Those, therefore, who, in spite of the fact that the productivity of modern communities has, relatively to their numbers, undergone an increase which is general, still maintain that the sole productive agency is labour, must seek for an explanation of this increase in some other fact than skill.
And without transgressing the limits which the theory of Marx imposes on us, such a further fact is very easy to find.

Adam Smith opens his _Wealth of Nations_ with a discussion of it.

The chief cause, he says, which in all progressive countries increases the productive power of the individual labourer, is not the development among a few of potentialities which are above the average, but a more effective development of potentialities common to all, in consequence of labour being divided, so that each man devotes his life to the doing of some one thing.

Thus if ten ordinary men were to engage in the business of pin-making, each making every part of every pin for himself, each man would probably complete but one pin in a day.

But if each man makes one part, and nothing else but that, thus repeating incessantly a single series of motions, each will acquire the knack of working with such rapidity that the ten together will make daily, not ten pins, but some thousands.