18 Evolution 



ring-theory adequate. Again the great object-lesson 

 of the heavens comes to our aid. Of the 120,000 nebulae 

 that we know, about half show what is called a spiral 

 structure. Vast arms of luminous matter stretch out 

 into space, in circles or semi-circles, from the centre of 

 the nebula. These, moreover, are the nebulae that seem 

 to be in process of formation into worlds. An incan- 

 descent gas gives, when its light is analysed in the 

 spectroscope, one or more vertical lines of light. An 

 incandescent liquid or solid gives a continuous rainbow 

 band of colour (unless vapour intervenes). Now the 

 great irregular nebulae, like the beautiful and immeasur- 

 able one in Orion, have a gaseous spectrum, and seem 

 to be comparatively at rest. The spiral nebulae — the 

 vast nebula in Andromeda, that you can see right over- 

 head on a winter's night with the naked eye, the fine 

 specimen in Canes Venatici, etc. — show the spectrum of 

 a more condensed condition. It is irresistible to think 

 that these spiral nebulae are the second stage, and that 

 the planets or attendant bodies of the central sun will 

 form from the great arms of the spiral. They are 

 irregular in texture, thus offering centres for condensa- 

 tion. In some cases the material of the arms is already 

 gathering into balls : in others the arms seem to have 

 shot right out from the central sun. 



It is probably wrong to seek one type of formation for 

 all the systems in the universe. They vary enormously. 

 The ring-system may have been verified in some cases. 

 The spiral seems to be the general mode of detaching 

 masses from the shrinking body. In other cases the 

 detachment may have taken place in yet other ways. 

 Our moon is now very largely believed to have budded 

 out at one end of the earth (like the knob at the end of 

 a lemon) or broken off in a tidal wave in its early plastic 

 stage, when it rotated much more rapidly than it does 



