370 THE ORIGIN OF SPECIES 



apparent exceptions to the rule are so few that they may fairly 

 be attributed to our not having as yet discovered in an inter- 

 mediate deposit certain forms which are absent in it, but which 

 occur both above and below: so in space, it certainly is the gen- 

 eral rule that the area inhabited by a single species, or by a group 

 of species, is continuous, and 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 liv- 

 ing within the same area, are often characterized by trifling 

 features in common, as of sculpture 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 whic'h 

 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 genera- 

 tion; in both cases the laws of variation have been the same, and 

 modifications have been accumulated by the same means of nat- 

 ural selection. 



