390 ABSENCE OF TERRESTRIAL 



tion holds good. For instance, Britain is separated by a 

 shallow channel from Europe, and the mammals are the 

 same on both sides ; and so it is with all the islands near 

 the shores of Australia. The West Indian Islands, on the 

 other hand, stand on a deeply submerged bank, nearly one 

 thousand fathoms in depth, and here we find American 

 forms, but the species and even the genera are quite distinct. 

 As the amount of modification which animals of all kinds 

 undergo partly depends on the 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 the islands 

 separated by deeper channels, we can understand how it is 

 that a relation exists between the depth of the sea separat- 

 ing two mammalian faunas, and the degree of their affinity, 

 a relation which is quite inexplicable on the theory of inde- 

 pendent acts of creation. 



The foregoing statements in regard to the inhabitants of 

 oceanic islands, namely, the fewness of the species, with a 

 large proportion consisting of endemic forms — the mem- 

 bers of certain groups, but not those of other groups in the 

 same class, having been modified — the absence of certain 

 whole orders, as of batrachian« and of terrestrial mammals, 

 notwithstanding the presence of aerial bats, the singular 

 proportions of certain orders of plants, herbaceous forms 

 having been developed into trees, etc., seem to me to accord 

 better with the belief in the efficiency of occasional means 

 of transport, carried on during a long course of time, than 

 with the belief in the former connection of all oceanic 

 islands with the nearest continent ; for on this latter view 

 it is probable that the various classes would have immi- 

 grated more uniformly, and from the species having entered 

 in a body, their mutual relations would not have been much 

 disturbed, and, consequently, they would either have not 

 been modified, or all the species in a more equable manner. 



I do not deny that there are many and serious difficulties 

 in understanding how many of the inhabitants of the more 

 remote islands, whether still retaining the same specific form 

 or subsequently modified, have reached their present homes. 

 But the probability of other islands having once existed as 

 halting-places, of which not a wreck now remains, must not 

 be overlooked. I will specify one difficult case. Almost 

 all oceanic islands, even the most isolated and smallest, are 

 inhabited by land-shells, generally by endemic species, but 



