reciprocity
n[U]
principleor practice of mutual exchange, esp of making concessions or granting privileges, etc in return for concessions or privileges received 相互交换的原则或实践; (尤指)互相让步或互惠
reciprocity in trade (between countries) (国与国之间的)贸易互惠.
Snakes did not acquire their poison for use against man but for use against prey such as rats and mice, and the effects on these of viperine poison is almost immediate.