I propose to confine attention to the dwarf stars[35] because it is among them that the upset has occurred. They form a well-defined series stretching from high surface-temperature to low surface-temperature, high luminosity to low luminosity, and the density increases steadily along the series. We now call this the Main Series. It comprises the great majority of the stars. To fix ideas let us take three typical stars along the series--Algol near the top, the Sun near the middle, and Krueger 60 near the bottom. The relevant information about them is summarized below: Mean Central density temperature Surface Mass (Water (million temperature Luminosity Star. (Sun = 1). =1). deg.). (deg.) Colour. (Sun = 1). Algol 4·3 0·15 40 12,000 white 150 Sun 1 1·4 40 6,000 yellow 1 Krueger 60 0·27 9·1 35 3,000 red 0·01 The idea of evolution is that these represent the stages passed through in the life-history of an individual star.[36] The increasing density in the third column should be noticed; according to our accepted criterion it indicates that the order of development is Algol→Sun→Krueger 60. A confusion between internal temperature and surface temperature is responsible for some of the mistakes of the older theories. To outward view the star cools from 12,000° to 3,000° in passing down the series, but there is no such change in its internal heat. The central temperature remains surprisingly steady. (No special reliance can be placed on the slight falling off apparently shown by Krueger 60.) It is very remarkable that all stars of the main series have a central temperature of about 40 million degrees as nearly as we can calculate. It is difficult to resist the impression that there is some unusual property associated with this temperature, although all our physical instincts warn us that the idea is absurd. But the vital point is the decrease of mass shown in the second column. _If an individual star is to progress any part of the way down the main series it must lose mass_. We can put the same inference in a more general way. Now that it has been found that luminosity depends mainly on mass, there can be no important evolution of faint stars from bright stars unless the stars lose a considerable part of their mass.