  This historical introduction is in two parts. The first is reprinted with
permission from ``A century of mathematics in America, Part II,'' Hist. Math.,
2, Amer. Math. Soc., 1989, pp.543-585. Virtually no change has been made to the
original text. In particular, Section 8 is followed by the original list of
references. However, the text has been supplemented by a series of endnotes,
collected in the new Section 9 and followed by a second list of references. If
a citation is made to the first list, then its reference number is simply
enclosed in brackets -- for example, [36]. However, if a citation is made to
the second list, then its number is followed by an `S' -- for example, [36S].
Further, if a subject in the reprint is elaborated on in an endnote, then the
subject is flagged in the margin by the number of the corresponding endnote,
and the endnote includes in its heading, between parentheses, the page number
or numbers on which the subject appears in the reprint below. Finally, all
cross-references appear as hypertext links in the dvi and pdf copies.
