The Descent of Man

et semblaient se complaire dans leur voisinage."
After the male has won his bride, he makes a
little disc of froth by blowing air and mucus out
of his mouth. He then collects the fertilised ova,
dropped by the female, in his mouth; and this
caused M. Carbonnier much alarm, as he
thought that they were going to be devoured.
But the male soon deposits them in the disc of
froth, afterwards guarding them, repairing the
froth, and taking care of the young when hatched.
I mention these particulars because, as we
shall presently see, there are fishes, the males of
which hatch their eggs in their mouths; and
those who do not believe in the principle of
gradual evolution might ask how could such a
habit have originated; but the difficulty is much
diminished when we know that there are fishes
which thus collect and carry the eggs; for if
delayed by any cause in depositing them, the
habit of hatching them in their mouths might
have been acquired.