Natural law is a controversial subject as it has a certain air of mystery – or even mysticism – about it. However, natural law can be observed in action around the world today as in the past. The idea basically is that when society makes laws that contravene the "nature" of human beings these laws will inevitably be contravened. They will be ignored and subverted.
This can be seen especially in modern laws having to do with criminalization of such practices as prostitution and the ingestion of various kinds of stimulants and or depressive agents (alcohol being one, marijuana being another). No matter how these "vices" are regulated, they nonetheless continue to be promoted and utilized from generation to generation.
It is against "human nature" to ban such fundamental commodities and in fact the history of human-kind shows that such goods and services have persisted for thousands of years. These examples illustrate how natural law collides with human instinct to create a situation where the law will be consistently subverted because it does not sufficiently acknowledge human nature.
Natural law is not common law, especially British Common Law, though the two are occasionally confused. British Common Law was a construction and codification of certain rights and judicial customs that evolved over the years. Some of these hewed directly to natural law and some departed.
Common law (lower case) that arose prior to British Common Law is probably closer to natural law; certainly natural law is little observed in the modern day. But the mechanisms of clan and tribal law certainly hewed to the nature of humankind and its central morality and biology. Common law (lower case) emphasized vendettas, payments and forfeitures for certain crimes and dealt a good deal with familial and sexual issues rather than with business and commercial issues.
Much of clan and tribal common law is focused on crime avoidance. Vendettas are a good example. If one knew that the murder or rape of an individual would generate a cycle of revenge that would last up to a hundred years or longer, one might think twice about taking a provocative action. Likewise, the emphasis on "honor duels" within a common law tradition had the effect of ensuring that society was mostly quite polite.
The US Declaration of Independence is a document often cited as observing natural law. And natural law has been much commented on by a variety of Western philosophers, especially those who espoused Anglican classical liberalism in the 1700s and 1800s. In fact, the Greeks were the first to make a distinction between natural law and "custom" or "convention" which might well depart from it. Aristotle postulated the idea of "natural rights" and has been cited as the "father of natural law." The great Catholic philosopher Thomas Aquinas elaborated on natural law and natural rights as he translated Aristotle from the original Greek.
Modern day Western law – both civil and criminal – has departed entirely from the common sense of natural law. As modern law becomes ever more complex and divorced from "natural" human actions, it gradually sinks into disrepute. This is the danger of avoiding or ignoring the reality of natural law. After a point, the old aphorism comes into play: "The law is an ass." This is, of course, dangerous for civil society; the more the law as a body of dictates sinks into disrepute, the easier it is for society to partake of fundamental lawlessness and corruption. This appears to be what's happening now.