250 



FOOT-NOTES TO EVOLUTION. 



be known, and when we know the stock of the tree, the 

 branches that came off in the higher Trias, Jura, and 

 Cretaceous will offer no difficulties. The most syste- 

 matic attempt to do this is Haug's paper, Les Ammo- 

 nites du Permien et du Trias;* but his classification is 

 based wholly on the character of the sutures, and 

 neglects other characters, such as sculpture and shape 

 of the whorls. Thus Haug places Eutomoceras with 

 the prionidian family TrachyceratidcB, disregarding its 

 ontogeny, which places it undoubtedly with the Tropitidcz. 

 But no classification based entirely on one character 

 can be truly genetic. f 



Study of the development of many species has shown 

 that similar characteristics do not always mean close 

 relationship ; they may often be developed in different 

 series coming from a common remote ancestor, and liv- 

 ing under similar conditions. They are in no sense 

 hereditary characters, but morphological equivalents 

 acquired from the action of the same stimulus. The 

 occurrence of orthoceran, cyrtoceran, gyroceran, nautil- 

 ian, and reversionary stages in both nautiloids and 

 ammonoids is a case in point. Compare the develop- 

 ment and reversion of Lituites (Plate V, Fig. 6), of the 

 nautiloid stock with that of Baculites (Plate V, Fig. 13) 

 of the ammonoids, and the analogy becomes evident ; 

 compare also the reversionary Crioceras (Plate V, Fig. 

 11) with the progressive Gyroceras (Plate V, Fig. 4). 

 Hyatt, J in his monographs on the ontogeny of ammonites, 



* Bull. Soc. Geol. France, ii ser., vol. xxii, 1894, No. 6. 



f Since the above was written, Haug's Etudes sur les Gonia- 

 tites, Mem. Soc. G^ol. France, 1898, has appeared, but could not 

 be used in this paper. 



X Bull. Mus. Comp. Zool., vol. iii, No. 5, 1872; and Smith- 

 sonian Contrib. to Knowledge, Genesis of the Arietidae, and 

 other papers. 



