CHAP. XIII.] LAST AND PRESENT CHAPTERS. 339 



numbers ; according or not, as those which entered happened to 

 come into more or less direct competition with each other and 

 with the aborigines ; and according as the immigrants were capable 

 of varying more or less rapidly, there would ensue in the two or 

 more regions, independently of their physical conditions, infinitely 

 diversified conditions of life, there would 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 en- 

 deavoured to show, why oceanic islands should have few inhabi- 

 tants, 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 rela- 

 tion 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 archi- 

 pelago, 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 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, 



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