Energy is conserved. Momentum is conserved. Charge is conserved. I propose that under admissible dynamics there exists a constrained quantity, structure, whose persistence obeys a conservation law modulo dissipation cost. I formalize structure as a path-dependent functional over state space and show that time evolution may be reinterpreted as constrained minimization of a structural cost under energy budget. Selection emerges as the measurable rate of structural filtering. I derive conservation conditions, dissipation bounds, and a generalized continuity equation for structural density.