CHAPTER XV 



VARIATION IN SPACE IN GEOLOGICAL TIMES 



Examples of variation : The Nassa of Piedmont, the Ammonites of 

 Crussol Regional races at the Cambrian, Liassic, and Pliocene 

 epochs Conclusions. 



IF from existing nature we go back to geological 

 times, with the object of studying in them the 

 variation of fossil species in space, that is to say, 

 in each of the epochs of the life of the globe, we shall 

 have to make, but less easily, observations quite 

 analogous to those above. Nature has not varied 

 its processes, however early the period in view. 

 Two examples, borrowed, one from Tertiary, the 

 other from Secondary times, will enable us to fix 

 our ideas with regard to this. 



In the Tertiary basin of Piedmont a clever palae- 

 ontologist, Bellardi, has described and drawn with 

 care the Miocene and Pliocene Gastropods of the 

 valley of the Po and of the Ligurian Apennines. 

 The genus Nassa in this basin is particularly rich 

 in varied forms, and Bellardi has made known no 

 less than three hundred and sixteen species or 

 varieties, of which more than three-quarters are 

 considered new, and receive, for the most part, 

 a specific name. But we are here really deal- 

 ing with a numerous series of superposed stages 

 with distinct faunas. Let us confine our analysis 



