The animals chosen for comparison in any study are likely to be quite different depending on which reason for comparison is operating. If the effects on the nervous system of adaptation to an arboreal mode of life are being studied, then animals that live in the trees but are not all closely related are chosen. Historical inference requires the most careful consideration of animal choices. It is perhaps in this realm that the greatest number of errors have been made. The goals of comparative studies have often been unclear or have been concerned primarily with demonstrating increasingly complex levels of organization “as one ascends the phylogenetic scale”. To make a valid inference of the history of the primate central visual pathways, living members of the primate lineage would have to be adequately sampled.