166 MAN'S PLACE IN THE UNIVERSE 



From the small amount of data yet collected it would 

 seem that our own nebula or galactic system is ex- 

 ceptionally large; it is even suggested that if the spiral 

 nebulae are "islands" the galactic system is a "con- 

 tinent". But we can scarcely venture to claim premier 

 rank without much stronger evidence. At all events 

 these other universes are aggregations of the order of 

 ioo million stars. 



Again the question raises itself, How far does this 

 distribution extend? Not the stars this time but uni- 

 verses stretch one behind the other beyond sight. Does 

 this distribution too come to an end? It may be that 

 imagination must take another leap, envisaging super- 

 systems which surpass the spiral nebulae as the spiral 

 nebulae surpass the stars. But there is one feeble gleam 

 of evidence that perhaps this time the summit of the 

 hierarchy has been reached, and that the system of 

 the spirals is actually the whole world. As has already 

 been explained the modern view is that space is finite — 

 finite though unbounded. In such a space light which 

 has travelled an appreciable part of the way "round the 

 world" is slowed down in its vibrations, with the result 

 that all spectral lines are displaced towards the red. 

 Ordinarily we interpret such a red displacement as sig- 

 nifying receding velocity in the line of sight. Now it is 

 a striking fact that a great majority of the spirals which 

 have been measured show large receding velocities often 

 exceeding iooo kilometres per second. There are only 

 two serious exceptions, and these are the largest spirals 

 which must be nearer to us than most of the others. 

 On ordinary grounds it would be difficult to explain why 

 these other universes should hurry away from us so fast 

 and so unanimously. Why should they shun us like 

 a plague? But the phenomenon is intelligible if what 



