The paper embraces a record of the great increase in the number of species and subspecies described since 1859, a definition of the word "speciation," and a discussion of the principles of continuous geographic isolation, intergradation, and speciation. Special reference is made to the molluscan genus Io, various fishes, amphibians, reptiles, birds, and mammals, and to the opinions of the specialists in these groups as to the value of isolation, insulation, and mutation in speciation. The author concludes that speciation is a normal and continuous process while mutation is abnormal and irregular in its mode of origin. Speciation is apparently always adaptive while mutation is not essentially an adaptive process, but rather a disturbance of the regular course of speciation, which governs the greater part of species origin.