SECTION III. 

 REPRODUCTION. 



CHAPTER I. 



ON THE NATURE OF REPRODUCTION, AND THE 

 ORIGIN OF PLANTS AND ANIMALS. 



THE process of reproduction is the most characteristic, and in 

 many respects the most interesting, of all the phenomena presented 

 by organized bodies. It includes the whole history of the changes 

 taking place in the organs and functions of the individual at suc- 

 cessive periods of life, as well as the production, growth, and de- 

 velopment of the new germs which make their appearance by 

 generation. 



For all organized bodies pass through certain well-defined epochs 

 or phases of development, by which their structure and functions 

 undergo successive alterations. We have already seen that the 

 living animal or plant is distinguished from inanimate substances 

 by the incessant changes of nutrition and growth which take place 

 in its tissues. The muscles and the mucous membranes, the osse- 

 ous and cartilaginous tissues, the secreting and circulatory organs, 

 all incessantly absorb oxygen and nutritious material from with- 

 out, and assimilate their molecules ; while new substances, produced 

 by a retrogressive alteration and decomposition, are at the same 

 time excreted and discharged. These nutritive changes correspond 

 in rapidity with the activity of the other vital phenomena ; since 

 the production of these phenomena, and the very existence of the 

 vital functions, depend upon the regular and normal continuance 

 of the nutritive process. Thus the organs and tissues, which are 

 always the seat of this double change of renovation and decay, 



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