122 
THE DESCENT OF MAN. 
Part I. 
shewn by almost all boys in climbing trees ; and this 
again reminds us how lambs and kids, originally alpine 
animals, delight to frisk on any hillock, however small. 
j Reversion. — Many of the cases to be here given 
might have been introduced under the last heading. 
Whenever 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, we may in one sense consider it as a case 
of reversion. The lower members in a group give us 
some idea how the common progenitor of the group was 
probably constructed ; and it is hardly credible that a 
part arrested at an early phase of embryonic develop- 
ment should be enabled to continue growing so as ulti- 
mately to perform its proper function, unless it had 
acquired this power of continued growth 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. There are other cases wdiich come more 
strictly under our present heading of reversion. Certain 
structures, regularly occurring in the lower members of 
the group to which man belongs, occasionally make 
their appearance in him, though not found in the normal 
human embryo; or, if present in the normal human 
embryo, they become developed in an abnormal manner, 
though this manner of development is proper to the 
lower members of the same group. These remarks will 
be rendered clearer by the following illustrations. 
In various mammals the uterus graduates from a 
double organ with two distinct orifices and two passages, 
as in the marsupials, into a single organ, showing no 
signs of doubleness except a slight internal fold, as in 
