ons hinge on the
question whether we really know how old the world is, and at what period
the various forms of life first appeared; and this may well be disputed.

The problem whether organisation on the whole has advanced is in
many ways excessively intricate. The geological record, at all times
imperfect, does not extend far enough back to show with unmistakable
clearness that within the known history of the world organisation has
largely advanced. Even at the present day, looking to members of the
same class, naturalists are not unanimous which forms ought to be ranked
as highest: thus, some look at the selaceans or sharks, from their
approach in some important points of structure to reptiles, as the
highest fish; others look at the teleosteans as the highest. The ganoids
stand intermediate between the selaceans and teleosteans; the latter
at the present day are largely preponderant in number; but formerly
selaceans and ganoids alone existed; and in this case, according to
the standard of highness chosen, so will it be said that fishes have
advanced or retrograded in organisation. To attempt to compare members
of distinct types in the scale of highness seems hopeless; who will
decide whether a cuttle-fish be higher than a bee--that insect which
the great Von Baer believed to be "in fact more highly organised than a
fish, although upon another type?" In the complex struggle for life it
is quite credible that crustaceans, not very high in their own class,
might beat cephalopods, the highest molluscs; and such crustaceans,
though not highly developed, would stand very high in the scale of
invertebrate animals, if judged by the most decisive of all trials--the
law of battle. Beside these inherent difficulties in deciding which
forms are the most advanced in organisation, we ought not solely to
compare the highest members of a class at any two periods--though
undoubtedly this is one and perhaps the most important element in
striking a balance--but we ought to compare all the members, high
and low, at two periods. At an ancient epoch the highest and lowest
molluscoidal animals, namely, cephalopods and brachiopods, swarmed in
numbers; at the present time both groups are greatly reduced,
while others, intermediate in organisation, have largely increased;
consequently some naturalists maintain that molluscs were formerly more
highly developed than at present; but a stronger case can be made out on
the opposite side, by consid