We examine Ralph W. Souter\u27s defence, in the 1930s, of Marshall\u27s Principles against Robbins\u27 attempt to recast economics as a \u27purely formal science of implications\u27. Souter elaborated on Marshall\u27s invocations progressively to increase the realism of economic science and contrasted this perspective on Marshall with Robbins\u27 atomistic bias, neglect of historical time and irreversibilities, arbitrary restrictions on the scope of economic science and emphasis on logical and mathematical form over content. Souter demonstrates that Robbins takes a Walrasian-inspired perspective on Marshall\u27s equilibrium concept whereas the \u27authentically Marshallian\u27 equilibrium notion generally incorporates potential for endogenous change. On this and other matters Souter has priority in drawing attention to Marshall\u27s incipient \u27evolutionary economics\u27
