change in language vs. mythology

In reference to Saussure and the nature of linguistics-- the sign is arbitrary. There is no reason why a tree should be called a tree. The signifier is brought upon the sign slowly, because of passing time, repetition, and tradition. Because language is based on arbitrary signs, there is no logical founding in the sign, and this renders language virtually impossible to discuss and/or change intentionally. Rather, language changes slowly, as a product of the slight day-to-day differences and modernizing community.

This makes perfect sense to me. One cannot purposefully change language, because there is no sensible reason for this change. Change comes about by chance. I am confused, however, as to how changes are brought about in mythology. Myth, as we have learned through the lectures and readings, seems to be a reflection of history; history is embedded into the structure of the existing mythology. If myth is supposed to be a product of history--a "radical denial or transformation" of history, then I am confused as to who, what, when and where the changing and embedding of history into the myth occurs.

Does this parallel the change that comes about in language? Or does it contradict it?