Now, if we examine the descriptions of the organic remains in the rocks, we find that in all ages of the world these four great classes of animals have existed. But in the earliest times, the three last classes--the mollusks, the articulated, and the radiated tribes--vastly preponderated, while the vertebral class had only a few representatives; and it is not till we rise as high as the new red sandstone, that we meet with any, except fishes, save a few batrachians in the old red sandstone, and the carboniferous group, detected alone by their tracks. Then the reptiles began to appear in abundance, with tortoises and enormous birds of a low organization, but no mammiferous animal is found, until we reach the oolite; and scarcely any till we rise to the tertiary strata, when they became abundant; but not so numerous as at present, though for the most part of larger size. Thus we find that the more perfect animals have been developed gradually, becoming more and more complex as we rise on the scale of the rocks. But in the three other classes, there does not appear to have been much advance upon the original types, although in numbers and variety there has been a great increase. The plants now growing upon the globe, amounting probably to nearly one hundred thousand species, are divided into two great classes, by a very decided character. Some of them have distinct flowers, and others are destitute of them. The former are called phenogamian, or flowering plants; and the latter cryptogamian, or flowerless plants. At present, the flowering plants very much predominate in the flora of every country. But in the earliest periods of organic existence, the reverse was the case. We find, indeed but very few flowering plants, and these of a character somewhat intermediate between flowering and flowerless; such as the coniferae and cycadeae, including the pine tribe. A few palms appeared almost as early, and some other monocotyledons. But most of the dicotyledons did not appear till the tertiary period, where more than two hundred species have been found. Of the three hundred species found in and beneath the carboniferous group, two thirds are tree ferns, or gigantic equisetaceae. More than one third of the entire flora of the secondary formation consists of cycadeae; whereas, this family of plants forms not more than the two thousandth part of the existing flora. In short, we find the more perfect plants as well as animals to be few in the earliest periods, and to have been gradually introduced up to the present time. But as to the flowerless plants, most of them seem to have been as perfect at first as they now are.