w
more prone to variation dependent on changes in the conditions by which they
are surrounded than others, whether we regard the variation observable in the
same individual or the same species at different periods of its history *. And,
further, if the \iews offered by me are correct, it is erident that such changes of
conditions must operate more frequently and powerfully at the surface than at
great depths. Now, since Ophiocoma granulata is found on the British and
Scandinavian coasts at depths varying from 10 to 50 fathoms, whilst in Greenland,
as already stated, it is met with at 200 fathoms, it is reasonable to suppose
that, if prone to variation, it would exhibit clear traces of it at the greater of
these depths. Such is not the case, however ; and hence the persistence of type
at the still more profound depth of 1260 fathoms becomes intelligible, although
nevertheless surprising.
I t has already been stated that Orphiocoma granulata ranges at the present
day from the confines of the arctic circle to our own shores. I failed to detect
it in the immediate neighbourhood of Eekiavik, but this was probably in consequence
of the peculiar scoriaceous nature of the bottom along the south-western
portion of the island. In the absence of data for determining whence the species
became diffused across such a wide longitudinal range, it would be useless to
speculate whether the isolated deep-sea area on which it has now been found
formed its genetic centre; whether the transport or migration of individuals
towards that area took place prior to its isolation and complete submergence ; or
whether it was colonized after isolation. From what has been stated regarding
the inability of the Ophiuridæ in their mature state to rise to any distance or
for any lengthened period from the bottom, it is apparent that transport of
individuals could only be effected by a powerful current. During the larval
condition, however, of certain species of Ophiuridæ, they sometimes float at the
surface of the sea, and, hence, under favourable circumstances, might be drifted
towards isolated areas fitted to receive them. In the Foraminiferous deposit of
the Agulhas bank, I dredged up larval Ophiuridæ, from a depth of 100 fathoms,
measuring not more than a quarter of an inch across the arms, whilst specimens
obtained from the immediate surface over the same deposit were of still more
minute dimensions. These had, in all probability, drifted from the adjacent
• According to Edward Forbes, no important variation has taken place in the Ophiuridæ since tho
extinction of the species found .fossil.
coast-line, and were destined to subside to the bottom as soon as their calcareous
structure became sufficiently consolidated to counterbalance the buoyancy of the
soft parts*. In the case of the Ophiocomæ of the sounding, the presence of ova
distinctly proves that the generative function was being carried on even at that
great depth ; and consequently the undetermined state of the question regarding
the genetic centre of the species is of very secondary importance ; for, whether
the submerged area was colonized by emigrants from adjoining coasts, prior to
its isolation, or by larval Ophiocomæ which had subsided upon it after it had
become partially or completely submerged, the phenomenon of subsequent
acclimatization (which is the one more particularly under consideration) remains
entirely unaffected.
Another very singular fact in connexion with the Star-fish sounding demands
notice, namely, its proximity to the circle of uniform temperature f . I t will be
recollected that the sounding was taken in lat. 59° 27' N., whereas the mean
latitude of the homothermal circle, according to Sir James C. Ross’s observations
in the southern hemisphere, is 5 6 °2 5 'J . The position corresponding most
closely, iu the Southern Sea, with that of the Star-fish sounding is in lat. 58° 36' S.,
long. 104° 40'AV.—a position at which it is stated to be “ about a degree higher
than in other parts of the ocean, and also rather further to the southward.” In
this situation the temperature of the air is stated to have been 32° F-, of the
surface 41°, and at 100 fathoms 40°’8 ^ ; whereas, in the locality of the sounding,
* In 1850, Mr. T. H. Stewart detected a larval Ophiocoma in abundance on a parasitic sponge brought
up from a depth of 53 fathoms in the British Channel. The same larval form was also met with in
pools left by the receding tide,—the fact of its being found both in shallow and deep water being
regarded as proof that the mature animal spawns iu both situations. (Annals and Magazine of Natural
History, 2nd scr. vol. xviii. p. 387.)
t I t is worthy of note that on the Agulhas bank, to the southward of the Cape of Good Hope, where
fish are abundant, the bottom consists of calcareous deposit, and that to the northward of Eockall
(where an extensive fishery has recently been re-discovered) the bottom is essentially of a similar character,
Echinoderm-débris being so plentiful in both localities as to render it probable th at the Echinoderms
form the food of the fish. Should shallow water stül exist in the vicinity of the reputed “ Sunk
land of Buss,” it is possible th at a more inexhaustible fish-bank than any yet known may be there
met with. I t is also remarkable that Loch Fyne, the Dogger Bank and Silver Pits, the Eockall Shoal,
and the Labrador and Newfoundland fisheries are all either on, or near, the circle of uniform temperature—
a coincidence in position which can hardly be regarded as accidental.
t See diagram at p. 99.
§ Tho mean latitude of the homothermal zone as computed by Sir James C. Ross, in the southern
hemisphere, is here given. (See ‘ Voyage to the Southern Seas,’ vol. ii. pp. 227 and 375.)