In Europe the chemotypes in different lichen groups tend to be distributed not randomly but along a north-south gradient. In the south more numerous and more complex chemotypes occur. In the north there are fewer chemotypes, often with fewer substances involved, or strains lacking certain compounds (0-strains). This gradient is established through chemical differentiation according to colourless substances of the medulla (group I), pigments of the cortex (group II), a combination of both in the same thallus (group III), and pigments in the medulla (group IV). The north-south gradient is found in lichens of very diverse families and genera, at least among species that are saxicolous, crustose or lobate but not sorediate. This gradient should not be interpreted as a consequence of relatively recent evolution but rather of increasing impoverishment in northern regions due to glaciation periods and the climatic conditions prevailing up to the present time.