20 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



even of the far-off regions of South America. 

 D'Orbigny has been very careful to thus set forth 

 his reasons himself. 



Each of the stages which follow one another in 

 the geological series possesses its own particular 

 fauna, distinct from that of the stages which pre- 

 cede or follow it ; and these different faunas are in 

 no way linked by forms passing from one to the other, 

 nor by gradual replacement of one by the other, but 

 always by sudden extinction. The fauna of one 

 stage stop at the late strata of that stage, and at 

 the first strata of the following stage animals quite 

 different from the first appear and constitute a new 

 fauna. The number of fossil species which pass 

 from one stage to another is exceedingly restricted, 

 and in Jurassic and Cretaceous soils the more 

 special object of d'Orbigny's studies this propor- 

 tion does not reach one per cent. These rare species, 

 common to two or three stages, must have passed 

 over the borders, either very accidentally when in 

 the living state, or more often in the condition of 

 dead shells. Shells provided with air chambers, 

 and therefore light such as those of the cephalo- 

 pods were better able than others to float after 

 the death of the animal, and to mix with the 

 littoral fauna of the following stage. 



These various propositions, notwithstanding the 

 strict and almost mathematical form given to them 

 by d'Orbigny, are yet far from offering so absolute 

 a character of certainty as he wished. The reason 

 is that they depend to a great extent on the wholly 

 personal and therefore somewhat arbitrary appre- 

 ciation which every naturalist makes of the limit 



