Abstract: Experimental results show:(1)The fecundity of species is subject to considerable variants, being chiefly to the size of species and seasons.(2)The fecundity of species appears to increase with increasing latitude, being generally higher in temperate than in tropical zones.(3)The fecundity of species is affected most markedly by temperature and food.(4)The fecundity of individuals of a species bears close relationship with body size(length or weight)of adults. There exists a positive eorrolation or linear relationship between them, but after attaining certain size or age, the fecundity tends to decrease owing to the decline of metabolie and reproductive rates in old age.