200 SUMMARY OF THE [Chap. XIII. 



be related, but less closely, to those of the nearest con- 

 tinent, or other source whence immigrants might 

 iiave 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 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 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 



