132 BIOLOGICAL LECTURES. 



In these as well as by the general experience of paleobiologists 

 in tracing groups by more empirical methods, phylogeny has 

 been found to be at first progressive. It has, like ontogeny, 

 a period of differentiation, when it spreads out into its many 

 different modifications, called species, genera, and so on, and 

 thus finally attains an acme of progress. This acme is fol- 

 lowed by a phase of retrogression in genetic series that have 

 what may be called a complete cycle, and this ultimately ends 

 in extinction. All genetic series do not have complete cycles, 

 any more than all individuals have complete existences, and 

 some certainly possess introduced adaptive stages (Echino- 

 dermata, Insecta) that complicate the record. Reference is 

 here, therefore, confined to those genetic stocks that did pass 

 through both progressive and retrogressive series of changes. 



There are other qualifications to such a brief statement that 

 cannot be discussed here, but in a general way it may be said 

 that this hypothesis of the correspondences of ontogeny and 

 phylogeny has been worked out by hard labor of minute com- 

 parison and the study of definite morphic modifications. The 

 method thus elaborated has been and is now in constant use 

 by a number of paleobiologists, and has proved in application 

 more helpful than its most enthusiastic advocate had hoped. 

 The correspondences are not simply between the youngest 

 stages and the remote ancestors, like those traced by embry- 

 ologists, but between all the stages of the ontogeny and all the 

 periods and phases of the phylogeny. The recapitulations of 

 the embryonic stages are in fact of no greater importance in 

 these researches than those of the epembryonic or later stages, 

 meaning by this term to include not only the young but also 

 the gerontic or senile metamorphoses. The nepionic stages 

 have characters derived from less remote ancestors than the 

 embryonic ; the neanic are similarly related to still nearer 

 progenitors ; the ephebic stages give the differentials elabo- 

 rated in the ontogeny at the acme of the evolution of the 

 stock. 1 Definite periods in the phylogeny characterized by 



1 " Bioplastology and the Related Branches of Scientific Research," Proc. Bost. 

 Soc. Nat. Hist., vol. xxvii ; " Phylogeny of an Acquired Characteristic," Proc. 

 Amer. Phil. Soc., Philadelphia, vol. xxxii, Nos. 143, 144, pp. 59-125. 



