SUMMARY 449 



present time the differences in different areas. We see this 

 in many facts. The endurance of each species and group of 

 species is continuous in time ; for the apparent exceptions to 

 the rule are so few, that they may fairly be attributed to our 

 not having as yet discovered in an intermediate deposit cer- 

 tain forms which are absent in it, but which occur both 

 above and below : so in space, it certainly is the general 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 be- 

 come extinct in the intermediate tracts. Both in time and 

 space species and groups of species have their points of maxi- 

 mum development. Groups of species, living during the 

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

 characterised by trifling features in common, as of sculpture 

 or colour. 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, 

 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 gen- 

 erally 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. 



O— lie XI 



