1880); Buller, Birds of New Zealand (2d ed., London, 1888); Newton, Dictionary of Birds (London and New York, 1893-96); Bryden, Nature and Sport in South Africa (London, 1897); Dixon, Lost and Vanishing Birds (London, 1898); Annual Report of Smithsonian Institution for 1888 (Washington, 1889). Consult, also, standard works of zoölogy, and the bibliography under Domestic Animals, and the various titles referred to in the body of the article.
EXTINCT BIRDS. Various birds may be called ‘extinct’ instead of ‘fossil,’ because their species have expired since the present geological era began, or, in several cases, since written records began to be made. In most cases these were birds that belonged to ancient and senescent races, such as the ratite moas (q.v.) and their kindred; or they were species of extremely limited range, or of degenerate powers, due to an insular habitat or other unfavorable surroundings. See Extinct Animals.
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
