REVERSION IN PURE BREEDS AND CROSSES 21 



the fig. of Mrs Blackburn's white cow on p. 19). Calves are 

 sometimes produced white with dark ears, like the ancient 

 wild cattle, when a red Shorthorn bull is put to West High- 

 land cows, or when a white Shorthorn is put to blue-grey cows. 



Reid says, in discussing bi-parental reproduction : " If 

 we cross varieties or species, the general tendency is ever 

 towards great reversion. All that distinguishes the one 

 variety or species from the other all the special characters 

 which have arisen along the diverging lines of evolution tend 

 to disappear. The ancestral form common to both varieties, 

 even if it be separated from the crossed descendants by 

 thousands of generations, tends to appear. Sometimes the 

 reversion is direct ; the ancestral form reappears immediately 

 in the offspring of the cross. At other times the offspring 

 blends in the first generation, but reverts in the subsequent 

 generations, or the descendants of the blend may revert, 

 not to the remote common ancestor, but to one or other of 

 the crossed varieties; the inheritance in this case being 

 exclusive." 



Cross-breds are often wilder and more restless than pure- 

 breds. 1 This is noticed particularly in sheep and cattle. 

 Cross sheep are not so easily fenced, and a dairy of cross 

 cows is much more difficult to train to stand for milking in 

 the field than pure Shorthorns. 



Sterility may follow changed conditions of life : for 

 example, many wild animals in confinement either breed 

 early and with difficulty or not at all. When the initial 

 difficulty has been overcome, animals of different species are 

 usually too far apart for their progeny to be fertile or to breed 

 freely. Mules from zebra and donkey stallions mated with 

 horse or pony mares are uniformly infertile. 



" With all highly-bred animals there is more or less 

 difficulty in getting them to procreate quickly, and all suffer 

 much from delicacy of constitution." " Half-wild British 

 cattle which have long interbred within the limits of the same 

 herd are relatively far less fertile " than ordinary animals. 



Domestication increases the fertility of animals, if they 

 are properly bred and not overfed. By bringing sheep into an 

 improving condition at the time of conception, the number of 



1 This rule refers to pure breeds of the same degree of wildness. If a 

 wild and a docile breed are mated, the progeny will be intermediate in 

 wildness. 



