no chord of that imperfect instrument, the human ear. They are too high or too grave. I have observed a flock of blackbirds occupying an entire tree-top—the tops of several trees—and all in full song. Suddenly—in a moment—at absolutely the same instant all spring into the air and fly away. How? They could not all see one another—whole tree-tops intervened. At no point could a leader have been visible to all. There must have been a signal of warning or command, high and shrill above the din, but by me unheard. I have observed, too, the same simultaneous flight when all were silent, among not only blackbirds, but other birds—quail, for example, widely separated by bushes—even on opposite sides of a hill.
"It is known to seamen that a school of whales basking or sporting on the surface of the ocean, miles apart, with the convexity of the earth between, will sometimes dive at the same instant—all gone out of sight in a moment. The signal has been sounded—too grave for the ear of the sailor at the masthead and his comrades on the deck—who nevertheless feel its vibrations in the ship as the stones of a cathedral are stirred by the bass of the organ.
