It follows from this that the capacity of breeding at an early age and at short successive intervals, as with pigeons, rabbits, &c., facilitates selection; for the result is thus soon made visible, and perseverance in the work is encouraged. It can hardly be {236} accidental that the great majority of the culinary and agricultural plants which have yielded numerous races are annuals or biennials, which therefore are capable of rapid propagation and thus of improvement. Sea-kale, asparagus, common and Jerusalem artichokes, potatoes, and onions, alone are perennials. Onions are propagated like annuals, and of the other plants just specified, none, with the exception of the potato, have yielded more than one or two varieties. No doubt fruit-trees, which cannot be propagated quickly by seed, have yielded a host of varieties, though not permanent races; but these, judging from pre-historic remains, were produced at a later and more civilised epoch than the races of culinary and agricultural plants. A species may be highly variable, but distinct races will not be formed, if from any cause selection be not applied. The carp is highly variable, but it would be extremely difficult to select slight variations in fishes whilst living in their natural state, and distinct races have not been formed;[580] on the other hand, a closely allied species, the gold-fish, from being reared in glass or open vessels, and from having been carefully attended to by the Chinese, has yielded many races. Neither the bee, which has been semi-domesticated from an extremely remote period, nor the cochineal insect, which was cultivated by the aboriginal Mexicans, has yielded races; and it would be impossible to match the queen-bee with any particular drone, and most difficult to match cochineal insects. Silk-moths, on the other hand, have been subjected to rigorous selection, and have produced a host of races. Cats, which from their nocturnal habits cannot be selected for breeding, do not, as formerly remarked, yield distinct races in the same country. The ass in England varies much in colour and size; but it is an animal of little value, bred by poor people; consequently there has been no selection, and distinct races have not been formed. We must not attribute the inferiority of our asses to climate, for in India they are of even smaller size than in Europe. But when selection is brought to bear on the ass, all is changed. Near Cordova, as I am informed (Feb. 1860) by Mr. W. E. Webb, C.E., they are carefully bred, as much as 200l. having been paid for a stallion ass, {237} and they have been immensely improved. In Kentucky, asses have been imported (for breeding mules) from Spain, Malta, and France; these "seldom averaged more than fourteen hands high; but the Kentuckians, by great care, have raised them up to fifteen hands, and sometimes even to sixteen. The prices paid for these splendid animals, for such they really are, will prove how much they are in request.