GEOGRAPHICAL DISTRIBUTION. 105 
such as are consistent with their economy. For, al- 
though their powers of progression are of a very low 
order, they are not by their instincts restricted to a 
particular local habitation, nor have they any regular 
places of breeding or of shelter, but on the contrary, 
seek such as may happen to suit their purposes, and be 
near when needed. Hence, although no individual 
animal can be supposed to have made any considera- 
ble progress, yet, as every one has receded further and 
further from the point of departure, it may be con- 
ceived that, in the course of the countless generations 
which have existed, they may have extended to vast 
distances from the original focus of the species, if indeed 
there was but one focus, a fact which is likely to remain 
undetermined. "VVe might expect then, a very wide and 
almost unlimited distribution of these animals, if there 
were no other counteracting circumstances to restrict it ; 
and, if such did not exist, our expectations would proba- 
bly be realized. But, as with the elevation of the ranges 
of mountains the atmospheric temperature is reduced, 
the character of the vegetation changes, and the geo- 
logical structure almost always assumes a new form, the 
conditions of life on high levels become very different 
from those existing below, and the influences resolve 
themselves mainly into those of climate. These operate 
in a very different manner from the obstacles we have 
before spoken of, and affect directly the means of 
procuring food, the power of reproduction, and the 
ability of sustaining life itself; and hence they are of 
vol. i. 28 
