184 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



larvcB, such as are more or less modified from ancestral 

 forms, and have continued to develop as free larvae since the 

 time when they constituted the adult forms ; (b) secondary 

 larvcB, such as have been introduced by kenogenesis into 

 the ontogeny of species that formerly developed by the foetal 

 process. If ancestral characters have been retained in the 

 ^^^1 then these secondary larvae may bear some palingenetic 

 characters, and thus be hard to distinguish from primary 

 larvae; otherwise they will be entirely adaptive, or keno- 

 genetic. A case in point is the development of most insects, 

 whose larval stages are supposed to be entirely secondary. 

 Study of individual development in a group of this sort can 

 throw no light on phylogeny. 



The student of larval stages must confine himself to the 

 primary sort, if he would correlate them with ancestral 

 genera. The development of the coelenterates, echinoderms, 

 brachiopods, most molluscs, and the lower crustaceans is 

 direct; thus larval stages of these groups may be bearers, 

 to a greater or less degree, of ancestral characters. But 

 since the free larvae of even these groups are exposed to 

 natural selection, secondary or kenogenetic characters will be 

 introduced, obscuring the resemblance to ancestral forms; 

 also characters that in the adult ancestral form were func- 

 tional and fully developed may in the representative larval 

 stage of the descendant be so little differentiated as to be 

 unrecognizable. 



But how can the morphologist who deals entirely with 

 living species know whether a character is primary, and 

 repeated by palingenesis in the larval history of the descend- 

 ant, or whether it is secondary, and introduced by keno- 

 genesis into that history? The answer to this lies wholly 

 within the domain of paleontology, for only by finding a 

 stage of growth represented by an ancestral form can the 

 morphologist know that the characters of that stage are 

 ancestral, and not secondary. Larval stages which may be 

 the bearers of ancestral characters must then be compared 

 with the adults of their predecessors, and the paleontologic 

 record must be invoked as a final resort — the court from 

 which there is no appeal. 



