"Marx did at one time clearly state a 'law' that wages tend to fall, in his series of 1847 lectures published age as 'Wage Labor and Capital.' Here he gave the opinion as the worker loses his specialized craft skills, competition between workers would force wage rates to decline. Within a few years, however, he was rejecting this [over]generalization, and condemning Lassalle for accepting the 'iron law of wages' which Ricardo had taken over from Malthus. When he sat down, in the fullness of developed thought, to discuss the fate of the worker in modern capitalist industry, in the chapter on capitalist accumulation [in Capital_Vol._1] mentioned above, he appeared to exclude falling wages from the galaxy of evils which he predicted, saying: 'The result is that, _in_proportion_ as_capital_accumulates, the condition of the worker, be his wages high or low, necessarily grows worse [my emphasis].
Marx's careful reservation [...] has quite naturally escaped wide notice, as the pounding of his artillery on the theme of 'growing misery' drowned it out. It must strike us today, after we have seen to what extent real wages can rise, as unnecessarily grudging. But, grudging though it may have been, his concern with this point of his theory was real enough, as was shown in 1875, several years after the publication of 'Capital,' when he turned the rough side of his tongue on the program elaborated by the German Social Democrats at the Gotha congress. Lassalle's followers had inserted therein the iron law of wages.' Marx called this 'a truly revolting retrogression.' He again repeats that his condemnation of 'wage slavery' does not depend on whether the laborer's pay is 'better or worse,' and adds: 'It is just as if among slaves who had at last penetrated the mystery of slavery, and had risen in rebellion, a slave, imbued with superannuated notions, inscribed on the program of the rebellion: 'Slavery must be abolished, because under the system of slavery the slaves' food can never exceed a certain low maximum'."
Harry Braverman, "Marx in the Modern World", American Socialist, May 1958
http://www.marxists.org/history/etol/newspape/amersocialist/AmerSoc_5805.htm