The principal goal of morphological studies has been to obtain insight into galaxy formation and evolution. Fundamental problems, such as the nature of S0 galaxies, the effects of environment, morphological segregation in clusters, the origin of bars, the driving mechanisms for spiral structure, the possibility of significant secular evolution of structure within a Hubble time, and the underlying factors which determined the various types at the time of galaxy formation, all require accurate knowledge of morphology in order to be addressed reliably. How effective morphology can be in addressing these problems depends on how well the relationships between the various types of galaxies are established, and the extent to which follow-up observations and theoretical analyses are carried out.
Morphology has been useful in spite of the fact that classification has been largely a subjective visual exercise. Its techniques, advantages, disadvantages, correlations, etc. have been discussed by many authors. The review by Sandage (1975) covers all of the older references (including those of Hubble, de Vaucouleurs, Holmberg, Morgan, and van den Bergh), while more recent papers include van den Bergh (1976, 1980a), van den Bergh, Pierce, and Tully (1990), Dressler and Sandage (1978), Sandage and Brucato (1979), Dressler (1980a, 1984), Sandage and Binggeli (1984), Sandage (1986), Kormendy (1979, 1982), Kennicutt (1981), Bothun (1982a, b), and Vorontsov-Velyaminov (1987). The problems of classification are fairly well-understood, but this has not prevented astronomers from uncritically accepting the types in catalogues without regard to source, or from blaming the classifiers or the classification systems for the inadequacy of type information for some classes of galaxies. The following five points help to place the problems of galaxy morphology into a general perspective:
1. In galaxy morphology, many distinct structures or "components" are seen: bars, rings, lenses, bulges, disks, spiral patterns, etc. These features presented in various combinations and at various inclinations leave the impression that galaxy morphology consists of an almost "impenetrable thicket of forms" (to borrow a phrase from Stephen Jay Gould, 1985, from an essay on biological taxonomy).
2. Galaxies have a wide range of surface brightnesses, luminosities, and other measured properties. This means that selection effects are always important. Details that are needed for classification can be easily missed on inadequate image material.
3. Galaxy structure is by and large continuous. In a multi-dimensional classification space, transition cases almost always exist between any two distinct and sufficiently common morphologies.
4. The distribution of morphologies in rich clusters is often much narrower than for field galaxies. This implies that environment may be important in determining the galactic form.
5. Galaxies have a large cross section for collisions, mergers, or interactions. This can lead, on one hand, to the possible evolution of rare or transient galactic forms (e.g., ring galaxies), but, on another hand, it could also be responsible for some of the more common forms (e.g., E galaxies).
These points highlight the fact that galaxy morphology is a complex problem. The goal of classification is to reduce that complexity somewhat by searching for order within the "utter chaos" (again, to borrow a phrase from S. J. Gould) of a wide range of forms whose relationships to each other may not be obvious. If a classification system eventually sheds light on these relationships, then it could provide the needed physical insight for addressing the ultimate goals of understanding galaxy formation and evolution.