SPECULATIVE THEORIES. % 
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 
eircumstances 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 
