Domestic varieties as a general rule certainly differ from each other in less important parts of their organisation than do species; and when important differences occur, they are seldom firmly fixed; but this fact is intelligible if we consider man's method of selection. In the living animal or plant he cannot observe internal modifications in the more important organs; nor does he regard them as long as they are compatible with health and life. What does the breeder care about any slight change in the molar teeth of his pigs, or for an additional molar tooth in the dog; or for any change in the intestinal canal or other internal organ? The breeder cares for the flesh of his cattle being well marbled with fat, and for an accumulation of fat within the abdomen of his sheep, and this he has effected. What would the floriculturist care for any change in the structure of the ovarium or of the ovules? As important internal organs are certainly liable to numerous slight variations, and as these would probably be inherited, for many strange monstrosities are transmitted, man could undoubtedly effect a certain amount of change in these organs. When he has produced any modification in an important part, it has generally been unintentionally in correlation with some other conspicuous part, as when he has given ridges and protuberances to the skulls of fowls, by attending to the form of the comb, and in the case of the Polish fowl to the plume of feathers on the head. By attending to the external form of the pouter-pigeon, he has enormously increased the size of the œsophagus, and has added to the number of the ribs, and given them greater breadth. With the carrier-pigeon, by increasing, through steady selection, the wattles on the upper mandible, he has greatly modified the form of the lower mandible; and so in many other cases. Natural species, on the other hand, have been modified exclusively for their own good, to fit them for infinitely
diversified conditions of life, to avoid enemies of all kinds, and to struggle against a host of competitors. Hence, under such complex conditions, it would often happen that modifications of the most varied kinds, in important as well as in unimportant parts, would be advantageous or even necessary; and they would slowly but surely be acquired through the survival of the fittest. Various indirect modifications would likewise arise through the law of correlated variation.
Domestic breeds often have an abnormal or semi-monstrous character, as the Italian greyhound, bulldog, Blenheim spaniel, and bloodhound amongst dogs,—some breeds of cattle and pigs, several breeds of the fowl, and the chief breeds of the pigeon. The differences between such abnormal breeds occur in parts which in closely-allied natural species differ but slightly or not at all. This may be accounted for by man's often selecting, especially at first, conspicuous and semi-monstrous deviations of structure. We should, however, be cautious in deciding what deviations ought to be called monstrous: there can hardly be a doubt that, if the brush of horse-like hair on the breast of the turkey-cock had first appeared on the domesticated bird, it would have been considered a monstrosity; the great plume of feathers on the head of the Polish cock has been thus designated, though plumes are common with many kinds of birds; we might call the wattle or corrugated skin round the base of the beak of the English carrier-pigeon a monstrosity, but we do not thus speak of the globular fleshy excrescence at the base of the beak of the male Carpophaga oceanica.
Some authors have drawn a wide distinction between artificial and natural breeds; although in extreme cases the distinction is plain, in many other cases an arbitrary line has to be drawn. The difference depends chiefly on the kind of selection which has been applied. Artificial breeds are those which have been intentionally improved by man; they frequently have an unnatural appearance, and are especially liable to loss of excellence through reversion and continued variability. The so-called natural breeds, on the other hand, are those which are now found in semi-civilised countries, and which formerly inhabited separate districts in nearly all the European kingdoms. They have been rarely acted on by man's
intentional selection; more frequently, it is probable, by unconscious selection, and partly by natural selection, for animals kept in semi-civilised countries have to provide largely for their own wants. Such natural breeds will also, it may be presumed, have been directly acted on to some extent by the differences, though slight, in the surrounding physical conditions.
It is a much more important distinction that some breeds have been from their first origin modified in so slow and insensible a manner, that if we could see their early progenitors we should hardly be able to say when or how the breed first arose; whilst other breeds have originated from a strongly-marked or semi-monstrous deviation of structure, which, however, may subsequently have been augmented by selection. From what we know of the history of the racehorse, greyhound, gamecock, &c., and from their general appearance, we may feel nearly confident that they were formed by a slow process of improvement: and with the carrier-pigeon, as well as with some other pigeons, we know that this has been the case. On the other hand, it is certain that the ancon and mauchamp breeds of sheep, and almost certain that the niata cattle, turnspit and pug-dogs, jumper and frizzled fowls, short-faced tumbler pigeons, hook-billed ducks, &c., and with plants a multitude of varieties, suddenly appeared in nearly the same state as we now see them. The frequency of these cases is likely to lead to the false belief that natural species have often originated in the same abrupt manner. But we have no evidence of the appearance, or at least of the continued procreation, under nature, of abrupt modifications of structure; and various general reasons could be assigned against such a belief: for instance, without separation a single monstrous variation would almost certainly be soon obliterated by crossing.
On the other hand, we have abundant evidence of the constant occurrence under nature of slight individual differences of the most diversified kinds; and thus we are led to conclude that species have generally originated by the natural selection, not of abrupt modifications, but of extremely slight differences. This process may be strictly compared with the slow and gradual improvement of the racehorse, greyhound, and gamecock. As every detail of structure in each species is closely adapted to its general
habits of life, it will rarely happen that one part alone will be modified; but the co-adapted modifications, as formerly shown, need not be absolutely simultaneous. Many variations, however, are from the first connected by the law of correlation. Hence it follows that even closely-allied species rarely or never differ from each other by some one character alone; and this same remark applies to a certain extent to domestic races; for these, if they differ much, generally differ in many respects.
Some naturalists boldly insist[[928]] that species are absolutely distinct productions, never passing by intermediate links into each other; whilst they maintain that domestic varieties can always be connected either with each other or with their parent-forms. But if we could always find the links between the several breeds of the dog, horse, cattle, sheep, pigs, &c., the incessant doubts whether they are descended from one or several species would not have arisen. The greyhound genus, if such a term may be used, cannot be closely connected with any other breed, unless, perhaps, we go back to the ancient Egyptian monuments. Our English bulldog also forms a very distinct breed. In all these cases crossed breeds must of course be excluded, for the most distinct natural species can thus be connected. By what links can the Cochin fowl be closely united with others? By searching for breeds still preserved in distant lands, and by going back to historical records, tumbler-pigeons, carriers, and barbs can be closely connected with the parent rock-pigeon; but we cannot thus connect the turbit or the pouter. The degree of distinctness between the various domestic breeds depends on the amount of modification which they have undergone, and especially on the neglect and final extinction of the linking, intermediate, and less valued forms.
It has often been argued that no light is thrown, from the admitted changes of domestic races, on the changes which natural species are believed to undergo, as the former are said to be mere temporary productions, always reverting, as soon as they become feral, to their pristine form. This argument has been well combated by Mr. Wallace;[[929]] and full details were given in the thirteenth chapter, showing that the tendency to reversion in feral