ASPECTS OF ONTOGENY IN AMMONITE EVOLUTION 143 



been accelerated; since the depressed early whorls are also pro- 

 longed the slender whorl stage is skipped entirely in the develop- 

 ment of this form. 



The reason for this skipping is perhaps best appreciated when 

 it is remembered that the Ammonite C was thus able to produce 

 the adult form directly, instead of developing from stout (embryo) 

 to slender (youth) and back to stout (adult). The tendency to 

 omit in development the stage 

 which appears unnecessary for 

 the production of the adult form 

 would perhaps be expected; the 

 most interesting fact is that this 

 skipping is accomplished not 

 simply by the acceleration of 

 the stout form of whorl but 

 partly by an apparent retarda- 

 tion of the early characters, not- 

 withstanding that the series is 

 distinctly progressive. 



Without discussing here the 

 biological interest of these obser- 

 vations, it is perhaps not out of place to notice that if lipopaHngenesis 

 is of frequent occurrence, the tracing of phylogeny from ontogenetic 

 evidence is not so easy as has been thought by some workers. It 

 is comparatively easy to follow cases of skipping of stages when an 

 early phylogenetic stage is omitted, but in cases Hke the one just 

 described, where the characters omitted are some of the more 

 advanced, there is obviously much difficulty in tracing descent 

 from developmental evidence alone. In such cases, descent can 

 only be proved by the finding of series of intermediate forms 

 connecting the various species. 



ABC 

 Fig. I. — Sections to show the changes 

 of whorl-shape in three ammonites from a 

 single lineage of the Family Liparocera- 

 tidae: 



A, a slender Capricorn ammonite, 

 C, an involute stout ammonite. 



B, a form intermediate between these 

 two. 



