SPIRAL NEBULAE — CURTIS. 127 
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 13 years/ Their form, and the evidence 
of the spectroscope, indicate, however, that they are in rotation. 
Knowing that their space velocities are high, tlie 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, notwithstanding, 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 avoid- 
ance just as logically as one of contiguity. It has been argued that 
the absolute avoidance wliich the spirals manifest 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 posi- 
tion 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 connection become overwhelming? 
It must be admitted that a distribution, which has placed three- 
quarters of a million objects around the poles of our galaxy, would 
be against all probability for a class of objects which would be ex- 
pected to be arranged at random, unless it can be shown that this 
peculiar grouping is only apparent, and clue to some phenomenon in 
our own galaxy. This point will be reverted to later. 
It has been shown that the factors of space velocity and space 
distribution separate the spirals very clearly from the stars of our 
galaxy ; from these facts alone and from the evidence of the spectro- 
scope the island-universe theory is given a certain measure of 
credibility. 
Another line of evidence has been developed within the past two 
years which adds further support to the island-universe theory 
of the spiral nebulae. 
s Curtis, H. D. The proper motion of the nebulae, Publ. Astron. Soc. Pacific 27 : 214. 
1915. 
