In an article by G.C Rota, i read (page 8 of www-groups.dcs.st-and.ac.uk/history/Extras/rota.pdf):
"in Euclidean $n$-dimensional space there are exactly $n + 1$ invariant measures, namely, the Euler characteristic and the intrinsic volumes corresponding to the $n$ elementary symmetric functions. The discovery of this fact is an achievement of mathematics in the latter half of the twentieth century."
What is the theorem he is referring to?