264 PROBLEMS OF HUMAN EVOLUTION 



species has become to a large extent permeated, then it loses its 

 power of resistance. As each new generation is born there is 

 less and less chance of reversion to the old type. At last the 

 absence of defect becomes exceptional, and what was abnormal 

 becomes normal. 



We must view this process in connection with the fact that 

 the loss in an individual is often total ; the sense of hearing may 

 be through congenital defect entirely wanting. A power which 

 took thousands of years to bring to perfection is gone at "one 

 blow. I have written above on the large variations by which 

 Natural Selection seems in some cases to work. But size is a 

 matter of degree and no one imagines that such an elaborate 

 organ as the stag's antlers could be developed in their complete- 

 ness suddenly without intermediate steps. And so with the 

 sense of hearing. But failure of heredity may suddenly and at 

 once cause the disappearance of all this accumulated perfection. 

 In earlier chapters I have contended that heredity is always 

 tending to fail, and is only maintained by the most stringent 

 Natural Selection (chap. ii. and chap. iv. sec. iv.). 



The variations, then, which pammixis finds ready to hand are 

 larger than those which Natural Selection has for constructive 

 purposes. And besides this, stability, when it has received a 

 number of shocks, is lost rapidly. These two facts must be 

 viewed in connection. Pammixis compared with Natural 

 Selection may be a clumsy manipulator, but it has more material 

 at its disposal. Total or great defect in individuals followed by 

 loss of stability in the group or species may clearly produce great 

 results. It is easier to pull down than to build. Pammixis is 

 able to sweep away all that has become superfluous and so 

 produce adaptations to new conditions. 



V 



NERVE STRAIN 



There is a great deal of talk about nerve strain as the dis- 

 tinguishing feature of modern life. Without, for the moment, 



