322 THE DESCENT OF MAX. 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. 



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 which 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 

 low r er 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 tw r o passages, 

 as in the marsupials, into a single organ, showing no 

 signs of doubleness except a slight internal fold, as in 



