Before revision:  Methods: We consider the geometric position of individuals of species, and base on that  we find the probability of observing at least one individual of the species , then we apply a translation of the well-studied problem of mixed salt-water in a tank to describe the formula of SAR. - Results: For a rectangular sample area the species-area relationship follows the pattern, with some simplification, S=c|A^{\beta}+a|^z, where S is the number of species in the area of size A and a,c,z, and \beta are constants with z<1 and \beta\leq1.
After revision:  Methods: We consider the geometric position of individuals of species, and base on that we find the probability of observing at least one individual of the species , then we apply a translation of the well-studied problem of mixed salt-water in a tank to describe the formula of SAR. For a rectangular sample area the species-area relationship follows the pattern, with some simplification, S=c|A^{\beta}+a|^z, where S is the number of species in the area of size A and a,c,z, and \beta are constants with z<1 and \beta\leq1.