20. LAW OF IRREVERSIBILITY 

Some resources (mineral, plant, or animal) do not renew themselves, because they are the result of a process (physical or biological) which has ceased to function in a particular habitat or landscape at the present time. 

Just like a tooth that's fallen out
And never never will grow back --
The overhunting of the whale
The trees we uproot, cut and hack,
Coal, tungsten, oil, amber and shale --
Sometime in the future, supplies will fail.
We can't reverse it, unfortunately,
The law's irreversibility.