It is generally believed that "species are the sum total of biological individuals with similar shapes, which can mate with each other and need similar environmental conditions". Someone has given a long definition of species: "Species is a unified form of continuity and discontinuity in the development of the biological world. The biological world develops in the form of species, with the formation of species as a certain stage of development: in sexual animals, species appear as a unified breeding group, which is composed of groups that occupy a certain space and are alternately distributed, but are isolated from other species groups in reproduction. " This definition is comprehensive, which explains the position of species in the process of development, that is, species is a discontinuous stage in the process of continuous development, and also explains an important feature of species, that is, individuals within species can have sexual reproduction, while there is reproductive isolation between different species.