D.W. Yarbrough quotes a Texan saying about coincidence which says that when you see a turtle on a fencepost, "you can be pretty damn sure he didn't get there on his own." This is an adaptation of a political joke about a politician resembling a turtle on a fencepost: he doesn't belong there, he didn't get there by himself, he can't get anything done while he's there, and you just want to get him down as soon as possible.
Yarbrough suggests the possibility that God has been going round putting turtles on fenceposts, to gain the attention of his human creation and make us think. It is typical of both Yarbrough and Jesuit teaching to turn a joke or proverb into a philosophical point.
Russell returns to the image a number of times. In a theological discussion with Emilio, Anne describes Emilio's growing sense that God was revealing himself and his will:
Like D.W. said, a whole hell of a lot of turtles showing up on a whole hell of a lot of fenceposts.
Later, when Marc Robichaux refers to the idea again, Yarbrough snorts and is "heartily sorry that he had ever mentioned turtles."
In the acknowledgements, Mary Doria Russell thanks her friend Molly Ivins, the Texan journalist and political commentator for "insight into Texans, turtles and armadillos." Ivins used the image of a turtle on a fencepost to describe President George W. Bush in a 2004 column (attached below), and it has been used of every president since.