CROSSING AND CLOSE-INTERBREEDING. 209 



per se, that, with the same degree of interbreeding, with 

 different individuals of the very same species, fertility 

 -graduates from zero to perfect fertility? 



Why is it, that, with horses, sheep, and cattle, very 

 little evil effects are observable? and, that, with some 

 individuals of those species, no evil whatever follows, 

 however long continued, and close the interbreeding 

 may be ? 



Why is it, on the contrary, that with pigeons, fowls, 

 and pigs, the greatest evil is the outcome of any 

 degree of interbreeding, even when there is no relation- 

 ship of blood? Why is it, that, with a short-beaked 

 tumbler, for instance, interbreeding is not possible, 

 without great evil resulting, even when the individual 

 is interbred with another of the same variety, which is 

 removed from it, in blood, for thousands of genera- 

 tions, and which was reared in a distant country ? 



Why is it, that, with cattle, sheep, and horses, vary- 

 ing degrees of evil are observable, in different individ- 

 uals of the same species, and in different individuals 

 of the same breed, from the same degree of close- 

 interbreeding; and that, in some individuals, there are 

 no evil effects? Why is it, that, with pigeons, fowls, 

 and pigs, varying degrees of evil are observable in 

 different individuals of the same species, and in differ- 

 ent individuals of the same variety, from the same 

 degree of interbreeding? 



Why is it; that with pigeons, fowls and pigs, the 

 more highly improved the varieties are, the greater 

 are the evil effects ; and, the less improved that the 

 varieties are, the less are the evil effects ? Why is it, 



