ge groups, called
_sub-kingdoms_. Each sub-kingdom is again divided into subordinate
groups termed _classes_. Each class is again divided into still more
subordinate groups called _orders_. Each order is again divisible into
_families_; each family into _genera_, and each genus into _species_,
while a zoological "species" may be provisionally defined as "a group of
animals which differ only by inconstant or sexual characters."

It could be wished that the reader should pursue his further inquiries
into the natural history of animals and plants, with a knowledge of
biological classification already acquired. But this is, unfortunately,
impossible, since biological classification reposes upon anatomical
facts, and cannot, therefore, be really understood until the main facts
of anatomy have been already mastered. Yet something in the way of a
classification, or at least of a definitely arranged catalogue, must be
even now attempted for the following reason:--

In the second of this series of Essays[7] we indicated the lines of
inquiry which must be followed up by any reader who would become
acquainted with the natural history of animals and plants. We saw that
their gross and minute structure, their very varied functions, their
relations to past time, and their geographical relations as well as
their relations to the physical forces and to their fellow organisms,
would all have to be successively considered. Obviously, however, it is
impossible to make known the facts of anatomy, physiology, and
hexicology[8] without constant references to animals and plants which
may be expected to be either altogether unknown, or at least very
incompletely known, to persons as yet unacquainted with zoological and
botanical science.

References to creatures so unknown or so little known would plainly be
of small profit and less interest, unless the reader was already
furnished with some mental images of such creatures and groups of
creatures--images calculated to sustain his attention and excite his
interest in the various kinds of animals and plants, otherwise unknown,
which will have to be again and again referred to. Accordingly, an
attempt must now be made to set before the reader a rough and general
sketch, or catalogue, of what the creatures and groups of creatures are,
the names of which will have so frequently to appear in the pages which
are to follow. In a word, as the preceding Essay[9] was devoted to
explaining what are the