Values of the Stages of Growth and Deeline. 881 
stage and with a general tendency to closer involution, which acted 
the same way in every series of forms, whether we select series of 
adults or of embryos for comparison. 
The use of a distinct term for the adult period becomes necessary 
not only on this account, and to separate its relations from those of 
preceding periods, but also because of the constant recurrence and 
importance of representative forms. The term Ephebology * has 
accordingly been adopted for the designation of the relations of the 
adult stages, and under this term can be classified also the represen- 
tation of similar forms in different groups or morphological equiva- 
lents. These are often so exact that it becomes very difficult to 
Separate them. They have been and will continue to be the most 
difficult and misleading obstacles to the student of genealogy and 
classification. 
In former essays we have described and defined the senile trans- 
formations 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 classifi- 
cation 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.? 
This nomenclature is similar to that adopted by Heckel, 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 development, 
Metaplasis ; third, those of decline, Cataplasis. He also appreciated 
and gave full weight to the general physiological correlations which 
are traceable between the history of a group and 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 period of decline 
in numbers of species, ete., as the Paracme. 
! “Egnfog, the age of puberty. 
* Tépas, old age, 
