"In a word: the Marxian law of value holds generally, as far as economic laws are valid at all, for the whole period of simple commodity production — that is, up to the time when the latter suffers a modification through the appearance of the capitalist form of production. Up to that time, prices gravitate towards the values fixed according to the Marxian law and oscillate around those values, so that the more fully simple commodity production develops, the more the average prices over long periods uninterrupted by external violent disturbances coincide with values within a negligible margin. Thus, the Marxian law of value has general economic validity for a period lasting from the beginning of exchange, which transforms products into commodities, down to the 15th century of the present era. But the exchange of commodities dates from a time before all written history — which in Egypt goes back to at least 2500 B.C., and perhaps 5000 B.C., and in
Babylon to 4000 B.C., perhaps to 6000 B.C.; thus, the law of value has prevailed during a period of from five to seven thousand years."
^^^^^ CB: So , your contention is that Marx disagreed with this claim by Engels, right ?
(Well, "written history" that Engels could understand. Ancient Egypt and Babylon had writing and written history at a certain point.)
Marx's discussion of universal equivalent and pre-money forms of value in early _Capital_ I is a discussion of the "barter" systems or "simple commodity production" that Engels refers to. That is Marx's more abstract discussion ( and "agreement" with) of Engels more concrete ( and more popular) discussion above.
http://www.marxists.org/glossary/terms/b/a.htm#barter
Barter
Barter is the exchange of commodities directly for one another, without the use of Money.
In barter, the Commodity relation and the Form of value is as yet undeveloped, and exchange of labour is necessarily marginal to the life of people. In societies where barter is practised, there is invariably some “universal equivalent” in which the value of each commodity offered for exchange is measured, so as to determine the appropriate ratio of exchange between any two given commodities. This universal equivalent will in the first place be a commodity that is widely used in the given society as well as being of a more or less uniform quality.
However, such a measurement of value is not enough to allow exchange of commodities to fully develop, and for the practice of exchange to freely develop, a commodity which is infinitely divisible, of absolutely uniform purity and quality, infinitely durable, able to store a large quantity of human labour in a small quantity of itself, is needed.
Historically gold and silver have taken on this role, and along this road, paper money and Credit, and inevitably the whole of bourgeois society arises.
Form of Value
http://www.marxists.org/glossary/terms/f/o.htm#form-value
The form of value is the historically evolved form through which the exchange-value of commodities are manifested in a given society.
“... the value of commodities has a purely social reality, and that they acquire this reality only in so far as they are expressions or embodiments of one identical social substance, viz., human labour, it follows as a matter of course, that value can only manifest itself in the social relation of commodity to commodity. In fact we started from exchange-value, or the exchange relation of commodities, in order to get at the value that lies hidden behind it. We must now return to this form under which value first appeared to us.” [Capital, Chapter 1]
The money-form of value did not appear ready-made but was the outcome of a long history in which one form successively replaced another, perfecting the form of value and evolving the money-form.
The value-form is the essence of the commodity. Value first makes its appearance in history with the accidental and occasional exchange of products between neighbouring people. Here the value of each product can be determined only in the immediate determinate situation in comparison exclusively with the product for which it is proffered in exchange.
Marx called this the “Elementary or Accidental Form of Value”, and in Chapter 1 of Capital, traced the 15 successive forms of value from this accidental form up to money. These forms of value track the gradual emergence of regular and socially determined practices of value-determination, beginning with more regulated forms of barter, up through the singling out of one commodity as a unit of measurement of all others.
The perfection of the form of value facilitates the expansion of trade, and without money, the industrial revolution would have been impossible. The money-form reached the pinnacle of its development at the end of the 19th century as gold emerged as the exclusive measure of value for international trade....