As far as I have been able to learn, after much research in natural history, the anthropoid apes do not show that they possess the sense of direction in a marked degree; thus we see that the immediate ancestors of pithecoid man had already begun to lose this sense, which in man is entirely wanting, and the absence of which should not be a matter of surprise in the slightest degree, but rather a result that should be expected.
Evidences of this sense are to be observed in animals of exceedingly low organization. On one occasion, while studying a water-louse, as I have already described elsewhere in this book, I saw the little creature swim to a hydra, pluck off one of its buds, then swim a short distance away and take shelter behind a small bit of mud, where it proceeded to devour its tender morsel. In a short while, much to my surprise, the louse again swam to the hydra, again procured a bud, and again swam back to its hiding-place. This occurred three times during the hour I had it under observation. The louse probably discovered the hydra the first time by accident; but when it swam back to the source of its food-supply the second time and then returned again to its sheltering bit of mud, it clearly evinced conscious memory of route and a sense of direction.
The common garden-snail is a homing animal, and it will always return to a particular spot after it has made an excursion in search of food. In front of my dwelling there is a brick wall capped by a stone coping; the overhanging edge of this coping forms a moist, cool home in summer for hundreds of snails. Last summer I took six of these creatures, and, after marking their shells with a paint of gum arabic and zinc oxide, I set them free on the lawn some distance away from the wall. In course of time, four of them returned to their homes beneath the stone coping; the other two were probably killed and eaten by blackbirds, numbers of which I noticed during the day feeding on the sward.
The centre of the sense of direction in snails is located at the base of the cephalic ganglion (brain); this ganglion lies immediately between and below the "horns" (eye-stalks), and is composed of several circumscribed and well-marked accumulations or corpuscles of nerve-cells and nerve-filaments.
This sense centre can easily be destroyed without inflicting injury on the circumjacent sense centres. Whenever this is done, the snail loses its sense of direction and locality, and cannot find its way back to its home when it is carried thence, and deposited amid new surroundings. It is not killed by the mutilation, for I have seen marked snails in which this sense centre had been destroyed, alive and apparently in good health, several weeks after having undergone this operation; they found temporary homes wherever they chanced to be.
The limpet is likewise a homing animal, and invariably returns to its home after journeys in search of food. Lieutenant L——, an officer in the British navy, once told me that he had repeatedly had specimens of this animal under observation for months at a time, and that they always had particular spots, generally depressions in rocks, which they regarded as homes, to which they would always return after excursions in search of sustenance. Romanes makes a similar statement.[103]
Some beetles have their homing sense highly developed; thus, in Mammoth Cave, the blind beetle (Adelops) has its particular home, and will always return to it even when it is set free at a considerable distance. Notwithstanding the fact these insects are blind, and that darkness reigns in this immense cavern, they have periods of rest corresponding with the diurnal rest-periods of kindred species living in daylight; hence, it is easy to study their habits at home and abroad.
I have frequently marked these beetles and then set them free some distance away from their domiciles; they would hide themselves at once beneath stones or clods of earth, but as soon as they had recovered from their fright they would turn towards home, and would not stop, if left unmolested, until they arrived at their particular and individual homing places. Truly a most wonderful exhibition of the homing sense!
At first, these beetles are, probably, directed and governed by their sense of direction alone, but as soon as they arrive among familiar surroundings, memory comes to their aid.
The agile flea is another "homesteader," and if marked, its favorite resting-place on a dog or cat can easily be determined. After feeding, it will invariably return to a certain spot in order to enjoy its nap in peace; for, strange as it may seem, fleas are sound sleepers, and, what is more, seem to require a great deal of sleep.[104]