28 FINAL CAUSES. 



following the progressive development of the organs, he ob- 

 serves them undergoing various modifications, as they are 

 assuming new forms, which characterize 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 individual being acting, in 

 its time, a variety of different parts; often reappearing 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 an- 

 tiquity. 



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 development, and has 

 united its energies with those which had been before per- 

 fected. Yet, however complete the arrangements that have 

 thus been established, it is still necessary, in order to pre- 

 serve the whole system in a state in which it may be capa- 

 ble of exercising the functions of life, th^t the materials 

 which compose its fabric should undergo a certain, slow, but 

 constant renovation; and the same circle of actions and re- 

 actions, which have brought it to its state of perfection, must 

 continue to be repeated, in order that a due proportion may 

 be maintained between the consumption and the supply of 

 these materials. In the course of a certain time, however, 

 even under the most favourable circumstances, this equili- 

 brium 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 dissi- 

 pated faster than they can be renewed; the channels through 

 which they circulate are more and more obstructed, 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 bod}', at an advanced age, the slightest additional 

 impediment that occurs will stop the movements of the 



