The Conditions of Existence as Affecting the Perpetuation of Living Beings | Page 4

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
and constant, and does not depend on sexual peculiarities, then all
naturalists agree in calling them two species; that is what is meant by
the use of the word species--that is to say, it is, for the practical
naturalist, a mere question of structural differences.*
[footnote]* I lay stress here on the 'practical' signification of "Species."
Whether a physiological test between species exist or not, it is hardly
ever applicable by the practical naturalist.
We have seen now--to repeat this point once more, and it is very
essential that we should rightly understand it--we have seen that breeds,
known to have been derived from a common stock by selection, may be
as different in their structure from the original stock as species may be
distinct from each other.
But is the like true of the physiological characteristics of animals? Do

the physiological differences of varieties amount in degree to those
observed between forms which naturalists call distinct species? This is
a most important point for us to consider.
As regards the great majority of physiological characteristics, there is
no doubt that they are capable of being developed, increased, and
modified by selection.
There is no doubt that breeds may be made as different as species in
many physiological characters. I have already pointed out to you very
briefly the different habits of the breeds of Pigeons, all of which depend
upon their physiological peculiarities,--as the peculiar habit of tumbling,
in the Tumbler--the peculiarities of flight, in the "homing" birds,--the
strange habit of spreading out the tail, and walking in a peculiar fashion,
in the Fantail,--and, lastly, the habit of blowing out the gullet, so
characteristic of the Pouter. These are all due to physiological
modifications, and in all these respects these birds differ as much from
each other as any two ordinary species do.
So with Dogs in their habits and instincts. It is a physiological
peculiarity which leads the Greyhound to chase its prey by sight,--that
enables the Beagle to track it by the scent,--that impels the Terrier to its
rat-hunting propensity,--and that leads the Retriever to its habit of
retrieving. These habits and instincts are all the results of physiological
differences and peculiarities, which have been developed from a
common stock, at least there is every reason to believe so. But it is a
most singular circumstance, that while you may run through almost the
whole series of physiological processes, without finding a check to
your argument, you come at last to a point where you do find a check,
and that is in the reproductive processes. For there is a most singular
circumstance in respect to natural species--at least about some of
them--and it would be sufficient for the purposes of this argument if it
were true of only one of them, but there is, in fact, a great number of
such cases--and that is, that,