This is a great book; it offers an outstanding intro to cladistics, or phylogenetic systematics. I knew this was a great book before I even got it. At the 2003 annual meeting of the Society of Vertebrate Paleontology, some prominent paleontologists pointed out that this book may be downloaded legally for free (!) at the following address:
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Why is it free? Because the folks at KU know how intellectually valuable this document is, not to mention how difficult it may be to come by.
This isn't a permanent offer; the link will only be valid until the Second Edition is published. So if you'd like to check it out, act now!