Jeans wrote: “The great nebulae exhibit an enormous difference of structural detail, but Hubble, who has devoted much skill and care to their classification, finds that most of the observed forms can be reduced to law and order.” This general statement applies in even stronger terms today, now that several decades of work have confirmed Hubble's intuition. In fact, “The conclusion is that the modern classification indeed describes a true order among the galaxies, an order not imposed by the classifier.”

The efforts to sharpen the empirical morphological classification are extremely important for our knowledge of the dynamics of normal galaxies. Indeed, “The ultimate purpose of the classification is to understand galaxy formation and evolution.” Thus the very existence of the morphological classification scheme proves that the observed morphology reflects a few intrinsic characteristics that vary with continuity along the Hubble sequence and that the overall structure is likely to be quasi-stationary. This plays a central role in the development of a dynamical framework for the classification of disk galaxies, for which the spiral structure is the most spectacular morphological property considered, as is outlined in Part III.

It has been noted that morphology changes significantly with the wave band of observation, with redder images generally found to be characterized by a higher degree of smoothness and regularity. For a dynamical theory aimed at bringing out the role of gravity in the classification, the recent near-infrared (near-IR) studies that probe the underlying evolved stellar disk thus become of primary importance, especially when they may show a contrast with the morphology based on standard optical images.