Ever since it became apparent that terrestrial animals ranging over large continental areas generally showed a certain degree of gradual geographic variation, attention has been focused on the colour variation of the Jay, Garrulus glandarius, in Europe. Surely the Jays belong to those species of palearctic land birds in which the formation of geographical differences must be considered to be exceptionally favoured: HARTERT (1903—1922; including HARTERT & STEINBACHER 1932) recognized as many as 10 European races of the Jay by name, whereas Kleiner (1935—38) in his monographic treatment of the species numbered as many as 9 races in the same region. In several instances of the geographic variation of the Jay the differences are exceedingly striking, e.g. between the reddish brown Jays from Ireland and the dark grey ones from northern and central Europe. Still, the intergradations are so gradual and the individual variation is so unexpectedly large, that the application of subspecific names as a method of expressing geographical variation has proved to meet with serious difficulties. The resulting confusion of names for years has stressed geographical differences being of only minor importance and has obscured others meriting a closer attention. However, it was not at all for nomenclatorial purposes that this study was started, nor in order to propose a new arrangement of the geographic races of the Jay in Europa. That, in spite of this, these topics have been dealt with in one of the following chapters of this paper must be explained from the fact that the author failed to see a possibility to avoid them. The main purpose of this study was to investigate instances of “clinal variation”, meaning the presence and the origin of geographical character gradients. “Character gradients in the frequencies or in the expression of variable characters” (DOBZHANSKY 1947, p. 67) occurring in continuous geographical areas have seriously attracted the attention of students of population genetics and of micro-evolution. Hence it seemed worth while to select a suitable subject for a comparison of local individual variation with geographical variation and to study the origin of the clines. This is what the author has tried to do in the course of the present study on Garrulus glandarius.