nd 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;
wherever the young are tended, it is almost always the father, not the
mother, who undertakes the duty of incubation. Only two instances
occur where the female fish assumes maternal functions towards her
young; about these I shall have more to say a little later on. We must
remember that reptiles, birds, and mammals are in all probability
descended from fish as ancestors, and it is therefore clear that the
habit of handing over the care of the young to the female alone
belongs to the higher grades of vertebrates--in other words, is of
later origin. We need not be astonished, therefore, to find that in
many cases among birds and other advanced vertebrates a partial
reversion to the earlier habit not infrequently takes place. With
doves, for example, the cock and hen birds sit equally on the eggs,
taking turns about at the nest; and as for the ostriches, the male
bird there does most of the incubation, for he accepts the whole of
the night duty, and also assists at intervals during the daytime.
There are numerous other cases where the father bird shares the tasks
of the nursery at least equally with the mother. I will glance first,
however, at one of the rare exceptions among fish where the main duty
does not devolve on the devoted father.

[Illustration: NO. 4. FEMALE TUBE-MOUTH.]

In No. 4 we have an illustration of the tube-mouth