438 ORIGIN OF SPECIES 



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 sepiarated by deeper channels, we can understand how 

 it is that a relation exists between the depth of the sea sep- 

 arating two mammalian faunas, and the degree of their 

 affinity, — a relation which is quite inexplicable on the theory 

 of independent 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 members 

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

 class, having been modified — the absence of certain whole 

 orders, as of batrachians and of terrestrial mammals, not- 

 withstanding the presence of aerial bats, — the singular pro- 

 portions of certain orders of plants, — herbaceous forms 

 having been developed into trees, &c., — 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 immigrated more uni- 

 formly, 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 in- 

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

 sometimes by species found elsewhere, — striking instances of 

 which have been given by Dr. A. A. Gould in relation to the 

 Pacific. Now it is notorious that land-shells are easily killed 

 by sea-water ; their eggs, at least such as I have tried, sink in 



