The Successive Development Theory. 3 



as the absence of cetacea, does not appear inexplicable in 

 the present imperfect state of our investigations, when we 

 consider the corresponding rarity and sometimes the absence 

 of the like remains observed in dredging the beds of existing 

 seas. 



Sixthly, the upper Silurian group contains amongst its 

 fossil fish cestraciont sharks, than which no ichthyic type is 

 more elevated. 



Seventhly, in the carboniferous fauna there have been re- 

 cently discovered several skeletons of reptiles of by no means 

 a low or simple organization, and in the Permian there are 

 saurian s of as high a grade as any now existing, while the 

 absence of terrestrial mammalia in the palaeozoic rocks 

 generally may admit of the same "explanation as our ig- 

 norance of most of the insects and all the pulmoniferous 

 mollusca, as well as of Helices and other land shells of the 

 same era. 



Eighthly, the fish and reptiles of the secondary rocks are 

 as fully developed in their organization as those now living. 

 The birds are represented by numerous foot-prints and copro- 

 lites in the Trias of New England, and by a few bones not 

 yet generically determined, from Stonesfield and the English 

 Wealden. 



Ninthly, the land quadrupeds of the secondary period are 

 limited to two genera, occurring in the inferior oolite of 

 Stonesfield ; the cetacea by one specimen from the Kimme- 

 ridge clay, the true position of which requires further inquiry, 

 while an indication of another is afi^orded by a cetacean 

 parasite in the chalk. But we have yet to learn whether in 

 the secondary periods there was really a scarcity of mam- 

 malia (such as may have arisen from an extraordinary pre- 

 dominance of reptiles, aquatic and terrestrial, discharging 

 the same functions), or whether it be simply apparent and 

 referable to the small progress made as yet in collecting the 

 remains of the inhabitants of the land and rivers, since we 

 have hitherto discovered but few freshwater, and no land 

 mollusca in rocks of the same age. 



Tenthly, in regard to the palaeontology of the tertiary 

 periods, there seems every reason to believe that the orders 



a2 



