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 the 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.
But, after all, how little do we know of these laws, which are only beginning to dawn on the minds of naturalists; and which the imperfections of our classification and nomenclature, and the defects in our knowledge of fossil species, render very dim and uncertain. All that appears settled is the existence of a definite plan, working over long ages, and connected with the most remarkable correlation of physical and organic change: going on with regular march throughout the Palæozoic, and then brought to a close to make room for another great succession. This following Mesozoic time must next engage our attention.
We may close for the present with presenting to the eye in tabular form the periods over which we have passed. The table on page 187, and the diagram (page 179), mutually illustrate each other; and it will be seen that each age constitutes cycle, similar in its leading features to the other cycles, while each is distinguished by some important fact in relation to the introduction of living beings. In this table I have, with Mr. Hull,[AB] for simplicity, arranged the formations of each age under three periods—an older, middle, and newer. Of these, however, the last or newest is in each case so important and varied as to merit division into two, in the manner which I have suggested in previous publications for the Palæozoic rocks of North America.[AC] Under each period I have endeavoured to give some characteristic example from Europe and America, except where, as in the case of the coal formation, the same names are used on both continents. Such a table as this, it must be observed, is only tentative, and may admit of important modifications. The Laurentian more especially may admit of division into several ages; and a separate age may be found to intervene between it and the Cambrian. The reader will please observe that this table refers to the changes on the continental plateaus; and that on both of these each age was introduced with shallow water and usually coarse deposits, succeeded by deeper water and finer beds, usually limestones, and these by a mixed formation returning to the shallow water and coarse deposits of the older period of the age. This last kind of deposition culminates in the great swamps of the coal formation.
[AB] “Quarterly Journal of Science,” July, 1869.
[AC] “Acadian Geology,” p. 137.
