68 THE MAMMALIA. 



the earth as it is nowadays. Specially ingenious 

 I consider his distinction between the adaptive 

 and inadnptive reduction of the limbs, which we 

 shall have to consider more in detail when discuss- 

 ing this point. 



Having now pointed out the direction in which 

 these investigators have worked, and their con- 

 ception of things in general as distinguished from 

 those of their numerous fellow-workers in the 

 domain of the higher animals, and having further 

 referred to the stimulus which their studies have 

 given to the theory of descent, I may now confine 

 myself to mentioning them only in so far as they 

 concern the palaeontology of the Old World. 



Within the last fifteen years a series of sur- 

 prising discoveries have been made concerning the 

 palaeontology of America ; these discoveries have 

 almost subverted, at all events completely modified, 

 the opinions that had hitherto prevailed as to the 

 distribution and derivation of animals, in so far as 

 they concern the exchange and succession between 

 the Old and New World. We have a summary of 

 the zoo-geographical inquiries into those primary 

 periods in a work of Eiitimeyer's, 1 not very com- 

 prehensive but rich in substance. He there says : 



die Hcrkunft unserer Thierwelt, 1803. 



