  We study the effect of time evolution on galaxy bias. We argue that at any
order in perturbations, the galaxy density contrast can be expressed in terms
of a finite set of locally measurable operators made of spatial and temporal
derivatives of the Newtonian potential. This is checked in an explicit third
order calculation. There is a systematic way to derive a basis for these
operators. This basis spans a larger space than the expansion in gravitational
and velocity potentials usually employed, although new operators only appear at
fourth order. The basis is argued to be closed under renormalization. Most of
the arguments also apply to the structure of the counter-terms in the effective
theory of large-scale structure.
