56 
the discovery of the two satellites of Mars, one of which revolves very much 
faster than Mars rotates on its axis. 
A theory that perfectly explains all the known facts may get a hearing 
and acceptance without any great amount of demonstration, but when many 
important facts appear at variance with a theory it becomes necessary to 
show how these facts may be accounted for by the theory, or to look with 
suspicion on the theory as a whole. 
There are many other facts than those just mentioned which cause 
distrust. Take for example the probable density of the ring that is supposed 
to have formed Neptune. If all the matter now in the Solar system were 
expanded till it formed a sphere the size of the orbit of this planet its average 
density would be about o1eumr non the present density of the sun. The 
density at the center would probably be many times that at the equator, 
which would make the density of the abandoned ring much less than 
21 Gos ban th of the present density of the sun. This would be many 
times as rare as the best vacuum yet obtained. To suppose that any such 
mass of matter, spread out in a ring whose diameter must have been at least 
thirty times the diameter of the earth’s orbit, ever collected in one place to 
form Neptune is a very great tax on the imagination. As a matter of fact 
it can be shown that this is physically impossible. This process involves 
long intervals of time and would make the outer planets much older than 
the earth, and other nearer planets. There is no observational data to 
support this idea; all that there are directly contradict it. On the supposi- 
tion that the sun has radiated heat in the past as it does now, and that the 
shrinkage of the sun is responsible for the development of its energy, it is 
possible to tell how many years ago the sun was large enough to fill the orbit 
of the earth. The earth must therefore be younger than this. All evi- 
dences in the earth itself point to an age of a least sixty million years, and 
on the above assumptions upon which the theory of La Place rests, the sun, 
sixty million years ago, was much larger than the earth’s orbit. The prob- 
ability is then that the assumptions are wrong. Other more technical ob- 
jections, some of which are even more conclusive, | must pass over. 
Another theory of Evolution based on tidal relations among sun, planets 
and satellites has been elaborated in more recent years, and either by itself 
or in connection with the foregoing has heen used to explain our present 
system. The application of this theory to the Earth—Moon system has 
been elaborated by Professor George Darwin. He supposes that the earth 
