From a letter in response to "Our Biotech Future", by Freeman Dyson:

The basic premise is that every step in evolutionary change is stochastic and adaptive. No new structure that appears in this way persists unless it is immediately adaptive, even if just one more step might produce something very superior. Thus green leaves dominate because they happen to have come along before black ones, and also because chance uncovered no route from green to black that was adaptive at every new step.