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 or Solenostoma, one
of the two known kinds of fish in which the female shows a sense of
her position as a mother. The tube-mouth, as you can see at a glance,
is a close relation of our old friend the seahorse, whose disguised
and undisguised forms in Australia and the Mediterranean we have
already observed when dealing with the question of animal
masqueraders. Solenostoma is a native of the Indian Ocean, from
Zanzibar to China. In the male, the lower pair of fins are separate,