Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Life does not cease here, for the products of this surface-life sink to greater depths and are fed upon by forms of sea animals that have become adapted to the dark and cold abyss of the ocean. Obviously, these deep-sea forms are a very distinct type of life, and constitute a fauna of the most pronounced kind, the abysmal fauna. Another distinct fauna occupies the open-ocean surface, the pelagic fauna. Still a third fauna occupies the shallow-water tract, whose bottom lies within the light zone—the photobathic zone—and embraces the animals that are dependent on the plants of this zone, or on its light and warmth, and that are more or less fixed to the bottom or confined to the zone because their food is there.

The physical plane of demarkation between the surface or pelagic fauna and the abysmal fauna is much more distinct and more fundamental than any that is found in ascending above the surface of the sea. The habitat of the shallow-water fauna is limited below by the darkness, limited above by the water-surface, limited at one side by the land, and limited on the other side by the deep sea. It is hemmed in vertically between two planes only a few hundred feet apart. Laterally, it is confined to a narrow belt about the borders of the continents and to the more or less land-girt epicontinental seas. Its vertical limits are fixed, but its lateral extent varies with the relations of the sea to the surface of the continental platforms.

This variation profoundly affects the development of the fauna. When a major deformation of the earth takes place which increases the capacity of the oceanic basins, the water is drawn down into them more fully, and correspondingly retreats from the continental shelf. The shore is thus carried out toward or to the border of the shelf, or even perhaps down to some line on the abysmal slope. In either case, the zone of shallow water suited to the photobathic life is narrowed, and at points it may be practically cut in two. There are, however, shelves and tracts that were below the light zone before, which now are brought within it by the lowering of the sea-level. Into these, as into harbors of refuge, the life migrates so far as it may. But these tracts are less prevalent and continuous than the typical continental shelf, and under the conditions supposed they would be but imperfectly connected with each other by available shallow-water tracts. (The steep shelving shore tracts, although furnishing a shallow-water connection possibly available for some species, would be unsuited to others and, under certain conditions of the sea-currents, would be an effective barrier.) To these limited tracts, therefore, the life of the photobathic type is restricted and measurably isolated, and develops into local and provincial faunas.

After a deforming movement has ceased, the seashore habitually advances, developing a new continental shelf, and in time new epicontinental gulfs and seas. In this it is assisted by the erosion of the continent and the filling of the sea, and probably by the slow settling of the continents. As the sea-shelf broadens, the isolated tracts, the harbors of refuge, become connected, and migration is facilitated. When the connection becomes general and broad, and when epicontinental seas have formed available tracts across the face of the continents, a general commingling of faunas follows, and a cosmopolitan fauna results.

In the same way, but more obviously, when the land is extended and connection between the continents becomes general, there is migration and commingling of the land faunas and floras, and cosmopolitan communities are the result.

It is obvious that the development on the land is the reciprocal of that in the sea. When the seas are extended and their life is tending toward cosmopolitanism, the lands are dissevered, and their life is tending toward provincialism, and vice versa. When, however, the land is greatly extended, it is usually accentuated by mountain ranges, and other products of the deformation which extended it, and these form barriers. Desert wastes and other inhospitable tracts, and even glaciation, are liable to develop as secondary consequences, and to interpose barriers, and hence the cosmopolitanism of the land-life is liable to be less complete than that of the sea-life.

Restrictive and expansional evolution.—It is obvious from the last discussion that if the picture of the earth’s movement above drawn be true, the areas available for particular classes of life may vary greatly from age to age. At times the shallow-water sea-life may be forced to retreat into a very narrow tract on the border of the land, and into chance expansions here and there. In being crowded into this limited tract, perhaps also less adapted for a habitat on account of the change, the life is subjected to severe competition and to hard conditions, and must experience in an intensified degree the effects of the struggle for existence. Whatever of evolutionary potency there may be in such a struggle under such restrictive conditions should be revealed in the modifications of the fauna that ensued.

On the other hand, when the shallow seas are generally extending themselves upon the land and the land is being base-leveled, and thus adapted to shallow submergence, the shallow-water life enjoys an enlarging realm, and should reveal the effects of evolution under expansional conditions. In affording a comparison between these opposite and alternating phases of restrictional and expansional evolution, geology makes one of its great contributions to the external causes and conditions of organic evolution. These will come under repeated consideration in the historical chapters.

INDEX.

VOLUME I.