Here's the passage <http://marxists.org/archive/marx/works/1847/wage-labour/ch09.htm>, from Wage-Labour & Capital:
Effect of Capitalist Competition on the Capitalist Class, the Middle Class and the Working Class
We thus see how the method of production and the means of production are constantly enlarged, revolutionized, how division of labor necessarily draws after it greater division of labor, the employment of machinery greater employment of machinery, work upon a large scale work upon a still greater scale. This is the law that continually throws capitalist production out of its old ruts and compels capital to strain ever more the productive forces of labor for the very reason that it has already strained them - the law that grants it no respite, and constantly shouts in its ear: March! march! This is no other law than that which, within the periodical fluctuations of commerce, necessarily adjusts the price of a commodity to its cost of production.
No matter how powerful the means of production which a capitalist may bring into the field, competition will make their adoption general; and from the moment that they have been generally adopted, the sole result of the greater productiveness of his capital will be that he must furnish at the same price, 10, 20, 100 times as much as before. But since he must find a market for, perhaps, 1,000 times as much, in order to outweigh the lower selling price by the greater quantity of the sale; since now a more extensive sale is necessary not only to gain a greater profit, but also in order to replace the cost of production (the instrument of production itself grows always more costly, as we have seen), and since this more extensive sale has become a question of life and death not only for him, but also for his rivals, the old struggle must begin again, and it is all the more violent the more powerful the means of production already invented are. The division of labor and the application of machinery will therefore take a fresh start, and upon an even greater scale.
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