o heads: we may consider structural
characteristics, and we may consider physiological characteristics.

In the first place, as regards structural characteristics, I endeavoured
to show you, by the skeletons which I had upon the table, and by
reference to a great many well-ascertained facts, that the different
breeds of Pigeons, the Carriers, Pouters, and Tumblers, might vary in
any of their internal and important structural characters to a very
great degree; not only might there be changes in the proportions of the
skull, and the characters of the feet and beaks, and so on; but that
there might be an absolute difference in the number of the vertebrae of
the back, as in the sacral vertebrae of the Pouter; and so great is the
extent of the variation in these and similar characters that I pointed
out to you, by reference to the skeletons and the diagrams, that these
extreme varieties may absolutely differ more from one another in their
structural characters than do what naturalists call distinct SPECIES
of pigeons; that is to say, that they differ so much in structure that
there is a greater difference between the Pouter and the Tumbler than
there is between such wild and distinct forms as the Rock Pigeon or
the Ring Pigeon, or the Ring Pigeon and the Stock Dove; and indeed
the differences are of greater value than this, for the structural
differences between these domesticated pigeons are such as would be
admitted by a naturalist, supposing he knew nothing at all about their
origin, to entitle them to constitute even distinct genera.

As I have used this term SPECIES, and shall probably use it a good deal,
I had better perhaps devote a word or two to explaining what I mean by
it.

Animals and plants are divided into groups, which become gradually
smaller, beginning with a KINGDOM, which is divided into SUB-KINGDOMS;
then come the smaller divisions called PROVINCES; and so on from a
PROVINCE to a CLASS from a CLASS to an ORDER, from ORDERS to FAMILIES,
and from these to GENERA, until we come at length to the smallest
groups of animals which can be defined one from the other by constant
characters, which are not sexual; and these are what naturalists call
SPECIES in practice, whatever they may do in theory.

If, in a state of nature, you find any two groups of living beings,
which are separated one from the other by some constantly-recurring
characteristic, I don't care how slight and trivial, so long as it is
defined