rule holds good as a broad generalisation--that
the lower groups, namely, precede the higher in point of time;
and though there are apparent exceptions to the rule, there are
none of such a nature as not to admit of explanation. Some of
the leading facts upon which this generalisarion is founded will
be enumerated immediately; but it will be well, in the first
place, to consider briefly what we precisely mean when we speak
of "higher" and "lower" groups.

It is well known that naturalists are in the habit of "classifying"
the innumerable animals which now exist upon the globe; or, in
other words, of systematically arranging them into groups. The
precise arrangement adopted by one naturalist may differ in minor
details from that adopted by another; but all are agreed as to
the fundamental points of classification, and all, therefore,
agree in placing certain groups in a certain sequence. What,
then, is the principle upon which this sequence is based? Why,
for example, are the Sponges placed below the Corals; these below
the Sea-urchins; and these, again, below the Shell-fish? Without
entering into a discussion of the principles of zoological
classification, which would here be out of place, it must be
sufficient to say that the sequence in question is based upon
the _relative type of organisation_ of the groups of animals
classified. The Corals are placed above the Sponges upon the
ground that, regarded as a whole, the _plan or type of structure_
of a Coral is more complex than that of a Sponge. It is not in the
slightest degree that the Sponge is in any respect less highly
organised or less perfect, as a Sponge, than is the Coral as a
Coral. Each is equally perfect in its own way; but the structural
pattern of the Coral is the highest, and therefore it occupies a
higher place in the zoological scale. It is upon this principle,
then, that the primary subdivisions of the animal kingdom (the
so-called "sub-kingdoms") are arranged in a certain order. Coming,
again, to the minor subdivisions (classes, orders, &c.) of each
sub-kingdom, we find a different but entirely analogous principle
employed as a means of classification. The numerous animals belonging
to any given sub-kingdom are formed upon the same fundamental
plan of structure; but they nevertheless admit of being arranged
in a regular series of groups. All the Shell-fish, for example,
are built upon a common plan, this plan representing the ideal
Mollusc; but the