X 



Chap. XXVlll 



CONCLUDING REMARKS. 



417 



we see in natural species of the variability of extremely modified 

 parts, that any structure, after remaining constant during a Ion" 

 series of generations, would, under new and changed conditions 

 of life, recommence its course of variability, and mio-ht again 

 be acted on by selection. Nevertheless, as Mr. Wallace 3 has 

 recently remarked with much force and truth, there must be 

 both with natural and domestic productions a limit to change in 

 certain directions; for instance, there must be a limit to the 

 fleetness of any terrestrial animal, as this will be determined bv 

 the friction to be overcome, the weight to be carried, and the 

 power of contraction in the muscular fibres. The Enough race 

 horse may have reached this limit; but it already surpasses in 

 fleetness its own wild progenitor, and all other equine species 



It is not surprising, seeing the great difference between many 

 domestic breeds, that some few naturalists have concluded that 

 all are descended from distinct aboriginal stocks, more especially 

 as th e> principle of selection has been ignored, and the high 

 antiquity of man, as a breeder of animals, has only recently 

 become known. Most naturalists, however, freely admit that 

 various 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 the 

 breeds arose. Yet these same naturalists will declare, with an 

 air of philosophical caution, that they can never admit that one 

 natural species has given birth to another until they behold all 

 tne transitional 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 

 pigeons are 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 

 dimcult to realise that slight changes added up during W 

 centuries can produce such results; but he who wishes to 

 understand the origin of domestic breeds or natural species must 

 overcome this difficulty. 



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 



'The Quarterly Journal of Science.' Oct. 1S67, p. 486 

 VOL. IT. ,L t . 



2 E 



