Frogs have good eyes, and the toad's eyes are "jewels." There is
evidence of precise vision in the neat way in which a frog catches a
fly, flicking out its tongue, which is fixed in front and loose behind.
There is also experimental proof that a frog discriminates between red
and blue, or between red and white, and an interesting point is that
while our skin is sensitive to heat rays but not to light, the skin of
the frog answers back to light rays as well. Professor Yerkes
experimented with a frog which had to go through a simple labyrinth if
it wished to reach a tank of water. At the first alternative between two
paths, a red card was placed on the wrong side and a white one on the
other. When the frog had learned to take the correct path, marked by the
white card, Prof. Yerkes changed the cards. The confusion of the frog
showed how thoroughly it had learned its lesson.
We know very little in regard to sense of smell or taste in amphibians;
but the sense of hearing is well developed, more developed than might be
inferred from the indifference that frogs show to almost all sounds
except the croaking of their kindred and splashes in the water.
The toad looks almost sagacious when it is climbing up a bank, and some
of the tree-frogs are very alert; but there is very little that we dare
say about the amphibian mind. We have mentioned that frogs may learn the
secret of a simple maze, and toads sometimes make for a particular
spawning-pond from a considerable distance. But an examination of their
brains, occupying a relatively small part of the broad, flat skull,
warns us not to expect much intelligence. On the other hand, when we
take frogs along a line that is very vital to them, namely, the
discrimination of palatable and unpalatable insects, we find, by
experiment, that they are quick to learn and that they remember their
lessons for many days. Frogs sometimes deposit their eggs in very
unsuitable pools of water; but perhaps that is not quite so stupid as it
looks. The egg-laying is a matter that has been, as it were, handed over
to instinctive registration.
[Illustration: _Photo: W. S. Berridge._