Chap. XIII.] LAST AND PRESENT CHAPTEES. 201 



other, whilst those in another class, or only in a different 

 section of the same order, differ greatly from each other. 

 In both time and space the lowly organised members 

 of each class generally change less than the highly 

 organised ; but there are in both cases marked excep- 

 tions to the rule. According to our theory, these 

 several relations throughout time and space are intelli- 

 gible ; for whether we look to the allied forms of life 

 which have changed during successive ages, or to 

 those which have changed after having migrated into 

 distant quarters, in both cases they are connected by 

 the same bond of ordinary generation ; m both cases 

 the laws of variation have been the same, and modifi- 

 cations have been accumulated by the same means of 

 natural selection. 



