THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 21 



the life of an individual, and in accordance with these ideas designated the pro- 

 gressive periods of expansion in the phylogenetic history of a group as the 

 Epacme, the period of greatest expansion in number and variety of species and 

 forms as the Acme, and the periods of decline in numbers of species, etc., as the 

 Paracme. 



Haeckel used also the term Anaplastology for the physiological relations of 

 the stages of progressive growth and those of the epacme of groups, Metaplas- 

 tology for those of the adult and the acme of groups, and Cataplastology for 

 those of the senile stages and the paracme of groups. These terms seem to 

 cover the same ground as those we have employed, but they were in reality 

 chosen for the purpose of classifying physiological relations. Thus the anaplastic 

 relations of the nsepionic and nealogic stages to the phenomena occurring dur- 

 ing the epacme of groups, the metaplastic relations of the ephebolic stages to the 

 phenomena occurring at the acme of groups, and the cataplastic relations of the 

 geratologic stages to the phenomena occurring during the paracme of groups, are 

 the functional relations of the structural modifications occurring in the ontogeny 

 of individuals to those which are characteristic of the phylogeny of groups. 



Theory of Radicals and Morphological Equivalence in 

 Progressive Forms. 



The simpler characters of the sutures in the adults of more ancient forms, as 

 compared with the more modern species of the same series, has been noticed by 

 Wiirtenberger, Zittel, Neumnyr, Waagen, and Branco, 1 in different groups of 

 Arnmonitinse. The first is very decided in his statement, that the Ammonitinae 

 he has studied form perpetually diverging series, which spring from certain 

 common ancestral forms. 



The constant repetition of discoidal and involute forms in series, which are 

 otherwise distinct in respect to their sutures and minor characteristics of develop- 

 ment and shell markings, produces a similarity in the succession of the forms. It 

 is practicable to compare the evolution of discoidal into more involute forms of 

 any one series with a similar genetic procession in any other series. Thus in the 

 General Summary, Plate XIV., we can compare the discoidal forms of Ver. Cony- 

 beari, Fig. 20, with Am. tardicrescens, Fig. 26, Cor. rotiforme, Fig. 30, and Ast. Tur- 

 ned, Fig. 36, and in the same way the involute forms of Ast. Gollenoti with Oxyn. 

 oxynotum, Gremoughii, and Lotharingum ; and these comparisons also hold good for 

 Schlot. BoucauUiana, and the terminal forms like Week. Emmerici and Psil. mesogenos, 

 which are also involute. In exceptional series the whorls do not become more 

 involute in the higher species, but are nevertheless modified in those character- 

 istics which usually accompany and correlate with increase of involution. Thus 

 the lateral diameter of the whorl decreases, the sides become more and more 



1 Wiirtenberger, Stammesgesoh. d. Amm., Darwinistische Schriften, Nr. 5, Leipzig, 1880, p. 91. Zittel, 

 Ueber Phylloeeras tatricum, Jahrb. d. k. k. geol. Keichsant., 1869, p. 65. Neuniayr, Die Phylloceraten d. 

 Dogger mid Malm, Ibid., 1S71, pp. 347, 348; also, Zeits. d. deutsch. geol. Gesellsoh., 1875, p. 866. Waagen, 

 Die Formenreihe d. Amm. subradiatus, Benecke's Geognost. paleont. Beitr., II. p. 202. Branco, Paleontogr., 

 XXVI., XXVII. 



