Page:Popular Science Monthly Volume 21.djvu/399

Rh to make the destroyer, with its victim, rise with increasing rapidity toward the surface, which they reach dead or in a dying condition."

It was also shown that, as the same species and genera are found in the most distant parts of the globe, the deep-sea fishes are not limited in their range, and consequently, as is admitted on other grounds, that the physical conditions of the ocean-depths must be much alike all the world over. That the deep-sea fishes are not of a peculiar order, however peculiarly organized, but are for the most part modified forms of surface-types, was another conclusion arrived at from the scattered evidence available before dredging at great depths was systematically practiced, and a conclusion that has since proved to be right. Nevertheless, it still remained to ascertain more precisely the bathymetrical horizons in which the different kinds lived, and this has been to some extent attained by observations made during the voyage of the Challenger; but these can not be received without further critical examination, for, unfortunately, no precaution seems to have been taken to keep the mouth of the dredge closed, and therefore it is probable, if not in some cases certain, that fishes were occasionally entrapped while the machine was passing through the surface-water. On the other hand, the majority of the examples taken in the dredge offer literally internal evidence that they were inhabitants of the abysses, being so organized as to be unable to live near the surface, and consequently that they were captured at the greatest depth to which the dredge reached, or nearly so.

The physical conditions of the deep sea, affecting the organization and distribution of these fishes, are thus formulated by Dr. Günther:

1. "—Probably the rays of the sun do not penetrate to, and certainly do not extend beyond, a depth of two hundred fathoms; therefore, we may consider this to be the depth where the deep-sea fauna commences. Absence of light is, of necessity, accompanied by modifications of the organs of vision, and by simplification of colors.

2. "—The absence of sunlight is in some measure compensated. for by the presence of phosphorescent light, produced by many marine animals, and also by numerous deep-sea fishes.

3. "—At a depth of five hundred fathoms the temperature of the water is already as low as 40° Fahr., and perfectly independent of the temperature of the surface-water; and from the greatest depths upward to about one thousand fathoms the temperature is uniformly but a few degrees above the freezing-point. Temperature, therefore, ceases to offer an obstacle to the unlimited dispersal of deep-sea fishes.

4. "—The pressure of the atmosphere, on the level of the sea, amounts to fifteen pounds per square inch of surface on the body of an animal; but the pressure amounts to a ton weight for every one thousand fathoms of depth.