15 



CHAPTER II. 



PRELIMINARY INQUIRIES, COntilUwd MODES OF DISPERSAL OF SPECIES. 



The mode in wliich, and the extent to whicli, species become dispersed, is another point which 

 must be settled at starting. Tlie j)rincipal question on which a difference of opinion exists is tlie 

 value to be attached to accidental or occasional dispersal as a means of distribution. Taking for 

 example the case of Oceanic Islands, there aa'e two ways in which their faunas and floras may 

 bo accounted for. One — that advocated by the late Professor Edward Forbes — by sujiposing 

 that at some period, more or less distant, the islands had been imited to the nearest land, to 

 whose faimas and floras their own was most akin. The other, which is that adopted by 

 Mr. Darwin for most of the cases which occur, tliat the}- had been colonized by chance visitants, 

 or what may be called the flotsam and jetsam of the ocean. 



For the purpose of testing the feasibility of his hj-pothesis, Mr. Darwin tried many ingenious 

 experiments as to the length of time for which seeds can float in the sea •\^'ithout losing their 

 vitality. He reckoned how fast ciu-rents carry them on their way. He also showed the many 

 different means by which they may be transported — as in earth about the roots of drift timber, 

 or adhering to the beaks or claws of birds, by icebergs, and by means even of fresh-water fish 

 swallowing seeds, and then being themselves swallowed by birds of prey. Edmund Bm-ke 

 objected to the instances of some of his political oiDponents, that " their examples for common 

 cases were all taken from the exceptions of most urgent necessity." The same objection apj)lies 

 to Mr. Darwin's illustrations. It is not to be denied that they are all possible, but they are also 

 all of an exceptional natm-e, and some of them very improbable, as, for example, the transport 

 of seeds by means of icebergs, or by birds of prey swallowing the stomachs of vegetable-feeding 

 fish. Where the icebergs leave their jjarent glacier there are few seeds of any kind, and fewer still 

 that woidd suit the climate of milder regions. As to the birds of prey, their digestion is so 

 notoriously rapid, that imless the bird set oif express immediately after its meal, and the islands 

 were not far ofi", there woidd be small chance of anj-thing it had swallowed ever reaching their 

 shores. The reader -n-ill remember numerous other instances of the actual difliision of jjlants 

 and animals in a similar way given by Sir C. Lyell ; * and Mr. Darwin, while he adds a few others, 

 has no doubt a multitude more in his armourj-. Notwithstanding this, I can come to no other 

 conclusion than that colonization or occasional dispersal is insiifiicient to account for the character 

 of the faimas and floras of Oceanic Islands ; and I believe that the normal mode in which 

 islands have been peopled, has been by direct continuity with the land at some fonner period, 

 or by contiguity so close as to be equivalent to j miction ; and that the exceptions to this, such 

 as St. Helena, have been excessively rare. That a slight intermixture due to Mr. Darwin's 



* Lyell, " Principles of Geology." 1st Edition. Vol. II. p. 10, and scq. 1832. 



