Darwin's theory (9)
I believe the answer is:
evolution
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">darwin's theory</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for evolution that I've seen before include "Darwinian theory" , "Darwin's theory" , "Slow change" , "Gradual change" , "Development - progress" .)