pecies which need to rest upon the shore,
do we fail to find that surface thickly tenanted with varied forms.
These are arranged according to the depth of the bottom. The species
of marine plants which are attached to fixed objects are limited to
the depth within which the sunlight effectively penetrates the water;
in general, it may be said that they do not extend below a depth of
one hundred feet. The animal forms are distributed, according to their
kinds, over the floor, but few species having the capacity to endure
any great range in the pressure of the sea water. Only a few forms,
indeed, extend from low tide to the depth of a thousand feet.

The greatest development of organic life, the realm in which the
largest number of species occur, and where their growth is most rapid,
lies within about a hundred feet of the low-tide level. Here sunlight,
warmth, and motion in the water combine to favour organic development.
It is in this region that coral reefs and other great accumulations of
limestone, formed from the skeletons of polyps and mollusks, most
abundantly occur. These deposits of a limy nature depend upon a very
delicate adjustment of the conditions which favour the growth of
certain creatures; very slight geographic changes, by inducing
movements of sand or mud, are apt to interrupt their formation,
bringing about a great and immediate alteration in the character of
the deposits. Thus it is that where geologists find considerable
fields of rock, where limestones are intercalated with sandstones and
deposits of clay, they are justified in assuming that the strata were
laid down near some ancient shore. In general, these coast deposits
become more and more limy as we go toward the tropical realms, and
this for the reason that the species which secrete large amounts of
lime are in those regions most abundant and attain the most rapid
growth. The stony polyps, the most vigorous of the limestone makers,
grow in large quantities only in the tropical realm, or near to it,
where ocean streams of great warmth may provide the creatures with the
conditions of temperature and food which they need.

As we pass from the shore to the deeper sea, the share of land
detritus rapidly diminishes until, as before remarked, at the distance
of five hundred miles from the coast line, very little of that waste,
except that from volcanoes, attains the bottom of the sea. By far the
larger part of the contributions which go to the form