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The overall characteristics of a galaxy are determined by its so-called integral properties, which include the morphological form under some classification system, color, total mass, luminosity, hydrogen mass, HII region abundance, radio continuum power, etc. By studying the correlations among these parameters we hope to learn something of the evolution of galaxies and the reasons for their different morphological forms.

Surveys of external galaxies in the 21-cm line have been made by a number of authors, and there are several excellent reviews of the integral properties of galaxies (e.g., Roberts, 1967, 1969). We shall merely note here some of the established trends of the integral properties.