[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, 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; 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