That does not make the least difference in the value of the highest results of that passion.

We might say the very same thing about any human emotion; every emotion can be evolutionally traced back to simple and selfish impulses shared by man with the lower animals.

But, because an apple tree or a pear tree happens to have its roots in the ground, does that mean that its fruits are not beautiful and wholesome?
Most assuredly we must not judge the fruit of the tree from the unseen roots; but what about turning up the ground to look at the roots?
What becomes of the beauty of the tree when you do that?
The realist--at least the French realist--likes to do that.

He likes to bring back the attention of his reader to the lowest rather than to the highest, to that which should be kept hidden, for the very same reason that the roots of a tree should be kept underground if the tree is to live.
The time of illusion, then, is the beautiful moment of passion; it represents the artistic zone in which the poet or romance writer ought to be free to do the very best that he can.

He may go beyond that zone; but then he has only two directions in which he can travel.