82 BREEDING 



of photography — the method of Daguerre and the 

 method of Fox-Talbot, who invented the negative. 

 By Daguerre's method a positive image was produced 

 on a silvered surface, and the picture could not be 

 copied except by an elaborate process of electrotypy. 

 A new daguerreotype could only be reproduced by 

 exposing a fresh plate. The negative, like the 

 reversionary hybrid, is useless in itself ; but it revolu- 

 tionised photography. Daguerre's might be called 

 the one-generation method, and Fox-Talbot's the 

 two-generation method of photography. A breeder 

 who threw away his first crosses because they did 

 not possess the character he was working for, would 

 be as foolish as a photographer who threw away 

 his negatives because the light parts in the objects 

 appeared dark, and the dark light, in them. 



First crosses are, however, not always bred because 

 they possess new characteristics, but because they 

 very often possess greater vigour than either of the 

 parents crossed. This is another reason why it 

 is better to raise those hybrids which possess 

 characteristics peculiar to themselves, such as roan 

 cattle, by repeating the cross than by breeding roan 

 cattle inter se. I do not know, and it does not much 

 matter, whether the excessive vigour of the first cross 

 is to be considered as a phenomenon of reversion, 

 but it is a fact that, in the case of my mice, reversion 

 to the ancestral condition in regard to disposition 

 is just as invariable a result of the cross as the rever- 

 sion in regard to colour. The difierence between 

 the disposition of the hybrid and that of the 



