I have thus far spoken of the changes in the vegetable growth alone as influenced by altitude and latitude, but the same is equally true of animals.  Every zone of the earth’s surface has its own animals, suited to the conditions under which they are meant to live; and with the exception of those that accompany man in all his pilgrimages, and are subject to the same modifying influences by which he adapts his home and himself to all climates, animals are absolutely bound by the laws of their nature within the range assigned to them.  Nor is this the case only on land, where river-banks, lake-shores, and mountain-ranges might be supposed to form the impassable boundaries that keep animals within certain limits; but the ocean as well as the land has its faunae and florae bound within their respective zooelogical and botanical provinces; 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.