GEOLOGICAL EVIDENCE. Our earth has had a history. Its age is approximately estimated to he about 50,000,000 years. Its history is divided by geologists into 'ages.' 'periods,' epochs.' etc. It is roughly estimated that about 30.000.000 years have elapsed since the deposition of the lowest fossiliferous rocks—those of Cambrian age. It is believed that this amount of time is suffi cient for the origin and development of all the forms of life with which we are thus far acquaint ed. The stratified are supposed to lie about 20 miles thick. the earth's crust about 100 miles in average thickne.,s.
During the enormous space of time since the Cambrian. the forces of life and nature have gone on much as at the present time, although the oceans and land masses down to the Glacial period practically had a subtropical climate. Yet there were revolutions, widespread changes of level in the relative distribution of land and water, so that the map of the world changed greatly at different periods. Hence there must have been successive changes of environment, the conditions of existence were unstable, there were vast migrations, and the founding of new col onies in regions opened up to migration resulting from the subsidence of one region and the elevation of another. Plateaus were elevated. mountain ranges formed, mountain peaks carved out of the mass of folded strata, and thus the entire plateau was finally worn down by the action of the rain and of rivers until the surface formed a peneplain. Such a his tory of topographical transformation occurred more than once on both the Atlantic and the Pacific coasts of the American continent. All these changes, these revolutions, such as the Ap palachian and those of the Glacial period, exerted a profound influence on the flora and fauna. The great lesson of geology is the immensity of time and the ceaseless elmnges which have taken place in the physical geography of our globe; and thee are of prime importance as respects the evolution of life on its surface and the varia tion of life forms; and yet there were long peri ods of rest, succeeded by local catastrophes and upheavals, though these so-called 'catastrophes,' however sudden geologically. may have extended through thousands of years. The breaks, as in dicated by local unconformities in the strata of different ages, were confined to comparatively limited areas. So that periods of what we call rapid extinction of life were also periods of the comparatively rapid evolution and specializa tion of plants and animals.
The changes of level, the great elevation of the land in the Northern and Southern Hemispheres, the widespread and profound change of climate which ushered in the Glacial period. and the effect which the geologically sudden lowering of the climate had on plant and animal life. causing extensive migrations and adaptations (as of the polar plants and animals) to their frigid environ ment, afford signal examples of the effect of geo logical changes on the extinction of some and the modification of other forms. So also the enor mous changes of level which occurred in Meso zoie and Tertiary times, when vast regions of the globe were earried up into the air, so to speak. and the climate changed from a tropical one to that of an elevated, cooler region. The very last changes of level which took place after the melting of the ice sheet, the drainage of inents, and the formation of extensive deserts. accompanied by the adaptation of much plant and animal life to them, should also be taken into account as producing variation.
EvmE,Ner; room PALEONTOLOGY. 1111Xley affirmed that the primary and direct evidence in favor of evolution can be furnished only by paleontology. and its evidence is. indeed. of the strongest na ture. Hie discoveries and conclusions of paleon tologists adding each year to the strength of the arsmment.
There are remains in the Cambrian reeks of fourteen elaS,Vti of marine invertebrate animals and traces of primitive plants. The Cambrian annelids, trilobites, crustaeeans, ;mil other class forms are highly developed. Some, as the trilo bites, are old-fashioned, generalized types; some of the crustacea are composite oi generalized types, as the Phyllocarida; but the annelids are as highly specialized as their representatives oi to-day. The earliest trilobites (q.v.) were blind or eyeless, though they may have descended from eyed forms. These and other facts strongly indi cate that the Precambrian, including the Hu ronian, and possibly the Upper Laurentian ocean, supported an abundant life, made up of proto zoans, sponges, and the ancestors of worms, mol lusks, arthropods, etc., and most probably of the vertebrates. The Precambrian time was a period of the rapid evolution of types: strati graphic geology shows that in this formative period there were widespread and rapid changes in the physical geography of the globe.