222 CURTIS: MODERN THEjORieS OP SPIRAL, NeBULAS 



pected in view of the fact that the average space velocity of 

 the spirals is about thirty times that of the stars. I have re- 

 peated all the earlier plates of the Keeler nebular program, and 

 was able to find no certain evidence of either translation or 

 rotation in these objects in an average time interval of thirteen 

 years. ^ Their form, and the evidence of the sp>ectroscope, 

 indicate, however, that they are in rotation. Knowing that 

 their space-velocities are high, the failure to detect any certain 

 evidence of cross motion is an indication that these objects 

 must be very remote. 



Even if the spiral is not a stage in stellar evolution, but a 

 class apart, is it still possible to assume that they are, notwith- 

 standing, an integral part of our own stellar universe, sporadic 

 manifestations of an unknown line of evolutionary development, 

 driven off in some mysterious manner from the regions of greatest 

 star density? 



A relationship between two classes of objects may be one 

 of avoidance just as logically as one of contiguity. It has 

 been argued that the absolute avoidance which the spirals mani- 

 fest for the galaxy of the stars shows incontrovertibly that they 

 must, by reason of this very relationship of avoidance, be an 

 integral feature of our galaxy. This argument has proved 

 irresistible to many, among others to so keen a thinker as Herbert 

 Spencer, who wrote: 



In that zone of celestial space where stars are excessively abundant 

 nebulae are rare; while in the two opposite celestial spaces that are 



furthest removed from this zone nebulae are abundant 



Can this be mere coincidence? When to the fact that the general mass 

 of the nebulae are antithetical in position to the general mass of the 

 stars, we add the fact that local regions of nebulae are regions where 

 stars are scarce does not the proof of a physical connec- 

 tion become overwhelming? 



It must be admitted that a distribution, which has placed 

 three-quarters of a milUon objects around the poles of our galaxy, 

 would be against all probability for a class of objects which would 

 be expected to be arranged at random, unless it can be shown 



' Curtis, H. D. The proper motion of the nebulae. Publ. Astron. Soc. 

 Pacific 27: 214. 1915. 



