Summarizing earlier work bearing on the sp. concept and combining therewith evidence from recent cyto-genetic study, the essentials of the sp. concept are: common structural characteristics which unite certain individual organisms into one group, and a common genetic basis for the group represented by a specific chromosome complex; characteristic features which distinguish such groups from one another, one of these frequently being the chromosome garniture; relative stability combined with variability within the group, this being made possible by high regularity in chromosome distribution, while inherited variations arise from occasional changes in genes and chromosomes; common descent of all individuals of the group from one or more preexisting sp. made possible by the known mechanism of heredity and genetic variation; free intercrossing and high inter-fertility among the individuals of the group expected in organisms possessing a high degree of genie homology; absence of free intercrossing and usually low fertility if not complete sterility in hybrids between different sp. (with occasional exceptions) resulting from accumulation of genie and chromosomal differences between, diverging groups of individuals within a sp.; and sub-specific groups, often occupying different geographic areas but connected by intergrading forms, which differ more from one another than do the individuals composing each subgroup, as a result of genetic variability within the sp. plus the influence of environment, isolation, and natural selection. Vavilov holds essentially the same concept when he says "a Linnaean sp. is a separate morpho-physiological system connected in its genesis with a definite environment and area.".