56 NOTICES OF BOOKS, 



-I 



Chapter 3 is devoted to the geographical distribution of the sums 

 of influencing temperatures. — ^Under this M. de CahdoUe states that 

 "The temperatures that influence each species, and that determine 

 each of its functions, are those above a certain degree, which degree 

 varies with every species and with each of its functions. * Tlie means 

 of these influencing temperatures, accumulated for days or for months, 

 year after year, differ in every locality, and are the elements of the 

 climate of each, so far as the phenomena of their vegetation is con- 

 cerned." 



Book II. Geographical Botany, or the study of species, genera, and 



families, under a geographical point of view. 



Chapter 4. On the limitation of species on plains and upon moun- 

 tains. — After some preliminary observations, chiefly occupied with de- 

 tailing some of the obstacles to the diffusion of plants, all of which M- 

 de CandoUe believes to be overcome by them if the climate is suited to 

 their dispersion, he proceeds to divide his subjects into 



§ 1, Limits of native species growing at the level of the sea; as ab- 

 solutely determined in their advance towards the poles or towards the 

 equator ; to the eastward, and to the westward ; and afterwards their 

 relative limits are discussed. The distribution of many annual, perennial- 

 rooted, and perennial-stemmed, etc., plants, is here given in detail, and 

 laws deduced from them ; or at least the conditions of climate apparently 

 necessary to their healthy existence, are accurately tabulated. Ex- 

 cluding a host of minor and disturbing causes, the limitation of species 

 towards the poles is summed up as indirectly due to the diminished 

 temperature, and more directly to the excessive cold of winter, to the 

 sudden check given in spring by accessions of cold to the young buds 

 or flowers, or to the absence of heat at certain times impeding parti- 

 cular functions. With regard to the equatorial limits of the same or 

 other species, they are generally determined by either heat or dryness, 

 the first of which causes is more or less directly appreciable, the second 

 is very complicated, and involves the action of Rain, Dew, Evapora- 

 tion, etc, 



§ 2 is devoted to the limitation of native species in ascending above 



the level of the sea. This is at once the most popular and perhaps 

 the best worked of all the branches of Botanical Geography or Geo- 

 graphical Botany; but yet M. de Candolle truly observes that the 

 subject is still involved and obscure, general laws are sought in vain 



