This survey may be opened with some apologetic remarks. The first is that to write a complete survey would mean to write a book. This has not been the intention of the editor, nor does it lie within the power of the present writer. Therefore, this survey is, in principle, incomplete, as will be easily seen by the many important authors whose work is not here discussed. The second remark is that a survey like this is, of necessity, one-sided and restricted. It is one-sided by reason of the special view of the writer on what is interesting and what is not; as, for example,this survey shows special interest in quantitative problems. It is restricted as a consequence of the fact that knowledge of nearly every individual about subjects more removed is less than about subjects in his direct neighborhood-both in a concrete and an abstract sense. The third remark relates to the subdivision of the material; it is difficult to represent, in a one-dimensional exposition, as the current text of a paper, a thing of multi-dimensional character.