18S7.] 405 [Hyatt. 



cult to separate them. They have been and will continue to be 

 the most difficult and misleading obstacles to the student of gene- 

 alogy and classification. 



In former essays we have described and defined the senile 

 transformations and their correlations with the degraded forms of 

 the same groups. The nature of these relations is, as has been 

 explained, quite distinct from those of the progressive and adult 

 stages, but the correlations are nevertheless equally important for 

 the classification and tracing of genealogies during the declining 

 period of a group, and in the case of degraded and aberrant forms. 

 We have, therefore, for some years past designated these relations 

 by the term Geratology. 15 



This nomenclature is similar to that adopted by Haeckel, but 

 is, when properly considered, also supplementary and based upon 

 morphological rather than physiological grounds. This eminent 

 author regarded the ontogeny of an individual to be divisible into 

 three periods : first, the stages of Anaplasis or those of progressive 

 evolution ; second, the stages of fulfilled growth and develop- 

 ment, Metaplasis ; third, those of decline, Cataplasis. He also 

 appreciated and gave full weight to the general physiological cor- 

 relations which are traceable between the history of a group and 

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

 ignated the progressive periods of expansion in the phylogenetic 

 history of a group as the Epacme, the period of greatest expan- 

 sion in number and variety of species and forms as the Acme, and 

 the period of decline in numbers of species, etc., as the Paracme. 



Haeckel used also the term Anaplastology for the physiologi- 

 cal relations of the stages of progressive growth and those of the 

 epacme of groups, Metaplastology 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 embryologic, silphologic and nealogic 

 stages to the phenomena occurring in the epacme of groups, and 

 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 par- 



16 Tepa?, old age. 



