The Descent of Man

Many of the cases to be here given, might have
been introduced under the last heading. When
a structure is arrested in its development, but
still continues growing, until it closely resembles
a corresponding structure in some lower
and adult member of the same group, it may in
one sense be considered as a case of reversion.
The lower members in a group give us some
idea how the common progenitor was probably
constructed; and it is hardly credible that a
complex part, arrested at an early phase of embryonic
development, should go on growing so
as ultimately to perform its proper function,
unless it had acquired such power during some
earlier state of existence, when the present exceptional
or arrested structure was normal. The
simple brain of a microcephalous idiot, in as far
as it resembles that of an ape, may in this sense
be said to offer a case of reversion. (38. In my
'Variation of Animals under Domestication'
(vol. ii. p. 57), I attributed the not very rare cases
of supernumerary mammae in women to