;
and a wall of granite is not more impassable to a marine animal than
that ocean-line, fluid and flowing and ever-changing though it be, on
which is written for him, "Hitherto shalt thou come, but no farther."
One word as to the effect of pressure on animals will explain this.

We all live under the pressure of the atmosphere. Now thirty-two feet
under the sea doubles that pressure, since a column of water of that
height is equal in weight to the pressure of one atmosphere. At the
depth of thirty-two feet, then, any marine animal is under the pressure
of two atmospheres,--that of the air which surrounds our globe, and of a
weight of water equal to it; at sixty-four feet he is under the pressure
of three atmospheres, and so on,--the weight of one atmosphere being
always added for every thirty-two feet of depth. There is a great
difference in the sensitiveness of animals to this pressure. Some fishes
live at a great depth and find the weight of water genial to them, while
others would be killed at once by the same pressure, and the latter
naturally seek the shallow waters. Every fisherman knows that he must
throw a long line for a Halibut, while with a common fishing-rod he will
catch plenty of Perch from the rocks near the shore; and the differently
colored bands of sea-weed revealed by low tide, from the green line of
the Ulvas through the brown zone of the common Fucas to the rosy and
purple hued sea-weeds of the deeper water show that the florae as well
as the faunae of the ocean have their precise boundaries. This wider
or narrower range of marine animals is in direct relation to their
structure, which enables them to bear a greater or less pressure of
water. All fishes, and, indeed, all animals having a wide range of
distribution in ocean-depths, have a special apparatus of water-pores,
so that the surrounding element penetrates their structure, thus
equalizing the pressure of the weight, which is diminished from without
in proportion to the quantity of water they can admit into their bodies.
Marine animals differ in their ability to sustain this pressure, just
as land animals differ in their power of enduring great variations of
climate and of atmospheric pressure.

Of all air-breathing animals, none exhibits a more surprising power of
adapting itself to great and rapid changes of external influences than
the Condor. It may be seen feeding on the sea-shore under a burning
tropical sun, and then, rising from its r