DE CANDOLLE'S GI^OGRAPHIE BOTANIQUE. 71 



relative positions of land and sea may afford ; that many of 

 these groups have become extinct, whilst others have increased, 

 at least so far as can be judged by comparing existing epochs 

 with preceding ones ; and lastly, that the latest geological 

 epoch, the Quaternary (that which preceded the existence of 

 man in Europe, and which followed the last elevation of the 

 Alps), has lasted many thousand years, during which impor- 

 tant geographical and physical changes have affected Europe 

 and some neighboring countries, whilst other regions of the 

 globe have suffered no change, or have been exposed to a dif- 

 ferent series of changes. 



" Thus the principal facts of geology and palaeontology, re- 

 duced to the most general and incontestable, suffice to explain 

 the facts of Botanical geography, or at least to indicate the 

 nature of the explanation, whieh it requires the progress of 

 many sciences to complete. 



" The most numerous, the most important, and often the 

 most anomalous facts in the existing distribution of plants, 

 are explained by the operation of causes anterior to those now 

 in operation, or by the joint operation of these and of still 

 more ancient causes, sometimes of such as are primitive (con- 

 nected with the earliest condition of the planet). The geo- 

 graphical and physical operations of our own epoch play but 

 a secondary part. I have shown that in starting from an 

 original fact, which it is impossible to understand, of the crea- 

 tion of a certain form, in a certain country, and at a certain 

 time, we ought to be able, and sometimes are able to explain 

 the following facts, chiefly by causes that operated previous to 

 our own epoch : 1, the very unequal areas occupied by natural 

 orders, genera, and species ; 2, the disconnection of the areas 

 that some of the species inhabit ; 3, the distribution of the 

 species of a genus or family in the area occupied by the genus 

 or family ; 4, the differences between the vegetations of coun- 

 tries that have analogous climates and that are not far apart, 

 and the resemblance between the vegetation of the countries 

 that are apart, but between which an interchange of plants is 

 now impossible. 



"The only phenomena explainable by existing circum- 



