212 LIFE ON THE EARTH. 



in proof of each later group being derived from some 

 earlier one by descent with modification. Take the 

 least incomplete series of forma, viz. the Mollusca, 

 and undoubtedly the most favourable of all the marine 

 groups for the application of hypothesis. 



The earliest known Mollusk is the Brachiopod 

 Lingula; which, as already observed, recurs in all 

 the systems of strata, and is still living. It gives 

 no generic branches. The next earliest are the Dimy- 

 arian genera, Ctenodonta and Cucullella, which can- 

 hot be regarded as descended from any conceivable 

 Brachiopod, or accepted as progenitors of Modiola, 

 Orthonota, Cardiola or Pleurorhynchus ; still less of 

 their Monomyarian companions, Ambonychia, Avicula 

 and Pterinea. It is inconceivable that from these or 

 any thing like them could be derived the Gaste- 

 ropod, Euomphali, Loxonemee, &c., or that the He- 

 teropod Bellerophon, the Pteropod Theca, or the 

 Cephalopod Orthoceras, are consanguineous, any one 

 of them with any other. All these great classes then 

 are according to the evidence equally aboriginal, 

 though not of equal antiquity. 



Without bringing in similar results from the other 

 invertebral classes we may boldly affirm that the 

 later series of Cambro-Silurian life cannot possibly 

 be derived from the earlier series, according to the 

 evidence preserved to us; but on the contrary re- 



