Any country schoolboy will tell you that worms can hear. He points to his simple experiment (pounding on the earth with a club) in proof of his assertion. For, as soon as he begins to pound the ground in a favorable neighborhood, the worms will come to the surface "to see what makes the noise." Darwin assumes that the worms feel the vibrations, which are disagreeable to them, and come to the surface in order to escape them. I do not deny the possibility or the probability of this assumption; I do deny, however, that it proves that worms are deaf.
If the third anal segment (abdominal aspect) of a worm be examined, two round, disk-like organs incorporated in the integument will be found; these organs are supplied with special nerves which lead to the central nerve-cord. Experiments lead me to believe that these are organs of audition.
When I tap the earth of my vermicularium with a pencil, the unmutilated worms will come to the surface; but, when the organs described above are removed, the worms so mutilated will not respond to the tapping, but will remain in their tunnel. The worms are not appreciably impaired by such mutilation; on the contrary, they seem to thrive as well as those to which the knife has not been applied.
In creatures which possess, in all probability, the senses of touch, taste, smell, sight, and hearing, we would naturally expect to find some evidences of conscious determination; and we do.
Certain leaves are the favorite food of earth-worms, while certain other leaves are eaten by them, but not with avidity. When these two kinds of leaves are given to worms, they will carefully select the favorite food and will ignore the other, thus unmistakably evincing conscious choice. Their avoidance of light is probably the result of conscious determination, and not reflex, as some observers maintain.
Oysters taken from a bank never uncovered by the sea, open their shells, lose the water within, and soon die; but oysters kept in a reservoir and occasionally uncovered learn to keep their shells closed, and live much longer when taken out of the water. This is an act of intelligence due directly to experience without even the factor of heredity.[33] It is an instance of almost immediate adaptation to surrounding circumstances.
A gentleman fixed a land-snail, with the mouth of the shell upward, in a chink of a rock. The animal protruded its foot to the utmost extent, and, attaching it above, tried to pull the shell vertically in a straight line. Then it stretched its body to the right side, pulled, and failed to move the shell. It then stretched its foot to the left side, pulled with all of its strength, and released the shell. There were intervals of rest between these several attempts, during which the snail remained quiescent.[34] Thus we see that it exerted force in three directions, never twice in the same direction, which fact shows conscious determination and no slight degree of intelligence.
A ground wasp once built a nest beneath the brick pavement in front of my door. The entrance of the nest was situated in the little sulcus, or ditch, between two bricks. While the wasp was absent, I stopped the entrance with a pellet of paper, and, when the little housekeeper returned, she was nonplussed for a moment or two, when she discovered that her doorway had been closed. The wasp, after examining the pellet of paper, seized it with her jaws and tried to pull it away; but, since she stood on the brick and pulled backwards (toward herself), the edge of the brick interposed, and she could not dislodge the obstacle. Finally, she got down into the little gully between the two bricks, and pulled the pellet away from the opening of the nest without any further trouble. Three times I performed the experiment, the wasp going through like performances each time. At the fourth time, however, she went at once into the little space between the bricks, and then removed the wad of paper without difficulty. I stopped the hole five or six times after this, but she had learned a lesson; she always got into the sulcus between the bricks before attempting to remove the paper. She had discovered the fact that she could not remove it when she stood upon the surfaces of the bricks, owing to the interposition of their sides, and that she could drag it away if she got down into the little ditch and pulled the paper in a direction where nothing opposed. In this instance there was not only conscious determination, but also a distinct exhibition of memory. It took the wasp some time to learn that she had to pull in a certain direction before she could remove the pellet of paper; but when she had once learned this fact, she remembered it. And this brings us to another quality of mind—memory—which will be discussed in the next chapter.
FOOTNOTES:
[23] "Sensorial impression is at the bottom of all our ideas, all our conceptions, though it may at first conceal itself in the form of a binary, ternary, quaternary compound; and, on our methodically pursuing the inquiry, it is easily recognizable—just as a simple substance in organic chemistry may be always summoned to appear, if we sit down with the resolution to disengage it from all the artificial combinations which hold it imprisoned."—Luys, The Brain and its Functions, p. 252.