h
Assuming, then, that carbonate of lime and silica are always present in seawater,
that carbonic acid increases in quantity with the depth, and that the
solvent power of water as regai-ds these two substances is due to the presence of
this acid that, on the deep-sea bed, in localities where carbonate of lime is said
to occur m such minute proportions at the surface as to be inappreciable even
by chemical tests, vast calcareous deposits are continuously being formed__
lastly, assuming that the increase of testaceous creatures stands in a direct relation
to the supply of the material of which their hard parts are composed *, we
are unavoidably led to the inference that pressure, so far from restricting animal
life to the higher zones of the ocean, may be regarded as one of tbe conditions
most essential to its existence at extreme depths.
In addition to the substances already enumerated as present in sea-water, there
are a few others which occur in still more minute quantities, or under such subtle
combinations as to have led to their being omitted in most analyses. Regarding
these, it is ouly necessary to state that iodine, fluorine, and phosphoric acid are
detected, the first in marine plants, the second in sea-water itself, and the third
in all organic remains as well as sea-water.
Of the conditions which determine the distribution of certain tribes of animals
in shallow water, the composition and configuration of the bottom is one of the
most important. Thus, rocky surfaces and sandy, gravelly, or muddy bottoms have
each their special inhabitants, whose structure, taken in conjunction with the
kinds of vegetable growth to be met with, induces them' to select the localities
best suited to them. There is no reason to suppose that the instinctive tendency
to select certain descriptions of sea-bed in preference to others is less powerfully
manifested or obeyed by creatures which live at great depths. For although we
are warranted in assuming that the general character of the deep-sea bed is much
more equable than that of shallower regions, there is ample evidence afforded by
the soundings to show that the universal layer of soft deposit which has been
declared by some writers to envelope its surface is very far from being constant,
and that no more fatal mistake can be committed than to take for granted that
mountain-ridges, abrupt precipices, and serrated rocks do not present themselves
even where volcanic disturbance is unknown.
* The soft parts of these organisms consist of “ sarcode,” a proteine-compoimd the elements of
which, oxygen, hydrogen, nitrogen, and carbon, are present in water.
It is obvious that deposit cannot take place universally over the bed of the
ocean in the absence of a universal depositing-agency. Now, no such agency
exists, but, on the contrary, there is clear proof that all the great calcareous
deposits are of a local character; and, from what has been already said regarding
the composition of sea-water at all depths and in all latitudes, it is evident that
this local character is not attributable to a deficiency of calcareous material in
the water, but to the absence (for reasons as yet beyond our comprehension) of
the organisms by whom the separation of the material from its chemical combinations
is effected.
The last condition which bears on the distribution of marine life is light—or
(paradoxical as it may appear) that total absence of light which has been regarded
by philosophers and poets alike, from time immemorial, as characteristic of the
Deep.
Although light can hardly be said to exert any material effect on the geographical
range of marine species, it constitutes an important element in determining
their bathymetrical limits within the superficial zones of the ocean. For,
notwithstanding it is the essential condition of vision, and it is impossible to
conceive that eyes should have been bestowed on creatures for any other purpose
than to see, we must bear in mind that the instinctive and well-marked preference
for particular degrees of light manifested not only by creatures wholly destitute
of visual organs, but by the vegetable kingdom in general, points to some more
subtle sense than that of sight, and proves that its exercise may not necessarily
be dependent on the light-giving rays of the solar spectrum.
We cannot tell why light affects different creatures in different ways; but its
varying influence is far too decided and constant to escape notice. Thus we find
certain forms living under the full glare of day, others avoiding i t ; some indifferent
to every degree of luminous influence, others susceptible to the slightest. But,
whatever the mode in which these idiosyncrasies manifest themselves, they pervade
all the individuals of the same species, and we simply recognize the fact
without knowing why or how it is brought about.
Again, although it has never been directly asserted that light is essential to
animal existence, such a view has been in some measure fostered by the promulgation
of two axioms,—namely, that light is essential to vegetation, and that animal
life is dependent on the previous manifestation of vegetable life. Both of these
s