The three-spined and two-spined sticklebacks live equally well in fresh
or salt water; the larger fifteen-spined stickleback is entirely marine.
In all three species the male fish makes a nest, in fresh or brackish
water in the first two cases, in shore-pools in the third case. The
little species use the leaves and stems of water-plants; the larger
species use seaweed and zoophyte. The leaves or fronds are entangled
together and fastened by glue-like threads, secreted, strange to say, by
the kidneys. It is just as if a temporary diseased condition had been
regularised and turned to good purpose. Going through the nest several
times, the male makes a little room in the middle. Partly by coercion
and partly by coaxing he induces a female--first one and then
another--to pass through the nest with two doors, depositing eggs during
her short sojourn. The females go their way, and the male mounts guard
over the nest. He drives off intruding fishes much bigger than himself.
When the young are hatched, the male has for a time much to do, keeping
his charges within bounds until they are able to move about with
agility. It seems that sticklebacks are short-lived fishes, probably
breeding only once; and it is reasonable to suppose that their success
as a race depends to some extent on the paternal care. Now if we could
believe that the nesting behaviour had appeared suddenly in its present
form, we should be inclined to credit the fish with considerable mental
ability. But we are less likely to be so generous if we reflect that the
routine has been in all likelihood the outcome of a long racial process
of slight improvements and critical testings. The secretion of the glue
probably came about as a pathological variation; its utilisation was
perhaps discovered by accident; the types that had wit enough to take
advantage of this were most successful; the routine became enregistered
hereditarily. The stickleback is not so clever as it looks.
[Illustration: _Photo: Imperial War Museum._