Text 1: The 'Gaia Hypothesis' proposes that living organisms and their inorganic surroundings evolve as a single, self-regulating system to maintain the conditions for life on Earth. Proponents argue that the stability of Earth's climate over millions of years is evidence of this planetary-scale biological regulation. Text 2: Critics of the Gaia Hypothesis, such as biologist W. Ford Doolittle, argue that for such a system to evolve, it would require group selection on a planetary scale, which is inconsistent with the principles of Darwinian evolution. Doolittle suggests that the apparent 'regulation' of Earth is more likely a series of feedback loops that have survived simply because they did not lead to total extinction.