In the higher animals each individual is either of the male or the female sex. A vast amount of ingenuity has been expended by zoologists in the attempt to ascertain what it is that determines sex. Many theories have been advanced, but no one of them has obtained anything like general acceptance, because its opponents are able to adduce facts which appear to be incompatible with it. It is tempting to try to interpret the phenomenon of sex on the assumption that the female-producing biological molecule or unit is an isomeride of the male-producing cell. Certain facts, however, seem to negative the idea, as, for example, the occasional appearance in an individual of one sex of characteristics of the other sex. Possibly the attempts to explain the phenomena of sex-production on a Mendelian basis may prove to be more successful. It seems not impossible that each fertilised egg contains material which is capable of developing into male generative organs and material which is capable of developing into female generative organs, but that only one kind of material, that which dominates, succeeds in developing. The number of what are known as “X-elements” that happen to be present in the fertilised egg appear to decide which kind of material is to be dominant. But the problem of the determination of sex, fascinating though it be, is not one that can be discussed adequately in a general work on evolution. Those interested in the subject are referred to Professor Thomson’s _Heredity_, and to the address given by Professor E. B. Wilson, of Columbia University, before the American Association for the Advancement of Science, which was fully reported in the issue of _Science_, dated January 8, 1909. Stated briefly, then, our conception is, that the fertilised egg is composed of a number of entities, to which we have given the name “biological molecules,” because in certain respects their behaviour is not unlike that of chemical molecules. The units which compose these molecules, being made up of protoplasm, are endowed with all the properties of life, including the inherent instability which characterises all living matter. We suggest that the continuous or fluctuating variations that appear in the adult organism may be the result of individual differences in the biological “atoms” that compose the molecule.