The basic unit is the SPECIES, e.g., housefly, musca domestics, and similar species are grouped into GENERA, (sing. Genus) e.g. Musca is the genus, and the domestica is the species.
- SPECIES: A species is a group of organisms which resemble each other in all important characteristics. The members of a species are capable of inter-breeding, e.g., all varieties of mango plants, (or all varieties of roses or all races of human beings) belong to the same species. Variations in size, shape and colour form varieties. A species may consist of several varieties or none at all.
- GENUS: A genus is a group of similar and related species. Thus orange, lemon and grape fruit which belong to different species are grouped into a genus Citrus; similarly, lion, tiger, and the common cat, are grouped into a genus—Felis.
- Similar genera are grouped into Family,
- Similar families are grouped into Order,
- Similar orders are grouped into Series,
Thus, Series under a Class, Classes under a Division and several Divisions form the Plant Kingdom.
In animal classification—
- Similar classes are grouped into PHYLA (sing. PHYLUM)