SECTION IV. 



REPKODUCTION. 



CHAPTER I. 



THE NATURE OF REPRODUCTION AND THE ORIGIN 

 OF PLANTS AND ANIMALS. 



T) EPRODUCTION is the process by which the different kinds of 

 JA; organized beings are perpetuated, notwithstanding the limited 

 existence of each individual. It includes the production, growth, and 

 development of new germs, as well as the history of successive changes 

 in the organs and functions, and the modifications of external form at 

 different periods of life. 



All organized bodies pass through various stages of development, 

 in which their structure and functions undergo corresponding altera- 

 tions. The changes of nutrition and growth, by which the animal 

 or plant is distinguished, correspond in activity with its other vital 

 phenomena ; since these phenomena depend on the continuance of the 

 nutritive process. Thus the organs and 'tissues are the seat of a double 

 change of renovation and decay, but retain nevertheless their original 

 constitution, and continue to exhibit their vital phenomena. 



This change, however, is not the only one which takes place. Al- 

 though the bodily structure appears to be maintained by the nutritive 

 process from one moment to another, or from day to day, yet examina- 

 tion at longer intervals will show that this is not precisely the case ; 

 since the changes of nutrition are progressive as well as momentary. 

 The composition and structure of the bones are not the same at the 

 age of twenty-five years that they were at fifteen. At the later period 

 they contain more calcareous and less organic matter than before ; their 

 solidity being consequently increased, while their elasticity is dimin- 

 ished. There is a notable difference in the quantities of oxygen and 

 carbonic acid inspired and exhaled at different ages ; and the irritabil- 

 ity of the muscles is diminished after some years, owing to a slow, but 

 steady and permanent, deviation in their intimate constitution. 



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

 ing structure ; and a time comes at last when they are perceptibly less 

 capable of action than before. The very exercise of the vital powers 

 is inseparably connected with the alteration of the organs to which 



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