Essay on the Principle of Population published by Thomas Robert Malthus, in which he warned about the dangers of unchecked population growth.

World population is one billion.

U.S. Department of the Interior established.

Henry David Thoreau publishes Walden, a work that inspired many people to live simply and in harmony with nature.

Yosemite in California becomes the first state park in the United States.

George Perkins Marsh publishes Man and Nature, described by some environmentalists as the fountainhead of the conservation movement.

Ernst Haeckel coins the term ecology to describe "the body of knowledge concerning the economy of nature."

Yellowstone in Wyoming becomes the first national park.

American Forestry Association founded to encourage wise forest management.

U.S. Geological Survey established.

Yosemite becomes a national park.

John Muir founds the Sierra Club to preserve the Sierra Nevada mountain chain.

Henry S. Salt publishes Animal Rights...