ORGANIC EVOLUTION. 59 



different islands all belong to the same or to 

 closely allied species. On this highly interesting 

 subject Darwin writes, " I have not as yet had 

 time to follow up this subject in all quarters 

 of the globe ; but as far as I have gone the 

 relation holds good. For instance, Britain is 

 separated by a shallow channel from Europe, 

 and the mammals are the same on both sides, 

 and so it is with all the islands near the shores 

 of America. The West Indian islands, on the 

 other hand, stand on a deeply submerged bank 

 nearly 1,000 fathoms in depth, and here we find 

 American forms, but the species, and even the 

 genera, are distinct. As the amount of modi- 

 fication which animals of all kinds undergo 

 partly depends on 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 separated by 



