And makes gaps even two can pass abreast.
The work of hunters is another thing:
I have come after them and made repair
Where they have left not one stone on a stone,
But they would have the rabbit out of hiding,
To please the yelping dogs. The gaps I mean,
No one has seen them made or heard them made,
But at spring mending-time we find them there.
I let my neighbor know beyond the hill;
And on a day we meet to walk the line
And set the wall between us once again.
We keep the wall between us as we go.
To each the boulders that have fallen to each.
And some are loaves and some so nearly balls
We have to use a spell to make them balance:
'Stay where you are until our backs are turned!'
Though they are working at the same time, together, they tend only to the rocks that are on their respective sides of the fence. 
 Again, the mending of the wall brings them together, however temporarily, but the basic purpose and outcome of the wall is separation. There is a sense here of “every man for himself.”
Mending Wall