The common law’s methodology makes it "evolutionary" in nature. It is very organic, growing out of its environment. A judge deciding a case must write her opinion as follows:

    1. The judge must examine the evidence that has been introduced during the trial and write a "statement of the facts."

    2. The Judge must then decide what legal rules (laws) will be applied to the facts to decide the case.

    3. The Judge then analyzes the case by taking those rules and applying them to the particular facts of the case.

    4. Based on this analysis, the judge must then come to a conclusion which disposes of the case.

Notice how this method is tied very closely to the specific facts of the case. The facts become the environment in which the law will evolve. Over time the fact patterns confronted by courts will change as society changes. Gradually, the law itself changes as its environment changes.

Each court is bound by the doctrine of stare decisis. http://en.wikipedia.org/wiki/Stare_decisis  This means that it must follow its prior decisions and the prior decisions of the courts directly above it. However, with regard to its own decisions, a court can overrule a prior decision, but it must explain in its written opinion why the prior decision should not longer be followed. This is an extraordinary measure taken by a court. A court may not overrule the opinion of a higher court.

More commonly, a court will "distinguish" a prior precedent of its own or of a higher court. To distinguish a precedent, the court must explain how the particular facts of the case justify the application of a different or modified legal rule in the case before the court. Upon Appeal, a higher court can then decide if the prior precedent should be distinguished and whether the legal rule should be modified. Again, the appellate court must explain its reasoning in a written decision, which is publicly available to all.

This requirement that each court must publicly explain any change or modification to a legal rule and be subject to review by a higher court makes the evolution of legal rules a very disciplined process. Thus, a good legal rule, which is well adapted to the current environment will tend to persist, while a poor legal rule which is not well adapted to the environment will tend to be quickly changed as judges and appellate courts adopt arguments, which explain the problem with the legal rule.

However, even a rule which is very well adapted to today's society, may become maladapted to society 20 years from now. Think of a legal rule from the past. A legally valid signature 20 years ago had to be a signature written in the person's own handwriting. At that time, the rule made good sense. Suppose that you are a court trying to apply this rule as commerce begins to be transacted over the Internet. Should you stick with the hand-written signature requirement, or should you consider applying a new definition of a "signature?" In this way, changes in the environment effect the way in which the law will be modified.

Also, consider the interpretation of the meaning of the "equal protection clause" of the 14th Amendment to the US Constitution. In 1896, when racism and the separation of races was generally accepted by society, the U.S. Supreme court held in Plessy v. Ferguson http://en.wikipedia.org/wiki/Plessy  that a requirement that different races use separate facilities did not violate the "equal protection clause." However, after different races fought together in World War II, and after the growth of civil rights movement was well underway, in 1954 the US, the Supreme Court changed the rule in the case of Brown v. Board of Education http://en.wikipedia.org/wiki/Brown_v._Board. The Court declared that separate was inherently unequal. The society had evolved and the legal rule evolved along with these changes in society. 

These examples highlight the evolutionary nature of the common law. One of its greatest strengths is that it adapts to changes in society, but in a very disciplined way.