Original: In his book Systematics and the Origin of Species (/O1942/O) he wrote that a species is not just a group of morphologically similar individuals, but a group that can breed only among themselves, excluding all others.
Simplified: Following his retirement, he went on to publish more than 200 articles, in a variety of journals - more than some reputable scientists publish in their entire careers; 14 of his 25 books were published after he was 65.