on the other hand, the good nurse must take care that the active young
fish do not stray far from the nest, and so expose themselves
prematurely to the manifold dangers of the outer world. Till they are
big enough to take care of themselves, he watches with incessant
vigilance over their safety; as soon as they can go forth with
tolerable security upon the world of their brook or pond, he takes a
last well-merited holiday.

It is not surprising under these circumstances to learn that
sticklebacks are successful and increasing animals. Their numbers are
enormous, wherever they get a fair chance in life, because they
multiply rapidly up to the extreme limit of the means of subsistence,
and develop as fast as food remains for them. There the inexorable
Malthusian law at last steps in: when there is not food enough for all
some must starve; that is the long and the short of the great
population question. But while provender is forthcoming they increase
gayly. Sticklebacks live mainly on the spawn of other fish, though
they are so careful of their own, and they are therefore naturally
hated by trout-preservers and owners of fisheries in general.
Thousands and thousands are caught each year; in some places, indeed,
they are so numerous that they are used as manure. It is their
numbers, of course, that make them formidable; they are the locusts of
the streams, well armed and pugnacious, and provided with most
remarkable parental instincts of a protective character, which enables
them to fill up all vacancies in their ranks as fast as they occur
with astonishing promptitude.

To those whose acquaintance with fish is mainly culinary, it may seem
odd to hear that the father stickleback alone takes part in the care
of the nursery. But this is the rule among the whole class of fish;