The New International Encyclopædia/Extinction of Species
EXTINCTION OF SPECIES. The extinction of species and higher groups has been due to two causes—first, changes in the physical geography and other environmental conditions of the globe during past geological time, and, second, to changes in the biological environment.
Geological Extinction—The primary factor, therefore, in the extinction as well as the origin of life-forms is geological changes. If we glance back through the geological ages, we shall see that there were instances of the comparatively rapid extinction of types or whole groups (orders and classes) of animals. The more remarkable were the death and disappearance of the trilobites and ammonites. Darwin remarks: “The extermination of whole groups, as of ammonites toward the close of the Secondary period, has been wonderfully sudden.” The trilobites as well as the important order of Eurypterida ceased to exist at the end of the Paleozoic era; the Silurian graptolites, that very considerable group of hydroids, disappeared with comparative suddenness. Coming down to the Mesozoic age, there was a remarkable extinction of types. The greater number of crinoids and brachiopods, and all the dinosaurs and ornithosaurs, as well as the pythonomorphs, these groups comprising the most highly organized reptiles which have ever lived, wholly perished toward or at the close of the Cretaceous period.
It should be borne in mind that these facts of comparatively rapid extinction have nothing in union with the Cuvierian catastrophic doctrine of sudden wholesale extinctions and recreations. But known facts of geology postulate long periods of quiet preparation, succeeded by more or less sudden crises, or radical changes in the physical structure of continents, resulting in catastrophes, both local and general, to certain faunas or group of animals, as well as individual species. These so-called catastrophes, though geologically sudden, may have required thousands of years for accomplishment.
There have been in the course of the earth's history a number of crises or revolutions, which were attended with the loss and extinction of types.
There were enormous changes in the relative distribution of land and sea in pre-Cambrian times. The strata of the lower and upper Huronian are unconformable to each other, the Keweenawan beds are unconformable to the Huronian. Between each two series is an unconformity representing an interval of time long enough for the land to have been raised above the seas, for the rocks to have been folded and to have lost by erosion thousands of feet, and for the land to have sunk below the surface of the ocean. Again, between the pre-Cambrian and Cambrian eras there was a great uplifting and folding of rock, succeeded by long-sustained erosion, over all the continental era.
At the end of the Paleozoic era occurred the Appalachian revolution. This was a period of mountain-building and of continent-making, and on the whole was the most extensive and biologically notable event in geological history. In its effects on life, whether indirect or direct, it was of vastly greater significance than any period since, for contemporaneous with, and as a probable consequence of, this revolution was the incoming of the vertebrates with limbs and lungs, adapted to a terrestrial life. The Appalachians of the Paleozoic tines were perhaps as high as the Sierra Nevada or Andean Cordillera of the present time. During this period the cryptogamous forests and their animal life may have been confined to the coastal plains and lowlands, while on the higher, cooler levels may have existed a different assemblage of life; and it is not beyond the reach of possibility that a scanty subalpine flora and fauna peopled the still cooler summits. But this process of mountain-building and erosion was not confined to the end of the Paleozoic era. Since that period there have been along the Atlantic border of the growing and changing continent several successive cycles of denudation extending down to the present time. The great Appalachian plateau with its lofty mountain ranges and peaks rising from the shores of the Atlantic probably presented during the Mesozoic era different climatic zones, from tropical lowlands with their vast swamps, to temperate uplands, stretching perhaps up to Alpine summits. New Zealand at the present day has a subtropical belt of tree-ferns, while the mountains bear glaciers on their summits. The Jurassic was a time of great denudation, when the high ranges of the Appalachian plateau were worn down, and the newly upheaved, tilted, and vaulted beds of the Trias were deeply eroded. During the Cretaceous period this region was a peneplain, the scenic features roughly recalling those of North Carolina and New England at the present day. Then there was a reëlevation, and in the Eocene Tertiary period the swelling and upheaval of the Appalachian dome began again.
We can in imagination see, as the result of these changes in a comparatively restricted portion of the earth's surface, resulting in the formation of separate basins or areas inclosed by mountain ranges, with different climates and zones on land, what a profound influence must have been exerted in the origination and also the extinction of species. In other parts of the world there were corresponding changes. The later revolutions, as those of Tertiary times, were perhaps less marked and extensive. Yet toward the close of this period the great mountain ranges of Asia and Europe, the Alps, Pyrenees, Caucasus, Himalayas, as well as the Atlas of North Africa and the Cordilleras of North and South America were upheaved. The western Alps rose to a height of 11,000 feet, and the Himalayas to a horizon 16,000 feet above the sea, while there were corresponding elevations in western North America and in the Rocky Mountain region.
The last great revolution, which, profound and widespread as it was in the Northern Hemisphere, did not apparently affect life and nature in the tropical zone, was the Glacial period. During this time there was, besides extensive migrations southward, and consequent modifications of species which could not resist the cold, a widespread extinction not only of numberless individuals, but of floras and faunas, a few forms becoming adapted to a circumpolar climate.
Biological Extinction Due to Competition. During all these changes, as the result of the struggle for existence, the competition between the outgoing and the incoming types and floras and faunas, there resulted vast biological changes, i.e. extinctions and re-creations.
In summing up the grand results of the Appalachian revolution and of the times immediately succeeding, Packard states that we should not lose sight of the fact that the changes in the earth's population were due to biological as well as geological and topographical factors. The process of extinction was favored and hastened by the incoming of more specialized forms, many of them being carnivorous and destructive. For example, nearly all fishes and reptiles live on other animals. The struggle for existence between those which became unadapted and useless in the new order of things went on more actively than at present. The process of extinction of the higher, more composite amphibians (the labyrinthodonts) was largely completed by the multitude of theromorphs and dinosaurs which overcame the colossal Cheirotherium, Mastodonsaurus, and their allies. Woodworth also states that “the exact cause of their decline is probably to be sought in the development of the more powerful reptiles.” The demise of the ornithosaurs or pterodactyls was assisted, says Packard, in two ways: Those with a feebler flight succumbed to the agile, tree-climbing dinosaurs; while the avian type, waxing stronger in numbers and powers of flight and exceeding in intelligence, exhausted the food-supply of volant insects, and drove their clumsier reptilian cousins to the wall, fairly starving them out; just as at the present day the birds give the bats scarcely a raison d'être.
At the close of the Jura-Trias period there was a widespread extinction of the peculiar coniferous plants of the Mesozoic, and they were succeeded by forests of deciduous trees of modern types. Vast forests of deciduous trees, such as the oak, sassafras, poplar, willow, maple, elm, beech, chestnut, and many others, as well as of conifers and palms, clothed the uplands, while in the jungles, on the plains, and in the openings of the forests, gay flowers bloomed. The flora must even then have been comparatively speaking, one of long existence, because highly differentiated composite plants, like the sunflower, occur in the Upper Cretaceous or Raritan clays of the New Jersey coast.
While the changes of level did not affect the abysses of the sea, the topography of the shallows and coast was materially modified, and to this was perhaps largely due the extinction of the ammonites and their allies.
In 1862 Wood more fully discussed this matter, and mentioned the same cause as suggested by Packard. “This disappearance,” says Wood, “of the Ammonitidæ, and preservation of the Nautilidæ, we may infer was due to the entire change which took place in the condition of the shores at the close of the Cretaceous period; and this change was so complete that such of the shore-followers as were unable to adapt themselves to it succumbed, while the others that adapted themselves to the change altered their specific characters altogether. The Nautilidæ having come into existence long prior to the introduction of the Ammonitidæ, and having also survived the destruction of the latter family, must have possessed in a remarkable degree a power of adapting themselves to altered conditions.” On the other hand, the dibranchiate cephalopods (cuttles or squids), living in deeper water, being ‘ocean-rangers,’ were quite independent of such geographical changes. Wood then goes on to say that the disappearance of the tetrabranchiate group affords a clew to that of the Mesozoic saurians, and also of cestraciont sharks, whose food probably consisted mainly of the tetrabranchiate cephalopods. “Now the disappearance of the Tetrabranchiata, of the cestracionts, and of the marine saurians, was contemporaneous; and we can hardly refuse to admit that such a triple destruction must have arisen either from some common cause or from these forms being successively dependent for existence upon each other.”
Woodworth suggests that mammalian life in the Mesozoic age was unfavorably affected by the nature of the peneplain of the Atlantic coast and by reptilian life.
“The weak marsupials or low mammals, which first appear in this country with Dromatherium in the tolerably high relief of the Trias, were apparently driven to the uplands by the more puissant and numerous reptilia of the peneplain. Their development seems also to have been retarded.” Again he says: “To sum up the faunal history of the Mesozoic alone, we have seen that pari passu with the creation of broad lowlands there was brought on to the stage a remarkable production of reptiles, a characteristic lowland life; and we note that the humble mammalia were excluded from the peneplain or held back in their development, so far as we know them by actual remains, during this condition of affairs until the very highest Cretaceous. At the close of the Mesozoic, the area of the peneplain was uplifted and there came into it the new life. Not only the changed geographic conditions, but the better titled mammalia also were probably factors in terminating the life of the peneplains.”
After the placental mammals once became established, as the result of favorable geographical conditions of migration, isolation, and secondarily of competition, the evolution as well as the elimination of forms, as is well known, went on most rapidly. Remains of over two thousand species of extinct mammals during Tertiary times which existed in America north of Mexico have been already described, where at present there are scarcely more than three hundred. This is an example of the amount of extinction which went on in a single class of animals. There must have been corresponding rates of extinction in the ease of birds, fishes, and insects.
The rapid summary we have given of the successive changes and revolutions in the earth's history, and the fact that they are accompanied or followed by the process of the extinction of the unadapted, and their replacement by the more specialized and better adapted, show that there is between these two sets of phenomena a relation of cause and effect. The subject is further illustrated by the extinction of life in South America.
The Andean plateau during the Quaternary period was paroxysmally elevated into the air some 12,000 feet. Packard calls attention to the possible results of such an enormous upheaval on the plants and animals of this region. Before and at the time this movement began, when the land was 12,000 feet lower than now, the Atlantic trade winds which now cross Brazil, impinge upon the Andes, and drop their moisture on the eastern slopes alone, then favored as well the western slopes and Pacific coast. The tropical flora and fauna now confined to the neighborhood of Guayaquil on the coast of Peru then probably spread over Bolivia, Ecuador, Peru, and Chile to Patagonia. At Riobamba, altitude 9200 feet, the climate and vegetation are temperate; here occur bones of the mastodon, horse, deer, and llama—animals which may have lived in a temperate climate. But was not their extinction, and that of the colossal sloths, armadillos, and other animals of the pampas largely due to a change of climate resulting from the elevation of the Andean plateau? As the land gradually rose, the atmosphere would become more rarefied and insupportable to tropical life; the animals and plants would either seek lower levels or undergo extinction, or in certain eases become modified into species suited to a temperate climate. As the plateau rose still higher, the air would become too cold and rarefied for even the mastodon and horse. Gradually an alpine zone became established, and finally the higher peaks of the Andes, at an elevation of 15,000 feet, became mantled with perennial snow, and on Chimborazo glaciers established themselves. We thus see how, within Quaternary times, temperate and alpine zones became established over the vast Andean plateau, originally, perhaps at the end of the Pliocene, a plateau of the third order, clothed with vast forests like those of Brazil and Venezuela.
Another, but more local cause of extinction, is seen in Great Salt Lake, Utah. Formerly this was a vast fresh-water lake, abounding in fish, insects, mollusks, and plants. When it was by elevation of the lake-basin transformed into a brine-pool, all life was extinguished, except a shrimp, a single species of fly, and an alga. So with deserts; when they are formed life is reduced to a relatively small proportion.
That there is a limit to the age of species as well as to individuals almost goes without saying. As there is in each individual a youth, manhood, and old age, so species and orders rise, culminate, and decline, and nations have risen, reached a maximum of development, and then decayed. The causes, however complex, are, in the case of plants and animals, apparently physical; they are general and pervasive in their effects, and have been in operation since life began; there have been critical periods in paleontological as well as geological history, and periods of rapid and widespread extinction as well as a continual, progressive dying-out of isolated species. Such extinction was, so to speak, a biological necessity, for otherwise there would have been no progress, no evolution of higher types.
Bibliography. Darwin, Origin of Species (London, 6th ed., 1882); Searles V. Wood, “On the Form and Distribution of the Land Tracts during the Secondary and Tertiary Periods, respectively, and on the Effects upon Animal Life which great changes in geographical configuration have probably produced,” in Philosophical Magazine, xxiii., p. 161 (Edinburgh, 1862); A. S. Packard, “Geological Extinction and some of its apparent causes,” in American Naturalist, xx. 29-40 (Philadelphia, January, 1886); “A Half-Century of Evolution, with special reference to the effects of Geological Changes on Animal Life,” in Proceedings of the American Association for the Advancement of Science (Boston, 1898); J. B. Woodworth, “Base Levelling and Organic Evolution,” American Geologist, xiv. (October, 1894).