DARWIN’S LAST book, written in 1881, was The formation of vegetable mould, through the action of worms, with observations on their habits. Across its 300-odd pages Darwin describes and explains the nature of worms, before concluding: ‘Worms have played a more important part in the history of the world than most persons would at first suppose.’
Privately, he noted the pleasure it had given him to elevate such humble animals. The book is full of fascinating observations, showing how Darwin was not only a great gatherer of facts, but also as diligent an experimenter as Faraday before him. On page 26 he begins describing his investigations of whether earthworms can hear, and their sensitivity to ground-borne vibrations:
Worms do not possess any sense of hearing. They took not the least notice of the shrill notes from a metal whistle, which was repeatedly sounded near them; nor did they of the deepest and loudest tones of a bassoon. They were indifferent to shouts, if care was taken that the breath did not strike them. When placed on a table close to the keys of a piano, which was played as loudly as possible, they remained perfectly quiet.
Although they are indifferent to undulations in the air audible by us, they are extremely sensitive to vibrations in any solid object. When the pots containing two worms which had remained quite indifferent to the sound of the piano, were placed on this instrument, and the note C in the bass clef was struck, both instantly retreated into their burrows. After a time they emerged, and when G above the line in the treble clef was struck they again retreated. Under similar circumstances on another night one worm dashed into its burrow on a very high note being struck only once, and the other worm when C in the treble clef was struck.
On these occasions the worms were not touching the sides of the pots, which stood in saucers; so that the vibrations, before reaching their bodies, had to pass from the sounding board of the piano, through the saucer, the bottom of the pot and the damp, not very compact earth on which they lay with their tails in their burrows. They often showed their sensitiveness when the pot in which they lived, or the table on which the pot stood, was accidentally and lightly struck; but they appeared less sensitive to such jars than to the vibrations of the piano; and their sensitiveness to jars varied much at different times.
The whole book can be read at the Darwin Online website.