Chap. XUI.] LAST AND PRESENT CHAPTERS. 201 



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; hut there are in both cases marked excep- 

 tions to the rule. According to our theory, these sev- 

 eral 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; in both cases the laws of 

 variation have been the same, and modifications 

 have been accumulated by the same means of natural 

 selection. 



