lntraspecific variation, particularly as this manifests itself in polymorphic species, is discussed as to its basis and especially its genetic determination. The concepts of form, variety, morph and subspecies are considered as to their application in monomorphic and polymorphic species in contrast. lntraspecific variation may in both cases be quantitative or qualitative, and cryptic or apparent, but polymorphic species show, in addition to the variation found in monomorphic species, the special alternative variation constituted by their morphs. Preferentially the name polymorphism (taxonomic) should be restricted to the case of alternative morphs of taxonomic or apparent nature, cryptic alternative variation belonging to pluralism (genetic). Allied concepts, including polyphenism, meristism, mosaicism (populational) and pseudo-polymorphism are also delimited to contrast them with (taxonomic or true) polymorphism. Application of these concepts to the case of the garden snail Helix aspersa is then considered in relation to genetic determination of the morphs of this species. Morphs are in general composed by characters each one with unitary alternative genetic determination; without such a basis being established the distinction of the component entities of morphs may be precarious. Genes as yet known, for shell pigmentary and allied structural characters of H. aspersa are mentioned. The genes may be of a dual type or of a multiple type as to their effects. Only two of the loci of the dual type, albino and especially uniform, have alleles which produce generalized, non-combinatory phenotypes; all the other loci whose effects we have studied produce combinatory morphs. lt is shown with examples that knowledge of the genetic basis of morphic characters allows that no confusion between polymorphism and pseudo-polymorphism results. lt is discussed how the effects of tanning of the periostracum, in the adults of H. aspersa may contribute to obliterate initially sharp distinctions of shell ground colour and bands, this leading to apparently lower degrees of polymorphism or even to false pseudo-polymorphism.