The stories collected here, written between August of 1987 and May of 1990, demonstrate that I still believe in the classical unities. Of course, what seems to us a unity now might not have appeared that way when H.G. Wells was writing his wonderful stories in the nineteenth century. Wells might have argued that my 'To the Promised Land' is built around two speculative fantastic assumptions, one that the Biblical Exodus from Egypt never happened, the other that it is possible to send rocketships to other worlds. But in fact we've sent plenty of rocketships to other worlds by now, so only my story's alternative-world speculation remains fantasy today. Technically speaking the space-travel element of the plot has become part of the given; it's the other big assumption that forms the central matter of the story.
--Robert Silverberg, from his Introduction