x.] GEOLOGICAL CONTEMPORANEITY. 187 



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 palaeontological 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 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 palaeontology, 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 

 \ve regard only the negative differences which separate the older 

 rocks from the more modern, and if we look upon specific and 

 generic changes as great changes, which from one point of view 

 they truly are. But leaving the negative differences out of con 

 sideration, and looking only at the positive data furnished by the 

 fossil world from a broader point of view from that of the com 

 parative anatomist who has made the study of the greater modi 

 fications of animal form his chief business a surprise of another 



1 &quot; Le plus grand service qu on puisse rendre a la science est d y faire 

 place nette avant d y rien construire.&quot; CUVIER. 



2 Anniversary Address for 1851, Quart. Journ. Geol. Soc. vol. vii. 



