166 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



extend through the whole or a great part of the 

 geological periods most often undergo but slight 

 modifications ; as, for instance, the Lingulas or 

 the Capuli. On the contrary, the short-lived 

 families, like the Trilobites of the Cambrian, the 

 Rudistse of the Cretacean, and the Arietitse of 

 the Lias, reveal a more energetic vitality, which 

 betrays itself by rapid changes in passing from 

 one stage to another, or even from one stratum to 

 another. Thus stratigraphers advantageously use 

 these rapidly evolving branches as a chronometric 

 scale, that is, as fossils characteristic of different 

 zones of the same stratum. 



It is, however, possible that the feeble longevity of 

 certain branches is much more apparent than real, 

 and is due to provisional gaps in our observations. 

 If, in fact, the dates of the extinction of each group 

 are almost always known with certainty, it is not 

 the same with the dates of their first appearance. 

 Later discoveries are, no doubt, preparing for us 

 the surprise of seeing that the origin of these 

 branches which are supposed to be very short, is 

 carried furtKer and further back into the depths of 

 the sedimentary strata of the earth's crust. The 

 study of the evolution of the Vertebrates will, 

 later on, furnish us with many proofs of this. 



