USE OF BIRDS IN NATURE. 161 



must move their wings so incessantly that they are 

 soon worn out, and fall to the earth. 



There is here a very beautiful chain of adaptations, 

 which is worthy of study in itself, besides being 

 intimately connected with the general economy and 

 structure of birds. All these natural trimmers of the 

 exuberance and removers of the waste of growing 

 nature are wanted, up to the full amount of their 

 powers. But they are so wanted only for a season ; 

 and though that season varies in length in different 

 latitudes and climates, there is not a spot on earth 

 where it could be perennial, or even of one whole 

 year's unbroken duration, unless the laws of the whole 

 system, that is, the qualities of the several parts of 

 which it is made up, were totally changed. The 

 vegetables could not bring all their "brairds" and 

 buds to maturity, nor would the earth supply sowing 

 ground for all their seeds ; and the creatures, of what- 

 ever kind, which keep down the superabundance of 

 these, would, in like manner, speedily overstock the 

 room that there is for them. But still they must 

 all have that elasticity by means of which they can 

 instantly adapt themselves to the changes of the 

 system. The earth consists of a definite quantity 

 of matter, occupying a definite space ; and to that 

 quantity and that space all the productions of the 

 earth must be capable of accommodating themselves, 

 otherwise the system would be imperfect. 



The earth itself is, perhaps, at once the best index 

 to the system of the earth's productions, and the best 

 illustration of the mode in which that system works. 

 It careers round the sun, altering its distance from 

 that luminary, and the rate of its motion, every mo- 

 ment, and differently affected as its own attendant 

 moon, or any other body in the solar system, is difr 



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