88 EVOLUTION AND ANIMAL LIFE 



high degree. The Southdown sheep are valued not for their 

 Southdown traits, but for the excellence of their mutton, a 

 trait with which middle length of wool, tawny legs, naked faces, 

 drooping ears, and absence of horns have nothing necessarily to 

 do. We value these race traits only for the other qualities 



FIG. 54. Skulls (in longitudinal section) of two breeds of domestic fowl, showing the 

 large modification in the cranium: upper figure, Polish cock; lower figure, Cochin 

 cock. (After Darwin.) 



which have been in a high degree associated with them in the 

 heredity of the race. 



Under crossing and selection, much bolder attempts are 

 possible. When parents widely divergent are crossed, many 

 very different results are attained. In general the progeny, at 

 least after the first generation, diverge very widely from one 

 another. Some will have the good traits of both parent stocks ; 

 some will have the undesirable ones; some will show a mosaic of 

 parental characters; some a more or less perfect blend of char- 

 acters, this blend being definable as a finer type of mosaic. 

 Some will diverge widely from either stock, often showing traits 

 either remotely ancestral or wholly new. From desirable vari- 

 ations of this sort new races may be developed, each succeeding 

 generation tending to give greater fixity. 



In general, wide crosses or hybrids are more successful with 

 plants than with animals, because the mutual adjustment 



