Almost everybody agrees that the past is fixed and immutable.
The actions of men are subject to general immutable laws expressed in statistics.
The fact that these things didn't happen means that the past is immutable.
At any moment in the history of a nation such customs seem, to a superficial observer, to be fixed and immutable.
It is also to be noted that he is here definitely opposing religion to magic, which he holds to be based on the (implicit) assumption " that the course of nature is determined, not by the passions or caprice of personal beings, but by the operation of immutable laws acting mechanically."