[Illustration: DIAGRAM SHOWING THE ADVANCE, CULMINATION, AND DECADENCE OF SOME OF THE LEADING TYPES OF PALÆOZOIC LIFE.] Fourthly, the progress in animal life in the Palæozoic related chiefly to the lower or invertebrate tribes, and to the two lower classes of the vertebrates. The oldest animal known to us is not only a creature of the simplest structure, but also a representative of that great and on the whole low type of animal life, in which the parts are arranged around a central axis, and not on that plan of bilateral symmetry which constitutes one great leading distinction of the higher animals. With the Cambrian, bilateral animals abound and belong to two very distinct lines of progress--the one, the Mollusks, showing the nutritive organs more fully developed--the other, the Articulates, having the organs of sense and of locomotion more fully organized. These three great types shared the world among them throughout the earlier Palæozoic time, and only in its later ages began to be dominated by the higher types of fishes and reptiles. In so far as we know, it remained for the Mesozoic to introduce the birds and mammals. In plant life the changes were less marked, though here also there is progress--land plants appear to begin, not with the lowest forms, but with the highest types of the lower of the two great series into which the vegetable kingdom is divided. From this they rapidly rise to a full development of the lowest type of the flowering plants, the pines and their allies, and there the progress ceases; for the known representatives of the higher plants are extremely few and apparently of little importance. Fifthly, in general the history tells of a continued series of alternate victories and defeats of the species that had their birth on the land and in the shallow waters, and those which were born in the ocean depths, The former spread themselves widely after every upheaval, and then by every subsidence were driven back to their mountain fastnesses. The latter perished from the continental plateaus at every upheaval, but climbed again in new hordes and reoccupied the ground after every subsidence. But just as in human history every victory or defeat urges on the progress of events, and develops the great plan of God's providence in the elevation of man; so here every succeeding change brings in new and higher actors on the stage, and the scheme of creation moves on in a grand and steady progress towards the more varied and elevated life of the Modern World.