526 NATURE OF REPRODUCTION. 



retain nevertheless their original constitution, and continue to be 

 capable of exhibiting the vital phenomena. 



The above changes, however, are not in reality the only ones 

 which take place. For although the structure of the body and the 

 composition of its constituent parts appear to be maintained in an 

 unaltered condition, by the nutritive process, from one moment to 

 another, or from day to day, yet a comparative examination of 

 them at greater intervals of time will show that this is not pre- 

 cisely the case ; but that the changes of nutrition are, in point of 

 fact, progressive as well as momentary. The composition and pro- 

 perties of the skeleton, for example, are not the same at the age of 

 twenty-five that they were at fifteen. At the later period it con- 

 tains more calcareous and less organic matter than before ; and its 

 solidity is accordingly increased, while its elasticity is diminished. 

 Even the anatomy of the bones alters in an equally gradual manner ; 

 the medullary cavities enlarging with the progress of growth, and 

 the cancellated tissue becoming more open and spongy in texture. 

 We have already noticed the difference in the quantity of oxygen 

 and carbonic acid inspired and exhaled at different ages. The 

 muscles, also, if examined after the lapse of some years, are found 

 to be less irritable than formerly, owing to a slow, but steady and 

 permanent deviation in their intimate constitution. 



The vital properties of the organs, therefore, change with their 

 varying structure ; and a time comes at last when they are per- 

 ceptibly less capable of performing their original functions than 

 before. This alteration, being dependent on the varying activity 

 of the nutritive process, continues necessarily to increase. The very 

 exercise of the vital powers is inseparably connected with the sub- 

 sequent alteration of the organs employed in them ; and the func- 

 tions of life, therefore, instead of remaining indefinitely the same, 

 pass through a series of successive changes, which finally terminate 

 in their complete cessation. 



The history of a living animal or plant is, therefore, a history of 

 successive epochs or phases of existence, in each of which the struc- 

 ture and functions of the body differ more or less from those in 

 every other. Every living being has a definite term of life, through 

 which it passes by the operation of an invariable law, and which, 

 at some regularly appointed time, comes to an end. The plant 

 germinates, grows, blossoms, bears fruit, withers, and decays. The 

 animal is born, nourished, and brought to maturity, after which he 

 retrogrades and dies. The very commencement of existence, by 



