ii8 VESTIGES OF THE 



with a reference to the kind of rock in connexion with 

 which they are found, it is observed that some strata are 

 attended by a much greater abundance of both species 

 and individuals than others. They abound most in 

 calcareous rocks, which is precisely what might be ex- 

 pected, since lime is necessary for the formation of the 

 shells of the mollusks and articulata, and the hard sub- 

 stance of the crinoidea and corals ; next in the carboni- 

 ferous series ; next in the tertiary ; next in the new red 

 sandstone ', next in slates ; and lastly, least of all, in the 

 primary rocks.* This may have been the case without 

 regard to the origination of new species, but more 

 probably it was otherwise ; or why, for instance, should 

 the polypiferous zoophyta be found almost exclusively in 

 the limestones? There are, indeed, abundant appear- 

 ances as if, throughout all the changes of the surface, 

 the various kinds of organic life invariably iwessed in, 

 immediately on the specially suitable conditions arising, 

 so that no place which could support any form of organic 

 being might be left for any length of time unoccupied. 

 Nor is it less remarkable how various species are with- 

 drawn from the earth, when the proper conditions for 

 their particular existence are changed. The trilobite, of 

 which fifty species existed during the earlier formations, 

 was extirpated before the secondary had commenced, 

 and appeared no more. The ammonite does not appear 

 above the chalk. The species, and even genera of all 

 the early radiata and mollusks were exchanged for 

 others long ago. Not one species of any creature which 

 flouiished before the tertiary (Ehrenberg's infusoria 

 excepted) now exists ; and of the mammalia which arose 

 (hiring tliai s«^ries. many forms are altogether gone, 



- See napci- l)y IV(»lcssor lulwanl Fork's, read to the Briti.sh 

 Atisocialidii, 1839. 



