If Nature does not wish that weaker individuals should mate with the stronger, she wishes even less that a superior race should intermingle with an inferior one; because in such a case all her efforts, throughout hundreds of thousands of years, to establish an evolutionary higher stage of being, may thus be rendered futile. - MEIN KAMPF by Adolf Hitler
If you read Alfred Rosenberg's The Myth of the Twentieth Century, the second most influential Nazi text after Mein Kampf, and the most explicit and extensive discussion of Nazi race philosophy, you discover that the Nazis believed human races to be primeval, with their distinct "race souls" created by God.
The Nazis weren't trying to "advance" evolution, as Ben Stein has claimed; they were trying to reverse and undo human evolution. Their goal was to restore the racial purity, the purity of "the blood" and the race soul carried in the blood, which God had created in the beginning.
The "evolution" which Nazis actually embraced was not Darwinian but pre-Darwinian. It invoked the pre-Darwinian meaning of "evolution" as "unfolding": the playing out of a prefigured plan of development. The Nazis believed the primeval races had distinct geniuses and distinct destinies. The supposed genius of the "Aryan" race was for conquest and rule, and therefore it's destiny was to conquer and enslave other races. This destiny had been foiled by "race mixing" and the dilution of racial purity. Thus reversing (normal) evolution and restoring racial purity was a necessary aspect of the Nazi program.