Introduction 3 



may permit satisfactory conclusions; but when greater 

 numbers and more complex forms are included, con- 

 clusions become not only difficult but sometimes impos- 

 sible. It is necessary, however, to organize facts, 

 as they are obtained, into some consistent theory that 

 will relate them to one another and that may be the 

 basis of further investigation. Progress in any complex 

 subject is marked by a series of explanations, each in 

 turn proving inadequate as facts multiply, but each in 

 turn helping to further progress. It should be under- 

 stood, therefore, that in the subsequent chapters the 

 theories presented are not to be regarded as final, but 

 rather as suggested explanations of the known facts. 

 It is certain that new facts will continue to be discovered 

 and that explanations will have to be modified to fit 

 them ; but the present explanations are necessary for co- 

 ordinating the facts we have. In other words, a sharp 

 distinction must be drawn between established facts 

 and proposed explanations. The former are permanent, 

 the latter are temporary. 



