Occasionally a breed may be retained for some particular quality in a nearly unaltered condition in the same country, together with highly modified offshoots or sub-breeds, which are valued for some distinct property. We see this exemplified in England, where the common tumbler, which is valued only for its flight, does not differ much from its parent-form, the Eastern tumbler; whereas the short-faced tumbler has been prodigiously modified, from being valued, not for its flight, but for other qualities. But the common-flying tumbler of Europe has already begun to branch out into slightly different sub-breeds, such as the common English tumbler, the Dutch roller, the Glasgow house-tumbler, and the long-faced beard tumbler, &c.; and in the course of centuries, unless fashions greatly change, these sub-breeds will diverge through the slow and insensible process of unconscious selection, and become modified, in a greater and greater degree. After a time the perfectly graduated links, which now connect all these sub-breeds together, will be lost, for there would be no object and much difficulty in retaining such a host of intermediate sub-varieties.
The principle of divergence, together with the extinction of the many previously existing intermediate forms, is so important for understanding the origin of domestic races, as well as of species in a state of nature, that I will enlarge a little more on this subject. Our third main group includes carriers, barbs, and runts, which are plainly related to each other, yet wonderfully distinct in several important characters. According to the view given in the last chapter, these three races have probably descended from an unknown race having an intermediate character, and this from the rock-pigeon. Their characteristic differences are believed to be due to different breeders having at an early period admired different points of structure; and then, on the acknowledged principle of admiring extremes, having gone on breeding, without any thought of the future, as good birds as they could,--carrier-fanciers preferring { 221} long beaks with much wattle,--barb-fanciers preferring short thick beaks with much eye-wattle,--and runt-fanciers not caring about the beak or wattle, but only for the size and weight of the body. This process will have led to the neglect and final extinction of the earlier, inferior, and intermediate birds; and thus it has come to pass, that in Europe these three races are now so extraordinarily distinct from each other. But in the East, whence they were originally brought, the fashion has been different, and we there see breeds which connect the highly modified English carrier with the rock-pigeon, and others which to a certain extent connect carriers and runts. Looking back to the time of Aldrovandi, we find that there existed in Europe, before the year 1600, four breeds which were closely allied to carriers and barbs, but which competent authorities cannot now identify with our present barbs and carriers; nor can Aldrovandi's runts be identified with our present runts. These four breeds certainly did not differ from each other nearly so much as do our existing English carriers, barbs, and runts. All this is exactly what might have been anticipated. If we could collect all the pigeons which have ever lived, from before the time of the Romans to the present day, we should be able to group them in several lines, diverging from the parent rock-pigeon. Each line would consist of almost insensible steps, occasionally broken by some slightly greater variation or sport, and each would culminate in one of our present highly modified forms. Of the many former connecting links, some would be found to have become absolutely extinct without having left any issue, whilst others though extinct would be seen to be the progenitors of the existing races.