Evolutionary ecology is concerned with the factors influencing animal numbers for which explanation has to be sought in the evolutionary history of the species. They should therefore be studied in the natural habitat of the species, and not in habitats much modified by man. As one ecological adaptation is often linked with another, evolutionary change may be slow. It is hard to test the survival value of many adaptive features in nature because unsuccessful variants have usually been eliminated. Where two variants exist, if hereditary they are probably about equally well adapted and if phenotypic they are probably adapted to different circumstances. Where an adaptive feature is similar in all members of one species, interspecific comparisons may be revealing, especially between closely related species. It is assumed that ecological adaptations are due to natural, not group, selection.