132 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1919. 
ing matter in our own galaxy, regarded as a spiral, would furnish 
an adequate explanation of the peculiar distribution of the spirals. 
There is considerable evidence of such occulting matter in our galaxy. 
An English physicist has cleverly said that any really good theory 
brings with it more problems than it removes. It is thus with the 
island-universe theory. It is impossible to do more than to mention 
a few of these problems, with no attempt to divine those which may 
ultimately be presented to us. 
While the data are too meager as yet, several attempts have been 
made to deduce the velocity of our own galaxy within the super-gal- 
axy. It would not be surprising if the space-velocity of our galaxy, 
like those of the spirals and the Magellanic Clouds, should prove to 
be very great, hundreds of miles per second. 
Further, what are the laws which govern the forms assumed, and 
under which these spiral whorls are shaped ? Are they stable struc- 
tures; are the component stars moving inward or outward? A be- 
ginning has been made by Jeans and other mathematicians on the 
dj^namical problems involved in the structure of the spirals. The 
field for research is, like our subject matter, practically infinite. 
