In Florida, in the spring when the mating-instinct is strong, I have
seen a flock of white ibises waltzing about the sky, going through
various intricate movements, with the precision of dancers in a ballroom
quadrille. No sign, no signal, no guidance whatever. Let a body of men
try it under the same conditions, and behold the confusion, and the
tumbling over one another! At one moment the birds would wheel so as to
bring their backs in shadow, and then would flash out the white of their
breasts and under parts. It was like the opening and shutting of a giant
hand, or the alternate rapid darkening and brightening of the sail of a
tacking ice-boat. This is the spirit of the flock. When a hawk pursues a
bird, the birds tack and turn as if linked together. When one robin
dashes off in hot pursuit of another, behold how their movements exactly
coincide! The hawk-hunted bird often escapes by reaching the cover of a
tree or a bush, but not by dodging its pursuer, as a rabbit or a
squirrel will dodge a dog. Schools of fish act with the same
machine-like unity.

In the South, I have seen a large area of water, acres in extent,
uniformly agitated by a school of mullets apparently feeding upon some
infusoria on the surface, and then instantly, as if upon a given signal,
the fish would dive and the rippling cease. It showed a unity of action
as of ten thousand spindles controlled by electricity.

How quickly the emotion of fear is communicated among the wild animals!
How wild and alarmed the deer become after the opening of the first day
of the shooting season. Those who have not seen or heard a hunter seem
to feel the impending danger.

The great flocks of migrating butterflies (the monarch) illustrate the
same law. In the fall they are all seized with this impulse to go South
and thousands of them march in one body. At night they roost in the
trees. I have seen photographs of them in which they appeared like a new
kind of colored foliage covering the trees. In the return flight in the
spring, the same massing again occurs. Recently the Imperial Valley in
California was invaded by a vast army of worms moving from east to west.
In countries that have been cursed with a plague of grasshoppers
witnesses of the spectacle describe them as moving in the same way. They
stopped or delayed railway trains and automobiles, their crushed bodies
making the rails and highways as slippery as grease would have made
them. Ten million or ten