Snails will also return to special dark hollows or crannies in the wall after their foraging excursions. Such behaviour in molluscs affords evidence of something more than instinct. In popular speech, we should say that there is memory of the locality. And in any case it is difficult to interpret the facts without the assumption that the animals are conscious, and that re-presentative states are evoked through the mediation of presentative sense-impressions. But how difficult, if not impossible, it is to form anything like a satisfactory conception of the rudimentary mental processes of a limpet!
The most highly developed molluscs are the cephalopods. They have long sensitive mobile arms with which they feel for and capture their prey. “Now Schneider observed,” writes Dr. Stout,[68] “a very young octopus seize a hermit-crab. The hermit-crab covers the shell in which it takes up its abode with stinging zoophytes. Stung by these, the octopus immediately recoiled and let its prey escape. Subsequently it was observed to avoid hermit-crabs. Older animals of the same species managed cleverly to pull the crab out of its house without being stung.” Such cases afford evidence of profiting by experience through the exercise of intelligence.
Darwin’s careful observations on the manner in which earthworms drag leaves into their burrows seem to show that these annelids act intelligently, and deal with leaves of different shapes in different ways. The leaves of Pine trees, consisting of two needles arising from a common base, were almost invariably drawn down by seizing this basal point of junction; while the leaves of the Lime were, in 79 per cent. of the cases examined, drawn down by the apex; in only 4 per cent. by the base; and in the remaining 17 per cent. by seizing some intermediate portion. On the other hand, the leaves of the Rhododendron, in which the basal part of the blade is often narrower than the apical part, were in 66 per cent. of the observations drawn down by the narrower base. Triangles of paper were in the majority of cases seized by the apex. Commenting upon his observations, carried out with great care under experimental conditions, Darwin says,[69] “As worms are not guided by special instincts in each particular case, though possessing a general instinct to plug up their burrows, and as chance is excluded, the next most probable conclusion seems to be that they try in different ways to draw in objects, and at last succeed in some one way;” that is to say, they profit by experience based on the method of trial and failure. But Darwin adds that the evidence he obtained shows “that worms do not habitually try to draw objects into their burrows in many different ways.” And he seems to attribute to them an almost rational power of dealing with the circumstances in the light of general conceptions. “If worms,” he says, “are able to judge, either before drawing or after having drawn an object close to the mouths of their burrows, how best to drag it in, they must acquire some notion of its general shape. This they probably acquire by touching it in many places with the anterior extremity of their bodies, which acts as a tactual organ. It may be well to remember how perfect the sense of touch becomes in a man when born blind and deaf, as are worms. If worms have the power of acquiring some notion, however rude, of the shape of an object and of their burrows, as seems to be the case, they deserve to be called intelligent; for they then act in nearly the same manner as would a man under similar circumstances.”