24 ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



ships he brings out may be explained much better on the basis of rever- 

 sion by arrest of development, as has already been shown under the head 

 of reversion. 



Convergence. 



It is impossible to conceive of the same species or genus as originat- 

 ing in different times, or in different places. But natural selection sorts 

 out certain characters, or environment calls them out, and so v^^e often 

 get very similar results from diverse materials. Similarity of habit pro- 

 duces external, but not fundamental, similarity of characters. In the 

 case of forms living together in time and place, convergence may well be 

 due to mimicry, and thus explained by natural selection. But where the 

 forms are separated by geologic ages, mimicry can not be appealed to. 



In the case of reversion by arrest of development we have a virtual 

 reappearance of generic types in widely separated epochs. Only, when 

 we know their history, we do not call the aggregation of characters by 

 the same generic names, especially since the reversionary forms are usu- 

 ally easily to be distinguished from the older types. Thus Arpadites, PL 

 VIII, figs. 1-10, and Beyrichites, PL VIII, figs. 14-23, both show a partial 

 reversion towards their ancestor Meekoceras, yet neither genus need be 

 confused with the ancient progenitor. 



Convergence is sometimes seen in widely separated stocks and in 

 widely separated times. Eutomoceras of the Middle Triassic, PL IX, figs. 

 5-7, the end genus of the Dalmatites-Hungarites stock, has been confused 

 with the Upper Triassic Discotropites (PL V, figs. 1-13), a late member 

 of the genetic series leading up from Gastrioceros-Columhites to the 

 Tropitidffi. Ontogeny shows the heredity of the two genera to be differ- 

 ent back to the Devonian. Their resemblance can hardly be due to at- 

 avism, for their development is not parallel, as both genetic series of 

 adults and ontogeny of each generic step show. It can also hardly be 

 due to natural selection, for along with these keeled members of each 

 stock there are numerous others without keel, as the geologic record shovv^s, 

 equally prosperous and prolific. It is also not due to the inheritance of 

 this character from a common ancestor, for the remote ancestors were not 

 common, and did not possess the keel, anyway. 



Again, we may have parallel development of very similar characters 

 in nearly related stocks. As an example of this may be cited the develop- 

 ment of the ventral keel in the Dalmatites-Hungarites-Eutomoceras phy- 

 lum, and the same thing in the Meekoceras-Ceratites line. Eutomoceras 



