CHAPTER XVI. 



EFFECTS OF SEXUAL REPRODUCTION. 



When an organ, as the brain, is used it acquires a functional 

 capacity as a result of such use, and the functional capacity acquired 

 is, within limits, proportional to the amount of use. The act of 

 using an organ causes the tissues of the organ to waste away, and 

 the rate of waste is proportional to the intensity of use. The mate- 

 rial wasted away by use is replaced by other material furnished by 

 the system of which the wasted organ is a part. The amount of 

 new material thus supplied is determined by the rate of waste. 

 When the waste is rapid in consequence of intense activity, the new 

 material not only equals, but slightly exceeds the amount wasted, 

 the surplus of new material being for the purpose of meeting similar 

 conditions of unusual activity. When the waste is very slow, the 

 amount of new material may, and generally does, not equal the 

 amount of waste, and the inactive organ diminishes in size because 

 of its inactivity. In this process of waste and repair the new 

 material takes on a functional capacity which is determined by the 

 functional activity of the repaired organ, and the functional 

 capacity of the organ as a whole is determined by the activity which 

 existed at the time when the various parts were built up by original 

 growth or by previous repair. 

 WASTE! AND REPAIR AS THEY PROBABLY EXIST IN THE GERM CELLS. 



A germ cell is the epitome of an individual of the species to 

 which it belongs. Within it are contained all of the elements which 

 characterize the adult, and it is from these elements that the future 

 adult develops. Such cells increase in number by a process of cell 



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