has
designedly produced by selection, are descended from a single common
parent-form, from one wild "true variety." The same is the case with
the numerous and highly differing varieties of pigeons. Domestic
pigeons and carrier-pigeons, turbits and cropper-pigeons, fantail
pigeons and owls, tumblers and pouters, trumpeters and laughing
pigeons (or Indian doves), and the rest, are all, as Darwin has
convincingly proved, descendants of a single wild variety, the
rock-pigeon (_Columba livia_). And how wonderfully various they are,
not only in general form, size, and colouring, but in the particular
form of the skull, the beak, the feet, and so forth! They differ much
more in every respect each from the others than the numerous wild
varieties which, in systems of ornithology, are recognised as true
varieties, and even as true species. It is the same with the different
artificial varieties of apples, pears, pansies, dahlias, and so on; in
short, of almost all the domestic varieties of animals and plants. We
would lay particular stress on the fact that these artificial species
which man has produced or created by artificial breeding and through
experimental transformation out of one original species, differ far
more one from another in physiological as well as in morphological
conditions than the natural species in a wild state. With these it is
self-evident that any proof by experiment of a common origin is wholly
impossible. For, so soon as we subject any wild variety of animal or
plant to such an experiment, we bring it under the conditions of
artificial breeding.

That the morphological conception of a Species is not a positive but
only a relative conception, and that it has no other absolute or
positive value than those other similar system-categories--sports,
varieties, races, tribes, families, classes--is now acknowledged by
every systematiser who forms an honest and unprejudiced judgment of
the practical systematic distinction of species. From the very nature
of the case there are no limits to arbitrary discretion in this
department, and there are no two systematists who are at one in every
instance; this one separating forms as true varieties which that one
does not. (Compare on this point "History of Creation," vol. i., p.
273.) The conception of variety or species has a different value in
every small or large department of systematic Zoology and Botany.

But the conception of species has just as little any fixed