Apart from natural conditions, such as fertility of the soil, etc., and from the skill of independent and isolated producers (shown rather qualitatively in the goodness than quantitatively in the mass of their products), the degree of productivity of labour, in a given society, is expressed in the relative extent of the means of production that one labourer, during a given time, with the same tension of labour power, turns into products. The mass of the means of production which he thus transforms, increases with the productiveness of his labour. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.438

Once given the general basis of the capitalistic system, then, in the course of accumulation, a point is reached at which the development of the productivity of social labour becomes the most powerful lever of accumulation. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.438

But whether condition or consequence, the growing extent of the means of production, as compared with the labour power incorporated with them, is an expression of the growing productiveness of labour. The increase of the latter appears, therefore, in the diminution of the mass of labour in proportion to the mass of means of production moved by it, or in the diminution of the subjective factor of the labour process as compared with the objective factor. This change in the technical composition of capital, this growth in the mass of means of production, as compared with the mass of the labour power that vivifies them, is reflected again in its value composition, by the increase of the constant constituent of capital at the expense of its variable constituent. This law of the progressive increase in constant capital, in proportion to the variable, is confirmed at every step (as already shown) by the comparative analysis of the prices of commodities, whether we compare different economic epochs or different nations in the same epoch. The relative magnitude of the element of price, which represents the value of the means of production only, or the constant part of capital consumed, is in direct, the relative magnitude of the other element of price that pays labour (the variable part of capital) is in inverse proportion to the advance of accumulation. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.438-439

The laws of this centralisation of capitals, or of the attraction of capital by capital, cannot be developed here. A brief hint at a few facts must suffice. The battle of competition is fought by cheapening of commodities. The cheapness of commodities demands, caeteris paribus, on the productiveness of labour, and this again on the scale of production. Therefore, the larger capitals beat the smaller. It will further be remembered that, with the development of the capitalist mode of production, there is an increase in the minimum amount of individual capital necessary to carry on a business under its normal conditions. The smaller capitals, therefore, crowd into spheres of production which Modern Industry has only sporadically or incompletely got hold of. Here competition rages in direct proportion to the number, and in inverse proportion to the magnitudes, of the antagonistic capitals. It always ends in the ruin of many small capitalists, whose capitals partly pass into the hands of their conquerors, partly vanish. Apart from this, with capitalist production an altogether new force comes into play – the credit system, which in its first stages furtively creeps in as the humble assistant of accumulation, drawing into the hands of individual or associated capitalists, by invisible threads, the money resources which lie scattered, over the surface of society, in larger or smaller amounts; but it soon becomes a new and terrible weapon in the battle of competition and is finally transformed into an enormous social mechanism for the centralisation of capitals. Commensurately with the development of capitalist production and accumulation there develop the two most powerful levers of centralisation – competition and credit. […] Whether this latter result is the consequence of accumulation or centralisation, whether centralisation is accomplished by the violent method of annexation – when certain capitals become such preponderant centres of attraction for others that they shatter the individual cohesion of the latter and then draw the separate fragments to themselves – or whether the fusion of a number of capitals already formed or in process of formation takes place by the smoother process of organising joint-stock companies – the economic effect remains the same. Everywhere the increased scale of industrial establishments is the starting point for a more comprehensive organisation of the collective work of many, for a wider development of their material motive forces – in other words, for the progressive transformation of isolated processes of production, carried on by customary methods, into processes of production socially combined and scientifically arranged. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.441