MECHANICS AND USEFUL ARTS. 20 



of what has been called a pro-embryo ; the evolution of antheridia and arche- 

 gonYa, or of male and female organs, from the former ; and the generation 

 from the archegouia of a frond bearing spores upon its under surface, is 

 analogous to what takes place hi flowering plants hi general ; where the seed, 

 when it germinates, produces stem, roots, and leaves; the stern for many 

 generations gives rise to nothing but shoots like itself: until at length a 

 rlower springs from it, which contains within itself for the most part the 

 organs of both sexes united, and, therefore, occasions the reproduction 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 hi his treatise on the Rejuvenescence 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 prirnceval type. 



blSTKIBUTIOX OF PLANTS. 



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

 sophical 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 subverted, by the investigations of subsequent writers ; amongst 

 whom Sir Charles Lyell, in his "Principles of Geology," and the younger De 

 Candolle, a worthy inheritor of his father's reputation, in his recently pub- 

 lished work on Botanical Geography, have especially signalized themselves. 

 But it is to the late Prof. Edward Forbes, and to Dr. Joseph Hooker, that we 

 have principally to attribute the removal 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 derivation of each species 

 from an individual, or a pair of individuals, created hi 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 hi 

 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 hi 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 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 Compositse, in spite of those feathery 

 appendages they possess, which are so favorable to the wide dissemination 

 of then* seeds, might be inferred, by their general absence from the fossil 

 Flora, to have diffused themselves in a less degree than many other families 



