"The Ancestor"

by August Delreth
originally published

by August Derleth
originally published The Survivor and Others, Arkham House, 1957

Henry, our narrator, has a cousin named Ambrose Perry, a retired physician.  It seems Perry had some radical ideas, not accepted by the medical establishment.   He has retired to a remote house in Vermont to pursue his unorthodox research.  He offers Henry a job as his secretary.

Henry finds Ambrose in debilitated shape - gaunt and spooky.  It seems Ambrose has found a way, using drug-and-environmentally-induced trances - to revert his mind as far back as his birth, and before his birth, into the mental fields of his ancestors, thusly he can take a gander at the world before he was born, even back to prehistoric times.

Ambrose starts getting weird.  He has bad table manners and starts locking himself away in his lab for days on end.  He stops eating.  His normally well-behaved dog starts acting up.  Henry thinks he spots a large animal in the nearby woods.  The house starts smelling "musky".

Henry finally breaks down the outside door to the lab.  The dog gets free and runs off that way first, and Henry hears the sound of a vicious struggle.  Inside the lab, the equipment has been wrecked and papers strewn around, and the remains of several half-eaten woodland creatures can be found.  Henry finds the dog having just finished killing a "sub-human caricature of a man".  But it's wearing Ambrose's clothes.  Clearly Ambrose had succeeded in regressing more than his mind...

A perfectly average Derleth story, not especially Lovecraftian.  Its very similar to Leonard Cline's "The Dark Chamber", an obscure 1920's horror novel that Lovecraft was fond of (and I was not), and I suspect that was the true inspiration, and not anything in HPL's "Commonplace Book".