278 LAWS OF ANIMAL DISPERSION. 
distance from the seat of luxuriant growth and 
great numerical increase, specimens having all the 
appearance of such as are found at the very verge 
of the geographical range of a species are constantly 
detected within short distances of, or absolutely with- 
in the metropolis itself. There are unquestionably two 
sets of causes in operative influence on animals in 
regard of distribution, and it is of great moment to 
refer the phenomena connected therewith to the 
right sources. The primary causes are quite un- 
known to us and are likely to continue so, though 
it would appear that the various parts of the organ- 
ized creation being ordained to counterbalance 
each other, that, as the laws of dependence pervade 
the world of living beings in all its parts, any 
determination or regulation such as the one men- 
tioned,—the diminution of numbers and deter- 
ioration in size and qualities of individuals— 
providedit were general and observed in all classes 
and species, need not excite surprise. 
The secondary causes appear to be temperature, 
food, situation, and the hostility of other species. 
The influence of these appears to be very consider- 
able, and though we cannot be altogether warranted 
in attributing the above-named circumstance of 
diminution of number and deterioration in size, &e. 
to these causes, however plausible it might seem 
to do so, they are undoubtedly the agents that 
cause deteriorations generally. These secondary 
causes become indeed of the greatest moment in 
investigating the zoology of a given district. Pri- 
mary laws can be seen and estimated only by 
reference to zoology as a whole, by taking into our 
view the phenomena exhibited by the entire series 
of animals ; secondary causes must be appreciated 
by examining the phenomena of animal geography 
on smaller scales. It is then we see temperature, 
food, situation, and other circumstances, operating 
