Why are cows sacred in the Hindu tradition?

From Marx's Capital, Volume 2, Chapter 12 (1885), (quoting observers).

"While the peasant farmer starves, his cattle thrive. Repeated showers had fallen in the country, and the forage was abundant. The Hindoo peasant will perish by hunger beside a fat bullock.

"The prescriptions of superstition, which appear cruel to the individual, are conservative for the community; and the preservation of the labouring cattle secures the power of cultivation, and the sources of future life and wealth.

"It may sound harsh and sad to say so, but in India it is more easy to replace a man than an ox." ...

"Desertion of life, without reward, for the sake of preserving a priest or a cow . . . may cause the beatitude of those base-born tribes."

One could almost suspect a conscious decision by high-caste Brahmans to create a religious rite protective of cattle, the foundation of an agrarian economy which found them at the apex.

I don't know to what extent these conditions of existence still apply, 130 years later, in rural India. Increasingly, however, the concept of the sacred cow will devolve into a natural experiment in cultural inertia.

Now what's the story on the pig?