sexes, or
various breeds, are called, in technical language, species. The English
lobster is a species, our cray fish is another, our prawn is another. In
other countries, however, there are lobsters, cray fish, and prawns, very
like ours, and yet presenting sufficient differences to deserve
distinction. Naturalists, therefore, express this resemblance and this
diversity by grouping them as distinct species of the same "genus." But
the lobster and the cray fish, though belonging to distinct genera, have
many features in common, and hence are grouped together in an assemblage
which is called a family. More distant resemblances connect the lobster
with the prawn and the crab, which are expressed by putting all these
into the same order. Again, more remote, but still very definite,
resemblances unite the lobster with the woodlouse, the king crab, the
water flea, and the barnacle, and separate them from all other animals;
whence they collectively constitute the larger group, or class,
_Crustacea_. But the _Crustacea_ exhibit many peculiar features in common
with insects, spiders, and centipedes, so that these are grouped into the
still larger assemblage or "province" _Articulata_; and, finally, the
relations which these have to worms and other lower animals, are
expressed by combining the whole vast aggregate into the sub-kingdom of
_Annulosa_.

If I had worked my way from a sponge instead of a lobster, I should have
found it associated, by like ties, with a great number of other animals
into the sub-kingdom _Protozoa_; if I had selected a fresh-water polype
or a coral, the members of what naturalists term the sub-kingdom
_Coelenterata_, would have grouped themselves around my type; had a snail
been chosen, the inhabitants of all univalve and bivalve, land and water,
shells, the lamp shells, the squids, and the sea-mat would have gradually
linked themselves on to it as members of the same sub-kingdom of
_Mollusca_; and finally, starting from man, I should have been compelled
to admit first, the ape, the rat, the horse, the dog, into the same
class; and then the bird, the crocodile, the turtle, the frog, and the
fish, into the same sub-kingdom of _Vertebrata_.

And if I had followed out all these various lines of classification
fully, I should discover in the end that there was no animal, either
recent or fossil, which did not at once fall into one or other of these
sub-kingdoms. In other words, every animal is organised u