466 SUMMARY. 



be some relation between the presence of mammals, in & 

 more or less modified condition, and the depth of the sea 

 between such islands and the mainland. We can clearly see 

 why all the inhabitants of an archipelago, though specifically 

 distinct on the several islets, should be closely related to 

 each 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 strik- 

 ing 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 governing 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 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 occa- 

 sional 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 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 throughout the world, we 

 find that species in certain classes differ little from each 

 other, while 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 organized members of each 

 class generally change less than the highly organized ; 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 



