IT is proper, I think, to say to the reader that what is here printed is, with slight corrections and enlargements, identical with an address which I prepared ten years or more ago. The address was delivered before several Illinois audiences- among others, before the faculty and students of the University of Illinois at Urbana. The language used is, therefore, less impersonal than would be proper in a monograph written for publication. I have, however, thought best not to change the form of its presentation, but let it go into print substantially as delivered.