Species means that there is normally no cross breeding in the wild. You can have two species which will reproduce in captivity, but in the wild are blocked by s

Species means that there is normally no cross breeding in the wild. You can have two species which will reproduce in captivity, but in the wild are blocked by some geographic feature such as mountains or water, or by some behavioral aspect such as a mating ritual. It's not uncommon for two species in the same genus to be able to have offspring, for example equines (mules) and felines (ligers). In some cases the resulting hybrid offspring are fertile, for example the "Savanna" cats which are domestic cats hybridized with servals. In the wild, when previously separated species are put in contact with each other, there can be instances of natural hybrids. This is what happened with the various Homo species, due to various migrations.