Evolution can be thought of as a filter for evolvable developmental architectures. Comparing neuron numbers in the retina, midbrain, thalamus and cortex for a variety of primates, and for rodents of unusual sizes, each cell class increases in number in a predictable manner with brain size, depending on each class’s position in overall order of neurogenesis and features of cell cycle kinetics. The large suite of features associated with nocturnal versus diurnal retinas appear to be organized by relatively minimal alterations in retinogenesis. Finally, birds show a quite distinct pattern of retinal and central variations compared to mammals, whose significance is as yet unknown.