Vein because they have valves to prevent back flow of blood.
Predators and prey are both in a constant struggle for survival, foxes with bigger ears tend to catch more mice, and live longer, because of this, foxes with smaller ears don't live as long, pretty soon all foxes have larger ears. The same goes for prey, Mice with padded feet live longer due to the foxes having more trouble hearing them, pretty soon all mice have padded feet, and the cycle continues.
Potential equation are root 3x+qp=qp
