EFFECTS OF SEXUAL REPRODUCTION. 249 



repair, and the cells enter upon a quiescent period equivalent to 

 disuse, during which the functional condition of the material of the 

 growing cells deteriorates. For the brain of man we may say that 

 this disuse continues for about one year, after which come those 

 glimmerings of intelligence that indicate the gradual revival of use. 

 Use increases as the child grows, and the amount of use necessary 

 to regain the functional capacity existing in the cell and the parent 

 before conception depends upon how great was the original func- 

 tional condition and how firmly fixed was the habit of repair. 

 Ordinarily it would be the amount normally occurring in an indi- 

 vidual prior to the period of complete maturity, but might be more 

 or less according to the degree of activity. 



REPRODUCTION AS TEMPORARY DISUSE. 



We may therefore say that in the chain of life reproduction is 

 temporary disuse existing for a period of a year and recurring at 

 irregular intervals, the length of which intervals is the time elapsing 

 between generations. Following each period of disuse there is a 

 period of slight use extending through several years of the child's 

 life. Under such conditions it will be evident that the extent to 

 which an organ like the brain may be developed will depend upon 

 the ratio of the length of the periods of use to the length of the 

 periods of disuse, and will be greatest when the variable period of 

 use is greatest. The circumstances are very similar to what would 

 exist if a man should exercise his arms for a few months and then 

 carry them done up in a plaster mold for thirty days, and repeat 

 these operations alternately in unending series. If the period in 

 the plaster mold were always thirty days, evidently the extent to 

 which he might develop his arms by exercise would be determined 

 by the distance between the plaster periods. When this distance 



