426 SUMMARY. 



be an almost endless amount of organic action and reaction, 

 and we should find some groups of beings greatly, and 

 some only slightly modified; some developed in great 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 

 endeavored 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 mam- 

 mals, should be absent from oceanic islands, while 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 difEerences iii different areas. We 

 see this in many facts. The endurance of each species 

 and group of species is continuous in time; for the appar- 

 ent 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 cer- 

 tainly is the general rule that the area inhabited by a 

 single species, or by a group of species, is continuous, and 



