269 



have imposed limits upon each ; and to very few 

 only has she given the power to live indifferently 

 in opposite circumstances, and under the varied 

 influences of different climates. We can easily 

 understand why quadrupeds which cannot tra- 

 verse the sea should be confined to particular 

 lands ; and we can conceive, too, that birds, 

 great and wonderful as are their migrations, 

 must from physical causes have a limit to their 

 flight ; but that fishes, inhabiting as they do, a 

 medium, the temperature of which is more uni- 

 form than that of the air, and with no obstacles 

 to prevent their progress, or to fix boundaries 

 to their journey ings, still confine themselves in 

 marked zones, beyond which they never pass, 

 except from rare and accidental causes, can be 

 accounted for only on the principle of a law of 

 nature. Some species, accordingly, live within 

 the tropics, under almost every meridian, but do 

 not pass these limits or at most to a very little 

 distance ; others seem to be proper to the tem- 

 perate or the frigid zones of the northern hemis- 

 phere, whilst others again, belong exclusively to 

 the south. 



