416 ABSENCE OF TERRESTRIAL 



can understand how it is that a relation exists between the 

 depth of the sea separating two mammalian faunas, and 

 the degree of their affinity, a relation which is quite inex- 

 plicable 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 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 batrachians 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 equa- 

 ble manner. 



I do not deny that there are many and serious difficul- 

 ties in understanding how many of the inhabitants of the 

 more remote islands, whether still retaining the same spe- 

 cific 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 diffi- 

 cult case. Almost all oceanic islands, even the most 

 isolated and smallest, are inhabited by land-shells, gener- 

 ally 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 

 it and are killed. Yet there must be some unknown, 

 but occasionally efficient means for their transportal. 

 Would the just-hatched young sometimes adhere to the 

 feet of birds roosting on the ground and thus get trans- 

 ported? It occurred to me that land-shells, when hyber- 



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