The writer has recently completed a survey of the Palaearctic members of the genus Parus, which it is hoped will be published in full elsewhere. In the course of the work the importance of the so-called (Bergmann's, Allen's, Gloger's and a rule for tail-length) became apparent, accounting straight away for most of the observed geographical variation of species with continuous ranges and some of that observed in species with discontinuous ranges. Bergmann's rule was, however, found to need some modification, and some amplification of all the rules was necessary when comparisons were made between species. The purpose of this paper is to give an account of these climatic trends in variation, and to discuss briefly their bearing on questions of adaptation. Since a fuller account is being prepared, the treatment here will be condensed. But it is hoped that the attention of systematists may be directed to the possibility of finding similar modifications and amplifications of the ecological rules in other groups. The genus Parus, comprising the tits and chickadees of the Old and New Worlds, contains 30-40 species of small, agile, hole-nesting birds with short beaks. which exploit the branches, twigs and leaves of trees for feeding. Many of them have very wide geographical ranges, and in many areas several species occur together, often as many as four or five in the same habitat. Their colours are varied but mostly subdued, the upper parts being of various shades of grey, blue-grey, brown, olive or green, the under parts usually pale, with some buff or yellow, and the head often with some pattern of black or white. They have long been a favourite genus for bird systematists, and the ecology and behaviour of some sp-ecies have been thoroughly investigated (e.g. Odum, 1941/2; Dixon, 1949; Gibb, 1950; Kluijver, 1951; Hinde, 1952). The Eurasian members of the genus fall rather naturally into several groups, which may be conveniently listed here: