456 Living in a World of Change 



more helpful to apply ourselves to a search for relationships 

 that we can understand in terms of " invariable ante- 

 cedent," or cause-and-effect, in the ordinary common- 

 sense way. 



There is another sense, however, in which we may speak 

 of purpose without the same danger of confusing our vision, 

 and without the need of avoiding the concept of mechanism 

 in nature. We may perhaps catch a glimpse of this by re- 

 calling the estimable old lady who wondered how it came 

 about that nature had placed the large rivers in the neighbor- 

 hood of the large towns. As an act of foresight, this jux- 

 taposition is truly marvelous. If, however, we take into 

 account something of the nature of man — his workings, 

 that is — and of the chronology of events, the fact seems 

 commonplace enough. In the same way we may under- 

 stand other adaptations. We do not need to say that the 

 ocean was made as it is to fit it as a habitation for sponges. 

 It is enough to say that sponges find it possible to live in the 

 ocean, but not in meadows. We do not need to say that 

 the earth was given its texture and composition to fit it for 

 worms. It is enough to note that worms find the soil con- 

 genial, but not the solid rocks, whereas the piddock finds it 

 possible to maintain life in the rocks at the tide level, but 

 not in the much softer earth. Living things turn the world 

 to their needs. They take of its materials and convert these 

 into their own substance. Evolution has meant, among other 

 things, the emergence of life forms that can more compre- 

 hensively make the world serve their purposes. There can 

 be no question of man's purpose, at least; and the evolutionary 

 point of view gives us confidence of growing mastery. 



Adaptation and Health 



On the positive side the concept of adaptation, empha- 

 sized by Darwin as an essential part of his principle of natural 

 selection, has opened up many lines of research that have 

 yielded important facts and general principles. In medicine 



