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88 THE BATHYMETRICAL LIMITS OF LIFE IN THE OCEAN.
conditions which, if sufficiently protracted in their operation, would have proved
fatal, and that they may have received a new vital impulse, as it were, from the
relapse towards those conditions which were normal,—the alternate tendency
towards extinction and re-establishment having been carried on through several
repetitions of these changes, and resulting in an augmented or a diminished
power to become permanent, as the digression from or recurrence to the normal
state assumed the preponderance.
I t is to Sir Charles Lyell that we are indebted for the first clear exposition of
the relation subsisting between the geographical distribution of organized beings
and the “ inorganic changes ” which are continually taking place over the
globe. So long ago as the year 1832 this all-important subject was treated by
him in the original edition of his ‘ Principles of Geology ’—a work which
constitutes at once the most compendious and the most perfect record of the
great physical laws which have through all time modified the exterior of the
globe. To Mr. Darwin’s researches much is also due—authenticated examples of
specific divergence having been collected and explained by him in a manner
that leaves no doubt on the mind of the facts, although his final inferences are
held to be “ unproven ” by some, and untenable by others.
Sir Charles Lyell remarks* that, however uniform may be the action of
“ inorganic causes,” its effect on the organic world is far from being uniform in
equal periods of time in different localities. There cannot be a doubt of the
correctness of this view with regard to inorganic agencies operating on land,
but there is good reason for supposing that when operating at sea their results
are of a far more uniform nature in like periods; for, assuming, as we are warranted
in doing, that over large equal areas of globe the amount of subterranean
disturbance is similar during vastly extended periods, it follows, from the much
greater area of sea as compared with land, that the effects would be diffused over
a much wider space, and hence sudden changes in distribution of the sea-bed
would be less frequent, whilst alterations in its depth would be much more slow
in their production.
“ We may fairly infer,” says Sir Henry De la Bechef, “ that the great mass of
organic remains, like the accumulation of detritus amongst which such remains
* See Lyell’s ‘ Principles of Geology.’ London, 1853: p. 691 et seq.
t ‘ Researches in Theoretical Geology,’ by Sir H. De la Beche, F.R.S., &c., 1834, p. 266.
THE BATHYMETRICAL LIMITS OF LIFE IN THE OCEAN.
occur, was accumulated in moderate depths around coasts, and either on or
around shoals at no considerable depth below the surface of the sea.” The
same view has been held by all subsequent writers; and this may explain, in
some measure, why it has been asserted that fossil species, which extend
through the greatest thickness of a deposit or series of deposits, have in all
probability also commanded the widest horizontal range,—the inference being
based on the supposition that, during the vast periods of elevation or subsidence
which rendered a change of position necessary, littoral species, in order to
endure, must have traversed successively formed zones. I f by horizontal range
it is meant that such species must have lived along very extended coastlines
to admit of their surmounting the obstacles which occasionally prevented
the maintenance of their normal depth, the view may be regarded as correct.
Otherwise it would appear that the geological range has been confounded with
the geographical, inasmuch as the measure of the horizontal area over which
such a fossil species may be found constitutes but the sum of an indefinite
number of geographical areas to each of which it was restricted for the time
being. And hence, apart from other evidence, the greatest vertical as well as the
greatest horizontal range of such a species, whilst attesting its duration, affords
no proof of its wide geographical distribution at any given period.
In the case of fossil deposits formed at the bed of the deep sea, however,
it may be fairly assumed that the horizontal range bears a much closer relation
to the actual geographical distribution of a species at any given period of its
history; for, although the sea-bed may be regarded as the reflex of the land,
softened down somewhat by the deposits continuously accumulating on its
surface, the very nature of the conditions present yields the creature living there
a much greater chance of escape from hurtful influences than pertains to littoral
species. Thus the latter, in event of elevation of a coast taking place, could
descend in one direction only, to maintain then- special bathymetrical zone;
and should a shoal or reef skirt the shore, advance in that direction would be
precluded. Therefore, unless the creature were capable of enduring a change
of conditions incident on removal to a higher zone, even no further elevation of
the coast-line took place, the result would be fatal to it.
In the case of deep-sea forms, excepting those located in circumscribed and
isolated areas, the possibility of migration to a locality at which the conditions
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