THE THEORY OF SUCCESSIVE CREATIONS 21 



of a species ^and its degree of variation. When 

 d'Orbigny speaks of species which, in an exceptional 

 state, pass through several geological stages, we are 

 rarely dealing with forms entirely identical at these 

 various levels. An attentive observer will nearly 

 always be able to perceive between these successive 

 forms of the same type appreciable variations 

 either in the size or the shape of the shell, or in the 

 details of its markings slight, but constant, varia- 

 tions, no doubt, which are sufficient to enable a 

 practised eye to recognize with every assurance 

 the precise level from which this variety comes, or 

 as we now say, following Waagen, this stratigraphic 

 mutation. 



When, on the other hand, d'Orbigny separates 

 under two distinct names two forms of the same 

 genus belonging to two successive geological stages, 

 it must not be thought that it is always a question 

 of considerable differences which force upon the 

 mind the idea of a distinct origin. In certain 

 zoological groups, of which the analytical study has 

 been carried especially far in the shells of Ammon- 

 ites, for example ribs more or less fine or more or 

 less numerous, more or less sinuous or more or less 

 bent inwards, spiral coils more or less close, 

 suffice to enable palaeontologists, even d'Orbigny 

 himself, to separate two species, because this dis- 

 tinction often has a great interest for the charac- 

 teristics of two stratigraphic zones. From this 

 point of view certain genera of molluscs for 

 example, the Perisphinctce or the Hoplites among 

 the Ammonites, the Trigonice among the Lamelli- 

 branchs, and most kinds of sea-urchins have been 



