Which idea explains darwin’s observations about galapagos islands finches?

The idea of species changing overtime explains Darwin's observation about the Galapagos Islands finches.

The theory of evolution by natural selection

Did you mean Marbury versus Madison?

It was the most important case in Supreme Court history, and the first time US Supreme Court case to apply the principle of "judicial review" - the power of federal courts to void acts of Congress in conflict with the Constitution