The value of a system of classification depends on its usefulness; the justification of the present attempt must lie in the direction of furnishing information additional to that of the classical Hubble system. It should be emphasized that the present system is put forward for the purpose of giving information additional to that included in Hubble's system.
This information can be considered principally to be in the field of stellar evolution, which has grown and developed so greatly over the past two decades. We may conclude (with a certain as yet unspecified probability) that as a class the "a" systems owe most of their light in the blue-violet region to B-, A-, and F-type stars; at the other extreme, we may conclude that as a group, the giant ellipticals and inner parts of systems like M 31 ("k" systems) owe most of their light to yellow and red giant stars. As far as the evidence goes, systems of intermediate form characteristics ("f," "g") have also population characteristics intermediate between the extremes. The revised system is therefore based on certain results bearing on the evolutionary history of stars - both singly and in clusters.
In the case of the S sequence, the contrast of the spiral arms reaches a maximum part way along the sequence from group aS to kS. This sequence appears to be of the greatest interest for further detailed investigation.
An interesting application of the new classification is in the comparative study of the space distribution of galaxies. An examination of the distribution over the sky of the systems classified in Table I shows that the different population groups are not well mixed; in the area from 10 hours to 12 hours of right ascension and between declinations +30° and +60°, the number of "a" systems among the nearer galaxies is outstanding. Perhaps the most interesting comparison is between the brightest members of the Ursa Major and Virgo clouds; the former contains a large percentage of "a" and "f" systems, and few - if any, ellipticals; the ratio of "a" + "af" systems to "k" systems is quite different in the two clusterings.