The formation of the tongue
in the frog is exceptional. The front part of the tongue, which in
other animals is detached, is tightly fixed in the frog as it is in
all fishes; but the part towards the pharynx is freely detached, and
may, so to speak, be spat outwards, and it is with this that it
makes its peculiar croak. The croaking that goes on in the marsh is
the call of the males to the females at rutting time; and, by the way,
all animals have a special cry for the like end at the like season, as
is observed in the case of goats, swine, and sheep. (The bull-frog
makes its croaking noise by putting its under jaw on a level with
the surface of the water and extending its upper jaw to its utmost
capacity. The tension is so great that the upper jaw becomes
transparent, and the animal's eyes shine through the jaw like lamps;
for, by the way, the commerce of the sexes takes place usually in
the night time.) Birds can utter vocal sounds; and such of them can
articulate best as have the tongue moderately flat, and also such as
have thin delicate tongues. In some cases, the male and the female
utter the same note; in other cases, different notes. The smaller
birds are more vocal and given to chirping than the larger ones; but
in the pairing season every species of bird becomes particularly
vocal.
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