 Read the following passage:

"Ninian bought one of those smallish, almost identical houses that mushroom on the fringes of a city after every war, particularly where intensive bombing has created a number of desirable building sites."

What, in that passage, is symbolic?
When it became clear that his mother was never going to show up again, Ninian bought one of those smallish, almost identical houses that mushroom on the fringes of a city after every war, particularly where intensive bombing has created a number of desirable building sites. There were hot water and heat all the time and a freezer well stocked with food—somewhat erratically chosen, for Ninian didn't know much about meals. We're virtually the same people ... although it is amazing what a difference two hundred odd years of progress and polish can make in a species, isn't it?" And, on those worlds, it's far cheaper to use native labor than to ship out all that expensive machinery.
 "houses that mushroom on the fringes of a city after every war, particularly where intensive bombing has created a number of desirable building sites," is symbolic of the way things like the atom bomb, that leaves a mushroom cloud in its wake, are thought to have been unleashed upon the world to preserve or "create" things that will be "desirable" for society.
 "houses that mushroom on the fringes of a city after every war, particularly where intensive bombing has created a number of desirable building sites," is symbolic of the way things like the atom bomb, that leaves a mushroom cloud in its wake, are thought to have been unleashed upon the world to preserve or "create" things that will be "desirable" for society.  "intensive bombing" is symbolic of what the descendants imagine they are doing to Martin to make sure they impart enough of their future society on him to make him agreeable to carrying on with their way of life.  The word "bought" is symbolic of the wealth Martin is to bring to future worlds.  "Smallish, almost identical houses," is symbolic of the way that Ninian blends in with current society.