GEOLOGICAL CONTEMPORANEITY 385 



mental conceptions, there must be no confusion between 

 what is certain and what is more or less probable.* But, 

 pending the construction of a surer foundation than 

 paleontology now possesses, it may be instructive, assum- 

 ing 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 paleontological 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,! 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 paleontology, 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 changes as great 

 changes, which from one point of view they truly are. 

 But leaving the negative differences out of consideration, 

 and looking only at the positive data furnished by the 

 fossil world from a broader point of view from that of 

 the comparative anatomist who has made the study of the 

 greater modifications of animal form his chief business 

 a surprise of another kind dawns upon the mind ; and 

 under this aspect the smallness of the total change becomes 

 as astonishing as was its greatness under the other. 



There are two hundred known orders of plants ; of these 

 not one is certainly known to exist exclusively in the 

 fossil state. The whole lapse of geological time has as 

 yet yielded not a single new ordinal type of vegetable 

 structure. $ 



The positive change in passing from the recent to the 



* " Le plus grand service qu'on puisse rendre a la science est d'y 

 faire place nette avant d'y rien construire." GUVIER. 



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



J See Hooker's Introductory Essay to the Flora of Tasmania, 

 p. xxiii. 

 66 M 



