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.
Germany's eugenic sterilization law, which went into effect on January 1, 1934, is no hasty improvisation of the Nazi regime. It has been taking shape gradually during many years, in the discussions of eugenicists. From one point of view, it is merely an accident that it happened to be the Hitler administration which was ready to put into effect the recommendations of specialists.
But Hitler himself - though a bachelor - has long been a convinced advocate of race betterment through eugenic measures. Probably his earlier thinking was colored by Nietzsche, but he studied the subject more thoroughly during his years in prison, following the abortive revolutionary movement of 1923. Here, it is said, he came into possession of the two-volume text on heredity and eugenics, by E. Baur, E. Fischer, and F. Lenz, which is the best-known statement of eugenics in the German language, and evidently studied it to good purpose. In his book, Mein Kampf, most of which was written during these prison years, and which outlines most of the policies since adopted by the Nazis as a political party, he bases his hopes of national regeneration solidly on the application of biological principles to human society.