The general rule for choosing between "which" and "that" is that the first should be used only for independent clauses.(see for instance these three sources 1, 2, 3) Yet, especially in old books, I often come across the word "which" in clauses that are not independent. For instance, I'm reading a book by an English scholar who seems to use ONLY the word which. Here a few examples:
"The Greek hero-legends are full of myths which have this ritual origin."
"On the other hand, the king is still regarded by others as a mere depositary of social authority—a temporary embodiment of a power which existed before him."
"In the mortal soul we find again the same combination of blood and mana which composed the sympathetic continuum of primary magic."
I really could go on forever with examples. This book is ridden with this use of "which". Can anyone tell me why? Is perhaps this use more archaic or literary?