MUTATION 273 



tion to the number of the same kinds of change that they 

 produce. The two following cases recorded by Stnrtevant 

 illustrate this relation: 



Two species, viz., Drosophila melanogaster and D, 

 funehris, have each produced a mutation called notch. 

 This character, notch, involves not only a notching at the 

 end of the wings but also the thickening of the second and 

 fifth veins of the wings, frequent reduction and roughen- 

 ing of the eyes, inequalities of the rows of hairs on the 

 thorax, frequent doubling of the anterior scutellar bristles, 

 and a recessive lethal effect. The character is also dom- 

 inant and sex-linked. It is one of the commonest muta- 

 tions in melanogaster and was the first to be picked out 

 in funebris. So many peculiarities in common make it 

 hard to believe that they do not represent the same genetic 

 change. Another mutant also found in D. funebris that 

 parallels one in D. melanogaster is called hairless, produc- 

 ing several similar effects in both. In both the factor is 

 an autosomal dominant; it affects the hairs, certain 

 bristles, and the second, fourth and fifth veins of the wings, 

 and has a recessive lethal effect. 



One of the most interesting ideas that De Vries brought 

 forward in his mutation theory is that groups of ''small 

 species'' or of varieties are made up of many common 

 genes and differ in a relatively small number of genes. 

 The genetic analysis of a group of smaller species would 

 consist in finding out how the different genes are dis- 

 tributed amongst the members of this group. Phylogene- 

 tic relationship comes to have a different significance 

 from the traditional relationship expressed in the descent 

 theory ; but this point of view is so novel that it has not 

 yet received the recognition which we may expect that 

 it will obtain in the future when relationship by common 

 descent will be recognized as of minor importance as 

 compared with relationship due to a community of genes. 



18 



