Chap. xni. last and present Chapters. 361 



force, some existing in scanty numbers — and this we do find in 

 the several great geographical provinces of the world. 



On these same principles we can understand, as I have endea- 

 voured to show, why oceanic islands should have few inhabitants, 

 but that of these, a large proportion should be endemic or peculiar ; 

 and why, in relation to the means of migration, one group of beings 

 should have all its species peculiar, and another group, even within 

 the same class, should have all its species the same with those in 

 an adjoining quarter of the world. We can see why whole groups 

 of organisms, as batrachians and terrestrial mammals, should be 

 absent from oceanic islands, whilst the most isolated islands should 

 possess their own peculiar species of aerial mammals or bats. We 

 can see why, in islands, there should be some relation between the 

 presence of mammals, in a 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 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 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 in- 

 habited 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 cir- 

 cumstances, 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 living 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 



