174 STRUCTURE OF FISH. 



bably, because they have no excess of heat to enable 

 them to decompose the air, and mix the oxygen with 

 the blood and the superfluous carbon. 



Fishes do not bear their change so easily. A salmon, 

 when caught in the open sea, dies if put into fresh 

 water ; and if one that has been for some months in 

 fresh water, be put into salt, it also dies. It is the 

 same with almost every fish. Hence the breathing ap- 

 paratus of a fish must undergo a change, every time 

 that it passes from the sea to fresh water, or from 

 fresh water to the sea. These changes are not imme- 

 diate ; and therefore the fish linger awhile in the estu- 

 aries, upon every journey, in order that, by the brackish 

 water, and by that alternate play of fresh and salt water 

 which is occasioned by the tides, they may prepare 

 themselves gradually for their new element. 



Though, generally speaking, the sea pasture tends 

 more to promote the growth, vigour, and fatness of the 

 fish, than the river pasture; yet it also demands the 

 stronger organization ; and thus, those fish that enter 

 the rivers for the purpose of spawning, are all of deli- 

 cate descriptions, and the young often linger so long 

 about particular parts of the estuaries, that they are not 

 unfrequently mistaken for distinct species. Still, all 

 this is in strict accordance with principle ; and affords 

 (as, in fact, every thing upon which we can reflect 

 affords) a proof that, though the works of creation be 

 many, the plan and the purpose are one. There is not 

 one power to adapt the fish to the water, and another 

 to adapt the water to the fish : the adaptation is reci- 

 procal, clearly proving that the power is one. The 

 whole is one complete machine, and no part can be 



