DISPERSAL OP REPTILES. 45 



tation found on many of them would serve with powerful effect in 

 the face of a wind. And while the majority of the animal inhab- 

 itants might be exterminated before the end of the voyage the safe 

 arrival on an island or distant shore of a very limited number of 

 individuals, embracing both males and females, would serve in a 

 short period, under favourable conditions, to stock the new land 

 with the species. That an absolute limit is set, however, to migra- 

 tion as effected in this manner is proved conclusively by the utter 

 absence in most of the oceanic islands of indigenous mammals, ex- 

 cepting bats. 



The same obstacle that is interposed by the ocean to the disper- 

 sion of the Mammalia presents itself in the case of the vast majority 

 of other terrestrial animals in which the power of flight is not at all, 

 or at best but feebly, developed. Thus, the serpents, although many 

 of them are fairly good swimmers, are, if we except the marine 

 forms, as incapable of passing oceanic barriers as are the quadrupeds, 

 and their transportation from continental areas to regions far remote 

 can only be effected by such or similar accidental means as that 

 just described. As might have been expected, therefore, they are 

 absent from nearly all oceanic islands. The Amphibia (frogs and 

 toads) are no more fortunate in passing broad arms of the sea than 

 are the serpents, despite the circumstance that in their young or 

 larval condition they are strictly aquatic in their habits. Salt water 

 proves fatal both to them and their eggs. Since moisture is a 

 necessary condition for the early existence of this class of animals, 

 it is evident that an extensive desert region will be an effectual 

 barrier to their distribution in fact, about as much so as an ocean. 

 Lizards, in their adult condition, are as incapable of traversing an 

 oceanic region as are the snakes and amphibians ; but it would ap- 

 pear that in some special way whether as effected by the oceanic 

 currents themselves or through the agency of birds their eggs 

 may be transported to very considerable distances out to sea, since 

 this order of animals is sufficiently represented in remote islands 

 where neither snakes nor amphibians have as yet been encountered. 

 That the ocean offers no insuperable obstacle to the broad disper- 

 sion of a very large body of birds is known from almost daily 

 observation. Birds are known to pass several hundreds of miles 

 on the wing without halting, and, indeed, it is not exactly im- 

 possible, or even improbable, that such unassisted flight may ex- 



