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 reversion. I was led to this as a probable conclusion, by the
additional mammae being generally placed symmetrically on the breast; and
more especially from one case, in which a single efficient mamma occurred
in the inguinal region of a woman, the daughter of another woman with
supernumerary mammae. But I now find (see, for instance, Prof. Preyer,
'Der Kampf um das Dasein,' 1869, s. 45) that mammae erraticae, occur in
other situations, as on the back, in the armpit, and on the thigh; the
mammae in this latter instance having given so much milk that the child was
thus nourished. The probability that the additional mammae are due to
reversion is thus much weakened; nevertheless, it still seems to me
probable, because two pairs are often found symmetrically on the breast;
and of this I myself have received information in several cases. It is
well known that some Lemurs normally have two pairs of mammae on the
breast. Five cases have been recorded of the presence of more than a pair
of mammae (of course rudimentary) in the male sex of mankind; see 'Journal