Wolfgang Kohler 
Mentality of Apes [PDF ebook] 

Wolfgang Koehler demonstrated that chimpanzees could solve problems by applying insight. His research showed that the intellectual gap between humans and chimpanzees was much narrower than previously thought. The work was revolutionary when originally published in 1917 in German, but it was largely ignored for decades because it violated the conventional wisdom that animal behavior is simply the result of instinct or conditioning.