Page:Popular Science Monthly Volume 51.djvu/571

Rh wonderfully this world which we find so beautiful has become adapted to us and we to it. We are too much accustomed to take the world as it actually exists for granted as something that always has been and that always will be. We are apt to forget that the whole human period is but as a narrow fringe upon the vast space of geologic time, and that the world before the advent of man was a very different world from that in which we live. We talk of the everlasting hills and of the primeval forest, but to the geologist the hills are not everlasting and the forest is but a creation of yesterday. The poet Tennyson has caught the true geological standpoint in the following fine verses of In Memoriam:

The earth, which we find to-day bright with varied hues, vocal with innumerable sounds, rich in fruits and fragrant with odors, lay for an almost incalculable period of time destitute, or all but destitute, of color, soundless save for the noise of wave and tempest, and with no promise as yet of the rich profusion of vegetable and animal forms that now diversify its surface and fill it with the thrill and manifold activities of life. We often speak of man as "the heir of all the ages," but not often, probably, do we pause to realize the significance of the word. We talk of evolution, but seldom make any due effort to grasp the plenitude and grandeur of the thought. These senses of which we have the use, and each of which brings a different world within our ken, whence are they? It seems so natural to see; it seems so natural to hear, to touch, to smell, to taste, that we forget through what slow processes, by what an incalculable number of slight accretions and delicate modifications these wonderful channels of knowledge and sensation have been made for us. We go back through the ages and we come to a sightless, voiceless world. For a period probably as long as all the rest of geological time the only forms of life were protozoa. Sight was developed among the wonderful crustaceans of the Silurian period, but as yet there were no organs of hearing. The first stridulation of an insect wing was heard (if it was heard) in the Devonian age, the birth epoch of the first vertebrates, fishes; but long ages had to pass before the first bee hummed over a flower or the first butterfly fluttered its wings in the sunshine. There were no flowers in the Devonian age nor yet in the ensuing Carboniferous, though in both there was a mighty vegetation.

 There rolls the deep where grew the tree. O Earth, what changes thou hast seen! There, where the long street roars, hath been The stillness of the central sea.

The hills are shadows, and they flow From form to form, and nothing stands; They melt like mist, the solid lands, Like clouds they shape themselves and go.

The earliest birds belong to the Cretaceous period—the classic age of reptiles. They were not songsters, however—far from it; nor were they beautiful to look upon, for they had strong points of affinity with the reptile tribe from which there is reason to believe they were developed. It was not till well into the Tertiary period that birds as we know them began to trill and twitter in the woods. It was in the same period that the mammals began to take masterful possession of the earth. The earliest mammalian forms were not the perfect organisms as regards form or activity with which the modern world is familiar, and many of them had but a comparatively short existence. In the Tertiary period, however, there was a vast out-break of insect, bird, and mammalian life, and now began in earnest the struggle for existence—that struggle