IX GEOLOGICAL CONTEMPORANEITY 289 



the firmness of which is thoroughly well assured. 

 Among its fundamental conceptions, there must 

 be no confusion between what is certain and what 

 is more or less probable.^ But, pending the con- 

 struction of a surer foundation than palseontology 

 now possesses, it may be instructive, assuming 

 for the nonce the general correctness of the 

 ordinary hypothesis of geological contemporaneity, 

 to consider whether the deductions which are 

 ordinarily drawn from the whole body of palse- 

 ontological facts are justifiable. 



The evidence on which such conclusions are 

 based is of two kinds, negative and positive. The 

 value of negative evidence, in connection with this 

 inquiry, has been so fully and clearly discussed in 

 an address from the chair of this Society ,2 which 

 none of us have forgotten, that nothing need at 

 present be said about it ; the more, as the con- 

 siderations which have been laid before you have 

 certainly not tended to increase your estimation 

 of such evidence. It will be preferable to turn to 

 the positive facts of palseontology, and to inquire 

 what they tell us. 



We are all accustomed to speak of the number 

 and the extent of the changes in the living popu- 

 lation of the globe during geological time as 



1 " Le plus grand service qn'on piiisse rendre k la science est 

 d'y faire place nette avant d'y rien construire." — Cuvier. 



- Anniversary Address for 1851, Quart. Joitrn. Geol. Soc. 

 vol. vii. 



205 



