The material here presented represents an elaboration on my Colloquium Lectures delivered before the American Mathematical Society at its September, 1940 meeting at Dartmouth College. The results of some of my own efforts together with a selection of those of other mathematicians relative to the subject chosen are presented in what is intended to be a coherent and approximately self-contained exposition, framed in the familiar topology of separable metric spaces. No attempt is made at comprehensive coverage either of the known work embraced by the title of the book or of the work of others, or even myself, which may be closely related to that included.