Permanence and Evolution. 159 



under European auspices), but an enormous 

 profusion of land shells surely as difficult to 

 transport as mammals. And we know as a 

 positive fact that large mammals of the 

 Chinchilla type existed on one and probably 

 more of the West Indian Islands. Now if the 

 islands were never connected with the mainland, 

 how did the large mammals get there ? If they 

 were once continental, how is it that they no 

 longer have a continental fauna ? It would 

 seem that there must be some cause tending to 

 extinguish mammals on islands after a certain 

 lapse of time, though they do well enough for 

 a short period. One cause may evidently be 

 the greater risk of extirpation within a small 

 area. We may take it as certain that mammals, 

 from their large size and comparative confine- 

 ment to one spot of ground, are amongst the 

 most easily extirpated of living things ; and, let 

 us ask, if the small islands of the Pacific had 

 once been thickly peopled with mammals, what 



