It is assumed that the rate of star formation for population I varies with a power n of the density of interstellar gas and that the initial luminosity function is time-independent. Direct evidence on the value of n is found in the relative distribution, perpendicular to the galactic plane, of gas and young objects. For various values of n, computations were made of the initial luminosity function, the rate of star formation, the exchange of gas between stars and interstellar medium, the number of white dwarfs and their luminosity function, and the abundance of helium. It is concluded, from a comparison of the results with observational data, that n is around 2. The present rate of star formation, then, is five times slower than the average rate. The interstellar gas, of which the surface density on the galactic plane was taken to be 11 per square parsec, loses 1.4 O/pc2 per 10 years by the formation of stars but gains about one-third of this by ejection of gas from evolving stars. The present helium abundance of the interstellar gas may be explained if a star has burned, on the average, 53 per cent of its original hydrogen into helium at the time that ejection takes place. The ejected material was assumed to have a composition equal to the average composition of the star. The effect of star formation on the gas density in the galactic system and other galaxies is briefly discussed.