Scott,
 
At first blush I would consider the possessive as incorrect in the usage you cite and that you are right in saying that Stevenson was over-Latinizing his English.
 
There are similar constructions or rather situations where the possessive has become ingrained in English.  Here the possessive indicates that the friend is one among a number of friends all belonging to the doctor:
 
This is a friend of the doctor's.
 
This construction suggest that perhaps Stevenson was thinking:
 
This hall was a pet fancy of his friend the doctor's many pet fancies. 
 
Then the more pedantic construction might be:
 
This hall was a pet fancy of his friend's many pet fancies -- the doctor's many pet fancies. 
 
This kind of paraphrase points out that it is likely a mixture of two different constructions, a blending of the two possessives: the so-called genitive-of and the double possessive. 
Bruce

>>> Scott Woods <[log in to unmask]> 01/24/08 11:10 AM >>>
List,
I found this sentence in Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde:
 
This hall, in which he was now left alone, was a pet fancy of his friend the doctor’s;
 
I see "the doctor's" as an appositive to friend, which is genitive showing the source of the fancy.  The appositive has been put in the same case, hence the appostrophe. Does my analysis look right?  Was Stevenson over Latinizing his English?  Is this the normal pattern?  Is it considered correct?  Would anyone on the list consider it an error in modern American usage? 
 
Thanks,
Scott Woods

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