304 MEANS OF DISPERSAL. [CHA 



changes within the period of existing species. It seems to me 

 that we have abundant evidence of great oscillations in the level 

 of the land or sea ; but not of such vast changes in the position 

 and extension of our continents, as to have united them within 

 the recent period to each other and to the several intervening 

 oceanic islands. I freely admit the former existence of many 

 islands, now buried beneath the sea, which may have served as 

 halting-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 continu- 

 ously, 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 oppo- 

 site sides of almost every continent, the close relation of the 

 tertiary inhabitants of several lands and even seas to their pre- 

 sent inhabitants, the degree of affinity between 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 

 proportions 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 ; if they had originally existed as continental moun- 

 tain 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 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 experi- 

 ments, it was not even known how far seeds could resist the 



