246 Darwin, and after Darwin. 



ness to leave no manner of doubt as to their affinities 

 with the fauna of that mainland. Lastly, no am- 

 phibians and no mammals (except bats) are ever 

 found on any oceanic islands. Yet, as we have seen, 

 on the theory of special creation, these islands must 

 all be taken to have been the theatres of the most 

 extraordinary creative activity, so that on only three 

 of them w^e found no less than 1258 unique species, 

 whereof 657 were unique species of land animals, to 

 be set against one single species known to occur else- 

 where. Nevertheless, notwithstanding this prodigious 

 expenditure of creative energy in the case of land- 

 birds, land-shells, insects, and reptiles, no single new 

 amphibian, or no single new mammal, has been 

 created on any single oceanic island, if we except 

 the only kind of mammal that is able to fly, and 

 the ancestors of which, like those of the land-birds 

 and insects, might therefore have reached the islands 

 ages ago. Moreover, with regard to mammals, 

 even in cases where allied forms occur on either 

 side of a sea-channel, it is found to be a general rule 

 that if the channel is shallow, the species on either 

 side of it are much more closely related than if it be 

 deep — and this irrespective of its width. Therefore 

 we can only conclude, in the words of Darwin — " As 

 the amount of modification which animals of all kinds 

 undergo partly depends on lapse of time, and as the 

 islands which are separated from each other or from 

 the mainland by shallow channels are more likely to 

 have been continuously united within a recent period 

 than islands separated by deeper channels, we can 

 understand how it is that a relation exists between 

 the depth of the sea separating two mammalian 



