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same direction; while, in the other, no race was evolved, because no such selection was exercised. A race is a propagated variety; and as, by the laws of reproduction, offspring tend to assume the parental forms, they will be more likely to propagate a variation exhibited by both parents than that possessed by only one. There is no organ of the body of an animal which may not, and does not, occasionally, vary more or less from the normal type; and there is no variation which may not be transmitted and which, if selectively transmitted, may not become the foundation of a race. This great truth, sometimes forgotten by philosophers, has long been familiar to practical agriculturists and breeders; and upon it rest all the methods of improving the breeds of domestic animals, which, for the last century, have been followed with so much success in England. Colour, form, size, texture of hair or wool, proportions of various parts, strength or weakness of constitution, tendency to fatten or to remain lean, to give much or little milk, speed, strength, temper, intelligence, special instincts; there is not one of these characters the transmission of which is not an every-day occurrence within the experience of cattle-breeders, stock-farmers, horse-dealers, and dog and poultry fanciers. Nay, it is only the other day that an eminent physiologist, Dr. BrownSéquard, communicated to the Royal Society his discovery that epilepsy, artificially produced in

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guinea-pigs, by a means which he has discovered, is transmitted to their offspring." But a race, once produced, is no more a fixed and immutable entity than the stock whence it sprang ; variations arise among its members, and as these variations are transmitted like any others, new races may be developed out of the pre-existing one ad infinitum, or, at least, within any limit at present determined. Given sufficient time and sufficiently careful selection, and the multitude of races which may arise from a common stock is as astonishing as are the extreme structural differences which they may present. A remarkable example of this is to be found in the rock-pigeon, which Mr. Darwin has, in our opinion, satisfactorily demonstrated to be the progenitor of all our domestic pigeons, of which there are certainly more than a hundred well-marked races. The most noteworthy of these races are, the four great stocks known to the “fancy” as tumblers, pouters, carriers, and fantails; birds which not only differ most singularly in size, colour, and habits, but in the form of the beak and of the skull: in the proportions of the beak to the skull; in the number of tail-feathers; in the absolute and relative size of the feet; in the presence or absence of the uropygial gland; in the number of vertebrae in the back; in short, in precisely those characters in which

o Weismann's Essays Upon Heredity, p. 310, et seq.

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the genera and species of birds differ from one

another. And it is most remarkable and instructive to

observe, that none of these races can be shown to have been originated by the action of changes in what are commonly called external circumstances, upon the wild rock-pigeon. On the contrary, from time immemorial pigeon-fanciers have had essentially similar methods of treating their pets, which have been housed, fed, protected and cared for in much the same way in all pigeonries. In fact, there is no case better adapted than that of the pigeons to refute the doctrine which one sees put forth on high authority, that “no other characters than those founded on the development of bone for the attachment of muscles” are capable of variation. In precise contradiction of this hasty assertion, Mr. Darwin's researches prove that the skeleton of the wings in domestic pigeons has hardly varied at all from that of the wild type; while, on the other hand, it is in exactly those respects, such as the relative length of the beak and skull, the number of the vertebræ, and the number of the tail-feathers, in which muscular exertion can have no important influence, that the utmost amount of variation has taken place.

We have said that the following out of the properties exhibited by physiological species would lead usinto difficulties, and at this point they begin

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to be obvious; for if, as the result of spontaneous
variation and of selective breeding, the progeny of
a common stock may become separated into groups
distinguished from one another by constant, not
sexual, morphological characters, it is clear that
the physiological definition of species is likely to
clash with the morphological definition. No one
would hesitate to describe the pouter and the
tumbler as distinct species, if they were found fossil,
or if their skins and skeletons were imported, as
those of exotic wild birds commonly are—and with-
out doubt, if considered alone, they are good and
distinct morphological species. On the other hand,
they are not physiological species, for they are
descended from a common stock, the rock-pigeon.
Under these circumstances, as it is admitted on
all sides that races occur in Nature, how are we to
know whether any apparently distinct animals are
really of different physiological species, or not,
seeing that the amount of morphological difference
is no safe guide 2 Is there any test of a physio-
logical species? The usual answer of physiologists
is in the affirmative. It is said that such a test is
to be found in the phaenomena of hybridisation—
in the results of crossing races, as compared with
the results of crossing species.
So far as the evidence goes at present, in-
dividuals, of what are certainly known to be mere
races produced by selection, however distinct they
may appear to be, not only breed freely together,

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but the offspring of such crossed races are perfectly fertile with one another. Thus, the spaniel and the greyhound, the dray-horse and the Arab, the pouter and the tumbler, breed together with perfect freedom, and their mongrels, if matched with other mongrels of the same kind, are equally fertile. On the other hand, there can be no doubt that the individuals of many natural species are either absolutely infertile if crossed with individuals of other species, or, if they give rise to hybrid offspring, the hybrids so produced are infertile when paired together. The horse and the ass, for instance, if so crossed, give rise to the mule, and there is no certain evidence of offspring ever having been produced by a male and female mule. The unions of the rock-pigeon and the ring-pigeon appear to be equally barren of result. IIere, then, says the physiologist, we have a means of distinguishing any two true species from any two varieties. If a male and a female, selected from each group, produce offspring, and that offspring is fertile with others produced in the same way, the groups are races and not species. If, on the other hand, no result ensues, or if the offspring are infertile with others produced in the same way, they are true physiological species. The test would be an admirable one, if, in the first place, it were always practicable to apply it, and if, in the second, it always yielded results susceptible of a definite interpretation. Unfortunately,

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