SPECULATIVE THEORIES. 7 



life at last becomes impracticable." * It has been fur- 

 ther added, that the conditions which regulate the geo- 

 graphic distribution of species appear to be limited to 

 circumstances connected with temperature, food, situ- 

 ation, and foes. 



(8.) This hypothesis pretends not to account for the 

 total difference in the genera and species of animals in 

 two countries,, which are yet under the same parallels of 

 latitude, of the same degree of temperature, and fur- 

 nished with the same means of supporting and enjoying 

 life : it leaves this question where it was, and might, 

 therefore, hardly deserve attention in an enquiry directed 

 principally to primary causes. The theory of a cir- 

 cular range being enjoyed by species, may possibly be 

 true in some few instances, although it would be dif- 

 ficult, perhaps, to name them : but, when applied to 

 animals generally, it is not only opposed by facts in- 

 numerable, but is destroyed by the very admission that 

 local circumstances exercise a primary influence on the 

 range of animals. The peregrine falcon is found in 

 America, Europe, and Australia, but it is totally un- 

 known throughout the whole continent of Africa, an 

 immense region thus intervening between two of its 

 habitats. The great bustard of Europe is another fa- 

 miliar example: its distribution is latitudinal; it is 

 found in the centre of England, through the heart of 

 Europe, and to the confines of Asia. Now, according 

 to the idea of animals enjoying a circular range, the first 

 of these birds should be found in Africa, and the latter 

 throughout a circle which would then comprise the 

 whole of northern and southern Europe, and Barbary. 



(9) The opinion that those conditions which re- 

 gulate the geographic distribution of species are limited 

 to circumstances connected with temperature, food, situ- 

 ation, and foes, is totally insufficient to account for the 

 phenomena of animal geography. We know, indeed, that 

 these causes, either singly or collectively, have great 



* Phil, of ZooL vol. i. p. 8. 

 B 4 



