. The
spines on his back here stand him once more in good stead: for small
as he is, the stickleback is not an antagonist to be lightly despised:
he can inflict a wound which a perch or a trout knows how to estimate
at its full value. But that is not all the good parent's duty. He
takes the eggs out of the nest every now and then with his snout, airs
them a little in the fresh water outside, and then replaces and
rearranges them, so that all may get a fair share of oxygen and may
hatch out about simultaneously. It is this question of oxygen, indeed,
which gives the father fish all the greatest trouble. That necessary
of life is dissolved in water in very small quantities; and it is
absolutely needed by every egg in order to enable it to undergo those
vital changes which we know as hatching. To keep up a due supply of
oxygen, therefore, the father stickleback ungrudgingly devotes
laborious days in poising himself delicately just above the nest, as
you see in No. 3, and fanning the eggs with his fins and tail, so as
to set up a constant current of water through the centre of the
barrel. He sits upon the eggs just as truly as a hen does; only, he
sits upon them, not for warmth, but for aeration.

[Illustration: NO. 3. MALE STICKLEBACK WATCHING EGGS IN NEST.]

For weeks together this exemplary parent continues his monotonous
task, ventilating the spawn many times every day, till the time comes
for hatching. It takes about a month for the eggs to develop: and then
the proud father's position grows more arduous than ever. He has to
rock a thousand cradles at once, so to speak, and to pacify a thousand
crying babies. On the one hand, enemies hover about, trying to eat the
tender transparent glass-like little fry, and these he must drive off:
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,
a