-Ai
3300 feet, where the pressm-e amounts to more than 100 atmospheres. Now, it
is quite as difficult, in reality, to believe that a creature constituted to live near
the surface, where the pressure does not exceed two or three atmospheres, could
adapt itself to sustain a pressure of 100 atmospheres, as it is to conceive that a
fish ordinarily living under this amount could sustain five times as much more
pressure; for the moment it is admitted that the same species may accommodate
itself to widely varying depths, the principle of progressive adaptation comes
into play, and we are warranted in assuming that very widely differing degrees
of pressure and other collateral conditions may be encountered, provided the
transition from one to the other be sufficiently gradual, and, further, that the
more simple the organism, the more fitted will it be to undergo them with
impunity.
I t has already been shown that the human frame is able to undergo a
diminution of nearly one-half the normal atmospheric pressure present at the
sea-level, if gradually encountered. Now, although the degree of tolerance
involved in this change of conditions seems to bear no comparison with that
which we suppose must be necessary to enable an animal to bear several hundred
times the normal pressure it is used to, there is a very important difference
between the primary conditions existing in the two cases, which renders
the comparatively slight reduction of atmospheric pressure much more determinate
in its effects on the one than the vastly increased amount of pressure on
the other. Thus, assuming the solids, fluids, and gaseous contents of the human
frame to be in equilibrium under the normal pressure at the sea-level, and that
each of these portions of the body is subject to a different rate of expansion and
contraction, it follows that this state of equilibrium must become more and more
disturbed as the atmospheric condition becomes more abnormal. Hence we can
readily account for those painful sensations which are experienced in suddenly
ascending to great elevations, or descending, as in the diving-bell, through depths
which engender even greater and much more sudden modifications.
On the other hand, in the case of all water-breathing creatures, inasmuch as
there are no gaseous contents present, and the circulating fluid is of the same or
very nearly the same specific gravity as the surrounding medium, and every
portion of the structure is completely pervious to fluids, either by its porosity
or through endosmotic action, the state of equilibrium remains undisturbed; and,
as already stated, no injury accrues, provided the transition from one degree of
pressure to the other be sufficiently gradual.
In the case of creatures generated at great depths, and migrating to higher
zones, the same law would apply, time being the essential element in rendering
the transition practicable and safe to the creature. The Ophiocomæ obtained
from a depth of 1260 fathoms (nearly a mile and a half) were not only alive
when they emerged from the water, but continued to move their rays freely for
a quarter of an hour afterward.s. I t is to be regretted that, in my anxiety to
preserve the specimens, no effort was made to keep some of them alive in their
natural element. But, even in the absence of frnther evidence, the one great
fact remains, that these starfishes retained their- vitality for nearly an hour,
during which period they passed through every gradation of pressure ranging
from a ton and a half on the square inch to only 15 lbs. Accordingly, since
there are no grounds for supposing that star-fishes, or any other creatures
constituted to live in the ocean, could undergo a transition from a high
to a low degree of pressure with greater safety than one from a low to a
high degree, and the abrupt change of conditions was borne in the present
case without producing instant death, it is but fair to assume that, under a
sufficiently gradual transition, any given amount of pressure might become
supportable.
Respiration may be regarded as the primary rital function of every animate
organism. Without it the independent life of the indiridual, whether animal
or plant, cannot have a beginning ; and on its permanent inteiTuption, existence
ceases. In so far as we know, and judging from all analogy, this law holds
good in the lowest as well as the highest order of being. The mechanism of
respiration may be more complex in one than in the other ; but, so far from the
phenomena being wonderful in proportion as they are complicated, the opposite
of this proposition is correct, and we find it difficult to comprehend how the
humblest tribes, both of the animal and vegetable kingdoms, in the absence of
anything like specialized structure, can nevertheless bring about the same results.
As is well known, in the case of air-breathing creatines the act of respiration
consists in the absorption into the blood of oxygen derived from the atmosphere,
and the exhalation of carbonic-acid gas—the interchange taking place by means
of the lungs, and the requh'ed effect being produced on the blood. In the case
Q