I t has been pointed out, at a previous page, that temperature is the most
importimt as well as the most uniformly operating condition in tho distribution
of terrestrial animal life, and, moreover, that it exerts an equivalent influence on
mai'ine life. AA hen, however, we estimate its effects on the bathymetrical
distribution of animals and plants in the ocean, we find them to be extremely
unequal; for, whereas mailne organisms are now known to exist at a depth of
at least 15,000 feet below the surface, no vegetable structures with which we
ai-e acquainted have been met with in a living state lower down than 2400 feet.
But, although it would be premature, with our present knowledge of the
deep-sea bed, to assert that this limit may not be considerably exceeded,
there is every reason to believe that, beyond that depth, a boundary is
rapidly attained at which a condition absolutely essential to plant-life in the
ocean fails, and hence renders its downward extension impossible. That condition
is light.
The two next conditions are elevation above and below the sea-level. A
small representative value is assigned to these, because, although associated
^vith the hitherto subtle and unexplained influences which at times determine
the appeai-ance of certain creatures in certain strata of the atmosphere, and
also of certain creatures in certain bathymetrical zones of the ocean, there is no
ground for the supposition that, per se, they exercise any effect of the kind.
AATiat these influences may be, there are at present no data whereupon to
foim a conjecture, since they have hitherto been recognized only by their
results. I t is clear, however, that they are not due to temperature, since the
same temperature at different periods does not produce similar results on the
organisms in question. In like manner, they are not attributable to rarefaction
or pressure, since under a similar amount of pressure or rarefaction the same
phenomena do not necessarily occur. I t is possible, and even probable, as
regards the ocean, that they may be in some way connected with the amount of
light present, inasmuch as Ught exercises a marked influence on the appearance
and disappearance of Ufe near the surface of the ocean, and of certain animal
forms in given strata of the atmosphere. But even here we meet with the same
inconstancy of action as in the former cases, as shall be shown hereafter—several
new and highly interesting facts tending to show that, whilst a certain quantity of
light is absolutely indispensable to the growth and colour of certain plants, it is
hy no means essential cither to the growth or to the colour of the animal forms
of the ocean.
Tt is obvious that rarefaction of the air, in the main, implies elevation above
the sea, and attendant decrease of temperature ; but it also involves something
more, since we find certain birds, as for example, the Tropic-bird (Fh/jMon),
almost invariably flying at a great elevation, even in the calmest weather.
Here temperature can exert no influence; for, were the object to be attained a
reduction of temperature, the bird would naturally frequent cooler latitudes,
further removed from the equator than those it is usually found in. From the
structure of the head and beak, the “ Tropic-bird ” is clearly an animal-feeder,
preying, like most birds that frequent the open ocean, on minute Animalcules,
Crustaceans, and Molluscoids. The selection of great heights, therefore, cannot
be held to be due to the search for necessary food, but to some instinctive
preference for a less dense atmosphere. In like manner, the Govinda Kite,
both in the mountains and plains of India, remains, for many hours daily, at
very great elevations in the atmosphere. The search for food cannot be the
incentive in this instance, since these birds soar as often after their meal as
before it, and frequently under circumstances which render it but too palpable
that food is in superabundance.
Some idea may be formed of the vastness of the pressure prevalent at great
depths from the fact (which may be readily proved by recourse to a few figures)
that at a depth of a mile it amounts to 2640 lbs., or 160 atmospheres, on eveiy
square inch ; whilst at 4000 fathoms, which is equivalent to about 4^ miles, it
amounts to 750 atmospheres; and water, which ranks amongst the least compressible
of all known compounds, would be compressed, at a depth of twentv
miles, to ^ t h of its original volume*. Computations of this kind have long been
in vogue ; and, as before stated, the extraordinary nature of the conditions they
involve has led to a sort of unqnestioniag belief that, where they are present,
all life is impossible. The chief fallacy has consisted in the introduction of
cases which are parallel in no essential particular. Thus, the effects of this
stupendous pressure has been viewed in relation to its effects on fishes having
closed air-bladders, or, at all events, bladders only capable of being cleared of
their contents by a very slow process ; and, as a matter of course, but one
* See ‘ Physical Geography,’ by Mi-s. Souierville, vol. i. p. 325.