PLANARIA ARETHUSA. 113 



easily comprehend why it may be so, than 

 we can hazard an explanation of its ascend- 

 ing to original conformation. 



This species of planaria is not so often 

 seen in a state of repletion as the rest ; 

 nor does it dwell indiscriminately among 

 them. I have on no occasion observed it 

 in stagnant water; whether from being 

 less congenial to it, or because its ordi- 

 nary food belongs to springs, I shall not 

 presume to determine. Certain bounda- 

 ries are assigned by nature to all her crea- 

 tures : the largest quadrupeds and the mi- 

 nutest insect are alike restrained within 

 the respective barriers of place and climate, 

 which may not be safely left behind. The 

 elk cannot dwell with the elephant; the foot 

 of the camel is formed for the parched and 

 sandy desert; while that of the goat is fitted 

 for climbing alpine mountains. The flight 

 of the tropic bird is confined within the li- 

 mits from which it derives its name ; and 

 many inhabitants of the vast Pacific Ocean 

 are banished from the northern seas. The 



