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the temperature of the air was 44°, of the surface 48°, and at 100 fathoms 48|°,—
clearly indicating the effect of the Gulf Stream both on the air and water.
I t would be futile to speculate on the probable influence which such a region
of uniform temperature from surface to bottom may have had upon the bathymetrical
distribution of animals, since the nature of the conditions under which
acclimatization commences presupposes a period so remote that the sea-bed
must have been elevated almost to the surface : but it is not unreasonable to
suppose that, as subsidence took place, the increasing uniformity in the temperature
and density of the water may have facilitated their acclimatization, and
eventually favoured their distribution over the bed of the homothermal sea.
I t only remains for me to mention that I have met with no example of any
of the Algae proper below 200 fathoms, and that the only recognizable vegetable
structures which occur amongst the deep-sea deposits belong to the lowest order
with which we are acquainted, namely, the Diatomacese. As already mentioned,
however, the appearance presented by the frustules of these, when obtained at
depths beyond 500 fathoms, indicates a molecular constitution of the protoplasmic
matter differing so materially from that observable in similar organisms
taken in a living condition in shallow water as to render it certain that all vegetable
life ceases at a limit far short of that to which animal life has now been
shown to extend.
Basing my arguments, then, on two facts which I venture to hope are unequivocally
proved in the preceding pages, namely, that highly organized creatures
have been captured in a living condition at depths vastly exceeding those to which
animal Ufe had previously been supposed to extend, and that their presence,
where captured, cannot be regarded as an accidental or exceptional phenomenon,
it has been my endeavour to establish the following important propositions :—
I. The conditions prevailing at great depths, although differing materially
from those which prevail near the surface of the ocean, are not incompatible
with the maintenance of animal life.
II. Assuming the doctrine of single specific centres to be correct, the occurrence
of the same species in shallow water and at great depths proves that it
must have undergone the transition from one set of conditions to the other with
impunity.
III. There is nothing in the nature of the conditions prevailing at great depths,
to render it impossible that creatures originally, or through acclimatization,
adapted to live under them should become capable of living in shallow water,
provided the transition be sufficiently gradual; and hence it is possible that
species now inhabiting shallow water may, at some anterior period, have been
inhabitants of great depths.
IV. On the one hand, the conditions prevailing near the surface of the ocean
render it possible for organisms to subside after death to the greatest depths,
provided every portion of their structure is freely pervious to fluid: on the other
hand, the conditions prevailing at great depths render it impossible for organisms
still constituted to live under them to rise to the surface, or for the remains of
these organisms after death to make their appearance in shallow water.
V. The discovery of even a single species living normally at great depths
warrants the inference that the deep sea has its own special fauna, and that it has
always had it in ages past; and hence that many fossiliferous strata, heretofore
regarded as having been deposited in comparatively shallow water, have been deposited
at great depths.
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