This chapter reveals the grid of transcription factors that balkanizes the nervous systems of bilaterian animals into modular districts. Many of those districts go on to form functionally distinct parts of the brain. Virtually all bilaterians use the same tricks to ensure accuracy in neural wiring. Trick #1 (the "Shotgun Strategy"): an excess of neurons sends a fusilade of axons to the target destination (e.g., retina to tectum), hence increasing the chance that every target will be hit at least once, and the remaining neurons, which fail to hit their target, kill themselves. This first trick is Darwininan insofar as the "fittest" connections are "selected" based on a live-or-die criterion. Trick #3 ("Passport Strategy"): axons that cross the midline are prevented from mistakenly ever re-crossing by means of a cell surface receptor (Robo) that switches its affinity for its ligand (Slit) when the midline is crossed. Trick #3: every neuron randomly selects one version of the cell surface receptor "Dscam," so that whenever two dendrites with the same Dscam bar code come into contact, they repel one another, thus ensuring that dendrites of each neuron spread out maximally.