208 THE NATURAL HISTOET REVIEW. 



probably have been classed with the more or less temporary intru- 

 ders, spoken of under the head of introduced plants. The three 

 others, Clematis recta, Coronilla juncea, and Arum arisarum, are 

 still not uncommon in some parts of Provence to the east, and in 

 Catalonia to the south-west, and are most probably species which in 

 fact are slowly but steadily losing ground. 



In investigating the causes of introduction and dispersion of new 

 species, Dr. Planchon has been unable to trace any results within 

 the last three centuries to the action of purely physical causes, such 

 as curreuts of water or wind, or to the action of animals independ- 

 ently of man. The Montpellier seaboard is not exposed to any 

 extensive maritime currents, which, on some of our own and other 

 Atlantic coast:^, bring seeds from great distances rapidly enough to 

 preserve their powers of germination, and if, as is most likely, some 

 are brought from smaller distances, and germinate and grow, they are 

 the same which have done so repeatedly during tens or hundreds of 

 centuries, and cannot be distinguished from those raised from native 

 seeds. Fresh water currents can still less have introduced foreign 

 plants, for every stream of the region rises within it. All that they can 

 do is occasionally to bring down mountain plants into the valleys, or 

 to disperse along their banks species otherwise introduced. The action 

 of winds and that of animals, always independently of human 

 agency, take effect chiefly within limited distances ; and in this 

 respect the same observation applies to them as to that of the sea, 

 that the present conformation and condition of the country has 

 lasted so long, that their force has long since had its full effect, and 

 they have no new plants to act upon, unless brought within their 

 influence by other means. The only new species which the author 

 thinks may possibly have been introduced by these means, is the 

 Erigeron canadense, which the winds may have brought into the 

 region from other parts of France, where it had previously established 

 itself It must, however, be recollected, that whatever be the means 

 by which a plant is first introduced into a region, it is, according to 

 the definition of the term, by the action of physical causes alone 

 that it can be naturalized — that is, maintained, propagated, and dis- 

 persed without human protection or aid. 



All means of introduction, independently of the above-mentioned 

 regular physical causes, are, unless some great change takes place in 

 the physical conformation, climate, or other condition of the country, 

 reduced to the direct or indirect influence of human agency. 



