SUMMARY, 427 



the exceptions, which are not rare, may, as I have 

 attempted to show, be accounted for by former migrations 

 under different circumstances, or through occasional means 

 of transport, or by the species having become extinct in 

 the intermediate tracts. Both in time and space species 

 and groups of species have their points of maximum 

 development. Groups of species, living during the same 

 period of time, or living within the same area, are often 

 characterized by trifling features in common, as of sculpt- 

 ure or color. In looking to the long succession of past 

 ages, as in looking to distant provinces throughout the 

 world, we find that species in certain classes differ little 

 from each other, while 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 organized 

 members of each class generally change less than the 

 highly organized; 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 tliey 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. 



