The term common law does not mean unwritten law. The common law is the non statutory law developed over time by court decisions. Much of English law (e.g., the law of property, torts, contracts and criminal law) was developed not by Parliament but by courts deciding one case at a time. Many of those opinions were written and served as precedent for use by courts deciding subsequent cases which concerned the same issue.
I will give you a famous example of how the common law developed. This famous case in the law of contracts is Hadley v. Baxendale decided in 1854. The facts of the case are fairly simple. The plaintiff owned a mill which required a crank shaft to operate. The crank shaft broke and the plaintiff sent the broken crank shaft to the defendant so that the defendant could make a new one just like it. The plaintiff's employee told the defendant that work at the mill was stopped because they lacked the needed crank shaft. Through some screw up the delivery of the new crank shaft was delayed. As a result, the plaintiff was delayed in reopening the mill and because of the additional down time the plaintiff lost additional profits from the delay.
The plaintiff sued the defendant and a jury awarded the plaintiff 25 pounds. The defendant appealed, claiming that even if the defendant breached the contract, the defendant should not be held liable for the plaintiff's lost profits because the damages were "too remote." So the question on appeal was what should be the proper measure of damages for a breach of contract? What will happen if we allow plaintiffs to recover lost profits in these kinds of cases? How will people in the position of the defendant be able to evaluate the risk of a breach of contract on his part if he has no idea how much might be his liability for a possible breach? Under those circumstances, how can crank shaft maker do business if he has absolutely no idea how much he might owe if there is a screw up? These are the policy questions that the court on appeal had to consider when creating a rule.
The important thing to notice is that the court itself is developing a rule here regarding the proper amount of damages for breach of a contract. The court is not relying upon a statute because there apparently was no statute that answered that question. Notice that it is the court that is creating this rule:
"Now we think the proper rule in such a case as the present is this:" and then the court announces this new rule.
This opinion was in writing and future courts deciding cases containing the same basic issue would be expected to follow that rule. In other words, the case became a precedent which could be relied upon by manufacturers, purchasers of new equipment and judges.
This case is an example of how most of the English laws on property, torts, contracts, etc. were developed. Laws created by courts in this matter are known as case law. It is the entire body of this case law that is known as the English common law (judge-made law).
The English common law is written. It was written by courts one case at a time. The rule in Hadley v. Baxendale regarding the proper measure of damages in the event of a breach of contract is still being taught to law students today in American law schools. It is a famous part of the English common law.
So you grasp the fact that there is no "natural born" statute?
Oh, and for rebuttal to this point, I give you James Madison. You may have heard of him.
What can he mean by saying that the Common law is not secured by the new constitution, though it has been adopted by the State Constitutions. The common law is nothing more than the unwritten law, and is left by all the constitutions equally liable to legislative alterations.