110 THE AMERICAN MIDLAND NATURALIST. 
dispersion, with corresponding opportunity for cooling would 
soon make of them liquid or solid particles revolving about 
the sun as their common center of gravity. These particles 
were the originals of the planetesimals, or as the word means, 
“little planets.” 
We now have the spiral nebula, ready for the final develop- 
ment into the solar system. In it are five elements which are 
to perform the leading parts in the evolution of a planetary — 
system from the spiral. They are: 
1. The great central mass (to become the sun.) 
2. The main knots in the arms (to become the planets.) 
3. Minor arm knots near the large knots, and more or less 
controlled by them (to become nuclei for the 
satellites.) 
4. Small, scattered knots (to become nuclei of the 
asteroids. ) 
Or 
_ scattered nebulous matter (to be added to the nuclei or 
sun. ) 
It is assumed that in the early spiral nebula the small par- 
ticles, or planetesimals, possessed elliptical orbits, as do the 
bodies of the solar system at the present time. All of these 
orbits would have as their gravitative center the sun, as would 
also the orbits of the nuclei. In the course of their passages 
through space the various bodies, both nuclei and planetesi- 
mals, would either pass near to each other or collide and as a 
result the small particles would be drawn to the larger par- 
ticles, and these in turn to the nuclei. We thus have the 
nuclei, or beginnings, of the planets gradually increasing in 
size by the aquisition of the scattered fine material of the 
nebula. How long this may have taken—how many thousands 
of millions of years the growth of a planet occupied—we have 
no means of telling, but without doubt it was many. 
How small the nucleus of the earth was we do not know, 
just as we do not know how rapidly it was built up. We know 
that the process has not yet ceased, for every year millions of 
meteors come within the atmosphere. Most of them become 
dust before reaching the surface of the earth, but the larger 
