This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
It was a well settled doctrine of Common Law that a contract between two parties was merged in a subsequent contract between the same parties, covering the same subject-matter whenever the second contract was of a higher nature than the first, and the creditor was not deprived of any pre-existing remedy by such merger. The great distinction at Common Law between contracts as to their nature or grade was between simple contracts which were of a lower grade, and formal contracts which were of a higher grade. Accordinglyv the simple contract is always merged in a subsequent formal contract entered into between the same parties and covering the same subject-matter. Among the formal contracts the contract of record was of a higher nature than the contract under seal and the latter merged in the former. The distinguishing feature of merger is that it operates without any dependence on the intention of. the parties that merger shall exist.