I am trying to think of an example of scientific explanation whose scope was in fact broader than we initially thought. The idea would be the following:
Initially, we used H (the explanation) to explain a certain phenomenon (call it x) and we took a range of phenomena to be relevantly similar to x in the sense that H would also apply to them. We latter discovered that the phenomena we took to be relevantly dissimilar to x (call it y) were not so and that the other explanation needed to explain them was in fact highly similar. We essentially discovered that H or H', a derivative of H, not only explained x but also y.
So, can anyone provide from the philosophy of science solid examples of the broadening of scope of scientific explanation and theory?