368 ON THE ORIGIN OF SPECIES 



doubtful species, and of distinct but representative 

 apecies. 



As the late Edward Forbes often insisted, there is a 

 striking parallelism in the laws of life throughout time 

 and space : the laws governing the succession of forms 

 in past times being nearly the same with those govern- 

 ing at the 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 exceptions to the rule are so few, that they may 

 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 inhabited 

 by a single species, or by a group of species, is con- 

 tinuous ; 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 characters in common, as of sculpture or colour. 

 In looking 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 be- 

 longing 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 connected by the same bond of ordinary gener- 



