EVOLUTION THE MASTER-KEY 



sand nebulae, of which about one-half are spiral in 

 form. This large proportion of the whole is suffi- 

 cient to exclude chance in their formation, and to 

 suggest that there must be a necessity in their 

 development. We are entitled to say that the 

 spiral nebulae constitute, next to the fixed stars, 

 the most important and characteristic objects in 

 the heavens. The first to be discovered was the 

 great nebula in Andromeda, which is still the 

 largest that is known. It was first seen by Lord 

 Rosse, and was one of the earliest of his rewards 

 for constructing his great telescope. The French 

 criticism passed at the time was that the astron- 

 omer had mistaken a spiral scratch, such as might 

 easily be produced in cleaning the lenses of a tele- 

 scope, for a celestial object. This, however, was 

 no more than ingenious. We now know that the 

 spiral nebulae constitute the second stage in the 

 evolution of a system, those which one may for 

 convenience style the "chaotic nebulae" constitut- 

 ing the first stage. 



The transition is not difficult of comprehen- 

 sion. The countless gaseous particles of which 

 the chaotic nebula is composed are subject to their 

 mutual gravitational influence. The nebula, there- 

 fore, shrinks. (Our sun the central mass of the 

 original solar nebula is shrinking at this hour at 

 the rate of about sixteen inches each year, and 

 has thereby produced the heat and light which 

 enabled me to write, you to read, and the plant 

 from which this paper is made to grow.) As the 

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