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28
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THE SEAS
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than
except the surface waters of the
open sea._ -' ThishaS an immediate effect in the distribution
ot plant life, for plants can only exist where there is light
which .is- necessary for the “ photosynthetic
”
action
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their green'pigment by means of which the carbonic acid
gas in the atmdsphere,‘ or in solution, is combined with
water to 'form starch, which, with the addition of salts
obtained from the soil or from solution, forms the food of
the plant.
The inuence of this ora of sea weeds on the
life of-the sea'shore is of great importance.
The population of the seaShore if it is to withstand these
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very variable conditions must be extremely hardy—how
hardy we realize When we discover that very slight changes
in thetemperature or salinit_v of the water will kill animals
used to the uniform conditions of the open-ocean.
The
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animals must be adapted in_many different ways, for protection, food collection, and reproduction, to mention only
three of the most important;
Yet in spite of these dif—
culties the population of the sea shore is one of thedensest
'and
on the surface of the earth. f So dense,
indeed, that themost striking features of shore life are the
perpetual struggle for existence, the constan‘t-scramble for
food in which the strongest or most subtle are the con—*
_
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querors,-iimurerable devices for ensuring the continuance
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of the race, the never ceasing pursuit
the more powerful
of
the weaker and smaller, the latter surviving to the extent
ro ‘Whichthey are able-to disguise or-hide themselves.
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We can “distinguish very denite associations of animals
_
living on the shore. ‘
=By-this-wè mean that in different sets of
conditions we.habitually nd the same types of animals, per—
haps .varying in species from place to place. These animals
may be of many different kinds—sCmeof themworms, others
starsh, crustaceans, mollus'cs and so on—_but they are all
adapted for life under thoSe particular conditions; they may
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