THE BIOGENETIC LAW 421 



Douville, Nickles, and others, the septum of adolescent ammon- 

 ites of this group is not more complex, but really less so, than 

 that of adults, although they are derived from Jurassic genera 

 with complex septa. Thus Douville derives the group Placenti- 

 ceras-Sphenodiscus from Hoplites ; the Pulchellidae, composed of 

 Pulchellia, Neolobites, and Tissotia, he derives from Oppelia of the 

 Jura. Since in each case the ancestral forms are more complex 

 than the descendants, the reduction in complexity of generic 

 evolution can be explained only by retardation or arrested 

 development. F. Bernard has in addition pointed out the fact 

 that the adult of Pulchellia is like the adolescent stage of the 

 ancestral Oppelia. Now if we define the law of acceleration of 

 development to mean that in a progressive series the young of 

 the descendants correspond to the adults of their more remote 

 ancestors, we find that this does not apply to a retrogressive 

 (retarded) series. In this latter case we must restate the law 

 as follows : the adults of descendants correspond to the young 

 of their more remote ancestors, the higher generic stages to 

 which these ancestors attained having been dropped away by 

 successive retardation or arrested development. The retarded 

 series themselves may become the radicals of new stocks, and 

 so we may have cases where the ontogeny of any one species or 

 genus can never give the full history of the race. 



Groups available for correlation. — We see then that the stu- 

 dent of morphogeny of animals has to be on his guard, first 

 against the loss of generic stages during the period while the 

 animal is in the egg ; then against the introduction of second- 

 ary larval stages when the ancestors lacked them ; then against 

 the introduction of secondary characters due to adaptation ; then 

 against unequal acceleration, bringing together in the ontogeny 

 of the descendant, characters that occurred in separate genera- 

 tions of ancestors ; and lastly, against retardation, by which 

 the form never reaches the full generic evolution of its ances- 

 tors, and where, if a new series starts out from the retarded form, 

 the complete family history is not recorded in ontogeny. 



