The vast plains of the planet Darwin IV are among the alien world's main environments. The plains of the northern hemisphere make up most of Planitia Borealis while the plains of the southern hemisphere make up most of Planitia Australis. The plains also continue through the Sinus Columbus, the great pass through the Equatorial Mountains. The habitats within them vary from grasslands to deserts.

These great open grasslands, which many believe to be ancient seabeds, are home to the overwhelming majority of Darwin IV's fauna. Many species have evolved into giants, a phenomenon believed to be a result of the planet's relatively low gravity (.6 of Earth) and the oxygen-rich atmosphere.

The plains support a wide assortment of fauna ranging in size from small ground-burrowing scavengers (like hoppercones), through the browsing tripedaliens and the predatory bipedalien liquivores, to the immense air-sifters, each delicately situated in its ecological niche.

On a beautiful afternoon, the rich, blue sky can be dotted with feathery cirrus clouds, and a gentle breeze can riffle through the rubbery brownish grass of a prairie that extends as far as the eye can see. For kilometers there can be mostly waving grass and drier flatlands. A dark line in the distance can indicate the outlying foothills of rougher terrain, such as a small chain of mesas.

Evidence in the form of countless fossilized tree stumps scattered throughout the plains had lead the First Darwinian Expedition's chief botanist, Dr. Dorothea Kay the human, to postulate that Darwin IV was once a far warmer and more humid planet.

Native life forms