MOLLUSCA.
I SHALL treat of the Mollusca before the Articulata, because as a group
their intelligence is not so high. Indeed, it is not to be expected that
the class of animals wherein the 'vegetative' functions of nutrition and
reproduction predominate so largely over the animal functions of
sensation, locomotion, &c., should present any considerable degree of
intelligence. Nevertheless, in the only division of the group which has
sense organs and powers of locomotion highly developed--viz., the
Cephalopoda--we meet with large cephalic ganglia, and, it would appear,
with no small development of intelligence. Taking, however, the
sub-kingdom in ascending order, I shall first present all the
trustworthy evidence that I have been able to collect, pointing to the
highest level of intelligence that is attained by the lower members.
The following is quoted from Mr. Darwin's MS.:--
Even the headless oyster seems to profit from
experience, for Dicquemase ('Journal de Physique,'
vol. xxviii. p. 244) asserts that oysters taken from a
depth never uncovered by the sea, open their shells,
lose the water within, and perish; but oysters taken
from the same place and depth, if kept in reservoirs,
where they are occasionally left uncovered for a short
time, and are otherwise incommoded, learn to keep
their shells shut, and then live for a much longer
time when taken out of the water.[9]
Some evidence of intelligence seems to be displayed by the razor-fish.
For the animals dislike salt, so that when this is sprinkled above their
burrows in the sand, they come to the surface and quit their
habitations. But if the animal is once seized when it comes to the
surface and afterwards allowed to retire into its burrow, no amount of
salt will force it again to come to the surface.[10]
With regard to snails, L. Agassiz writes: 'Quiconque a eu l'occasion
d'observer les amours des limaçons, ne saurait mettre en doute la
séduction déployée dans les mouvements et les allures qui préparent et
accomplissent le double embrassement de ces hermaphrodites.'[11]
Again, Mr. Darwin's MS. quotes from Mr. W. White[12] a curious exhibition
of intelligence in a snail, which does not seem to have admitted of
mal-observation. This gentleman 'fixed a land-shell mouth uppermost in a
chink of rock; in a short time the snail protruded itself to its utmost
length, and, attaching its foot vertically above, tried to pull the
shell out in a straight line. Not succeeding, it rested for a few
minutes and then stretched out its body on the right side and pulled its
utmost, but failed. Resting again, it protruded its foot on the left
side, pulled with its full force, and freed the shell. This exertion of
force in three directions, which seems so geometrically suitable, must
have been intentional.'
If it is objected that snail shells must frequently be liable to be
impeded by obstacles, and therefore that this display of manoeuvring
on the part of their occupants is to be regarded as a reflex, I may
remark that here again we have one of those incessantly recurring cases
where it is difficult to draw the line between intelligence and
non-intelligence. For, granting that the action is to a certain extent
mechanical, we must still recognise that the animal while executing it
must have remembered each of the two directions in which it had pulled
ineffectually before it began to pull in the third direction; and it is
improbable that snail shells are so frequently caught in positions from
which a pull in only one direction will release them, that natural
selection would have developed a special instinct to try pulling
successively in three directions at right angles to one another.
The only other instance that I have met with of the apparent display of
intelligence in snails is the remarkable one which Mr. Darwin gives in
his 'Descent of Man,' on the authority of Mr. Lonsdale. Although the
interpretation which is assigned to the fact seems to me to go beyond
anything that we should have reason to expect of snail intelligence, I
cannot ignore a fact which stands upon the observation of so good an
authority, and shall therefore quote it in Mr. Darwin's words:--
These animals appear also susceptible of some degree
of permanent attachment: an accurate observer, Mr.
Lonsdale, informs me that he placed a pair of
land-snails (_Helix pomatia_), one of which was
weakly, into a small and ill-provided garden. After a
short time the strong and healthy individual
disappeared, and was traced by its track of slime over
a wall into an adjoining well-stocked garden. Mr.
Lonsdale concluded that it had deserted its sickly
mate; but after an absence of twenty-four hours it
returned, and apparently communicated the result of
its successful exploration, for both then started
along the same track, and disappeared over the
wall.[13]
In this case the fact must be accepted, seeing that it stands on the
authority of an accurate observer, and is of so definite a kind as not
to admit of mistake. Consequently we are shut up to the alternative of
supposing the return of the healthy snail to its mate a mere accident,
and their both going over the wall into the well-stocked garden another
mere accident, or acquiescing in the interpretation which Mr. Darwin
assigns. Now, if we look closely into the matter, the chances against
the double accident in question are certainly so considerable as to
render the former supposition almost impossible. On the other hand,
there is evidence to prove, as I shall immediately show, that a not
distantly allied animal is unquestionably able to remember a particular
locality as its home, and habitually to return to this locality after
feeding. Therefore, in view of this analogous and corroborative case,
the improbability of the snail remembering for twenty-four hours the
position of its mate is very much reduced; while the subsequent
communication, if it took place, would only require to have been of the
nature of 'follow me,' which, as we shall repeatedly find, is a degree
of communicative ability which many invertebrated animals possess.
Therefore, in view of these considerations, I incline to Mr. Darwin's
opinion that the facts can only be explained by supposing them due to
intelligence on the part of the snails. Thus considered, these facts are
no doubt very remarkable; for they would appear to indicate not merely
accurate memory of direction and locality for twenty-four hours, but
also no small degree of something akin to 'permanent attachment,' and
sympathetic desire that another should share in the good things which
one has found.[14]
The case to which I have just alluded as proving beyond all doubt that
some Gasteropoda are able to retain a very precise and accurate memory
of locality, is that of the common limpet.
Mr. J. Clarke Hawkshaw publishes in the Journal of the Linnæan Society
the following account of the habits in question:--
The holes in the chalk in which the limpets are often
to be found are, I believe, excavated in a great
measure by rasping from the lingual teeth, though I
doubt whether the object is to form a cavity to
shelter in, though the cavities, when formed, may be
of use for that purpose. It must be of the greatest
importance to a limpet that, in order that it may
insure a firm adherence to the rock, its shell should
fit the rock accurately; when the shell does fit the
rock accurately, a small amount of muscular
contraction of the animal would cause the shell to
adhere so firmly to a smooth surface as to be
practically immoveable without fracture. As the shells
cannot be adapted daily to different forms of surface,
the limpets generally return to the same place of
attachment. I am sure this is the case with many; for
I found shells perfectly adjusted to the uneven
surfaces of flints, the growth of the shells being in
some parts distorted and indented to suit
inequalities in the surface of the flints. . . .
I noticed signs that limpets prefer a hard, smooth
surface to a pit in the chalk. On one surface of a
large block, over all sides of which limpets were
regularly and plentifully distributed, there were two
flat fragments of a fossil shell about 3 inches by 4
inches, each embedded in the chalk. The chalk all
round these fragments was free from limpets; but on
the smooth surface of the pieces of shell they were
packed as closely as they could be. I noticed another
case, which almost amounts, to my mind, to a proof
that they prefer a smooth surface to a hole. A limpet
had formed a clearing on one of the sea-weed-covered
blocks before referred to. In the midst of this
clearing was a pedestal of flint rather more than one
inch in diameter, standing up above the surface of the
chalk; it projected so much that a tap from my hammer
broke it off. On the top of the smooth fractured
surface of this flint the occupant of the clearing had
taken up its abode. The shell was closely adapted to
the uneven surface, which it would only fit in one
position. The cleared surface was in a hollow with
several small natural cavities, where the limpet could
have found a pit ready made to shelter in; yet it
preferred, after each excursion, to climb up to the
top of the flint, the most exposed point in all its
domain.[15]
It appears certain from these observations, which to some extent were
anticipated by those of Mr. F. C. Lukis,[16] that limpets, after every
browsing excursion, return to one particular spot or home; and the
precise memory of direction and locality implied by this fact seems to
justify us in regarding these actions of the animal as of a nature
unquestionably intelligent.
Coming now to the cephalopoda, there is no doubt that if a larger sphere
of opportunity permitted, adequate observation of these animals would
prove them to be much the most intelligent members of the sub-kingdom.
Unfortunately, however, this sphere of opportunity has hitherto been
very limited. The following meagre account is all that I have been able
to gather concerning the psychology of these interesting animals.
According to Schneider,[17] the Cephalopoda show unmistakable evidence
of consciousness and intelligence. This observer had an opportunity of
watching them for a long time in the zoological station at Naples; and
he says that they appeared to recognise their keeper after they had for
some time received their food from him. Hollmann narrates that an
octopus, which had had a struggle with a lobster, followed the latter
into an adjacent tank, to which it had been removed for safety, and
there destroyed it. In order to do this the octopus had to climb up a
vertical partition above the surface of the water and descend the other
side.[18] According to Schneider, the Cephalopoda have an abstract idea
of water, seeking to return to it when removed, even though they do not
see it. But this probably arises from the sense of discomfort due to
exposure of their skin to the air; and if we can call it an 'idea,' it
is doubtless shared by all other aquatic Mollusca when exposed to air.
FOOTNOTES:
[9] This fact is also stated by Bingley, _Animal Biography_, vol. iii.
p. 454, and is now turned to practical account in the so-called
'Oyster-schools' of France. The distance from the coast to Paris being
too great for the newly dredged oysters to travel without opening their
shells, they are first taught in the schools to bear a longer and longer
exposure to the air without gaping, and when their education in this
respect is completed they are sent on their journey to the metropolis,
where they arrive with closed shells, and in a healthy condition.
[10] Bingley, _loc. cit._, vol. iii. p. 449.
[11] _De l'Espèce et de la Classe_, &c., 1869, p. 106.
[12] _A Londoner's Walk to Edinburgh_, p. 155 (1856).
[13] _Descent of Man_, pp. 262-3.
[14] The facts, however, in order to sustain such conclusions, of course
require corroboration, and it is therefore to be regretted that Mr.
Lonsdale did not experimentally repeat the conditions.
[15] _Journal Linn. Soc._ vol. xiv. p. 406 _et seq._
[16] _Mag. Nat. Hist._ 1831, vol. iv. p. 346.
[17] _Thieresche Wille_, § 78.
[18] _Leben der Cephalopoden_, s. 21.