It's just not direct experimental evidence within the scope of current experiments. (discovermagazine.com)
A new, 20 - year study, led by University of Illinois at Urbana — Champaign ecologist David Philipp, «provides the first direct experimental evidence that vulnerability to angling is a heritable trait,» the authors wrote. (scientificamerican.com)
Our results provide direct experimental evidence for a significant increase in the Earth's greenhouse effect that is consistent with concerns over radiative forcing of climate. (nature.com)