Summary.1. Previous work on the behaviour of genes in interspecific crosses is discussed, and it is concluded that allelomorphic relationships exist between the genes of crossable species whether these are in the same genus or in reputedly different genera2. Genetical experiments on interspecific hybrids in six species of the genusGossypiumenable the following main conclusions to be drawn:(a) Although allelomorphic relationships exist throughout the six species in respect of all genes examined, cases of identity (apparent or real) are practically confined to recessive genes only. Geographical isolation over a long period of time has resulted in the production of new alleles at most loci. These may be termed “species alleles”. Species endemic in the Galapagos and Hawaiian Islands are characterised by species alleles not found in mainland species.(b) The introduction of genes from one species into another indicates that species differ not only in the mode of distribution of alleles functioning as main genes, but also in the modifier complexes accompanying such alleles. The degree to which modifier complexes differ in species is of primary importance from a taxonomic point of view.(c) The bearing of experiments with the “Crinkled” mutant on the Fisher theory of dominance is discussed. It is shown that a number of normal alleles of crinkled exist, which are distinguishable only by their dominance potency, that the dominance relation is due to the interaction of a normal allele of specific potency with a modifier complex to which it is precisely adjusted, and that the genes constituting the dominance modifier complex have been preserved not because of their function as modifiers of initially disadvantageous heterozygotes, but because of their selective value on their own account. It is believed, however, that on the appearance of the crinkled mutant, selection probably ensued in favour of alleles with greater dominance potency,i.e.that the “Haldane effect” has been operative.(d) Examples are given of three different ways in which homologous characters can be genetically constructed in the genusGossypium.It is pointed out that the conception of continuous change with time in the genetical architecture of an organ is probably of profound evolutionary significance.3. The Darwinian process of evolution by natural selection, involving mere gene substitution, has probably been the main mechanism involved in the profound changes induced in the genusGossypiumthrough geographica