1. The term cline is proposed as an auxiliary taxonomic term, to denote graded spatial variation within a population. 2. This is in no way intended to replace the current methods of taxonomic specification by named areal groups. It may, however, supplement them usefully (a) by stressing continuity as against discontinuity, (b) by effecting synthesis in relating the characters of separate groups to general trends, (c) by obviating the need for assigning names to groups which are only slightly distinct. 3. Clines may be of two rather distinct types: a) compound or inter-group clines, connecting the means of characters of distinct groups (subspecies of a polytypic species, species of an Artenkreis). (b) internal clines, within a whole interbreeding group or any section of it. 4. Clines may concern size, colour, pattern, physiological resistance, the ratio between two or more distinct varieties, etc. 5. Some clines are adaptively correlated, either directly or indirectly, with corresponding environmental gradients or with other environmental factors such as colour of background. Others appear to be correlated with migration from a centre of distribution, still others with the production and subsequent migration of a mutant with positive selective value. Another type not adaptively correlated with environment is that of intermediacy between two distinct subsubspecific types across an interbreeding zone. 6. This last type of cline may be called a genocline. Other main types are geoclines, over large geographical areas, and ecoclines through a range of ecological habitats. 7. Some clines are related to development, in that the greater development of a character is related to greater intensity of rate-genes which determine it. 8. Quantitative figures for a few size geoclines in birds give changes varying from 0.5° to 2° N. lat. per 1 % change in size of part affected. 9. The principle of harmoniously stabilized gene-complexes, deduced by R. A. FISHER and others and empirically established by TIMOFEEFF-RESSOVSKY, will account for the extension of the range of particular genecombinations beyond the areas for which they were initially selected, and the restriction of intermediates to narrow zones between the ranges of the favoured stabilized combinations. 10. Owing to this principle, the existence of sufficient environmental diversity between different regions of a population’s range will establish partial biological discontinuities, the population being broken up into relatively uniform subspecies covering larger areas and separated by interbreeding zones which are kept narrow by selection. 11. Even in the event of subsequent range-change, the intergrading zones will remain narrow. 12. The production of partial discontinuities will be facilitated by the existence (a) of regions of relatively rapid environmental change, (b) of regions of relatively low population density (partial isolation). 13. The tendency to produce a regular internal cline will thus be overridden, and replaced by a stepped cline. It is theoretically probable that the subspecific groups in such cases will show internal clines of slight slope provided that environmental conditions differ sufficiently across the subspecific range. 14. Geographical and physiological (ecological) isolation will also tend to introduce discontinuities into regular clines. The resultant discontinuous intergroup cline will tend to be steeper than the original continuous internal cline, while the internal clines within the various isolated groups will tend to be less steep.