132 Nature's Sifting 



of the larval body or in absorbing the products of 

 their disintegration. Similarly not a few familiar 

 changes, such as those involved in the disappearance 

 of a tadpole's tail, would certainly be classed as 

 pathological degenerative processes if we did not 

 know that they turn out all right in the long run. 



No good service is ever done by making different 

 things seem the same, and we are not hiding the fact 

 that disease-changes are self -contradictory and dis- 

 integrative, whereas health-changes make for greater 

 harmony and unity of life. But our points are two — 

 (1) that what would be a disease-change in one 

 organism may be normalized in another, and (£) 

 that it is not surprising to find among the multitude 

 of bull's eyes that some of the arrows of variation miss 

 the target altogether. We call these misses constitu- 

 tional diseases. 



§15. (D) Selection. 



The fourth great idea in Darwinism is Natural Selec- 

 tion. This means the sifting of the new departures 

 that living creatures show, the winnowing of their ex- 

 periments. The course of Natural Selection has been 

 a long commentary on the maxim: Prove all things 

 and hold fast that which is good. Yet Darwin warned 

 those who followed his argument that Natural Selec- 

 tion was not such an easy idea as it looked. For the 

 sifting takes place in the struggle for existence, and 

 that, as we have seen, is complex and manifold. 



living creatures are insurgent, urged by "a will 

 to live"; they find themselves hemmed in by diffi- 



