324 MEANS OF DISPERSAL. [Chap. XII. 



places for plants and for many animals during their migration. In 

 the coral-producing oceans such sunken islands are now marked by- 

 rings of coral or atolls standing over them. Whenever it is fully 

 admitted, as it will some day be, that each species has proceeded 

 from a single birthplace, and when in the course of time we 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 continuously, 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 be- 

 tween the mammals inhabiting islands with those of the nearest 

 continent, being in part determined (as we shall hereafter see) by 

 the depth of the intervening 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 pro- 

 portions of the inhabitants of oceanic islands are likewise opposed 

 to the belief of their former continuity with continents. Nor does 

 the almost universally volcanic composition of such islands favour 

 the admission that they are the wrecks of sunken continents ; — it 

 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 dis- 

 tribution. I shall here confine myself to plants. In botanical 

 works, this or that plant is often 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 87 kinds, 64 germinated after an 

 immersion of 28 days, and a few survived an immersion of 137 

 days. It deserves notice that certain orders were far more injured 

 than others : nine Leguminosa3 were tried, and, with one exception, 

 they resisted the salt-water badly ; seven species of the allied orders, 

 Hydrophyllaceas and Polcmoniacea?, were all killed by a month's 



