220 PROCEEDINGS OF THE AMERICAN ACADEMY, 



The terms anaplasis, etc., and their correspondents, phylanaplasis, are 

 the structural correlatives of dynamical terms, epacme, etc., and will be 

 found useful when the statical jjhenoraena or structures are mentioned or 

 contrasted with the dynamical phenomena, or with periods of time in 

 which they occur, since the terms epacme, acme, and paracme also refer 

 to time. Terms of the ontogeny are placed opposite to their correlatives 

 in the column of i)hylogenetic terms, but in reading the table it should be 

 clearly understood that the individual whose life history is represented 

 by the first three columns is supposed to have been taken from the midst 

 of those that lived during the acme of the phylum and belonged to a 

 phylephebic species. In studying the development of such an individual 

 it has repeatedly been observed that tlie embryo repeated the adult char- 

 acters of the most ancient representatives of the 2:)hylum, which are here 

 called, in accordance with this evidence, phylembryonic. 



It has also been ascertained that there are full-grown types in the 

 epacme and acme of groups which correspond to the transient uepionic or 

 baby stage of those that occur later in time ; these are the phylone- 

 pionic ; others have similar correspondences with the neanic stages, and 

 are properly designated as phyloneanic types or forms. The structures 

 of the ephebic (adult) stage are essentially the ditferentials of the time 

 and fauna in which they occur, and necessarily have no correlations with 

 the past. Their relations are obviously and wholly with the present, 

 except in so far as they represent the consummations of evolution in 

 structures. The structural changes in the gerontic stage of the individual 

 are repeated with sufficient accuracy in the adult, and often even in the 

 neanic stages of types that occur in the paracme of the evolution of a 

 phylum, so that one is forced seriously to consider whether they may not 

 have been inherited from types that occur at the acme of the same group. 

 The fact that these changes occur first in the ontogeny during the geron- 

 tic stage does not necessarily imply that they first make their appearance 

 after the reproductive period. No gerontic limit is known to the repro- 

 ductive time in the lower animals, arid it may well be that the continual 

 recurrence of gerontic stages in individuals during the epacme of groups 

 may lead to their finally becoming fixed tendencies of the stock or heredi- 

 tary in the phylum, and thus established as one of the factors that 

 occasion the retrogression or paracme of groups. The paracme may also 

 be considered as occasioned by changes in the surroundings from favor- 

 able, as they must have been up to acmatic time, to unfavorable during 

 the succeeding paracmatic period in evolution. Still a third supposition is 

 also possible, namely, that the type, like the individual, has only a limited 



