NOTICES OF BOOKS. 59 



adopted for expressing the facts examined into, and some of the causes 

 that appear to lead to isolation or aggregation are detailed, 



The causes of aggregation are divided into those that depend on the 



constitution of the species and those that arise out of the conditions 

 of the station or locality. As a question quite apart from this, the 



subject of the general diffusion or rarity of a species over a great 

 extent of country, and over the whole area inhabited by the species, 

 is discussed ; and tables are given, derived from the Flora of France, 

 from which it a]i pears that there is a larger percentage of very common 

 Dicotyledons than of Monocotyledons, of biennials than of annuals, of 

 annuals than of perennial-rooted plants, and of the latter than of bushes 

 and trees ; and that of the principal Natural Families the species of 

 ChenopodiacetB are the most widely distributed, the Orobanchece least 

 so ; that of LabiatfBy Polygone^y Juncets^ and Amentacece, upwards of 

 30 per cent, are very common plants, whilst of Orchidece, LiUacecs^ and 

 CampanulacecB ^ less than 10 per cent, in each are very common; of 



Orcliidem only 2'7 per cent. 

 The effect of a series of years in changing the relative abundance of 



w 4- 



species next occupies M. de Candolle's attention ; and under this head 

 the important subject of the replacement of species is discussed, and a 

 number of very curious facts on the alternation of species detailed. 



Chapter 7. On the area occupied by species. — The difficulty of de- 

 termining the amount of species occupying a considerable area is very 

 great, and three methods of doing so are proposed : the first, suggested 

 by Brown, is taking the species common to two countries the furthest 

 removed from one another, as Australia and Europe, and assuming that 

 they are common to all or most intermediate countries ; the second 

 consists in taking local Floras or Monographs, and finding the number 

 of species limited to the area of which they treat, and of those that 

 are found elsewhere ; the third consists in dividing the surface of the 

 globe into a certain number of regions as precisely defined as possible, 

 and arranging the species into those found in one, two, or more of 

 these regions. 



♦ M. de Caudolle proceeds to divide the globe into fifty such regions, of neces- 

 sarily very unequal geographical dimensions, but nnfortunat^ly of not sufficiently 

 equal value a. botanical regions either ; this however is the ^ost difficult part of the 

 work, and we apprehend that the necessary data for he subdividing thn globe mo 

 provinces characterized by approximately equivalent differences of vegetation hardly 

 eiist.— Ed. J. B. 



