The Descent of Man

in two weeks; those of the fowl in three; those
of the duck in four; those of the goose in five;
and those of the ostrich in seven weeks. As far
as we can judge, a recurrent period, if approximately
of the right duration for any process
or function, would not, when once gained,
be liable to change; consequently it might be
thus transmitted through almost any number of
generations. But if the function changed, the
period would have to change, and would be
apt to change almost abruptly by a whole week.
This conclusion, if sound, is highly remarkable;
for the period of gestation in each mammal,
and the hatching of each bird's eggs, and many
other vital processes, thus betray to us the primordial
birthplace of these animals.), resembling
the larvae of existing Ascidians. These
animals probably gave rise to a group of fishes,
as lowly organised as the lancelet; and from
these the Ganoids, and other fishes like the Lepidosiren,
must have been developed. From
such fish a very small advance would carry us