266 STUDIES IN EVOLUTION 



in the life of each individual, they do not represent condi- 

 tions of growth, or the possession of characters which always 

 agree, stage for stage, in the species of one family or of 

 different families. 



Other distinctions to be made whenever possible are («) 

 whether certain characters (natural or acquired) belong to a 

 species by inheritance, or (^) are mere adaptations to special 

 conditions of environment arising at any time in its history. 

 A clear understanding of the first will lead to the true phylog- 

 eny of a species or genus, but to reach this the characters of 

 the second category must be excluded. Thus in the series 

 of ScJdzocrania, Orbiculoidea, and Discinisca, already cited, 

 there is an apparent genetic connection in the facts as stated. 

 The contrary must be the case with shells like Liv(/ula 

 compla7iata Williams and L. riciniformis Hall, which initiate 

 a holoperipheral * mode of growth in the ephebic period, for 

 this agreement in the method of concrescence with adult 

 Orbiculoidea here appears in the mature stages of this species, 

 and being absent in the early members of the genus cannot 

 therefore be an ancestral character. It is a morphological 

 equivalent, which may or may not be continued in the later 

 species of the series. 



Whenever features are present which can be referred to an 

 ancestral origin, their elimination can take place only by the 

 process of acceleration of development. On the other hand, 

 there may be secondary characters of dynamical or homo- 

 plastic origin which appear simultaneously or independently 

 in different groups belonging to diverse genetic lines, as the 

 deltidial plates of the Rhynchonellidfe, Terebratulida), and 

 Spiriferidte. Further, many such secondary features may 

 occur anywhere in the geological history of the groiq), as 

 the liigh hinge-area of Orthisina, Spirifet\ S^rinf/oth//ri,s, and 

 Thecidium. These statements are in full accord with what 

 Hyatt has determined in the Cephalopoda, and the applica- 

 tion of such ideas affords a fertile field of research. 



* 8\oy wliolc, and ntpKpfpfia ciroumferciice. 



