FINAL CAUSES. 15 



developement of the organs, he observes them un- 

 dergoing various modifications, as they are assum- 

 ing new forms, which characterise certain definite 

 epochs in the general growth of the system. In a 

 great number of instances, especially among the 

 lower orders of animals, he witnesses the same indi- 

 vidual being acting, in its time, a variety of dif- 

 ferent parts ; often re-appearing on the stage of life 

 with new organs, new faculties, and new conditions 

 of existence, and undergoing metamorphoses as 

 complete as any that have been depicted in the 

 fables of antiquity. 



The period at length arrives when the animal, 

 having completed its growth, attains the maturity 

 of its being, and acquires the full possession of its 

 powers. Every organ in succession has received 

 its entire developement, and has united its energies 

 with those which had been before perfected. Yet, 

 however complete the arrangements that have thus 

 been established, it is still necessary, in order to 

 preserve the whole system in a state in which it 

 may be capable of exercising the functions of life, 

 that the materials which compose its fabric should 

 undergo a certain slow, but constant renovation ; 

 and the same circle of actions and reactions, which 

 have brought it to its state of perfection, must con- 

 tinue to be repeated, in order that a due proportion 

 may be maintained between the supply and the 

 consumption of these materials. In the course of 

 a certain time, however, even under the most fa- 

 vourable circumstances, this equilibrium begins to 

 fail ; the energies of the system decline, and the 

 processes of nutrition are insufficient to repair the 

 waste in the substance of the body. The fluids are 



