90 THE GEOLOGICAL SUCCESSION [Chap. XL 



but, as Bronn has remarked, neither the appearance 

 nor disappearance of the many species embedded in 

 each formation has been simultaneous. 



Species belonging to different genera and classes have 

 not changed at the same rate, or in the same degree. 

 In the older tertiary beds a few living shells may still 

 be found in the midst of a multitude of extinct forms. 

 Falconer has given a striking instance of a similar fact, 

 for an existing crocodile is associated with many lost 

 mammals and reptiles in the sub-Himalayan deposits. 

 The Silurian Lingula differs but little from the living 

 species of this genus; whereas most of the other Silurian 

 Molluscs and all the Crustaceans have changed greatly. 

 The productions of the land seem to have changed at a 

 quicker rate than those of the sea, of which a striking 

 instance has been observed in Switzerland. There is 

 some reason to believe that organisms high in the scale, 

 change more quickly than those that are low: though 

 there are exceptions to this rule. The amount of or- 

 ganic change, as Pictet has remarked, is not the same in 

 each successive so-called formation. Yet if we compare 

 any but the most closely related formations, all the spe- 

 cies will be found to have undergone some change. 

 When a species has once disappeared from the face of 

 the earth, we have no reason to believe that the same 

 identical form ever reappears. The strongest apparent 

 exception to this latter rule is that of the so-called 

 " colonies " of M. Barrande, which intrude for a period 

 in the midst of an older formation, and then allow the 

 pre-existing fauna to reappear; but Lyell's explanation, 

 namely, that it is a case of temporary migration from a 

 distinct geographical province, seems satisfactory. 



These several facts accord well with our theory. 



