Heredity and the Origin of Variations. 171 



that afforded by animals, we may consider first that pre- 

 sented by domesticated breeds. They might be expected 

 to afford exceptionally good examples. Their modifiability 

 and the readiness with which they interbreed are two of 

 the determining causes of their selection for domestication. 

 They have, moreover, been placed under new conditions of 

 life, and they undoubtedly exhibit changes of structure, 

 many of which Darwin * regarded as attributable to the 

 effects of use and disuse. In domestic ducks, the relative 

 weight and strength of the wing-bones have been diminished, 

 while conversely the weight and strength of the leg-bones 

 have been increased. The bones of the shoulder-girdle have 

 been decreased in weight and "the prominence of the crest 

 of the sternum, relatively to its length, is also much re- 

 duced in all the domestic breeds. These changes," says 

 Darwin, "have evidently been caused by the lessened use 

 of the wings." The shoulder-girdle and breast-bone of 

 domestic fowls have been similarly reduced. After a care- 

 ful consideration of numerous facts concerning the brains 

 of rabbits, Darwin concluded that this " most important 

 and complicated organ in the whole organization is subject 

 to the law of decrease in size from disuse." And Sir J. 

 Crichton Browne has recently shown that, in the wild duck, 

 the brain is nearly twice as heavy in proportion to the body 

 as it is in the comparatively imbecile domestic duck. In 

 pigs, the nature of the food supplied during many genera- 

 tions has apparently affected the length of the intestines ; 

 for, according to Cuvier, their length to that of the body 

 in the wild boar is as 9 to 1, in the common domestic boar 

 as 13-5 to 1, and in the Siam breed as 16 to 1. With 

 regard to horses, Darwin tells us that " veterinarians are 

 unanimous that horses are affected with spavins, splints, 

 ring-bones, etc., from being shod and from travelling on 

 hard roads, and they are almost unanimous that a tendency 

 to these malformations is transmitted." 



These are samples of the effects of domestication. It 

 has been suggested, however, that, quite apart from any 

 * The instances cited are from " Animals and Plants under Domestication." 



