For "some academic economists" read "Engels":
"According to the Ricardian law of value, two capitals employing equal quantities of equally paid living labour all other conditions being equal, produce commodities of equal value and likewise surplus-value, or profit, of equal quantity in equal periods of time. But if they employ unequal quantities of living labour, they cannot produce equal surplus-values, or, as the Ricardians say, equal profits. Now in reality the opposite takes place. In actual fact, equal capitals, regardless of how much or how little living labour is employed by them, produce equal average profits in equal times. Here there is therefore a contradiction of the law of value which had been noticed by Ricardo himself, but which his school also was unable to reconcile. Rodbertus likewise could not but note this contradiction. But instead of resolving it, he made it one of the starting-points of his utopia. (Zur Erkenntnis, p. 131.) Marx had resolved this contradiction already in the manuscript of his Zur Kritik. According to the plan of Capital, this solution will be provided in Book III. Months will pass before that will be published. Hence those economists who claim to have discovered in Rodbertus the secret source and a superior predecessor of Marx have now an opportunity to demonstrate what the economics of a Rodbertus can accomplish. If they can show in which way an equal average rate of profit can and must come about, not only without a violation of the law of value, but on the very basis of it, I am willing to discuss the matter further with them. In the meantime they had better make haste. The brilliant investigations of the present Book II and their entirely new results in fields hitherto almost untrod are merely introductory to the contents of Book III, which develops the final conclusions of Marx's analysis of the process of social reproduction on a capitalist basis. When this Book III appears, little mention will be made of the economist called Rodbertus..."
And also read "Marx": Capital, volume III, part 1...