 Indeed, some gradations have been considered so ubiquitous that they have been labelled ecological “rules”.One commonly cited example of a gradient in morphology is Gloger's Rule, named after Constantin Gloger, who observed in 1833 that environmental factors and the pigmentation of avian plumage tend to covary with each other, such that birds found in arid areas near the Equator tend to be much darker than those in less arid areas closer to the Poles.Since then, this rule has been extended to include many other animals, including flies, butterflies, and wolves.