2.2. Physical foundations of the Hubble sequence
Figure 1 shows the familiar ``tuning fork'' diagram, representing Sandage's definitive exposition  of the system described by Hubble in his book Realm of the Nebulae . I assume the reader is familiar with the basic organization of the classification system, and I will focus here on issues of physical interpretation. Because of limited space, this section only touches upon the details of the Hubble system - in particular I do not have space to describe the important extensions to the Hubble sequence developed by de Vaucouleurs . The reader can consult several excellent reviews for more comprehensive descriptions of these issues [52, 74, 85]. Furthermore, I will not be able to consider alternative classification systems in this lecture - in my view the most important of these is the Yerkes system developed by Morgan [63, 64], based on central concentration of galaxian light.
Figure 1. The ``tuning fork'' of the classical Hubble sequence . As described below, the correlation between position on this sequence and the physical properties of galaxies suggests a strong underlying link between morphology and galaxy evolution.