8 ON THE GEOGRAPHY OF ANIMALS. 
infiuence on Jocal distribution, but they have nothing to 
do with the geographic distribution of animals indi- 
genous to large continents: nor will they even explain 
the local distribution of some of the commonest birds. 
It may be said, indeed, that the absence of the night- 
ingale in Northumberland and Scotland is to be at- 
tributed to the greater coldness of those parts, compared 
with the milder air of southern England. But how 
are we to account for this bird being common in the 
more northern kingdoms of Sweden and Germany ? 
Climate, in this case, can have no influence ; neither can 
food, since insects and their larve, of the same species, 
are found in all these localities: the thickets of Scot- 
land are as favourable for breeding in as those of 
Sweden ; and in regard to foes, no reason can possibly be 
devised. In what way, also, can the circular range of the 
nightingale be made out? Again, we will allow that 
these causes are sufficient to account for the fire-crested 
warbler (Sylvia ignicapilla) being found in the Parisian 
gardens, while it is a stranger to England. ‘The dif- 
ference of temperature, we will say, is the reason: 
England is colder than France. But how are we to 
account for two species of these gold-crested warblers 
being common in North America, in precisely the same 
latitudes, yet totally distinct from those of Europe? It 
is by such questions, of which every class of animals 
will furnish innumerable examples, that closet theories 
must be tried: at the same time, they will at once point 
out the very distinct nature of local dispersion from that 
of geographic distribution, properly so called. 
(10.) Another theory supposes that the same spe- 
cies of animal or plant has been originally placed in 
many different regions: in other words, to have been 
at the period of their first existence locally diffused in 
countries widely distant. Hence it is, that nearly every 
country in the old world has a particular breed or race 
of the horse, ox, sheep, and dog ; all of which, in com- 
mon language, are termed the original breed of the par- 
ticular countries wherein they are found. Little need 
