FINAL CAUSES. 10 



structed, and at length cease to be pervious : 

 and the solids gradually become hard and rigid. 

 As in a machine of which the wheels are worn, 

 and the springs have lost their elastic force, so 

 in the animal body, at an advanced age, the 

 slightest additional impediment that occurs will 

 stop the movements of the whole system : and, 

 when once stopped, their renewal is impossible. 

 Nature has thus assigned to every living being a 

 certain period as the utmost extent of its dura- 

 tion. Even when exempt from external inter- 

 ference, all are doomed to perish, sooner or 

 later, by the slow but unerring operation of the 

 same internal causes which originally effected 

 their developement and growth, and which are 

 inseparably interwoven with the conditions of 

 their existence. 



Numerous, however, are the extraneous and 

 accidental causes that may hasten or precipitate 

 their destruction, long before the period of natural 

 decay. How striking is the contrast, on those 

 occasions, between the scene we have just beheld 

 of an animal in the full vigour of its powers, 

 either rapidly bounding across the plain, or glid- 

 ing beneath the wave, or soaring in the elevated 

 regions of air, and the spectacle of the same 

 animal lying, the next moment, extended at our 

 feet, bereft at once of activity and of sense 

 of all the faculties and powers that constitute 

 life. Can we contemplate without amazement 



