Final answer:
The ratio in Stilk v Myrick establishes that additional wages promised for the performance of an existing contractual obligation do not provide valid consideration to modify said contract.
Step-by-step explanation:
The ratio in the case of Stilk v Myrick is a fundamental principle in contract law relating to the modification of contracts. In this 1809 English case, sailors were promised extra wages if they agreed to do more work after some of the crew deserted. However, the court held that the sailors' promise to do extra work could not be considered valid consideration for the additional wages because they had only performed their existing duties, albeit under more arduous conditions. This case set an important legal precedent that a promise to perform an existing contractual obligation is not sufficient consideration to modify a contract.