GEOGRAPHICAL DISTRIBUTION 369 



nearly the same physical conditions, should often be inhabited 

 by very different forms of life; for according to the length of time 

 which has elapsed since the colonists entered one of the regions, 

 or both; according to the nature of the communication which al- 

 lowed certain forms and not others to enter, either in greater or 

 lesser 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 ca- 

 pable 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 al- 

 most 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- 

 deavored to show, why oceanic islands should have few inhabit- 

 ants, 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, 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 in- 

 habitants 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 con- 

 tinent, or other source whence immigrants might have been de- 

 rived. We can see why, if there exist very closely allied or repre- 

 sentative 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 



