"Inside of a dog"

[Laurence Horn]

I've always been fond of the apothegm in question--

"Outside of a dog, a book is man's best friend. Inside of a dog, it's
too dark to read."

that I assumed was indeed uttered/written by Groucho Marx.  There was
always that question of whether "inside of a dog" is really truly
possible, but on balance, it's still an immortal line.  Only maybe
it's not Groucho's after all?

I'm at the Seattle airport at the moment, where there doesn't seem to
be a copy of Fred's YBOQ on hand, but what struck me is that a recent
addition to the "Cognitive Science looks at canines" bookshelf,
_Inside of a Dog: What Dogs See, Smell, and Know_ (by Alexandra
Horowitz, Scribner, 2009) indicates that the eponymous epigraph is
"attributed to Groucho Marx", which leads my Gricean antennae to
infer a lack of success on Horowitz's part in pinning down an actual
source. (Not that any of the amazon reviewers shy away from crediting
Groucho for it.)

Are there any definitive takes on this one?

LH

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