Correlation of Variation in the Wrentit with Environmental Gradients

Abstract

Correlations between environment and animal size or color have long aroused the interest of biologists and have stimulated the formulations of ecogeographical rules and the evaluation of the validity of these principles in many different animals. The questions that arose concerning the mechanisms governing correspondence between environment and the animal led to early, unsuccessful experiments to establish direct effects of humidity, temperature, and color of surroundings The geographical principle concerned with high levels of melanism found in vertebrates inhabiting humid regions, namely Gloger' s rule, has been described in general terms many times. However, there is a paucity of evidence causally relating humidity and other environmental factors with color expression. It is even stated in a recent paper (Hamilton, 1958:336) that the adaptive significance of increase in pigment deposition with increases in environmental humidity is unknown.