The following is a quote from "C.S. Lewis" by Roger Lancelyn Green
(Bodleigh Head, London, 1963).

"Many readers have wondered whether Lewis had drawn out a scheme
for the whole Narnian series before writing `The Lion, the Witch
and the Wardrobe': the answer, on the only reliable authority -
his own - is that he had not. The earliest sketch for the first 
book was made in 1938; it was very different from the final 
version, and Aslan did not appear in it. After it had grown to 
its final form in 1949, the next few stories followed in natural
sequence. Then Lewis turned back to seek for the origin of the
Witch, the Wardrobe - and the Lamp Post. From this quest grew
`The Magician's Nephew', which ran away with its creator to make
perhaps the most symmetrically perfect story of the series. In
doing so, as careful readers will discover, the inspiration led
Lewis to contradict one assumption or suggestion made in the two
earliest stories, that there were no human beings from our world
in Narnia before the children paid their first visit. (In the 
first draft of the story the `Adam' and `Eve' of Narnia came from 
the lost world of Charn; both Lewis and a friend who read the 
manuscript agreed that these characters were unsatisfactory. In
casting about for a replacement the cabman and his wife stepped
suddenly and inevitably into their place - and were not to be
ousted."