GEOGRAPHICAL DISTRIBUTION 3SS 



and then having become extinct at intermediate points. But the 

 wide distribution of fresh-water plants, and of the lower animals, 

 whether retaining the same identical form, or in some degree 

 modified, apparently depends in main part on the wide dispersal 

 of their seeds and eggs by animals, more especially by fresh- 

 water birds, which have great powers of flight, and naturally 

 travel from one piece of water to another. 



ON THE INHABITANTS OF OCEANIC ISLANDS 



We now come to the last of the three classes of- facts, whick 

 I have selected as presenting the greatest amount of difficulty 

 with respect to distribution, on the view that not only all the in- 

 dividuals of the same species have migrated from some one area, ^ 

 but that allied species, although now inhabiting the most distant 

 points, have proceeded from a single area, the birthplace of their 

 early progenitors, I have already given my reasons for disbelicT- 

 ing in continental extensions within the period of existing species 

 on so enormous a scale that all the many islands of the several 

 oceans were thus stocked with their present terrestrial inhabitants. 

 This view removes many difficulties, but it does not accord witJi 

 all the facts in regard to the productions of islands. In the fol- 

 lowing remarks I shall not confine myself to the mere question of 

 dispersal, but shall consider some other cases bearing 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 continental areas: Alph. 

 de Candolle admits this for plants, and Wollaston for insects. 

 New Zealand, for instance, with its lofty mountains and diversi- 

 fied stations, extending over 780 miles of latitude, together with 

 the outlying islands of Auckland, Campbell, and Chatham, con- 

 tain altogether only 960 kinds of flowering plants; if we compare 

 this moderate number with the species which swarm over equal 

 areas in Southwestern Australia or at the Cape of Good Hopcj, 

 we must admit that some cause, independently of different physi- 

 cal conditions, has given rise to so great a difference in number. 

 Even the uniform county of Cambridge has 847 plants, and the 

 little island of Anglesea 764, but a few ferns and a few intro- 

 duced plants are included in these numbers, and the comparison 

 in some other respects is not quite fair. We have evidence that 

 the barren island of Ascension aboriginally possessed less than 

 half a dozen flowering plants; yet many species have now be- 

 come naturalized on it, as they have in New Zealand and on 



