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Show 416 CONCLUDING REMARKS. C11AP. XXVIII. animals and plants has been greatly exaggerated, though no doubt to a certain extent it exists. It would be opposed to all · the principles inculcated in this work, if domestic animals, whe:U exposed to new conditions and compelled to. struggle for the.Ir own wants against a host of foreign competitors, were not m the course of time in some manner modified. I.t should al~o be remembered that many characters lie latent m all. orgamc beings ready to be evolved un,der :fitting condi~ions; a~d I~ breed.s modified within recent times the tendency to reversiOn IS particularly strong. But the antiquity of various breed~ clear!! ?roves that they remain nearly constant as long as their conditiOns of life remain the same. It has been boldly maintained by s~me aut~ors that. the amount of variation to which our domestiC ~roducti.ons are h~ble is strictly limited; but this is an asse.rtiOn resti~g on l~ttle evidence. Whether or not the amount m any partiCular direction is :fixed, the tendency to general variability seems unlimited. Cattle, sheep, and pigs have been domesticated and have varied from the remotest period, as shown by the researches of Hiitimeyer and others, yet these animals have, within quite :ec~nt times, been improved in an unparalleled degree; and this Implies continued variability of stru~ture. Wh~at,. as ~e know from the remains found in the Swiss lake-habitatiOns, IS one of the most anciently cultivated plants, yet at the present day nevy and better varieties occasionally arise. It may be that an ox will never be produced of larger size or :finer proportions than our present animals, or a race-horse fleeter than Eclipse, or a gooseberry larger than the London variety ; but he would be a bold man who would assert that the extreme limit in these respects has been finally attained. With flowers and fruit it has repeatedly been asserted that perfection has been reached, but the standard has soon been excelled. A breed of pigeons may never be produced with a beak shorter than that of the present short-faced tumbler, or with one longer ~ha~ that of the English carrier, for these birds have weak constitutiOns and are bad breeders; but the shortness and length of the beak are the points which have been steadily improved during at least the last 150 years; and some of the best judges deny that the goal has yet been reached. We may, also, reasonably suspect, from what CnAP. xxvm; CONCLUDING REMARKS. 417 we see in natural species of the variability of extremely modified par.ts, that any s~ructure, after remaining constant during a long sen~s of generatiOns, would, under new and changed conditions of hfe, recommence its course of variability, and might again be acted on by selection. N evertbeless, as Mr. Wallace 3 has recent!! remarked with much force and truth, there must be both ;VIth. nat~ral and d~mestic productions a limit to change in certam directiOns; for mstance, there must be a limit to the fleetn~ss. of any terrestrial animal, as this will be determined by the frwtwn to be overcome, the weight to be canied, and the power of contraction in the ·muscular :fibres. The English racehorse ma! have re~ched this .limit; but it already surpasses in fleet~ess Its own Wild progemtor, and all other equine species. It Is .not surprising, seeing the great difference between many domestw breeds, that some few naturalists have concluded that all are de~ce~ded from dist.inct aboriginal stocks, more especially as ~.he. prmCiple of selectiOn has been ignored, and the high antlqmty of man, as a breeder of animals, has only recently bec?rne known. Most naturalists, however, freely admit that varwus extremely dissimilar breeds are descended from a single stock, although they do not know much about the art of breeding cannot show the connecting links, nor say where and when th~ breeds arose. Yet these same naturalists will declare with an air of philosophical caution, that they can never admi; that one natural s~~cies has given birth to another until they behold all the transiti~nal steps. But fanciers have used exactly the same language With respect to domestic breeds ; thus an author of an excellent treatise says he will never allow that carrier and fantail pigeo.n~ a.re the descendants of the wild rock-pigeon, until the transitiOns have "actually been observed, and can be repeated "whenever man chooses to set about the task." No doubt it is difficu~t to realise that slight changes added up during long centunes can produce such results; but he who wishes to understand the origin of domestic breeds or natural species must overcome this di:ffieulty. The causes inducing and the laws governing variability have been so lately discussed, that I need here only enumerate the leading points. As domesticated organisms are much more 8 'The Quarterly Journal of Science,' Oct. 1867, p. 486. VOL. II. 2 E |
