60 ILLINOIS ACADEMY OF SCIENCE 
centers of the knots. The movement of these minute bodies 
was of the same type as that of the planetoids and planets and 
hence they have been called planetesimals, and the theory of 
which they are the distinctive working element, the plane- 
tesimal hypothesis. Those minute bodies that, in addition to 
revolving about the solar center, revolved also about the grav- 
ity-centers of the knots, may be called satellitesimals if one 
wants to be specially precise, but the term planetesimal is the 
generic one and covers the whole class of small bodies revolv- 
ing in orbits similar to those of the planets. 
We have now the working mechanism for the remainder of 
the evolution. All the rest follows as a matter of celestial 
mechanics, and the details need not be pursued here. The 
knots, as the natural centers of growth, gathered in the haze 
and grew to planets, planetoids or satellites. The eccentric- 
ities, obliquities and irregularities of the original planetesimal 
orbits involved many crossings and thus facilitated accretion. 
The shiftings of the orbits by the mutual attractions within 
the system led on constantly to new relations and further aided 
aggregation until most of the planetesimals were gathered 
into the growing planets, planetoids and satellites. Perhaps 
some planetesimals remain unassembled and contribute to the 
phenomena of the Zodiacal Light and the Gegenschein, The 
combination of a multitude of bodies in somewhat eccentric 
and irregular orbits led inevitably to fewer and more circular 
orbits. The nuclei that grew most came to have the most 
circular orbits as an obvious consequence, and this is well 
exemplified in the present planetary system. The planetoids 
are especially eccentric, while the small planets, Mercury and 
Mars, have more eccentric orbits than the great planets. As 
a mechanical effect, each nucleus moved into a larger or a 
smaller orbit in proportion as more planetesimals of larger or 
of smaller orbits were added to it, so that the growing planets 
tended to space themselves out automatically towards the 
less occupied feeding grounds. Probably the distribution of 
mass in the nebula gave rise to stable and unstable zones, 
and the planetesimal orbits were more or less bunched in the 
stable zones on account of this and thus also the growth and 
position of the planetary nuclei were influenced. Bode’s law 
thus perhaps comes to have a physical meaning. 
