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Supposing that I were to ask the evidence for macroevolution across species, the reply I'd get back would be very similar to James': Check the entire body of work we've done. When I asked, "Like what," no single exhibit, taken in isolation, would be at all convincing.
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AS A GENERAL RULE you don't want to accept argument on authority ALONE, and you're absolutely right that James himself preaches (too humbly) that nobody should give him any extra credit for his erudition.
AS IT APPLIES TO A SENSE OF HISTORICAL PROPORTION it would be pusillanimous to deny James' feel for what and what is not typical in baseball history. It's not the end of the discussion, but it's worthy of consideration.