fifth Jovian satellite with its small mass. The feebleness of its
tide-raising power obliged it to remain behind its companions; for there
is no sign of its being more juvenile than the Galilean quartette.

The yielding of plastic bodies to the strain of unequal attractions is a
phenomenon of far-reaching consequence. We know that the sun as well as
the moon causes tides in our oceans. There must, then, be solar, no less
than lunar, tidal friction. The question at once arises: What part has
it played in the development of the solar system? Has it ever been one
of leading importance, or has its influence always been, as it now is,
subordinate, almost negligible? To this, too, Professor Darwin supplies
an answer.

It can be stated without hesitation that the sun did _not_ give birth to
the planets, as the earth has been supposed to have given birth to the
moon, by the disruption of its already condensed, though viscous and
glowing mass, pushing them then gradually backward from its surface into
their present places. For the utmost possible increase in the length of
the year through tidal friction is one hour; and five minutes is a more
probable estimate.[1181] So far as the pull of tide-waves raised on the
sun by the planets is concerned, then, the distances of the latter have
never been notably different from what they now are; though that cause
may have converted the paths traversed by them from circles into
ellipses.

Over their _physical_ history, however, it was probably in a large
measure influential. The first vital issue for each of them
was--satellites or no satellites? Were they to be governors as well as
governed, or should they revolve in sterile isolation throughout the
aeons of their future existence? Here there is strong reason to believe
that solar tidal friction was the overruling power. It is remarkable
that planetary fecundity increases--at least so far outward as
Saturn--with distance from the sun. Can these two facts be in any way
related? In other words, is there any conceivable way by which tidal
influence could prevent or impede the throwingoff of secondary bodies?
We have only to think for a moment in order to see that this is
precisely one of its direct results.[1182]

Tidal friction, whether solar or lunar, tends to reduce the axial
movement of the body it acts upon. But the separation of satellites
depends--according to the received view--upon the attainment of a
disruptive rate of rotatio