CONCLUSION. 449 



superior strata, Ammonites in those beneath, and Producti with 

 numerous Encrini in the lowest"* of the secondary strata ; 

 yet the beginnings of the class manifest themselves in the 

 formations of the earlier division of geological time. 



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

 mian, and the Opis 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 branchiate have superseded the Palliobranchiate bivalves.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, Producta, 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 

 and asiphonate Gastropods, have left their remains in secon- 

 dary strata, is not more reasonable, than to conclude that the 

 proportion of mammalian life may have been as great in 

 secondary as in tertiary strata, because a few small forms of 

 the lowest orders have made their appearance in triassic and 

 oolitic beds. 



The proportion of the known forms of extinct life may be 

 very small compared with that which remains for future 

 discovery ; but the sum of what is known yields the legiti- 

 mate deduction, that there has been a succession of species 

 illustrating in the main the progressive perfection of the 

 nervous system, and the concomitant predominance of mind 

 over matter. 



If, turning from a retrospect into past time to the prospect 

 of that to come, we may speculate on the future course of 

 vital phenomena on this planet, the guide-post of Palaeontology 



* A generalization of William Smith's. 



2 a 



