NOTICES OF BOOKS. 87 



existing species, and further, that since the creation of the latter no 

 intermediate. large islands or archipelagos have existed. 



4. The most active transport has been between the opposite coasts 

 of America and Africa, and is due to the currents and the slave-trade. 



5. The agency of man has hitherto been rather involuntary than 

 voluntary. 



6. The majority of the species are such as are easily propagated. 



7. The species are very frequently littoral, or affect cultivated or 

 artificial localities. 



8. Certain families are very largely represented in the list ; they 

 are MaUacem^ Tiliace^, Leguminosce^ Convolvulacemy Jjahiata^ Amaran- 

 tacets^ and Nyctaginece, Of these, the Malvacere, Leguminosce^ and Con- 

 volvulacecBy have seeds capable of retaining their vitality during long 

 exposure to immersion ; others have seeds adapted to cling to foreign 

 bodies, as Tiliacece, Labiat(B, and Nyctagine^e. 



9. Most of the species are annual or woody. 



A very curious description is given on the subject of plants which 

 might be expected to have become naturalized, but which are not so, or, 

 as M. de Candolle entitles it, " Exemples de Naturalisation manques.'' 



The difficulty of naturalizing plants at all is here forcibly dwelt upon, 

 and the extremely small proportion of the many thousand species that 

 have been introduced into our gardens, which eventually propagate 

 themselves beyond those limits. Of a vast number which have been 

 tried at the Bois de Boulogne, the Fotentilla Pennsylvanica is the only 

 one which is positively known to have established itself. In the neigh- 

 bourhood of Geneva, one of M. de CandoUe's friends has been in the 

 habit during the last eighteen years of annually scattering many hun- 

 dreds of seeds collected in the Botanical Gardens, but hitherto with- 

 out any appreciable result. 



A comparison between the facility with which plants are naturalized 

 and the extent of the areas over which they are indigenous, affords no 

 result in the present state of our knowledge. The last section of this 

 Chapter is devoted to the contraction of the area occupied by a species. 

 Artificial causes of course operate largely in diminishing the number of 

 individuals, and hence of the area occupied by a species, and of these 

 the most important are the draining of marshes and the cutting down 



of forests. 



The data for facts of this class are necessarily rare, and arc chiefly 



