410 GEOGRAPHICAL DISTRIBUTION. Chap. XII. 



fairly be attributed to our not having as yet discovered 

 in an intermediate deposit the forms which are therein 

 absent, but which occur above and below : so in space, 

 it certainly is the general rule that the area inha- 

 bited 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 

 migration at some former period under different con- 

 ditions or by occasional means of transport, and 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, belonging either to a certain period of time, or 

 to a certain area, are often characterised by trifling cha- 

 racters in common, as of sculpture or colour. In look- 

 ing to the long succession of ages, as in now looking to 

 distant provinces throughout the world, we find that 

 some organisms differ little, whilst others belonging to a 

 different class, or to a different order, or even only to a 

 different family of the same order, differ greatly. In 

 both time and space the lower members of each class 

 generally change less than the higher; but there are 

 in both cases marked exceptions to the rule. On my 

 theory these several relations throughout time and space 

 are intelligible ; for whether we look to the forms of life 

 which have changed during successive ages within the 

 same quarter of the world, or to those which have 

 changed after having migrated into distant quarters, in 

 both cases the forms within each class have been con- 

 nected by the same bond of ordinary generation ; and 

 the more nearly any two forms are related in blood, the 

 nearer they will generally stand to each other in time 

 and space ; in both cases the laws of variation have been 

 the same, and modifications have been accumulated by 

 the same power of natural selection. 



