Chap. XII. SUMMARY. 409 



loped in great force, some existing in scanty numbers — 

 in the different great geographical provinces of the 

 world. 



On these same principles, we can understand, as I 

 have endeavoured to show, why oceanic islands should 

 have few inhabitants, but of these a great number 

 should be endemic or peculiar ; and why, in relation to 

 the means of migration, one group of beings, even within 

 the same class, should have all its species endemic, and 

 another group should have all its species common to 

 other quarters of the world. We can see why whole 

 groups of organisms, as batrachians and terrestrial mam- 

 mals, should be absent from oceanic islands, whilst the 

 most isolated islands possess their own peculiar species of 

 aerial mammals or bats. We can see why there should 

 be some relation between the presence of mammals, in 

 a more or less modified condition, and the depth of 

 the sea between an island 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 likewise be related, 

 but less closely, to those of the nearest continent or 

 other source whence immigrants were probably derived. 

 We can see why in two areas, however distant from each 

 other, there should be a correlation, in the presence of 

 identical species, of varieties, of doubtful species, and of 

 distinct but representative species. 



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 exceptions to the rule are so few, that they may 



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