302 SUMMARY". [Chap. XIIL 



looking to distant provinces throughout the world, we find that 

 species in certain classes differ little from ouch 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 exceptions 

 to the rule. According to our theory, these several relations 

 throughout time and space are intelligible ; 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. 



