A century ago astronomy was concerned with little beyond the sun, moon and planets—the small colony we have described as the sun's family. To-day it is mainly engaged in studying in detail the various other stars and colonies of stars, such as the three stars which form the system of Alpha Centauri, our nearest neighbours in space. The aggregate of all such stars and colonies constitutes the Galactic System, the vast conglomeration of stars whose rim is the Milky Way. At the same time, astronomy has discovered that even this huge system is only one of a vast number of somewhat similar systems. The present situation may be perhaps summed up in the three statements:

1. (1) The earth is only one member of the sun's family.

2. (2) The sun's family is only one member of the Galactic System.

3. (3) The Galactic System is only one member of the system of star-cities in space.

This is the furthest that astronomy has travelled so far, but we may well wonder what the situation will be, say, a thousand years hence. Will the above three statements still suffice, or will they have been supplemented by more statements of the same kind? In other words, shall we find that the whole system of star-cities only forms one unit in a still vaster assembly, and this assembly perchance a mere unit in something vaster still?