ADDRESS. 



ixi 



DeCandolle in 1820, in his celebrated article on the Geography of Plants, 

 contained in the ' Dictionnaire des Sciences Naturelles,' where the derivation 

 of each species from an individual, or a pair of individuals, created in one 

 particular locality, was made the starting-point of all our inquiries. 



These anomalies were of two different kinds, and pointed in two opposite 

 directions: for we had in some cases to explain the occurrence of a peculiar 

 flora in islands cut off from the rest of the world, except through the medium 

 of a wide intervening ocean ; and in other cases to reconcile the fact of the 

 same or of allied species being diffused over vast areas, the several portions of 

 which are at the present time separated from each other in such a manner, as 

 to prevent the possibility of the migration of plants from one to the other. 

 Indeed, after making due allowances for those curious contrivances by which 

 Nature has in many instances provided for the transmission of species 

 over different parts of the same continent, and even across the ocean, and 

 which are so well pointed out in DeCandoUe's original essay, we are com- 

 pelled to admit the apparent inefficiency of existing causes to account for the 

 distribution of the larger number of species ; and must confess that the 

 explanation fails us often where it is most needed ; for the Compositae, in 

 spite of those feathery appendages they possess, which are so favourable to 

 the wide dissemination of their seeds, might be inferred, by their general 

 absence from the fossil flora, to have diffused themselves in a less degree than 

 many other families have done. And on the other hand, it is found, that 

 under existing circumstances, those Composite, which are disseminated 

 throughout the area of the Great Pacific, belong in many cases to species 

 destitute of these auxiliaries to transmission. 



But here Geology comes to our aid ; for by pointing out the probability of 

 the submergence of continents on the one hand, and the elevation of tracts 

 of land on the other, it enables us to explain, the occurrence of the same 

 plants in some islands or continents now wholly unconnected, and the exist- 

 ence of a distinct flora in others too isolated to obtain it under present cir- 

 cumstances from without. In the one case we may suppose the plants to 

 have been distributed over the whole area before its several parts became 

 disunited by the catastrophes which supervened ; in the other, we may re- 

 gard the peculiar flora now existing as merely the wreck, as it were, of one 

 which once overspread a large tract of land, of which all but the little patch 

 upon which it is now found had since been submerged. 



Upon this subject, however, our opinions may in some measure be swayed 

 by the nature of the conclusions we arrive at with respect to the length of 

 time during which seeds are capable of maintaining their vitality ; for if after 

 remaining for an indefinite period in the earth they were capable of germi- 

 nating, it would doubtless be easier to understand the revival, under favour- 

 able circumstances, of plants which had existed before the severance of a 

 tract of land from the continent in which they are indigenous. An inquiry 

 has accordingly been carried on for the last fifteen years under the auspices of, 

 and with the aid of funds supplied by, this Association, the results of which, 

 it is but fair to say, by no means corroborate the reports that had been 

 from time to time given us with respect to the extreme longevity of certain 

 seeds, exemplified, as it was said, in the case of the mummy-wheat and other 

 somewhat dubious instances ; inasmuch as they tend to show, that none of 

 the seeds which were tested, although they had been placed under the most 

 favourable artificial conditions that could be devised, vegetated beyond a 

 period of forty-nine years ; that only twenty out of 288 species did so after 

 twenty years ; whilst by far the larger number had lost their germinating 

 power in the course of ten. 



