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.
CHAPTER III.
ANTS.
Within the last ten or twelve years our information on the habits and intelligence of these insects has been so considerably extended, that in here rendering a condensed epitome of our knowledge in this most interesting branch of comparative psychology, it will be found that the chapter is constituted principally of a statement of observations and experiments which have been conducted during the short period named. The observers to whom we are mainly indebted for this large increase of our knowledge are Messrs. Bates, Belt, Müller, Moggridge, Lincecum, MacCook, and Sir John Lubbock. From the fact that these naturalists conducted their observations in different parts of the world and on widely different species of ants, it is not surprising that their results should present many points of difference; for this only shows, as we might have expected, that different species of ants differ considerably in habits and intelligence. Therefore, in now drawing all these numerous observations to a focus, I shall endeavour to show clearly their points of difference as well as their points of agreement; and in order that the facts to be considered may be arranged in some kind of order, I shall deal with them under the following heads:—Powers of special sense; Sense of direction; Powers of memory; Emotions; Powers of communication; Habits general in sundry species; Habits peculiar to certain species; General intelligence of various species.
Powers of Special Sense.
Taking first the sense of sight, Sir John Lubbock made a number of experiments on the influence of light coloured by passing through various tints of stained glass, with the following results. The ants which he observed greatly dislike the presence of light within their nests, hurrying about in search of the darkest corners when light is admitted. The experiments showed that the dislike is much greater in the case of some colours than in that of others. Thus under a slip of red glass there were congregated on one occasion 890 ants, under green 544, under yellow 495, and under violet only 5. To our eyes the violet is as opaque as the red, more so than the green, and much more so than the yellow. Yet, as the numbers show, the ants had scarcely any tendency to congregate under it: there were nearly as many under the same area of the uncovered portion of the nest as under that shaded by the violet glass. It is curious that the coloured glasses appear to act on the ants in a graduated series, which corresponds with the order of their influence on a photographic plate. Experiments were therefore made to test whether it might not be the actinic rays that were so particularly distasteful to the ants; but with negative results. Placing violet glass above red produces the same effect as red glass alone. Obviously, therefore, the ants avoid the violet glass because they dislike the rays which it transmits, and do not prefer the other colours because they like the rays which they transmit. Sodium, barium, strontium, and lithium flames were also tried, but not with so much effect as the coloured glass.
It has just been observed that the relative dislike which Sir John Lubbock's ants showed to lights of different colours seems to be determined by the position of the colour in the spectrum—there being a regular gradation of intolerance shown from the red to the violet end. As these ants dislike light, the question suggests itself that the reason of their graduated intolerance to light of different colours may be due to their eyes not being so much affected by the rays of low as by those of high refrangibility. In this connection it would be interesting to ascertain whether ants of the genus Atta show a similarly graduated intolerance to the light in different parts of the spectrum; for both Moggridge and MacCook record of this genus that it not only does not shun the light, but seeks it—coming to the glass sides of their artificial nests to enjoy the light of a lamp. Possibly, therefore, the scale of preference to lights of different colours would be found in this genus to be the reverse of that which Sir John Lubbock has found in the case of the British species.
As regards hearing, Sir John Lubbock found that