How have the courts interpreted a parol agreement where a landlord was induced to sign a lease with a promise that rabbits would be destroyed?

In Morgan v. Griffith (1871), L.R. 6 Ex. 79, 40 L.J. Ex. 46, 23 L.T. 783, the plaintiff Griffith hired some grass land to the defendant, who entered on the land and found it overrun with rabbits. The lease was presented to him for signature, and he refused to sign it if he was to be annoyed by the rabbits in the future. He again complained when paying his rent in the following March of the annoyance, and the defendant promised to destroy the rabbits. After that the lease was again tendered to the plaintiff for signature, and he again complained of the number of rabbits and refused to sign it, or continue to hold the land for the current year. The defendant then said, “I promise you faithfully that they shall be destroyed,” and the plaintiff requested that a term to that effect should be inserted in the lease. This the defendant refused. He again promised that the rabbits should be destroyed, and the plaintiff accordingly signed the lease. It was held there that this was a collateral parol agreement and was binding; and the consideration for it is clear, viz.: he was induced to sign the lease by the express promise of the defendant that the rabbits should be removed.