 For example, it has been observed that in Australia, birds generally become smaller the further towards the north of the country they are found.In contrast, the intensity of their plumage colouration follows a different geographical trajectory, being most vibrant where humidity is highest and becoming less vibrant further into the arid centre of the country.Because of this, clines were defined by Huxley as being an “auxiliary taxonomic principle”; that is, clinal variation in a species is not awarded taxonomic recognition in the way subspecies or species are.