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1. INTRODUCTION

The first step in the development of most sciences is a classification of the objects under study. Its purpose is to look for patterns from which hypotheses that connect things and events can be formulated by a method proposed and used by Bacon (1620). If the classification is useful, the hypotheses lead to predictions which, if verified, help to form the theoretical foundations of a subject.

Simple description, although not sufficient as a final system, is often an important first step. In the study of galaxies, Wolf's (1908) and Vorontsov-Velyaminov's (1962, 1963, 1964, 1968) descriptions are examples of a system of this first type. But as a classification develops, a next step is often to group the objects of a set into classes according to some continuously varying parameter. If the parameter proves to be physically important, then the classification itself becomes fundamental, and often leads quite directly to the theoretical concepts.

It is too early to judge if the classification of galaxies has reached this stage because no theory of their origin and evolution is yet certain. But the systems of Hubble (1926) (as extended by Holmberg 1958; de Vaucouleurs 1956, 1959a; van den Bergh 1960a, b; and others) and of Morgan (1958, 1959) are based on continuously varying parameters and therefore constitute classifications of the second kind.

The classification criteria for the Hubble system are (1) the size of the nuclear bulge relative to the flattened disk, (2) the character of the spiral arms, and (3) the degree of resolution into stars and H II regions of the arms and/or disk. Item (1) is likely to be related to the angular-momentum distribution of the original protogalaxy, and to the timing of earliest star formation relative to the collapse time. The other two criteria are probably related to the present rate of conversion of gas into stars in a rotating galaxy.

Both the Hubble and the Morgan systems appear to be more than simple descriptions because many galaxian properties such as the integrated color, the composite spectral type, and the density of free H I gas vary systematically along the sequence of forms. These classifications may therefore be fundamental in the above sense because they provide connective relations, based on form alone, that may be related to the initial conditions of formation and subsequent temporal change.