the Vegetation of a Country. 4)47 



situated, including, for example, an extent of 1000 square leagues, 

 we shall have perhaps to add 200 species to the Flora, but not more 

 than 2 or 3 genera, and hardly a family; this would give 1*2 species^ 

 0*4 genera, and 0*1 family to the square league of the province, 

 and 2*9 species for each genus, and 11 "8 to each family. If the 

 surface be still further extended, and we imagine for example a 

 vast country including this province and several others, making 

 in all 20,000 square leagues, the Flora will perhaps possess 

 2000 species, 500 genera, and 103 or 104 families. The pro- 

 portions will be, O'l species, 0*02 genera, and 0*005 families to 

 each square league, and 4 species to each genus, 19 to each 

 family. Thus, the more extensive we suppose the surface of a 

 country to be, the greater (the vegetation remaining homo- 

 geneous in other respects) will be the diminution in the number 

 of species, genera and families to the square league, and this will 

 take place more rapidly in proportion in groups of higher rank ; 

 the larger the country also, the more will the number of species 

 in each genus and family be increased. 



We might confer upon these arithmetical relations the form of 

 more precise general laws, by employing the average values of 

 the areas of the species, genera and families, such as our re- 

 searches have shown them to be, but this would be of little 

 use, because the diiferent countries and different groups of pha- 

 nerogamous plants always depart more or less from the average 

 values founded upon the totality. A mathematician would 

 perhaps see with pleasure the changes which the areas combined 

 with the surfaces introduce into the relations, but naturalists 

 prefer a demonstration founded upon particular cases. 



With this object I shall compare these Floras, included the 

 one within the other, and of which I have already made use ; 

 that of the department of Maine-et-Loire (the ancient Anjou) by 

 M. Guepin (ed. 3. 1845), that of the centre of France, including 

 this department with several others, by M. Boreau, and that of 

 the whole of France according to the ' Botanicon Gallicum ' of 

 M. Duby. 



After deducting the cultivated species from each work, and 

 reducing the families to a uniformity with those of the ' Botani- 

 con,' I obtain the following numbers : — 



Surface* in 



leagues. Species. Genera. Families. 



Maine-et-Loire . . . 365 1304 473 88 



Centre of France . . 2600 1530 535 90 



France 27,000 3615 739 103 



* The surface of Maine-et-Loire, and that of the departments included 

 in M. Bureau's Flora, are given by the authors themselves. The surface 

 of France in leagues is derived from the new * Geogi-aphical Dictionary ' 

 of M. Langlois. 



