ANIMAL AND PLANT BREEDING 



319 



certainly there are very many cases in which the combination is not 

 successful and the hybrids are weakly or even inviable, while in the 

 best cases they are often sterile or nearly so, even though sturdy. 



An increase of vigour on crossing, though often found, is perhaps 

 not so general among animals as it is in plants, and this is probably 

 correlated with the fact that the results of inbreeding are not so uni- 

 formly unfortunate. Animal populations do not seem to be so permeated 

 with harmful recessives as are cultivated plants, and it is often feasible 



Fig. 1 32. The Pedigree of a Famous American Bull, Comet. — Notice the large 

 amount of inbreeding in the ancestry of this superlatively fine animal. 



(After Rice.) 



Comet 



Favorite (bull) 



r Foljambe 

 Bolingbroke J 



I Phoenix 



Barker's Bull 

 Haugl 



[Young Strawberry {^^^1° 

 Foljambe 



\Haughton 



Dalton Duke 

 ite 



/Barker's Bull 

 \Haughton 



Young Phoenix' 



Favorite 

 (bull) 



''Phoenix 



Favorite 

 f Bolinbroke 



Phoenix 

 r Foljambe 

 L Favorite 



{ 



Alcock's Bull 



X 



r Foljambe 



Young Strawberry 



Foljambe 

 Favorite 



Barker's Bull 

 Haughton 



Alcock's Bull 



X 



to practice quite close inbreeding without harmful results. In fact, close 

 inbreeding is an extremely valuable method of stock improvement and 

 has been involved in the building up of nearly all the best animal 

 stocks. It is the only method available for creating a homogeneous 

 population, and if combined with selection gives a population homo- 

 zygous for favourable genes. We owe the development of longhorn 

 cattle, Leicester sheep, and shire horses largely to Bakewell, who, in 

 the middle of the eighteenth century, dared to go against the prevaiUng 

 prejudice against "incest" and practice inbreeding. Even full brother 

 and sister matings, which is as close inbreeding as is possible in a 

 bisexual organism with separate sexes, has been shown to have no bad 

 effects for four generations in a certain line of pigs. ^ 



^ For a detailed study of inbreeding in rats, see King 191 8, 1919. 



