GEOGRAPHICAL DISTRIBUTION 349 



migration of the same aquatic species. \Y r e should not 

 forget the probability of many species having formerly 

 ranged as continuously as fresh-water productions ever 

 san range, over immense areas, and having subsequently 

 become extinct in intermediate regions. But the wide 

 distribution of fresh - water plants and of the lower 

 animals, whether retaining the same identical form 

 or in some degree modified, I believe mainly depends 

 on the wide dispersal of their seeds and eggs by animals, 

 more especially by fresh-water birds, which have large 

 powers of flight, and naturally travel from one to 

 another and often distant piece of water. Nature, like 

 a careful gardener, thus takes her seeds from a bed of 

 a particular nature, and drops them in another equally 

 well fitted for them. 



On the Inhabitants of Oceanic Islands. — We now 

 come to the last of the three classes of facts, which I 

 have selected as presenting the greatest amount of 

 difficulty, on the view that all the individuals both of 

 the same and of allied species have descended from a 

 single parent ; and therefore have all proceeded from a 

 common birthplace, notwithstanding that in the course 

 of time they have come to inhabit distant points of the 

 globe. I have already stated that I cannot honestly 

 admit Forbes's view on continental extensions, which, 

 if legitimately followed out, would lead to the belief 

 that within the recent period all existing islands have 

 been nearly or quite joined to some continent. This 

 view would remove many difficulties, but it would not, 

 I think, explain all the facts in regard to insular pro- 

 ductions. In the following remarks I shall not confine 

 myself to the mere question of dispersal ; but shall 

 consider some other facts, which bear on the truth of 

 the two theories of independent creation and of descent 

 with modification. 



The species of all kinds which inhabit oceanic islands 

 are few in number compared with those on equal con- 

 tinental areas: Alph. de Candolle admits this for plants, 

 and VVollaston for insects. If we look to the large 



