412 CONCLUDING REMARKS. Chap. XXVIII. 



is one of the most anciently cultivated plants, yet at the 

 present day new and better varieties frequently arise. It 

 may be that an ox will never be produced of larger size and 

 finer proportions, or a racehorse fleeter, than our present 

 animals, 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 than that of the English carrier, for these 

 birds have weak constitutions and are bad breeders ; but short- 

 ness and length of beak are the points which have been 

 steadily improved during the last 150 years, and some of 

 the best judges deny that the goal has yet been reached. 

 From reasons which could be assigned, it is probable that 

 parts which have now reached their maximum development, 

 might, after remaining constant during a long period, vary 

 again in the direction of increase under new conditions of 

 life. But there must be, as Mr. Wallace has remarked with 

 much truth, 3 a limit to change in certain directions both 

 with natural and domestic productions; for instance, there 

 must be a limit to the fleetness of any terrestrial animal, 

 as tliis will be determined by the friction to be overcome, 

 the weight to be carried, and the power of contraction in the 

 muscular fibres. The English racehorse may have reached 

 this limit ; but it already surpasses in fleetness its own wild 

 progenitor and all other equine species. The short-faced 

 tumbler-pigeon has a beak shorter, and the carrier a beak 

 longer, relatively to the size of their bodies, than that of any 

 natural species of the family. Our apples, pears and goose- 

 berries bear larger fruit than those of any natural species 

 of the same genera ; and so in many other cases. 



It is not surprising, seeing the great difference between 

 many domestic breeds, that some few naturalists have con- 

 cluded that each is descended from a distinct aboriginal stock, 

 more especially as the principle of selection has been ignored, 

 3 ' The Quarterly Journal of Science,' Oct. 1867, p. 486. 



