Cartilaginous fish sleep at times so soundly that they may be
caught by hand. The dolphin and the whale, and all such as are
furnished with a blow-hole, sleep with the blow-hole over the
surface of the water, and breathe through the blow-hole while they
keep up a quiet flapping of their fins; indeed, some mariners assure
us that they have actually heard the dolphin snoring.
Molluscs sleep like fishes, and crustaceans also. It is plain also
that insects sleep; for there can be no mistaking their condition of
motionless repose. In the bee the fact of its being asleep is very
obvious; for at night-time bees are at rest and cease to hum. But
the fact that insects sleep may be very well seen in the case of
common every-day creatures; for not only do they rest at night-time
from dimness of vision (and, by the way, all hard-eyed creatures see
but indistinctly), but even if a lighted candle be presented they
continue sleeping quite as soundly.
Of all animals man is most given to dreaming. Children and infants
do not dream, but in most cases dreaming comes on at the age of four
or five years. Instances have been known of full-grown men and women
that have never dreamed at all; in exceptional cases of this kind,
it has been observed that when a dream occurs in advanced life it
prognosticates either actual dissolution or a general break-up of
the system.
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