GEOGRAPHICAL DISTIUBUTION. ^Qr^ 



sucli as are consistent "with their economy. For, al- 

 though their poAvers 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 hkely to remain 

 undetermmed. We 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 wich the elevation of the ranges 

 of mountains the atmospheric temperature is reduced, 

 the character of the vegetation changes, and the geo- 

 logical structm-e 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. 13 



