In the final chapter of his "Evolution the modern synthesis" he defines evolutionary progress as "a raising of the upper level of biological efficiency, this being defined as increased control over and independence of the environment. "Natural selection plus time produces biological improvement... Improvements in biological machinery... the limbs and teeth of grazing horses... the increase in brain-power... The eyes of a dragon-fly, which can see all round [it] in every direction, are an improvement over the mere microscopic eye-spots of early forms of life". "[Over] the whole range of evolutionary time we see general advance — improvement in all the main properties of life, including its general organization. [But] improvement is not universal. Lower forms manage to survive alongside higher".