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axioms are undoubtedly true in the case of terrestrial organisms; but the last is
wholly unsupported as regards the lowest orders of creatures inhabiting the sea;
for not only does every trace of light become finally extinguislied at a depth comparatively
speaking insignificant, but at a point considerably short of that at which
some of the most highly organized littoral species habitually live. Thus, it is
supposed that at 700 feet below the surface the realm of total darlcness commences;
and yet we know that many creatui-es indigenous to the deep-coral
zone, and more especially certain fishes, enjoy a downward range of from three
to five times that depth—that is to say, 2000 feet beyond the reach of the most
attenuated sunbeam.
But whilst animal life has recently been shown to be present in vast abundance
along the deep bed of the ocean, no vegetable life of any kind analogous to that
with which we are acquainted has hitherto been discovered in a similar position.
Vegetable structures unquestionably occur, and at times in considerable profusion;
but there is no reliable evidence to show that these have lived elsewhere than
near the surface. On the contrary, the appearance of the soft parts, in such
as have been raised from depths exceeding 500 fathoms, indicates a molecular
constitution differing materially from that observable in similar organisms which
have been taken alive in shallow water. All analogy teaches us that plants cannot
live in the absence of light. Under its influence the absorption of carbonic acid
goes on, and the evolution of oxygen *. In default of it, a reverse process is set
up. Light exerts no influence of this kind on animals; but, as already stated,
since all terrestrial animals are ultimately dependent on vegetables for their food,
and vegetables are thus shown to be dependent on Ught for their principal vital
function, we are driven to the admission that ultimately light is essential to terrestrial
animals. In the sea, the conditions under which plants respire are still
unchanged. Light is essential to the absorption of carbonic acid and the evolution
of oxygen. Even in the case of the lowest protophyte, this fact admits of
demonstration; and it must be remembered that the vegetable forms met with
in the deep-sea deposits belong exclusively to the latter class. Their occurrence
* I t is certain th at the organs of respiration of plants are confined to their green surfaces.
Humboldt is stated to have found “ a species of sea-wced of a bright-green tint, although it had grown
at a depth of 190 feet,” where it was computed no more than of the solar rays could penetrate.
(See ‘ Manual of Physiologj-,’ Professor Carpenter, p. 57.)
at' extreme depths in a living condition would, therefore, involve the introduction
of an exceptional law, for which there is no kind of precedent.
I t is one of the most important offices of plants to purify the atmosphere of
the element which would be poisonous to animals were it allowed to accumulate.
How then, it may be asked, are the densely peopled waters of the ocean purified
of the same poisonous element, if vegetation and hght cease at the same limit ;
and under what other conditions than exceptional ones can marine animal life
be maintained without the previous manifestation of vegetable life, as must be
the case if it exists at extreme depths 1
The answer to the first of these questions is obvious. The waters of the ocean
seize on the carbonic acid exhaled by animals, and, as already shown when discussing
the saline constitution of sea-water, the quantity of carbonate of lime
present is sufficient to convert so much of it into a non-poisonous compound that
the remainder is as innocuous as the carbonic-acid gas invariably present in the
atmosphere. On the other hand, the indispensable oxygen is principally derived
from the atmospheric air which has been absorbed by the water, the portion of
it exhaled by marine plants being unimportant in comparison with the area and
mass of the ocean.
I readily admit the difficulty of furnishing a solution to the second question,
without appealing to a process of nutrition for which there is no acknowledged
precedent. At the same time I submit that in the majority of the marine Protozoa—
as, for instance, in the Foraminifera, Polycystina, Acanthometræ, Thalas-
sicollidæ, and Spongidæ—there is much stronger ground for supposing that the
process referred to is that by which nutrition is effected, than that complex
functions of a kind analogous to those observable in creatures endowed with
specialized organs, should be performed, by the simplest order of beings with
which we are acquainted, in the absence of anything approaching to specialized
structure. This subject will be more fully discussed in another portion of
this work : at present it is only necessary to state that no exceptional law is
invoked, but, on the contrary, that the proof of these organisms being endowed
with a power to convert inorganic elements for their own nutrition rests on the
undisputed power which they possess of separating carbonate of lime or silica
from waters holding these substances in solution. In other words, if it can be
shown that these organisms are able to produce a chemical separation of inorganic
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