166 MUTATION. 



taneous creation, or even the spontaneous acquisition of new 

 genes, real positive mutation. 



Other cases of dominant novelties in pure-bred stock are 

 Morgan's cases in Drosophila. Drosophila, although it furnishes 

 excellent material for genetic studies, has furnished all sorts of 

 complications, irregularities in the ordinary segregation of 

 genes, in some respects somewhat similar to the case of 

 Oenothera. Then, we have to remember that dominance is 

 relative, and that by loss of one gene, individuals maijr be pro- 

 duced which have a character, dominant to a corresponding 

 character of others, which look like the original stock. In chick- 

 ens, white animals may produce coloured off-spring, for in- 

 stance, white Leghorns may produce brown ones. Such brown 

 fowls have a colour, dominant to the white colour of the Silky, 

 or of the white Rosecomb bantam. According to Punnett there 

 are black rabbits that may produce agoutis, it they are heter- 

 ozygous for a gene which they have more than the agoutis. 

 Such agouti is dominant over the black of most black rabbits. 



In other words, the production of such dominant novelties 

 as pigmented animals in chickens or agouti rabbits, may be 

 due to loss, or at least removal of an additional gene, which 

 made their progenitors look hke the recessives. In addition, the 

 production of dominant novelties as the result of combination 

 of genes, not heretofore present together, is possible in a more 

 or less pure-bred strain. Such a combination may be noted only 

 after several generations, if both individuals carrying A and 

 individuals carrying B are relatively rare. 



Only if we are absolutely certain of the purity of the individ- 

 uals of a given family, we may without further tests call mu- 

 tation the sudden production of unexpected novelties. If, how- 

 ever, we can be sure of the absolute homozygosity of a group 

 of plants or animals, we can feel free of the burden of further 

 tests. 



In these cases where we have to demand the observance of 

 the most rigorous precautions before we can accept the reality 

 of an alleged mutation, we have an analogy to the controversy 



