EXTINCTION OF SPECIES. The extinc tion of species and higher groups has been due to two causes—first, changes in the physical geog raphy 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: "['he extermina tion of whole groups, as of ammonites toward the close of the Secondary period, has been won derfully 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 sudden ness. Coming down to the Mesozoic age, there was a remarkable extinction of types. The great er number of crinoids and brachiopods. and all the dinosaurs and o•nithosaurs, 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 common with the Cuvierian catastrophic doctrine of sudden wholesale extinctions and recreations. But known facts of geology postulate long peri ods 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 Huro nian are unconformable to each other, the Ke weenawan beds are uneonformable to the 'Biro nian. Between each two series is an "inconformity representing an interval of tune lung enough' for the land to have been raised above the scar, for the rocks to have been folded and to have lost by erosion thou:still& 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 contincnt-making, and on that whole WZI s the most extensive mid bio logically 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 prob able consequence of. this revolution was the in
coming of the vertebrates with limbs and lungs, adapted to a terrestrial life'. The Appalachians of the Paleozoic times were perhaps as high as the Sierra Nevada or Andean Cordillera of the present time. During this period the cryptog anions forests and their animal life may have been confined to the coastal plains and lowlands, while on the higher, cooler levels may have ex isted 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 reiIlevation, 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 por tion of the earth's surface, resulting in the for mation 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. Tn other parts of the world there were corresponding changes. The later revolutions, as chose 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 west ern Alps rose to a height of 11,000 feet, and the Himalayas to a horizon Moue feet above the sea, while there were corresponding elevations in western North America and in the Rocky Moun tain region.
The last great revolution, which, profound and widespread as it was in the Northern Hemi sphere, 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 modifica tions 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.