What does "appetency" mean?

Appetency: Specifically: An instinctive inclination or propensity in animals to perform certain actions, as in the young to suck, in aquatic fowls to enter into water and to swim; the tendency of an organized body to seek what satisfies the wants of its organism. These lacteals have mouths, and by animal selection or appetency the absorb such part of the fluid as is agreeable to their palate. E. Darwin.