Excerpt from Karl Marx, Capital, Volume I, Chapter 6: The Buying and Selling of Labour Power.
“The owner of labour-power is mortal. If then his appearance in the market is to be continuous, and the continuous conversion of money into capital assumes this, the seller of labour-power must perpetuate himself, “in the way that every living individual perpetuates himself, by procreation.”  The labour-power withdrawn from the market by wear and tear and death, must be continually replaced by, at the very least, an equal amount of fresh labour-power. Hence the sum of the means of subsistence necessary for the production of labour-power must include the means necessary for the labourer’s substitutes, i.e., his children, in order that this race of peculiar commodity-owners may perpetuate its appearance in the market. 
In order to modify the human organism, so that it may acquire skill and handiness in a given branch of industry, and become labour-power of a special kind, a special education or training is requisite, and this, on its part, costs an equivalent in commodities of a greater or less amount. This amount varies according to the more or less complicated character of the labour-power. The expenses of this education (excessively small in the case of ordinary labour-power), enter pro tanto into the total value spent in its production. “
Full text available at http://www.marxists.org/archive/marx/works/1867-c1/#n9