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Sea; tlie higher specific gravity at the surface than
at g’leatei depths in the Atlantic; the higher specific
gravity of the surface-Avater in the Atlantic to the
north than to the soutli of tlie equator), it is probable
tbat the general circulation is kept up chiefly
by an excess of evaporation in the region of tbe
North xitlautic, balancing a corresponding excess
of precipitation over evaporation in tbe water-
liemispbere.
The distribution and nature o f the deep-sea fauna.—
Tbe most prominent and remarkable biological result
of tbe recent investigations is the final establishment
of tbe fact tb a t tbe distribution of living
beings lias no deptb-limit ; but th a t animals of all
the marine invertebrate classes, and probably fishes
also, exist over tbe Avbole of the floor of tbe ocean :
and some of the most interesting of tbe problems
Avbicb are noAv before us have reference to the nature
and distribution of the deep-sea fauna, and to its
relations Avitb tbe fauna of shalloAver Avater, and with
tbe faunæ of past periods in the earth’s history.
Tins is however precisely the class of questions
AA'liicb we are as yet least prepared to enter into,
for everything depends upon the careful study and
the critical determination of tbe animal forms which
have been procured ; and tbis task, which will occupy
many specialists for several years, has been only ju st
commeuced.
My present impression is that although life is tbus
universally extended, tbe number of species and of individuals
diminishes after a certain depth is reached,
and that at tbe same time tbeir size usually decreases.
This latter observation is not, however, true for all
groups ; a peculiar family of tbe Holothuridea, very
AA'idely distributed in deep water, maintain the full
dimensions of the largest of tbeir class, and even
exhibit some forms of unusual size. Of tbe value
of our present impressions on such questions I am
by no meaus sure. Using all precautions, and Avitb
ample poAA'er and tbe most complete appliances, it is
extremely ditficult to work either with the dredge or
witb the traAvl at depths approaching or exceeding
3,000 fatlioms. A single dredging operation in snch
dejitbs takes a long time ; the dredge is p u t over at
daybreak, and it is usually dark before it is recovered,
so tb a t tbe number of such operations must be comparatively
small. I t is necessary to take every
precaution to keep tbe sbip as nearly as possible
in the same place ; and as tbis can never be done
absolutely, it is unsafe to ru n tbe risk of adding
to any motion whicli tbe dredge may already have
acquired, by attempting to drag it for any distance
over the ground. Tbe consequence is, tb a t in tbose
cases wbere the dredge does reach the hottom, it
probably too often sinks at once into the soft ooze
and remains clogged with a single ‘ mouthful ’ until
it is hauled up again. Sometimes a slight excess
of movement in the vessel, from currents or from
wind-drift, seems to give a vibratory motion to the
enormous length of rope, and to keep the dredge
tripping over the ground, so th a t only a feAV things
are picked up hy the tangles or clinging to the
outside of the net. MY must therefore bear in
mind th a t only an infinitesimally small portion of
the floor of the ocean at depths over 2,500 fathoms
has yet been explored.