a planet
between Mars and Jupiter which burst, the explosion must have produced
just such clusters of bodies and classes of phenomena as we actually
find?

And what is the objection? Merely that if such an explosion occurred it
must have occurred many millions of years ago--an objection which is in
fact no objection; for the supposition that the explosion occurred many
millions of years ago is just as reasonable as the supposition that it
occurred recently.

It is, indeed, further objected that some of the resulting fragments
ought to have retrograde motions. It turns out on calculation, however,
that this is not the case. Assuming as true the velocity which Lagrange
estimated would have sufficed to give the four chief planetoids the
positions they occupy, it results that such a velocity, given to the
fragments which were propelled backwards by the explosion, would not
have given them retrograde motions, but would simply have reduced their
direct motions from something over 11 miles per second to about 6 miles
per second. It is, however, manifest that this reduction of velocity
would have necessitated the formation of highly-elliptic orbits--more
elliptic than any of those at present known. This seems to me the most
serious difficulty which has presented itself. Still, considering that
there remain probably an immense number of planetoids to be discovered,
it is quite possible that among these there may be some having orbits
answering to the requirement.

NOTE V. Shortly before I commenced the revision of the foregoing essay,
friends on two occasions named to me some remarkable photographs of
nebulae recently obtained by Mr. Isaac Roberts, and exhibited at the
Royal Astronomical Society: saying that they presented appearances such
as might have been sketched by Laplace in illustration of his
hypothesis. Mr. Roberts has been kind enough to send me copies of the
photographs in question and sundry others illustrative of stellar
evolution. Those representing the Great Nebulae in Andromeda and Canum
Venaticorum as well as 81 Messier are at once impressive and
instructive--illustrating as they do the genesis of nebulous rings round
a central mass.

I may remark, however, that they seem to suggest the need for some
modification of the current conception; since they make it tolerably
clear that the process is a much less uniform one than is supposed. The
usual idea is that a vast rotating nebulous spheroid arises before