of Science in regard to Evolution.
- In the Auk for April, 1890, is an essay on the Horned Larks of North America, by Jonathan Dwight, Jr. Mr. Dwight's conclusions are based on 2,012 specimens; those of Mr. H. W. Henshaw, above given, on 350. To the forms mentioned above, Mr. Dwight adds var. adusta, small, and "scorched pink" in general hue, from southern Arizona and northern Mexico; var. menilli, large and dusky, in Idaho and neighboring regions; and var. pallidus, very small and pale, from Lower California.
- Dr. Allen says, in a recent paper: "We arbitrarily define a species as a group of individuals standing out distinct and disconnected from any similar group, within which, though occupying different parts of the common habitat, we recognize other forms characteristic of and restricted to particular areas. These reach a maximum degree of differentiation at some point in the habitat, and thence gradually shade into other con-specific forms geographically contiguous."—On the Recognition of Geographical Forms; The Auk, January, 1890, p. 1.