Identify one of Thoreau's themes or main, ideas in this selection.

Thoreau began his life at Walden, when the Industrial Revolution was in full force. Its impact upon life is best illustrated in Walden by the locomotive which passes daily by the pond, its whistles and rumbling contrasting with the natural sounds of the birds. Village life now runs at a faster pace, "railroad time," leaving even less time for the contemplation of self and nature which Thoreau desires. Thoreau believes that this kind of "progress" has a negative impact upon people's lives and upon the environment, the purity of which it pollutes and destroys.