Fitzgerald's short story is a challenge to understand because there are so many moving parts to it.  I think that one aspect which is evident is the idea of how there is struggle in this life.  Tanner struggles with helping his wife with the delivery, as she struggles in bringing life into this world.  There is struggle in the birth of the first child.  Then, there is additional struggle in the discarding of the second child in the afterbirth, and the doctor's saving this child.  There seems to be struggle in the very element of life's existence and Fitzgerald brings this out.  Within this struggle, literally, wanders in Brinkman and his belief or need for dinner to be made.  Confronted with some of the most profound issues of life, death, and the struggle in human consciousness, Fitzgerald gives insight to the wanderer:

Two more women born into the world.  It must have seemed to him that if this sort of thing went on, there should be a good chance, in the end, for him to acquire one for himself.  Meanwhile, they would have to serve dinner sometime. 

This might be one of Fitzgerald's points to make.  While survival and consciousness involves struggle, there is a pattern to being that almost regulates it.  This makes being one in which individuals are part of what amounts to be a finely made watch, where gears and levers mesh into a functioning being.  Struggle becomes part of this configuration, and one towards which human direction is geared.