THE BIOGENETIC LAW 4 1 5. 



larvae : [a) primary larvce, 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 ; ($) secondary 

 larva, such as have been introduced by cenogenesis into the 

 ontogeny of species that formerly developed by the fcetal process.. 

 If ancestral characters have been retained in the egg, 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 cenogenetic. A case in point is the 

 development of most insects, whose larval stages are supposed 

 to be largely secondary. Study of individual development in a 

 group of this sort can throw little light on phylogeny. 



The student of larval stages must confine himself to the pri- 

 mary sort, if he would correlate them with ancestral genera. The 

 development of the ccelenterates, echinoderms, brachiopods, most 

 mollusks, 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 cenogenetic 

 characters will be introduced, obscuring the resemblance to 

 ancestral forms ; also characters that in the adult ancestral form 

 were functional 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 descendant, or whether 

 it is secondary, and introduced by cenogenesis 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. 



