90 THE GEOLOGICAL SUCCESSION [Chap. XI. 



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 organic 

 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 species 

 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 hitter rule is that of the so-called "colonies " of 

 \L Barrande, which intrude for a period in the midst of 

 an older formation, and then allow the pre-existing 

 fauna to reappear ; but LyelTs explanation, namely, that 

 it is a case of temporary migration from a distinct geo- 

 graphical province, seems satisfactory. 



These several facts accord well with our theory, which 



