920 



pared together, with respect to the influence of levels, unless equal in 

 regard to soil. The region of the plains can almost never be legiti- 

 mately assimilated to the superior regions established for the mountains. 



Independently of the three principal agents of distribution indicated 

 above, there are divers conditions that circumscribe the area of species 

 in general or of certain species in particular, as the casual limit, every 

 plant ceasing partially, — the topographical limit, notably the chains 

 of mountains, — the limit imposed by the extreme sociability of certain 

 species, &c. On the contrary, other conditions extend the area of 

 distribution, such as certain facilities of mechanical transport at con- 

 temporary epochs, historical or even geological. These divers con- 

 ditions, singly or in combination, produce certain facts of distribution 

 that, in obeying the requirements of latitude, of levels and of soils, are 

 not however in exclusive dependence upon them. These facts, in a 

 limited country, may be salient, and over-ride in some measure the 

 phyto-statistical generalities attributable to the three chief agents 

 above mentioned. But they are most usually too few to alter mate- 

 rially the aspect of the generalities. 



The flora and the vegetation are then two essentially different 

 things. The flora may be rich and the vegetation scanty, or the re- 

 verse. The number of species of each order is a bad comparative 

 test between two neighbouring countries. The census {role) of each 

 species, regarded in its quantity of distribution, ought to be the chief 

 element of that comparison. One characteristic and widely-spread 

 species modifies more the vegetable landscape than numerous rare 

 species. Groups of species characteristic of a region of altitude may 

 represent or rather characterize sufficiently the relative composition 

 of the vegetable landscape in different districts. These species should 

 be selected among those that contrast by their presence, their absence, 

 or their degree of distribution, between different soils and levels. 



All the facts of distribution presented in our field of study are ex- 

 plicable on the principles thus summed up. They are all essentially 

 dependent on the united influence of latitude, of levels, and of the 

 mechanical qualities of the subjacent rocks. There is no exception 

 in this last respect, unless for certain salts soluble in water, whether 

 of mineral or of animal origin. 



N.B. — The preceding summary is translated from a very elaborate 

 and important work, in two volumes, published at the end of 1849, by 

 Mons. Jules Thurmann, entitled ' Essai de Phytostatique applique a 

 la chaine du Jura et aux contrees voisines ; ou Etude de la dispersion 



