“There is a presumption in favor of every existing institution. Many of these (we will suppose the majority) may be susceptible of alteration for the better; but still the "Burden of proof" lies with him who proposes an alteration; simply, on the ground that since a change is not a good in itself, he who demands a change should show cause for it. No one is called on . . . to defend an existing institution, till some argument is adduced against it; and that argument ought in fairness to prove, not merely an actual inconvenience, but the possibility of a change for the better.”