NOTICES or BOOKS. 249 



^ original ideas as led Lyell first to conceive and then to prove that 

 species may be older than the lands which they now inhabit, and that 

 led Edward Forbes to seek in the distribution of the fossil remains of 

 existing British species a key to their present diffusion. 



It remains to say a few words upon the general subject of Botanical 

 Geography. It is no fault of M. de Candolle's work that we lay it 

 down more impressed than ever with the vagueness of its principles, 

 the inexactness of its methods, the puzzling complexity of its phenomena, 

 and the purely speculative character of those hypotheses upon which all 

 mquirers base their efforts to explain its facts and develope its laws. 



Much stress is laid upon the value of Meteorological observations, 

 but there is no method of tabulating these that offers a prospect of their 

 being applied to the solution of any one general question in the distri- 

 bution of species. Certain plants will not survive temperatures above or 

 below a given number of degrees; or in other words, certain suras of 

 temperatures are necessary to the fulfilment of their functions : this all 

 the world knows ; but the tabulation of these temperatures has hitherto 

 led to no general laws, for not every family of plants, nor every genus, nor 

 even every species, but often every variety or race, must have its own sum 

 of degrees to ensure its continued existence. Nor is this all : the sum 

 of degrees must extend annually over a certain definite period of the 

 year, and must be accompanied with so many favourable conditions 

 of soil, light, moisture, and purity of air, that the mere question of 

 temperature becomes a very subordinate element, however accurately 

 ascertained. So far, then, as Meteorological observations are concerned, 

 ive must consider that, however accurate they be, they have hitherto 

 admitted of no exact practical application with reference to the distri- 

 bution of species, nor have they even indicated a theoretical approxi- 



mation to it. 



Next with regard to the limitation of species, genera, and families, 

 within certain areas ; this again is subject to no appreciable laws ; 

 plants are no doubt governed in their diffusion by conditions of climate 

 and soil, and are dependent for their diffusion on their own powers of 

 endurance, on the time that has elapsed since they first existed as 

 species, on the elements, on the motions of animals, and on geological 

 changes ; but we not only know nothing in any <^ise of the t.me 

 elapsed, and next to nothing of the geological changes they may have 

 survived, but all our attempts have failed to regulate thr^r distnbuUon 



VOL. VIII, 



