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http://www.nyfolklore.org/pubs/voic40-1 … trees.html
The link provides a long extract from Van Valen's article. If you want to read the entire article, it costs $3. I can't print it here because it's under copyright protection. David S. Cohen (Yes, that one!) wrote a critical review of this article which can be accessed here: www.academia.edu . (You will have to sign in to google or facebook to find it and read it.) I am currently writing a critique of both the article and Cohen's review. That will appear in the Book Review section of the Forum, as soon as I finish it.