How do forces merge?
At low temperatures different forces act differently, but at higher temperatures, it turns out that the different forces are really the same thing- not just the same strength, but really the same force, acting in the same way on the same kinds of particles. The math describing exactly how this happens is beyond this course (it involves group theory and symmetries), but it can be understood as a kind of phase transition. Your text uses a water analogy: just as water changes its behavior with temperature, so can a force. Unlike water though, which is just one substance in different forms as temperature changes, a force can split into two separately acting forces as temperature decreases (and two forces merge together as temperature increases).