Along with his analysis of pheasant phylogeny and his studies of life in the Galápagos Islands, Beebe regarded one of his most important contributions to the field of evolutionary biology to be his hypothesis that the ancestors of birds passed through what he referred to as a “Tetrapteryx stage”, with wings on both their front and hind limbs. Beebe based this theory on his observation that the hatchlings and embryos of some modern birds possess long quill feathers on their legs, which he regarded as an atavism; he also noticed vestiges of leg-wings on one of the specimens of "Archaeopteryx". Beebe described his idea in a 1915 paper published in "Zoologica", titled "A Tetrapteryx Stage in the Ancestry of Birds".