Allen's rule
/ˈalənz ˌruːl/Zoology
noun
An empirical rule according to which the appendages of a warm-blooded animal are smaller in populations occupying the cooler parts of its geographical range.
Origin
1930s; earliest use found in Warder Clyde Allee (1885–1955). From the genitive of the name of Joel A. Allen, United States zoologist, who first formulated the rule in 1877 (Radical Rev. 1 116) + rule.