x.] GEOLOGICAL CONTEMPORANEITY. 215 



distinctly before the minds of geologists for the last 

 thirty years ; and if,, at the present time, it has seemed 

 desirable to give them more definite and systematic 

 expression, it is because palaeontology is every day 

 assuming a greater importance, and now requires to 

 rest on a basis 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. 1 But, pending the 

 construction of a surer foundation than palaeontology 

 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 palseontological facts are justifiable. 



The evidence on which such conclusions are based is 

 of two kinds, negative and positive. The value of 

 negative evidence, in connexion 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 considerations 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 palceontology, 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 population of 

 the globe during geological time as something enormous ; 

 and indeed they are so, if we regard only the negative 

 differences which separate the older rocks from the 

 more modern, and if we look upon specific and generic 



1 " Le plus grand service qu'on puisse rendre k la science est d'y faire place 

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



2 .Anniversary Address for 1S51, Quart. Journ. Geol. Soc. vol. vii. 



