A method of testing whether or not galaxies evolve with time. It involves calculating the volume of a sphere with the Earth at its centre and a given galaxy or quasar on its perimeter. The calculated volume is then divided by the volume of a sphere for which the object, if it were on the sphere’s perimeter, would be just too faint to detect. The calculation is repeated for each member of a sample of galaxies or quasars. The average value of this ratio for the complete sample should be 0.5 if there is no evolution in the properties of the sources. Quasar samples give a value of about 0.7, which indicates that there are many more quasars at high redshift than would be expected without evolution.