INHERITANCE, VARIATION AND SELECTION. 47 



ment, and the very rapid development of functional power in some 

 one organ can only be at the expense of the proper development 

 of some other organ or quality. 



Growth is distinguished from repair in that it involves increase 

 in size without the incorporation of force or power in the growing 

 organ. On the other hand, repair is a process of rebuilding a wasted 

 organ so as to incorporate in it a functional power that it did not 

 have hefore. Growth and repair sometimes accompany each other 

 and sometimes do not. Thus, the muscles of a child both grow and 

 are repaired; the same muscles in an adult, when used uniformly 

 for a long time, are repaired but do not grow ; the hair grows, but 

 is not repaired. In organs which both grow and are repaired the 

 ratio of growth and repair to each other is continually changing. 

 In the embryonic stage there is growth with but little or no repair ; 

 in youth the two are nearly equal ; and in the adult we have repair 

 with but little or no growth. 



