The author reviews and discusses cytologic, genetic, and physiologic evidence, treating [male] and [female] digamety, sex-linked inheritance, non-disjunction, parthenogenesis and cyclic sexuality, gynandromorphism, hermaphroditism, genie balance, intersexes, and sex reversal. The strongest support of the chromosome theory of sex is in exceptional types of sex behaviorism and production. The work of Morgan and Bridges on sex mosaics in Drosophila is considered the most decisive evidence in favor of the idea of irregularities in distribution of X-chromosomes as explaining gynandromorphism. Difficulties in the case of birds are discussed. Evidence bearing on hermaphroditism is presented and it is concluded that difficulties encountered here are no greater than in the case of differentiation of any somatic character. The theory of genie balance was developed by Bridges from chromosome irregularities correlated with sex intergrades, etc., in Drosophila, and physiologic ideas regarding the nature of sex-determining substances were formulated by Gold-schmidt, who crossed different races of Lymantria. It is concluded that in vertebrates the sex-differentiating hormones are produced in gonads, but in insects, in various somatic tissues. Development of secondary sex characters in the latter is therefore independent of gonads.