PART II 



The irreversibility of degenerative evolution 



MOST authorities on the subject are agreed that 

 evolution is not reversible, 1 and that institutions 

 or organs which have disappeared or been reduced 

 to rudiments do not reappear and develop afresh. 

 It would be a useless extension of this volume 

 to cite many facts in favour of a view which is 

 almost without supporters, but it will be useful to 

 examine the exceptions, real or apparent, and to 

 discuss 



1. If an institution or organ which has dis- 



appeared may reappear. 



2. If an institution or organ which has been 



reduced may resume its primitive function. 



3. If an institution or organ which has been 



reduced may redevelop and assume a func- 

 tion other than its original function. 



1 L. Dollo, Les lois de Involution (Soc. Belg. Gtol. Pal&mt. 

 Hydr., t. vii., 1893, proces-verbaux, pp. 164-166. 



