To find solid ground on which to base an appreciation of the behaviour
of fishes, it is necessary to experiment, and we may refer to Miss
Gertrude White's interesting work on American minnows and sticklebacks.
After the fishes had become quite at home in their artificial
surroundings, their lessons began. Cloth packets, one of which contained
meat and the other cotton, were suspended at opposite ends of the
aquarium. The mud-minnows did not show that they perceived either
packet, though they swam close by them; the sticklebacks were intrigued
at once. Those that went towards the packet containing meat darted
furiously upon it and pulled at it with great excitement. Those that
went towards the cotton packet turned sharply away when they were within
about two inches off. They then perceived what those at the other end
were after and joined them--a common habit amongst fishes. Although the
minnows were not interested in the tiny "bags of mystery," they were
even more alert than the sticklebacks in perceiving moving objects in or
on the water, and there is no doubt that both these shallow-water
species discover their food largely by sense of sight.
The next set of lessons had to do with colour-associations. The fishes
were fed on minced snail, chopped earthworm, fragments of liver, and the
like, and the food was given to them from the end of forceps held above
the surface of the water, so that the fishes could not be influenced by
smell. They had to leap out of the water to take the food from the
forceps. Discs of coloured cardboard were slipped over the end of the
forceps, so that what the fishes saw was a morsel of food in the centre
of a coloured disc. After a week or so of preliminary training, they
were so well accustomed to the coloured discs that the presentation of
one served as a signal for the fishes to dart to the surface and spring
out of the water. When baits of paper were substituted for the food, the
fishes continued to jump at the discs. When, however, a blue disc was
persistently used for the paper bait and a red disc for the real food,
or _vice versa_, some of the minnows learned to discriminate infallibly
between shadow and substance, both when these were presented alternately
and when they were presented simultaneously. This is not far from the
dawn of mind.
In the course of a few lessons, both minnows and sticklebacks learned to
associate particular colours with food, and other associations were also
formed. A kind of larva that a minnow could make nothing of after
repeated trials was subsequently ignored. The approach of the
experimenter or anyone else soon began to serve as a food-signal. There
can be no doubt that in the ordinary life of fishes there is a process
of forming useful associations and suppressing useless responses. Given
an inborn repertory of profitable movements that require no training,
given the power of forming associations such as those we have
illustrated, and given a considerable degree of sensory alertness along
certain lines, fishes do not require much more. And in truth they have
not got it. Moving with great freedom in three dimensions in a medium
that supports them and is very uniform and constant, able in most cases
to get plenty of food without fatiguing exertions and to dispense with
it for considerable periods if it is scarce, multiplying usually in
great abundance so that the huge infantile mortality hardly counts,
rarely dying a natural death but usually coming with their strength
unabated to a violent end, fishes hold their own in the struggle for
existence without much in the way of mental endowment. Their brain has
more to do with motion than with mentality, and they have remained at a
low psychical level.
Yet just as we should greatly misjudge our own race if we confined our
attention to everyday routine, so in our total, as distinguished from
our average, estimate of fishes, we must remember the salmon surmounting
the falls, the wary trout eluding the angler's skill, the common
mud-skipper (Periophthalmus) of many tropical shores which climbs on the
rocks and the roots of the mangrove-trees, or actively hunts small
shore-animals. We must remember the adventurous life-history of the eel
and the quaint ways in which some fishes, males especially, look after
their family. The male sea-horse puts the eggs in his breast-pocket; the
male Kurtus carries them on the top of his head; the cock-paidle or
lumpsucker guards them and aerates them in a corner of a shore-pool.
§ 3