A NOTE ON THE PSYCHOLOGY OF FISHES[21]
Numerous facts witness in a vague way to the ability of fishes to profit
by experience and fit their behavior to situations unprovided for by
their innate nervous equipment. All the phenomena shown by fishes as a
result of taming are, of course, of this sort. But such facts have not
been exact enough to make clear the mental or nervous processes involved
in such behavior, or simple enough to be available as demonstrations
of such processes. It seemed desirable to obtain evidence which should
demonstrate both the fact and the process of learning or intelligent
activity in the case of fishes and demonstrate them so readily that any
student could possess the evidence first hand.
Through the kindness of the officials of the United States Fish
Commission at Woods Holl, especially of the director, Dr. Bumpus, I was
able to test the efficiency of some simple experiments directed toward
this end. The common Fundulus was chosen as a convenient subject, and
also because of the neurological interest attaching to the formation of
intelligent habits by a vertebrate whose forebrain lacks a cortex.
The fishes studied were kept in an aquarium (about 4 feet long by 2 feet
wide, with a water depth of about 9 inches) represented by Fig. 24. The
space at one end, as represented by the lines in the figure, was shaded
from the sun by a cover, and all food was dropped in at this end. Along
each side of the aquarium were fastened simple pairs of cleats, allowing
the experimenter to put across it partitions of wood, glass or wire
screening. One of these in position is shown in the figure by the dotted
line. These partitions were made each with an opening, as shown in Fig.
25. If now we cause the fish to leave his shady corner and swim up to
the sunny end by putting a slide (without any opening) in behind him at
_D_ and moving it gently from _D_ to _A_ and then place, say slide _I_,
across the aquarium at 1, we shall have a chance to observe the animal’s
behavior to good purpose.
[Illustration: FIG. 24.]
[Illustration: FIG. 25.]
This fish dislikes the sunlight and tries to get back to _D_. He reacts
to the situation in which he finds himself by swimming against the
screen, bumping against it here and there along the bottom. He may stop
and remain still for a while. He will occasionally rise up toward the
top of the water, especially while swimming up and down the length of
the screen. When he happens to rise up to the top at the right-hand end,
he has a clear path in front of him and swims to _D_ and feels more
comfortable.
If, after he has enjoyed the shade fifteen minutes or more, you again
confine him in _A_, and keep on doing so six or eight times a day for a
day or so, you will find that he swims against the screen less and less,
swims up and down along it fewer and fewer times, stays still less and
less, until finally his only act is to go to the right-hand side, rise
up, and swim out. In correspondence with this change in behavior you will
find a very marked decrease in the time he takes to escape. The fish
has clearly profited by his experience and modified his conduct to suit
a situation for which his innate nervous equipment did not definitely
provide. He has, in common language, _learned_ to get out.
This particular experiment was repeated with a number of individuals.
Another experiment was made, using three slides, _II_, _III_, and
another, requiring the fish to find his way from _A_ to _B_, _B_ to _C_,
and from _C_ to _D_. The results of these and still others show exactly
the same general mental process as does the one described—a process which
I have discussed at length elsewhere.
Whatever interest there is in the demonstration in the case of the
bony fishes of the same process which accounts for so much of the
behavior of the higher vertebrates may be left to the neurologists.
The value of the experiment, if any, to most students will perhaps
be the extreme simplicity of the method, the ease of administering
it, and its possibilities. By using long aquaria, one can study the
formation of very complex series of acts and see to what extent any
fish can carry the formation of such series. By proper arrangements the
delicacy of discrimination of the fish in any respect may be tested.
The artificiality of the surroundings may, of course, be avoided when
desirable.