EVOLUTION. 330 EVOLUTION. 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 iitions 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 tiie 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 changes which have taken place in the physical geography of our globe; and these are of prime importance as respects the evolution of life on its surface and tin- varia- tion of life forms; and yet there were long peri- ods of rest, succeeded by local catastrophe* 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 tin- effect which the geologically sudden lowering of the climate had on plant and animal life, causing extensive migrations and adaptations (as of the polar plant- 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- zoic ami Tertiary ti s. when va-t regions of (lie globe «cn- carried up into the air. so to -peak, 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, tin 1 drainage oi continent., and (lie formation of extensive deserts. accompanied by the adaptation of much plant and animal life to them, should also be taken into
- n eounl as producing variai ion.
Evidence fbom Paleontology. Euxley affirmed thai the primary and direct evidence in favor of evolution can be furnished only by paleontology, and its evidence i-. indeed, oi tie' strongest na- ture di trie and eonclu -ion- of paleon- adding each year t< ngth of the tent. The. iii the Cambrian rocks of fourteen classes of marine invertebrate animals nrnl priiiiii iv brian annelids, trilohites, crustaceans, and othei forms are highly developed. .Some, as the trilo- hites, are old-fashioned, generalized types; some of the Crustacea are composite 01 generalized types, as the Phyllocarida ; but the annelids arc a- highly specialized as their representatives of to-day. The earliest trilohites (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. Another period of the apparently rapid evolu- tion of life forms was the time of the Appalachi- an revolution, when vertebrates with lungs and limbs appeared, and the forerunners of reptiles, birds, and mammals probably originated. In these early times the Precambrian, as well as the opening ages of the Mesozoic, animal types were more plastic than now; dynamic evolution and use-inheritance did their work in the origination of class and ordinal types with comparative sud- denness. Paleontology teaches the fact of the rise, cul- mination, and death of types; the origin of life from generalized forms and their gradual modi- fication and specialization. It is safe to say that the ancestral forms of most, if not all, the classes of animals began with composite or syn- thetic types. The geological succession of the arthropod classes, a- well as those of the verte- brate phylum, all tell the same story. What morphology and embryology strongly suggest is emphatically confirmed by the series of fossil re- mains. The origin of reptiles, of birds, of mammals, and of man from generalized types is now placed beyond a reasonable doubt. Familiar examples of those principles or laws of organic evolution are afforded by the genealogy of the horse family (see HOESE, FOSSIL; Camei.id.e: . the ox. deer. cat. and other families and orders of vertebrates. And so it is with the phyla into which the arthropods will have to lie divided. There are lines of development which have undergone a continual course of modifica- tion by the rapid development by exercise of the brain, limbs. an d teeth, and the reduction or atrophy of digits or teeth and other hard and 50ft parts. On the other hand, certain types have never made any progress, and show little advance over their Paleozoic ancestors; such are the Forami- nifera. the sponges, the coral-, certain mollusks, as nautilus, king-crabs. Lingula, and even Cera- t.iilus and llatteria. Certain arthropod-, as Peripatus, Scolopendrella, and Campodea, are probably persisteni tj pes. Geological extinction ha- been iluc to obvioui causes, such a- changes in climate, the eleva lion of one area and the subsidence of another. also to the competition with other types. If these causes are quite obvious in their result follows that the same cause- which led to the ex tinction of some form- exerted an influence in modifying other-. It should he observed that iii, imperfection of the geological record is still
