CHAPTER XII. 



Heredity {continued)— Reversion— Its Two Chief Causes — Peculiarity in the 

 Evolutionary Process and Partial Dissolution of a Mature Tissue. 



Beversion. — In the disappearance of a recently acquired 

 character, whether physiological or pathological, we have an 

 example of what is technically termed reversion. This prin- 

 ciple must now be considered somewhat in detail. 



The following I hold to be the three great causes of rever- 

 sions : — 



1. Peculiarities of ante-partem E, causing arrest or per- 

 version of development. 



2. The union of parents having uncombinable tendencies 

 — such uncombinable tendencies being for the most part of 

 recent origin, and of divergent character. 



3. Peculiarities of E, ante or j^ost partem, whereby a mature 

 tissue is compelled to revert to one of inferior order. 



In the first two varieties the reversion is due to a peculiarity 

 in the process of development — to a peculiarity, namely, in 

 the grand evolutionary process ; in the last, it is due to an 

 undoing of evolution, or, to speak more accurately, to the 

 partial undoing of a more or less completely evolved tissue. 



Peculiarities of ante-partem E may lead to reversion in a 

 twofold way — (a) by interference with the due sequence of 

 embryonic events : in this way the development of certain 

 organs and tissues may be arrested, the individual being born 

 with characters belonging to remote ancestors ; (b) the peculiar 

 E may lead to such perversion of embryonic processes that 

 development proceeds more or less on remotely ancestral lines. 



As an example of arrest, we may instance the occasional 

 persistence of a branchial cleft, and many forms of idiocy in 

 which far-off ancestral mental traits may appear. Thus 

 Maudesley has recorded several cases of idiocy where the indi- 



