THE ORGANIC TENDENCY 181 



their expression depends on the opportunity ex- 

 tended by the external environment. 



So it is with the other fluctuating characters. An 

 organism, according to the theory of natural selec- 

 tion, may be adapted to a given mode of life but 

 competition forces it to use other inherent possi- 

 bilities and to change its habits, or to perish. Even 

 the normally inherited characters develop accord- 

 ing to the opportunity given to the normal heritage 

 by the developing body. A normal mental heritage 

 in man finds expression in the internal environment 

 provided partly by a normal thyroid gland, but 

 the thyroid cannot function normally if the ex- 

 ternal environment does not provide enough iodine. 



The astonishing uniformity of a species through 

 many generations is a tribute to the fact that 

 the environment has provided it with adequate 

 opportunity. The living substance has been 

 able gradually to appropriate new things from 

 its environment, to build from them a heritage of 

 ever-increasing complexity, and at every step to 

 make the consummation of its normal life a little 

 more certain and a little more independent of 

 fluctuating external conditions. With each step 

 new possibilities have been realized, as the doc- 

 trine of emergent evolution states, and so still 

 further steps have been made possible. As Child 

 has expressed it: "Actually, however, it becomes 

 more and more evident that life is largely, if not 



