In his book Systematics and the Origin of Species -LRB- NUMBER@1 -RRB- 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 .

In his book Systematics and the Origin of Species -LRB- published in NUMBER@1 -RRB- 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 .
