One of my greatest fears as a writer is that I will unconsciously plagiarize. When I was growing up I read so incessantly and rapidly that I retained almost no memory of individual books. They all ran together into a confused mixture. Consequently, my mind is full of other people's ideas. I can't tell where my own ideas begin, or if I have any. I can never be sure anything I write is even slightly original.

Mary and I purposely avoid reading historical mysteries set in ancient times so that we don't risk copying them and can't be accused of doing so. As far as we know there haven't been many books set during 6th century and no mysteries.

Lately, however, I have been wondering exactly where the line is between stealing ideas and making a book marketable by fitting it into a niche.

For example, I had until recently been blissfully unaware of what is called urban fantasy. There appear to be a lot of books about worlds which are just like our own, except that the fellow behind the cash register at WalMart might be a vampire or a werewolf, and no big deal. Now the concept of a world like the one we live on, enlivened simply by the addition of vampires and/or werewolves seems fairly specific. Not quite as broad as, say, time travel. I would have been hesitant to employ the concept myself. It's been done, I would have thought. But there seem to be quite a few series using that idea.

Then, too, authors begin to mix in wizards and zombies and gorgons and centaurs (oh my!) who knows what else. So there you have a marketing niche, a subgenre of speculative fiction. If you can write a new time machine story where the traveler visits an era that hasn't been used to death, that's something new, even though the time travel concept isn't. Populate our world with a mixture of creatures that haven't been overdone, that's new, even though the legendary creatures in our current world concept isn't. Does that make sense?

Recently though I read Simon R. Green's Something From the Nightside and Neal Gaiman's Neverwhere back to back and was dumbfounded. Green imagines there is another London "beneath" the London we know, entered only by secret ways, a place where time is jumbled, magic works, populated by a fantastic variety of creatures and strange cultures. A delightful idea really. But I was surprised then to read Neverwhere, based on a television series predating the Nightside books by six or seven years. Gaiman had already imagined that there was a secret London, exactly identical in concept, if not in particulars, to Green's Nightside. One of Gaiman's major characters even had the ability to open secret doors, one of the main abilites, in effect, of Green's protagonist, who can find things, including, quite often secret doors. (I also can't help thinking of Stephen Saylor's Gordianus the Finder and John D. MacDonald's Travis McGee, a salvage expert who retrieved things but that's another issue.)

I very much enjoyed both novels, but I couldn't have written Green's without feeling guilty of copying Gaiman's earlier effort.

Not being well versed in modern speculative fiction I don't know what Gaiman might have "copied." I was long ago intrigued by the world of mixed-up eras which Murray Leinster first advanced (perhaps) in Sidewise in Time and also enthralled by the idea of magical worlds impinging, usually invisibly on our own, as set forth, for instance, in The King of Elfland's Daughter by Lord Dunsany. But to me these sorts of ideas, quite evident in Something From the Nightside and Neverwhere seemed to have already been accounted for. I would have hesitated to venture into such territories.

I wonder if my definition of plagiarism has been overly broad. As far as I know Gaiman and Green aren't the only authors to write about a secret London. It amazes me, really. Is that a subgenre? Secret Londons? I suppose in a sense it dates back to all those books from the Victorian era where writers revealed to their middle-class readers the fantastic London of the poor and the criminal which existed, all but unseen, right under their noses. Books with titles like London at Midnight, London Shadows and, yes evenThe Night Side of London.

And then there is the remarkable new mystery The City and the City by China Mieville in which two cities co-exist in the same space and time.

Mieville's city is, at least, not London. It (or they) are fictional. Perhaps that is the genre: Co-existing cities, with the subgenre being co-existing Londons. How about co-existing New Yorks? Or Chicago, or Tokyo? Hoboken anyone?

Can we use Leinster's idea and endlessly juxtapose different asortments of eras? Or maybe take Dunsany's Elfland and plunk it down here, there and everywhere, good for a new series with every geographic move?

I'm not sure. I don't whether there's a co-existing cities genre to which a writer may contribute without being accused of plagiarism. All I know for certain is that I'm lost.