CROSSING, 63 



very good example. Nearly every cross between animals and 

 plants, which are not too closely related produces new charact- 

 ers as its result. 





\m^M' 



••oooop^f; 



Fig. 8. 

 Inheritance of albinism in a family of Malayan field-rats. 



We do not think it probable, that new breeds of animals or 

 plants, other than those which are habitually self-fertilized, are 

 directly derived from hybrid individuals. Even in the history 

 of the tame species of fowls, not many instances are known, 

 in which a new breed was deliberately produced by crossing, 

 out of the variable off-spring of hybrid birds. The common way 

 in which new breeds of fowls are made, is by breeding a new 

 character into an old species. Species with a colour, new for the 

 group, are often produced, by breeding an animal of the desired 

 colour with good typical representatives of the species, and b^ 

 continuing to breed the hybrid back to the old species until 

 the desired result is obtained. It is especially easy to introduce 

 a gene, not heretofore present, as the desired character which 

 in this species greatly depends upon the presence of the gene, 

 is not lost sight of. In our size-inheritcmce work with mice, we 

 find it extremely easy to retain any one gene present in one 

 species during a fbur or five times repeated crossing back, 

 "grading up" to a second species. By breeding back to an 

 albino strain, without more selection than retaining any pig- 



