PARALLELISM. 201 



The superposition of characters which constitutes 

 evolution, means that more numerous characters are 

 possessed by the higher than the lower types. This 

 involves a greater number of changes during the on- 

 togenetic growth of each individual of the higher type. 

 In other words, characters acquired during the phylo- 

 genetic history are continually assumed by the pro- 

 gressive form at earlier and earlier periods of life. 

 This process has been metaphorically termed by Pro- 

 fessor Alpheus Hyatt and myself "acceleration." All 

 progressive organic evolution is by acceleration, as 

 here described. Retrogressive evolution may be ac- 

 complished by a retardation in the rate of growth of 

 the taxonomic characters, so that instead of adding, 

 and accumulating them, those already possessed are 

 gradually dropped ; the adults repeating in a reversed 

 order the progressive series, and approaching more 

 and more the primitive embryonic stages. This pro- 

 cess I have termed "retardation." Retardation is not 

 however, always exact, even in retracing a true phylo- 

 genetic line, whence in such instances the process may 

 not be correctly described as retardation. Professor 

 Hyatt has applied to such types the term "senile," 

 and gerontic ; and to the resulting condition, the term 

 "senility." His observations on this subject have been 

 made on Mollusca, and principally on the Cephalo- 

 poda, and are of fundamental importance in this con- 

 nection. 



The history of a type which has passed through a 

 full cycle of life, from its earliest appearance to its ex- 

 tinction, is divided by Haeckel into three stages, viz.: 

 those of its rise ; full vigor, as displayed by predomi- 

 nance of variations and numbers ; and decadence. 

 For these stages he uses the expressions Anaplasis, 



