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 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. as 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 HORSE, FOSSIL; CAMELID.E;
ete.). 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 be 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. and teeth, and the reduction or atrophy of digits or teeth and other hard and soft parts.
nn the other hand. certain types have never made any progress. and show little advance over their Paleozoic ancestors; such are the Forami nifera, (lie sponges. the corals, certain mollusks, as nautilus, king-crabs. Lingnla, and even Cent tod us :end Hatt eria. Curtain arthropods, as Peripat us. Scolopendrella. and Campodea, arc probably persistent types, Cileologieal has been due to obvious ea wws. such as changes in climate, the eleva tion of one area and the subsidence of another, as also to the competition with oilier types. If these causes are quite obvious in their results. it follows that the same causes which led to the ex. Unction of some forms exerted an intim-nee in modifying others. It slomld be observed that the imperfection of the geological record is still marked, hut many gaps have in late years been closed. See ExiaNcTioN mi. SPECIES.