(1) Polymorphism is the occurrence together in the same habitat of two or more distinct forms of a species in such proportions that the rarest of them cannot be maintained by recurrent mutation. (2) If a genetically controlled form occurs in even a few per cent, of a population it must have been favoured by selection. (3) Polymorphism may either be transient, in which a gene is in process of spreading through a population unopposed, or balanced, in which it is maintained at a fixed level by a balance of selective agencies. (4) Owing to the recurrent nature of mutation, transient polymorphism is generally due to changes in the environment, which make the effects of a previously disadvantageous gene beneficial. (5) A notable instance of transient polymorphism is provided by industrial melanism. (6) The explanation of this phenomenon, either on a basis of simple selection for the black forms in industrial country or of induced mutation, produced by mineral salts in the food, cannot be supported. (7) On the contrary, industrial melanism is probably due to the spread of genes improving the viability of the species, but those responsible for excess melanin production could not be used in normal circumstances, for this destroys cryptic colouring. It is now known that the genes giving rise to industrial melanism occur as rarities in the ordinary population, and that they confer a greater viability than the normal forms which they have been unable to replace. Such genes can, however, be used in industrial areas, where black colouring is a less serious handicap. (8) The variability involved in balanced polymorphism may either be environmental or genetic. (9) A balanced polymorphism generally involves a high degree of permanence in the ratios of the respective forms. (10) Variations in the relative frequencies of polymorphic forms may, however, follow cycles in absolute numbers. (11) Polymorphic forms may be distributed as clines, which may or may not lead from groups which are monomorphic. (12) Though balanced polymorphism necessitates the existence of a switch mechanism determining alternative types, these are themselves capable of genetic adjustment within the framework of the polymorphism. (13) A special kind of balanced polymorphism is associated with close linkage dependent upon sectional translocations of the chromosomes. (14) The polymorphism of heterostyled plants is due rather to the physiological than the morphological differences between the forms. (15) Sex itself is a true balanced polymorphism, environmentally or genetically controlled. (16) The human polymorphisms include the ability to taste phenyl-thio-urea in low concentrations and the blood groups (and conditions related to them). The relative claims of the transient and the balanced systems of polymorphisms are considered for each of them.