Ch. xxvii.j old fossiliferous periods. 459 



The only incongruity between the geological results, and those which 

 our dredging experiences might have led us to anticipate a priori^ con- 

 sists in the frequency of fossil reptiles, and the comparative scarcity of 

 mammalia. It would appear that during all the secondary periods, not 

 even excepting the newest part of the cretaceous, there was a greater 

 development of reptile life than is now witnessed in any part of the globe. 

 The preponderance of this class over the mammalia depended probably 

 on climatal and geographical conditions, for we can scarcely refer it to 

 " progressive development," by which the vertebrate type was steadily 

 improving, or becoming more perfect, as Time rolled on. We cannot 

 shut our eyes to the positive proofs now obtained of the creation of mam- 

 malia before the excess of reptiles had ceased, — nay, apparently before it 

 had even reached its maximum. 



In conclusion, I shall simply express my own conviction that we are 

 still on the mere threshold of our inquiries ; and that, as in the last fifty 

 years, so in the next half century, we shall be called upon repeatedly to 

 modify our first opinions respecting the range in time of the various classes 

 of fossil Vertebrata. It would therefore be premature to generalize at 

 present on the non-existence, or even on the scarcity of Vertebrata, 

 whether terrestrial or aquatic, at periods of high antiquity, such as the 

 Silurian and Cambrian.'^' 



* For observations on the rarity of air-breathers ia the coal, see above, p. 401. 



