MEANS OF DISPERSAL. $60 



a single birthplace, and when in the course of time ^e 

 know something definite about the means of distribution, 

 we shall be enabled to speculate with security on the former 

 extension of the land. But I do not believe that it will 

 ever be proved that within the recent period most of our 

 continents which now stand quite separate, have been con- 

 tinuously, or almost continuously united with each other, 

 and with the many existing oceanic islands. Several facts 

 in distribution — such as the great difference in the marine 

 faunas on the opposite sides of almost every continent — » 

 the close relation of the tertiary inhabitants of several 

 lands and even seas to their present inhabitants — the 

 degree of affinity between the mammals inhabiting islands 

 with those of the nearest continent, being in oart deter 

 mined (as we shall hereafter see) by the depth ox the inter 

 vening ocean — these and other such facts are opposed to 

 the admission of such prodigious geographical revolutions 

 within the recent period, as are necessary on the view 

 advanced by Forbes and admitted by his followers. The 

 nature and relative proportions of the inhabitants of oceanic 

 islands are likewise opposed to the belief of their former 

 continuity of continents. Nor does the almost universal!)' 

 volcanic composition of such islands favor the admission 

 that they are the wrecks of sunken continents ; if they had 

 originally existed as continental mountain ranges, some at 

 least of the islands would have been formed, like other 

 mountain summits, of granite, metamorphic schists, old 

 fossiliferous and other rocks, instead of consisting of mere 

 piles of volcanic matter. 



I must now say a few words on what are called accidental 

 means, but which more properly should be called occasional 

 means of distribution. I shall here confine myself tr> 

 plants. In botanical works, this or that plant is oftei* 

 stated to be ill adapted for wide dissemination; but the 

 greater or less facilities for transport across the sea may be 

 said to be almost wholly unknown. Until I tried, with Mr. 

 Berkeley's aid, a few experiments, it was not even known 

 how far seeds could resist the injurious action of sea-water. 

 To my surprise I found that out of eighty-seven kinds, 

 sixty-four germinated after an immersion of twenty-eight 

 days, and a few survived an immersion of 137 days. It 

 deserves notice that certain orders were far more injured 

 than others; nine Leguminosse were tried, and, with one 

 exception, they resisted the salt water badly ; seven specie! 



