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 Zrachyceratide, disregarding its 
ontogeny, which places it undoubtedly with the Zropitide. 
But no classification based entirely on one character 
can be truly genetic. 
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 Lztudtes (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 Crzoceras (Plate V, Fig. 
11) with the progressive Gyroceras (Plate V, Fig. 4). 
Hyatt,{ in his monographs on the ontogeny of ammonites, 
* Bull. Soc. Géol. France, ii ser., vol. xxii, 1894, No. 6. 
+ Since the above was written, Haug’s Etudes sur les Gonia- 
tites, Mém. Soc. Géol. France, 1898, has appeared, but could not 
be used in this paper. 
¢ Bull. Mus. Comp. Zool., vol. iii, No. 5, 1872; and Smith- 
sonian Contrib. to Knowledge, Genesis of the Arietidae, and 
other papers. 
