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CHAPTER III. 



DECLINE OF THE SYSTEM. 



To follow minutely the various steps by which Nature 

 conducts the individual to its state of maturity, would en- 

 gage us in details incompatible with the limits of the pre- 

 sent work. I shall only remark, in general, that during the 

 period when the body is intended to increase in size, the 

 powers of assimilation are exerted to prepare a greater abun- 

 dance of nourishment, so that the average supply of mate- 

 rials rather exceeds the consumption: but when tlie fabric 

 has attained its prescribed dimensions, the total quantities 

 furnished and expended being nearly balanced, the vital 

 powers are no longer exerted in extending the fabric, but 

 are employed in consolidating and perfecting it, and in qua- 

 lifying the organs for the continued exercise of their re- 

 spective functions, during a long succession of years. 



Yet, while every function is thus maintained in a state of 

 healthy equilibrium, certain changes are in progress, which, 

 at the appointed season, will inevitably bring on the decline 

 and ultimate destruction of the system.* The process of 



* It would appear, from the researches of De CandoUe, that the vegetab?e 

 system is not, like the animal, subject to the destructive operation of internal 

 causes; for the agents which destroy vegetable life are always extitmeous to 

 its economy. Each individual tree is composed of an accumulation of the 

 shoots of every successive year since the commencement of its growth; and 

 although, from the continued deposition of lignin, and the consequent obli- 

 teration of many of its cells and vessels, the vitaUty of the interior wood may 

 be destroyed, and it then becomes liable to decay by the action of foreign 

 agents, yet the exterior layers of the liber still vegetate with umiiininishcd 

 vigour; and unless injured by causes extraneous to its own system, the life of 

 the tree will continue to be sustained for an indefinite period. If, on the 



Vol. II. 55 



