Original: In his book Systematics and the Origin of Species (1942) 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: In his book Systematics and the Origin of Species (published in 1942) he wrote that a species is not just a group of individuals that look similar, but a group that can breed only among themselves, excluding all others.