108 THE AMERICAN MIDLAND NATURALIST 
indicate that they are the products of combined outward and 
rotatory movement. The former is indicated by the pro- 
tuberance of the arms, the latter by their pronounced coiling. 
Such a supposition calls for the existence of an earlier body 
that embraced the whole mass, and from which the present 
nebula is descended.. We are forced to look, not only for the 
ancestor of the solar system, but for the ancestor to that an- 
cestor. Such exploration must, of course, possess a good deal 
of uncertainty, and its results must be taken, as the saying is, 
with a grain of salt. Nevertheless, where there are thousands 
of known cases similar to that under consideration—i. e., the 
spiral nebula—and in all of these cases the same results are 
evident, it is not out of the way to suppose that the same 
causes apply throughout. What the scientist must do in such 
a case is to determine what conditions might produce the re- 
‘sults observed, and take the most satisfactory of the hypo- 
theses as the one on which to base his further work. 
This is precisely what Professor Chamberlin did. The body 
most apt to produce a nebula of any sort is a sun, and of these 
there are more than 100,000,000 known, besides an unknown 
multitude of dark bodies which move through space, and of 
whose existence we have no definite knowledge. Among such 
a throng of celestial bodies it is almost inevitable that 
collisions should have occurred during the billions of years 
which the universe has been in existence. These collisions 
would naturally occur in the regions where stars are thickest, 
and it is worthy of note that in such a region, the Milky Way, 
the number of new stars—stars which appear where none 
were before—and also the number of bright line or free- 
molecular nebulae, are the greatest. This does not mean 
that the new stars and the bright line nebulae necessarily 
arise from the collision of two celestial bodies, but it does 
give weight to the statement that such collisions occur. 
If collisions between stars occur, as it is almost certain they 
do, there must be much greater probability of close approach 
of the stars to one another or to dark bodies. There are 
several astronomical considerations which make it probable 
that close approach rather than actual collision is responsible 
for the origin of the spiral nebulae, and it was therefore 
selected by Chamberlin. It must be remembered, however, 
