Gloger's rule is an ecogeographical rule which states that within a species of endotherms, more heavily pigmented forms tend to be found in more humid environments, e.g. near the equator. It was named after the zoologist Constantin Wilhelm Lambert Gloger, who first remarked upon this phenomenon in 1833 in a review of covariation of climate and avian plumage color. Erwin Stresemann later noted that the idea had been expressed even earlier by Peter Simon Pallas in Zoographia Rosso-Asiatica (1811). Gloger found that birds in more humid habitats tended to be darker than their relatives from regions with higher aridity. Over 90% of 52 North American bird species studies conform to this rule.