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 considering the vast reduction of brachiopods, and the fact that our
existing cephalopods, though few in number, are more highly organised than
their ancient representatives.
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