BBITISH ASSOCIATION EOR THE ADVANCEMENT OE SCIENCE. 55 



of a frond bearing spores upon its under surface, is analogous to what takes place 

 in flowering plants in general ; where the seed, when it germinates, produces stem> 

 roots, and leaves ; the stem for many generations gives rise to nothing but shoots 

 like itself; until at length a flower springs from it, which contains within itself for 

 the most part the organs of both sexes united, and, therefore, occasions the repro- 

 duction of the same seed with which the chain of phenomena commenced. This is 

 the principle which a learned Professor at Berlin has rather obscurely shadowed 

 out in his treatise on the Kejuvenescence of Plants, and which may perhaps be 

 regarded as one, at least, of the means by which Nature provides for the stability of 

 the forms of organic life she has created, by imparting to each plant a tendency to 

 revert to the primeval type. 



To the elder De Candolle we are also indebted for some of our most philosophi- 

 cal views with respect to the laws which regulate the distribution of plants over 

 the globe, — views which have been developed and extended, but by no means sub- 

 verted, by the investigations of subsequent writers; amongst whom Sir Charles 

 Lyell, in his ' Principles of Geology,' and the younger De Candolle, a worthy inher- 

 itor of his father's reputation, in his recently published work on Botanical Geogra- 

 phy, have espescially signalized themselves. But it is to the late Prof. Edward 

 Forbes, and to Dr. Joseph Hooker, that we have principally to attribute the remov- 

 al of those anomalies, which threw a certain degree of doubt upon the principles 

 laid down by De Candolle in 1820, in his celebrated article on the Geography of 

 Plants, contained in the ' Dictionnaire des Sciences Naturelles,' where the deriva- 

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

 ticular 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 con- 

 trivances 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 De Candolle's original essay, we are compelled 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 Composite in spite of those feathery appendages 

 they possess, which are so favorable to the wide dissemination of their seeds, might 

 be inferred, by their general absence from the fossil Flora, to have diffused them- 

 selves 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 ex- 

 plain the occurrence of the same plants in some islands or continents now wholly 

 unconnected, and the existence of a distinct Flora in others too isolated to obtain it 

 under present circumstances from without. In the one case we may suppose the 

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



