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particle is, in time, bound to come close to many others, and eventually to 
collide with many. 
If any one particle were large enough to start with, it would therefore 
grow by collision with other particles, and the more it grew the more power of 
growing it would have by reason of its increasing mass. It seems likely 
then, that loose, widely extended nebulae of this sort must eventually come 
into a system of small bodies revolving about a large central mass. It can 
be shown that a mass revolving in this way and suffering collision with other 
masses must move in an orbit whose eccentricity is continually diminishing. 
We should therefore expect to find, if our system has been formed in this 
way, that the more massive planets have the least eccentric orbits and that 
the smaller ones have the greatest eccentricity. As a matter of fact all of 
the large outer planets have low eccentricity and the smaller planets a 
higher amount. The greatest eccentricity is found among the planetoids, 
or asteroids, many of which are only a few miles in diameter. 
It has also been shown that a close approach of two masses in the arms 
of the spiral might not result in collision, but under conditions which might 
easily arise, the smaller might be made to revolve in an elliptical orbit about 
the larger, thus giving rise to a satellite or system of satellites, and these 
satellites might revolve in one direction as easily as another. We can 
therefore account for the retrograde motion of the satellite of Neptune, 
those of Uranus, for the fact that Jupiter has some going in one direction and 
others in the reverse direction, for the widely scattered zone of the Asteroids 
and even for the very rapid motion of the inner satellite of Mars. 
These, and many other features are not speculations as to what may have 
happened. They have all been made the subject of rigorous mathematical 
calculations, and with the supposed initial conditions are all entirely possible. 
As to whether these initial conditions that we have supposed, actually 
existed or not—whether or not our earth and the other bodies revolving 
about the sun ever developed from a spiral nebula, we can not be so sure. 
Here it is a question of what is most probable. We are practically certain 
that it did not come about as La Place supposed. There are too many 
things mathematically impossible about that. By this theory, the develop- 
ment into the present system was entirely possible, and certainly no more 
probable evolution has been proposed. 
La Place did not and could not account for his nebula. On this plan we 
can. I have said that the spirals far outnumber any other class in the sky. 
