SUCCESSION OF CLASSES 4>1 1 



strongly in the retrospect as the prospect. And not only as 

 respects the Vcrtebrata, but the sum of the animal species at 

 each successive geological period has been distinct and pecu- 

 liar to such period. 



Not that the extinction of such forms or species was sudden 

 or simultaneous: the evidences so interpreted have been but 

 local. Over the wider field of life at any given epoch, the 

 change has been gradual ; and, as it would seem, obedient to 

 some general, continuously operative, but as yet, ill-compre- 

 hended, law. In regard to animal life, and its assigned work 

 on this planet, there has, however, plainly been " an ascent 

 and progress in the main." 



Although the mammalia, in regard to the plenary develop- 

 ment of the characteristic orders, belong to the Tertiary division 

 of geological time, just as " Echini are most common in the 

 superior strata; Ammonites in those beneath, and Produdi 

 with numerous Encrini in the lowest"* of the secondary 

 strata, yet the beginnings of the class manifest themselves in 

 the formations of the earlier preceding division of geological 

 time. 



We are not entitled to infer from the Lucina of the per- 

 mian, and the Opts of the trias, that the Lamellibranchiate 

 Mollusks existed in the same rich variety of development at 

 those periods as during the tertiary and present times ; and no 

 prepossession can close the eyes to the fact that the Lamelli- 

 branchiate have superseded the Palliobranchiate bivalves. 



On negative evidence, Orthisina, Theca, Produda, or Spirifer 

 are believed not to exist in the present seas: on negative evi- 

 dence the existing genera of siphonated bivalves and univalves 

 are deemed to have been very rare in permian, triassic, or 

 oolitic times. To suspect that they may have then abundantly 

 existed, but have hitherto escaped observation, because certain 

 Lamellibranchs with an open mantle, and some holostomatous 



* A generalization of William Smith's. 



