94 LECTURES ON EVOLUTION m 



imagine any two groups more definitely and dis- 

 tinctly separated, notwithstanding certain charac- 

 ters which they possess in common. 



As we trace the history of birds ba,ck in time, we 

 find their remains, sometimes in great abundance, 

 throughout the whole extent of the tertiary rocks ; 

 but, so far as our present knowledge goes, the birds 

 of the tertiary rocks retain the same essential char- 

 acters as the birds of the present day. In other 

 words, the tertiary birds come within the definition 

 of the class constituted by existing birds, and are as 

 much separated from reptiles as existing birds are. 

 Not very long ago no remains of birds had been 

 found below the tertiary rocks, and I am not sure 

 but that some persons were prepared to demonstrate 

 that they could not have existed at an earlier period. 

 But, in the course of the last few years, such remains 

 have been discovered in England ; though, unfortu- 

 nately, in so imperfect and fragmentary a condition, 

 that it is impossible to say whether they differed 

 from existing birds in any essential character or not. 

 In your country the development of the cretaceous 

 series of rocks is enormous ; the conditions under 

 which the later cretaceous strata have been de- 

 posited are highly favourable to the preservation of 

 organic remains ; and the researches, full of labour 

 and risk, which have been carried on by Professor 

 Marsh in these cretaceous rocks of Western 

 America, have rewarded him with the discovery of 

 forms of birds of which we had hitherto no concep- 



