If we construct an euler class, why is it that it behaves functoriality (Naturality)? That is:

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Here is a very simple question that I can't seem to figure out for some reason. If we construct an Euler class as in Milnor's Characteristic classes, why is it that it behaves functorially (Naturality)? That is, why

$$e(F) = f^*e(E)?$$

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By functoriality of cohomology and uniqueness (and existence) of the Thom Class presented in Thm 9.1. in Milnors Characteristic classes the naturality of the euler class follows. Just write down the commutative diagramm arising from the bundle map $f: F \to E$ (which also yields the map $(F,F_0) \to (E,E_0)$) and look where the Thom class goes and hence where the euler class is forced to go.