TEANSPOETATION OF LAND-SKAILS. 287 



This mixture of the true Philippine fauna with the two 

 foreign elements at the opposite ends of this chain of islands can, 

 as it seems to me, be satisfactorily accounted for only by the 

 assumption that the marine currents, which in those seas are 

 dependent on the monsoons — or at any rate are greatly influenced 

 by them — have been the essential means of transporting animal 

 forms.^"^ For animals from China could by their aid reach only 

 the most northern islands, while those from the Indian and 

 Australian islands must first reach Palawan and Mindanao. 

 This, as I have shown, is precisely the case ; and the almost 

 entire absence of such forms, which would easily betray their 

 foreign origin, from the middle region of the group confirms the 

 idea that in the north immigration has taken place from the 

 west and in the south from still farther south. 



The general conclusion to be drawn from this — i.e., that 

 the constant currents combined with the changes of the mon- 

 soons have been the principal agents in the peculiar distribution 

 of many of the land moUusca of the Philippines — is still further 

 strengthened by the following considerations. If land-snaOs in 

 general can be conveyed from one island to another at all, it 

 certainly can only be by currents, and presumably by the inter- 

 vention of trees on which they are borne, and their eggs may be 

 transported in the same manner. But it is self-evident that in 

 this way a selection will be effected between the different forms, 

 according as they are qualified to endure an ocean voyage or not. 

 Large species and such as live in the highest branches of trees 

 and lay their eggs there, like all the species of Gochlostyla, 

 are obviously far more difficult to transport than small species 

 which can creep into the rifts in trees or between the roots ; the 

 species of a group which, like Helix ^imilaris^]xvei on the ground 

 among stones and earth, will be almost as well protected on the 

 sea-voyage as the operculated snails which have the mouth of the 

 shell closed by a lid or shield, which protects the soft part of the 

 creature almost perfectly from contact with the salt water. 

 Hence we may expect to find in islands a greater multitude of 

 different species in those genera which are most easily trans- 

 portable. We know that the constant introduction of the parent 

 forms in any numbers into a new colony will prevent the 



