idea of these two isothermal curves, and also of the temperature of the
water from surface to bottom, in the locality at which the Stai'-iishes were
captured,—a most important fact, to which I shall have to refer particularly
hereafter.
Although a certain amount of heat is constantly being given off from the
interior of the earth, its quantity is so small as to exercise no perceptible
influence on climate. On the other hand, the quantity of heat annually received
from the sun, and upon which the temperature of the earth’s surface may be
said entirely to depend, is so great that, were an equal amount not radiated into
space, the temperature would very speedily become insupportable. The quantities
given off from the first of these soimces, and received through the last, have been
computed by M. Peltiers and M. Elie de Beaumont with reference to their capability
for dissolving a certain bulk of ice,—it being computed that the quantity
of central heat which reaches the surface of the earth in the course of a year
would suffice to melt a layer of ice a quarter of an inch in thickness, covering the
entire globe, whereas the quantity of heat received directly from the sun in the
course of a single year would suffice, in a like period, to melt a layer of ice
46 feet in thickness, covering the same area. But, notwithstanding the amount
of solar heat thus imparted to the earth, its influence does not extend deeper
under the surface than from 60 to 90 feet ; whilst below this point there is a
regular increase of temperature, arising from the earth’s central heat, amounting
to 1° for every 55 feet of descent.
I t follows that, at any given depth of sea-bottom, the heat emanating from
the earth’s centre is the same as would occur at the same depth of solid crust,
but that, owing to the rapidity with which the temperature communicated is
diffused through the entire mass, all accumulation of heat is prevented. Hence,
although the line of uniform temperature in the ocean is unquestionably
modified to a certain extent by the heat received from below, its depth below
the surface is entirely regulated by the amount of solar heat absorbed, coupled
with the amount of radiation, incident on the degree of latitude. In shallow
seas or along coast-lines, where the disturbing action of tides, currents, and
inequalities of the bottom is felt, as a matter of course, the isothermal line
vanishes, and the temperature of the various vertical zones of water is determined
by a new set of conditions.
For these reasons, whilst temperature may justly be regEirded as the most
important element in regulating the distribution of life within the bathymetrical
limits hitherto insisted on as extreme, it becomes highly probable that, in those
profounder abysses, extending from a quarter of a mile below the surface to
depths yet unfathomed by the sounding-machine, this uniformity constitutes the
pre-eminent condition of animal existence, and that a number of creatures
formed to live under it will yet be found equally distributed throughout the vast
area of the sea-bed.
I t has long since been ascertained that the number of types, as also of genera
and species, of the animal and vegetable kingdoms materially diminish as we
advance from equatorial to polar regions; and hence this decrease cannot be
regarded otherwise than as indicative of dependence on reduction of temperature.
Until a little more is known of the deep-sea fauna, it would be injudicious to
speculate on the relative numerical proportion of its members in different
latitudes. But we are warranted in assuming, even from the scanty data already
at our command, when taken in conjunction with the established uniformity of
climate prevalent through the length and breadth of the deep-sea bed, that no
such marked variation exists as that just referred to in the case of the terrestrial
and the superficial marine fauna. And further, if it be legitimate to estimate
the profusion of deep-sea forms generally by the number of individuals belonging
to the same species which were brought up from a single spot of the sea-bed,
there is every reason to believe that it falls short in no degree of that observable
amongst species frequenting the superficial zones.
I t is a very significant fact, and one the importance of which was fully appreciated
by Edward Forbes and Professor Loven, that the greatest profusion of
typical Invertebrata is to be met with, not in the superficial, but in the deeper
zones of the Arctic and Boreal provinces, and, moreover, that the downward
range of the four bathymetrical zones is much more extended than in the Celtic
or Lusitanian provinces. AVant of time and opportunity unfortunately prevented
me from carrying out my intended series of observations on the bathymetrical
range of the Greenland Invertebrata; and, as noted in my diary, my dredgings
at Goodhaab were altogether confined to the fiords near the anchorage, where
the depth of water did not exceed 200 fathoms. But even this limited experience
left no doubt on my mind that the abundance of some of the forms met