The taxonomy and the species
A species is a group of more similar living organisms in the shape that can reproduce to give birth of the new fertile individuals which are able to reproduce and keeping the existence of the species.
The species is the basic classification unit for the living organisms, Linnaeus used the species as a fundamental unit of the natural classifying system, African, European and Asian humans whatever their colour, race or home are also belonging to one species (Human).
The taxonomy is a branch of biology that searches for the similarities and the differences among the living organisms, and it places the similar ones in groups according to a certain system in order to ease their study.
An intercourse may happen between some of the coupled species, but the product offspring is sterile.
For example when there is an intercourse occurring between a donkey and a horse, the product offspring will be a sterile female called “Mule”, The intercourse between a zebra, and a donkey produces a sterile female called “Zonkey”.
If you look at the dogs, the cats and the rabbits, You will notice that although the cats differ from each other, they differ more than the rabbits, and the dogs, So it is impossible for the cats to mate with the rabbits.
Mating can take place among any couple of cats whatever the difference in the shape or the size, so, all the cats are placed in one species, while the rabbits are classified in another different species and so dogs.