Page:Popular Science Monthly Volume 63.djvu/503

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O matter what particular theory he may hold as to the origin and distribution of life over this globe of ours, the thoughtful naturalist or the thoughtful student in any sphere of research, if he stops to question at all, must ponder with deepest interest the problems connected with the wide dispersal of animal life and its adaptations to most diverse conditions. From darkest depths of abyssal ocean to lofty mountain peak, life abounds and even the high regions of ethereal air are traversed by one or another of the organisms which represent the great aggregate of animal life.

It is true that these limits, viewed from a certain standpoint, are very narrow, for a moment's consideration will reveal the fact that life on this sphere now, as in all time past, is confined to a comparatively thin stratum at its surface. From the deepest habitable reaches of ocean to the highest point attainable by bird (a few miles, indeed, of vertical elevation) is a slight range in the radius of the earth, and the densely populated stratum of land and sea—the stratum actually capable of supporting life continuously is, in reality, limited to a very few feet—an exceedingly thin layer on a gigantic ball. One almost trembles at the thought of how narrow the habitable limits and how slight a change in conditions of atmospheric or other physical environment might extinguish the vital spark which has characterized mother earth through untold stretches of years. Consider a moment how little below the surface any animal can live, how slightly above it is existence possible.

But my purpose here is to touch upon some of the routes of development of the shifting forms of animal life which have drifted hither and thither over sea and land in the great struggle for perpetuity and expansion.

This effort we can clearly see in the movement taking place under our own eyes, and, more largely, within historic times, as evidenced by the host of animals introduced and spread in the new world from the old, the displacement or extinction of certain forms and perhaps most emphatically in man himself, the dominant animal of the present age, whose struggle has now become not so much a struggle with other species as a struggle for dominance of race over race, or nation