I have said in a previous article that the general unity of character prevailing throughout these three divisions, so that, taken from the broadest point of view, each one seems a unit in time, justifies the application to them of that term, Age, by which we distinguish in human history those periods marked throughout by one prevailing tendency;—­as we say the age of Egyptian or Greek or Roman civilization,—­the age of stone or iron or bronze.  I believe that this division of geological history into these great sections or chapters is founded upon a recognition of the general features by which they are characterized.

Passing over the time when the first stratified deposits were accumulated under a universal ocean in which neither animals nor plants existed, there was an age in the physical history of the world when the lands consisted of low islands,—­when neither great depths nor lofty heights diversified the surface of the earth,—­when both the animal and vegetable creation, however numerous, was inferior to the later ones, and comparatively uniform in character,—­when marine Cryptogams were the highest plants, and Fishes were the highest animals.  And this broad statement holds good for the whole of that time, even though it was not without its minor changes, its new forms of animal and vegetable life, its variations of level, its upheavals and subsidences; for, nevertheless, through its whole duration, it was the age of low detached lands,—­it was the age of Cryptogams,—­it was the age of Fishes.  From its beginning to its close, no higher type in the animal kingdom, no loftier group in the vegetable world, made its appearance.

There was an age in the physical history of the world when the patches of land already raised above the water became so united as to form large islands; and though the aspect of the earth retained its insular character, yet the size of the islands, their tendency to coalesce by the addition of constantly increasing deposits, and thus to spread into wider expanses of dry land, marked the advance toward the formation of continents.  This extension of the dry land was brought about not only by the gradual accumulation of materials, but also by the upheaval of large tracts of stratified deposits; for, though the loftiest mountain-chains did not yet exist, ranges like those of the Alleghanies and the Jura belong to this division of the world’s history.  During this time, the general character of the animal and vegetable kingdoms was higher than during the previous age.  Reptiles, many and various, gigantic in size, curious in form, some of them recalling the structure of fishes, others anticipating birdlike features, gave a new character to the animal world, while in the vegetable world the reign of the aquatic Cryptogams was over, and terrestrial Cryptogams, and, later, Gymnosperms and Monocotyledonous trees, clothed the earth with foliage.  Such was the character of this second age from its opening to its close;