As compared with later faunas, the animals of the Cambrian were primitive, but their diversity--every subkingdom of invertebrates being represented--is positive evidence that they were not the first inhabitants of the waters. Considered from the point of view of development, this fauna stands at least half-way, and some students of the ancient history of the earth place it as far as nine-tenths of the way, up the life column--that is, the time from the first appearance of life on the earth to the beginning of the Cambrian was at least as long and possibly nine times as long as the time that has since elapsed. This is a sufficient promise that many records of life, and it seems safe to predict as varied an assemblage of organisms as the at present known Cambrian fauna, will ultimately be discovered in the Algonkian or lower rocks. The Paleozoic era witnessed the first appearance of vertebrate life. The earliest known forms were fish-like in character and were succeeded in sequence by batrachians and reptiles. In this connection the most important contribution to the world's knowledge, from the study of the American records, include the discovery of a large number of fishes, or fish-like forms, some of them of gigantic size, in the Devonian and Carboniferous rocks of the Ohio region, by J. S. Newberry; numerous batrachians in the Coal Measures of Ohio, by E. D. Cope; of batrachians and probably reptiles in rocks of similar age in Nova Scotia, by J. W. Dawson and O. C. Marsh. During the Paleozoic era land plants appeared, and before its close the continent was densely clothed with forests consisting of flowerless plants such as ferns and club-mosses, together with a less abundance of trees related to the existing conifers. Great additions to the world's knowledge of the varied and beautiful floras of the swamps in which the coal-beds of Pennsylvania, Ohio, Nova Scotia, etc., were accumulated have been made by H. D. Rogers, J. S. Newberry, Leo Lesquereux, J. W. Dawson, I. C. White, David White, and others. The Mesozoic era is characterized among other events by the first appearance and rapid development of flowering plants, the cycads being especially numerous, and of our ordinary broad-leaved trees, such as the oak, willow, sassafras, etc., and by the coming in of palms; and in the animal kingdom by the culmination of reptilian life and the advent of birds and mammals. The American Mesozoic rocks have yielded a rich store of fossil plants, as is well known from the painstaking studies of J. S. Newberry, Leo Lesquereux, W. M. Fontaine, L. F. Ward, F. H. Knowlton, and others. These same students of the progress of plant life on the continent have also made extensive and critical studies of the Cenozoic floras.