I don't wish to engage in a long, hair-splitting debate with you, Gerry, but could you perhaps be more precise with your terminology?  I believe that monogenesis and polygenesis refer to the origin of language in general.  Monogenesis is the concept that ALL languages (okay, ALL human languages, for sake of argument) are ultimately derived from a single root source.  Polygenesis, on the other hand, is the idea that didferent language families are derived from different root sources:  for example, under polygenesis, Nilo-Saharan would have a different ultimate ancestor than Sino-Tibetan, which would have a different ultimate ancestor than, say, Indo-European.  I may be off-base with this observation, but to me, language polygenesis almost seems like a reflection of racism in linguistics.  These next TWO sentences are NOT an expression of my personal beliefs but an observation to try to clarify my previous comment:  Negroid people and asian people look different than caucasian people, so they must have different ancestors.  Therefore, if the people are descended from different ancestors, then their languages can't possibly have the same common ancestor as the languages of caucasian people. Personally, I think that that is pure hogwash, but that's my opinion of polygenesis.  Based on what I've read of the "Out-of-Africa" theory, I think it covers the anthropological and linguistic bases pretty well.  An exodus of anatomically modern people from Africa into the Middle East about 100K years ago, and from there, dispersing to the rest of the world, taking their language(s) with them.  The languages would evolve grammar, syyntax and lexicon over time, and, as they were further and further isolated from the original point of dispersal, they would resemble the languages of other people going in different directions. 
An individual language (or dialect-group, to head off your quibble), by definition, is monogenetic.  Individual languages CANNOT be polygenetic.  With the possible exception of som pidgins, each language is descended from a single preceding language.  For example, Modern English is descended from MIddle English, which is descended from Old English, which is descended from Northwest Germanic, which is descended from Proto-Germanic, which is descended from Proto-Indo-European, which is hypothetically descended, possibly through one or more intermediate stages, from Proto-Nostratic.  At no point does another language family intrude into the fundamental evolution of English.  Even the massive French and Latin influence that came in the Middle English period cannot change the fact that English is essentially a Germanic language.  Even if the French/Latin influx had  fundamentally altered English, it still would not be an argument for polygenesis of English, since Latin, and French by derivation, are still Indo-European languages.
To summarize:  one can argue whether human spoken language has a monogenetic origin or a polygenetic origin.  One cannot argue whether individual languages are polygenetic in origin.  I personally believe in monogenesis.
Andy Howey
-----Original Message-----
From: nostradafemme [mailto:waluk@...]
Sent: Thursday, July 22, 2004 08:13
To: Nostratica@yahoogroups.com
Subject: [Nostratica] Monogenetic vs Polygenetic Language

A monogenetic language originates from a single source as opposed to
polygenetic language which originates from many different sources.
Identifying a protolanguage is actually constructing a manmade
monogenetic structure.