532 
MR, G. H. DARWIN ON THE 
If the tidal retardation is sufficiently great, the increase of rotation due to con¬ 
traction will he so far counteracted as never to permit an epoch of instability to occur. 
Now the rate of solar tidal frictional retardation decreases rapidly as we recede from 
the sun, and therefore these considerations accord with what we observe hi the solar 
system. 
For Mercury and Venus have no satellites, and there is a progressive increase in the 
number of satellites as we recede from the sun. Moreover, the number of satellites is 
not directly connected with the mass of the planet, for Venus has nearly the same 
mass as the earth and has no satellite, and the earth has relatively by far the largest 
satellite of the whole system. Whether this be the true cause of the observed distri¬ 
bution of satellites amongst the planets or not, it is remarkable that the same cause 
also affords an explanation, as I shall now show, of that difference between the earth 
with the moon, and the other planets with their satellites, which has caused tidal 
friction to be the principal agent of change with the former but not with the latter. 
In the case of the contracting terrestrial mass we may suppose that there was for 
a long time nearly a balance between the retardation due to solar tidal friction and 
the acceleration due to contraction, and that it was not until the planetary mass 
had contracted to nearly its present dimensions that an epoch of instability could 
occur. 
It may also be noted that if there be two equal planetary masses which generate 
satellites, but under very different conditions as to the degree of condensation of the 
masses, then the two satellites so generated would be likely to differ in mass ; we 
cannot of course tell which of the two planets woidd generate the larger satellite. 
Thus if the genesis of the moon was deferred until a late epoch in the history of the 
terrestrial mass, the mass of the moon relatively to the earth, would be likely to differ 
from the mass of other satellites relatively to their planets. 
If the contraction of the planetary mass be almost completed before the genesis 
of the satellite, tidal friction, due jointly to the satellite and to the sun, will thereafter 
be the great cause of change in the system, and thus the hypothesis that it is the sole 
cause of change will give an approximately accurate explanation of the motion of the 
planet and satellite at any subsequent time. 
That this condition is fulfilled in the case of the earth and moon, I have endeavoured 
to show in the previous papers of this series. 
At the end of the last of those papers the systems of the several planets were 
reviewed from the point of view of the present theory. It will be well to recapitulate 
shortly what was there stated and to add a few remarks on the modifications and 
additions introduced by the present investigation. 
The previous papers were principally directed to the case of the earth and moon, 
and it was there found that the primitive condition of those bodies was as follows :— 
the earth was rotating, with a period of from two to four hours, about an axis inclined 
at 11° or T2 C to the normal to the ecliptic, and the moon was revolving, nearly in 
