Chap. X. GEOLOGICAL SUCCESSION. 313 



nor disappearance of their many now extinct species lias 

 been simultaneous in each separate formation. 



Species of different genera and classes have not 

 changed at the same rate, or in the same degree. In 

 the oldest 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, 

 in an existing crocodile associated with many strange 

 and 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 

 change at a quicker rate than those of the sea, of which 

 a striking instance has lately been observed in Switzer- 

 land. There is some reason to believe that organisms, 

 considered high in the scale of nature, change more 

 quickly than those that are low : though there are ex- 

 ceptions to this rule. The amount of organic change, 

 as Pictet has remarked, does not strictly correspond 

 with the succession of our geological formations ; so that 

 between each two consecutive formations, the forms of 

 life have seldom changed in exactly the same degree. 

 Yet if we compare any but the most closely related for- 

 mations, 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 reason to believe 

 that the same identical form never 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 tempo- 

 rary migration from a distinct geographical province, 

 seems to me satisfactory. 



P 



