GEOGRAPHICAL DISTRIBUTION 209 



other: and should likewise be related, but less closely, 

 to those of the nearest continent, or other source whence 

 immigrants might have been derived. We can see why, 

 if there exist very closely allied or representative species 

 in two areas, however distant from each other, some 

 identical species will almost always there be found. 



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 spe- 

 cies 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 certain 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 inhab- 

 ited 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 former 

 migrations under different circumstances, or through occa- 

 sional means of transport, or by the species having be- 

 come extinct in the intermediate tracts. But 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 sculpture or color. In looking to the long succession 

 of past ages, as in looking to distant provinces through- 

 out the world, we find that species in certain classes dif- 

 fer little from each other, while those in another class, 



