Ethology
Earlier phylogenetic stages and (pre-) conditions which persist often determine the form of more modern characteristics. For instance, the vertebrate eye (including the human eye) has a blind spot, whereas octopus eyes do not. Once the vertebrate eye had evolved, the only way it could improve was to minimise the effect of the blind spot. We don't notice it ordinarily, because with binocular vision what one eye misses, the other fills in.