Nest-building

Definitions

"A very different and fairly common quasi-human kind was sometimes produced by planets rather larger than the Earth. Owing to the greater strength of gravitation, there would first appear, in place of the familiar quadruped, a six-legged type. This would proliferate into little sextuped burrowers, swift and elegant sextuped grazers, a sextuped mammoth, complete with tusks, and many kinds of sextuped carnivora. Man in these worlds sprang usually from some small opossum-like creature which had come to use the first of its three pairs of limbs for nest-building or for climbing. In time, the forepart of its body thus became erect, and it gradually assumed a form not unlike that of a quadruped with a human torso in place of a neck. In fact it became a centaur, with four legs and two capable arms. It was very strange to find oneself in a world in which all the amenities and conveniences of civilization were fashioned to suit men of this form."