exist in the atmospheres of the Class M and N stars,
respectively. The presence of these oxides indicates a
relatively low temperature, and this is what we should expect
from stars so far advanced in life.

The period of existence succeeding the very red stars has
illustrations near at hand, we think, in Jupiter, Saturn,
Uranus and Neptune, and in the Earth and the other small
planets and the Moon: bodies which still contain much heat, but
which are invisible save by means of reflected light.

The progression of stellar development, which we have
described, has been based upon the radiation of heat. This is
necessarily gradual, and the corresponding changes of spectrum
should likewise be gradual and continuous. It is not intended
to give the impression that only a few types of spectra are in
evidence: the variety is very great. The labels, Class B, Class
A, and so on to Class N, are intended to mark the miles in the
evolutionary journey. The Harvard experts have put up other
labels to mark the tenths of miles, so to speak, and some day
we shall expect to see the hundredths labeled. Further, it is
not here proposed that heat radiation is the only vital factor
in the processes of evolution. The mass of a star may be an
important item, and the electrical conditions may be concerned.
A very small star and a very massive star may develop
differently, and it is conceivable that there may be actual
differences of composition. But heat-radiation is doubtless the
most important factor.

The evolutionary processes must proceed with extreme