Having made the birds of the United States his study for several years, the writer glanced over the bird carvings in the most cursory manner, being curious to see what species were represented. The inaccurate identification of some of these by the authors of The Ancient Monuments of the Mississippi Valley led to the examination of the series as a whole, and subsequently to the discussion they had received at the hands of various authors. The carvings are, therefore, here considered rather from the stand-point of the _________________.
What is the most critical completion of the passage?
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