1857.] REVIEWS, 185 



eighth, ninth, and tenth volumes of the ' Prodromus/ as offering 

 sufficient illustration of his subject, and finds that, of the 8495 

 species contained in these three volumes, there are only 116 

 which present any very marked differences (four times at least) 

 between the lengths of the opposite diameters of their areas. 

 The remaining 8372 species appear to occupy more or less circu- 

 lar areas, — a remarkable fact. It must be observed however that 

 this examination refers exclusively to certain orders of the Corol- 

 liflora, whose distribution can hardly be taken as fairly represen- 

 tative of that of phanerogamous plants in general. Of the 116 

 species selected by M. de Candolle {sic) 68 extend east and west, 

 and 48 in a narrow line north and south. One should hardly 

 have a priori anticipated so nearly equal a division of the num- 

 ber ; it seems so much more natural that a species should extend 

 along the parallels of latitude, or at least in the isothermal lines, 

 than along those of longitude. We are thus taught that other 

 causes than those of annual temperature powerfully influence the 

 natural dispersion of plants. The most potent, probably, are 

 moisture and exposure to certain winds. Of the 48 species enu- 

 merated as having a north and south distribution, only two are 

 natives of Britain, viz. Pinguicula lusitanica, which extends along 

 the Atlantic shores from Portugal to the north of Scotland, but 

 which is not found anywhere far from a western coast, and Ery~ 

 thrcea latifolia, likewise a coast species, found from Norway to 

 Portugal. 



The distribution of individuals in the areas occupied by the 

 species is next discussed with considerable detail, showing how 

 local causes modify the frequency or non-frequency or the luxu- 

 riant development of each species. The nature of the soil, ex- 

 posure, supply of moisture, and other obvious modifying causes 

 are indicated, and a list is given, after Mohl, of species which 

 are characteristic of primitive rocks, and of those which only 

 occur in calcareous soil. M. De Candolle seems to think also 

 that something like a "^"^ rotation of crops'^ exists naturally among 

 wild plants, and that a species shifts its soil (especially an annual 

 one) from year to year, from causes similar to those that force 

 the farmer to vary his crops when cultivating the same soil. 

 " One cannot doubt," he says, " that the existence of a species, 

 and especially its prolonged existence, becomes a cause unfavour- 

 able to the life of that same species, or of analogous species, in 



N. S. VOL. II. 2 b 



