i86 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



be only specifically distinct, although any prolonged 

 study and comparison of the young would have shown 

 that they were widely separated in development and 

 really only morphic equivalents evolved from entirely 

 distinct ancestors. 



"A good example of this is the Eudoceratidae^ in- 

 cluding the Silurian and Devonian Eudoceras and Trip- 

 teroceras, and probably gyroceran form Edaphoceras 

 of the Carboniferous and the close-coiled nautilian 

 shells of Endolobus of the Carboniferous. The pe- 

 culiar forms of this series and their remarkable sutures 

 enable the observer to follow the line both in the grada- 

 tions of the adults and by means of the parallelisms 

 of the development. 



"Another good series easily distinguished by the re- 

 markable sculpture of the shells is Zittelloceras of the 

 Silurian with cyrtoceran forms, and the gyroceran and 

 nautilian Halloceras of the Devonian. 



"One of the best is Thoraceras, a rough spinous 

 cone of the Silurian, Devonian, and Carboniferous, 

 which has straight and cyrtoceran shells ; the gyroceran 

 Triboloceras of the Carboniferous, and the nautilian 

 shells of Vestinautilus and its allies in the same period, 



"There is no possible explanation of the parallel- 

 isms of development of these nautilian shells and the 

 adult stages of others except heredity in the same gen- 

 etic series. It is useless to waste time in discussion 

 unless the facts are specifically denied after having 

 been properly reexamined. 



"When the ammonoids are taken up, it is easy to 

 demonstrate^ by the study of the young of the Gonia- 



1" Genera of Fossil Cephalopods," Proc. Bost. Soc. Nat. Hist,, p. 287. 

 2 See " Genera of Fossil Cephalopods," Proc. Bost. Soc. Nat. Hist., XXII., 

 1883, p. 303. 



