2o 



THE AMEBIC AN NATURALIST [Vol. LII 



The three groups of F 2 tails (Fig. 11) show 126 doubles, 

 36 singles, 46 none. The expectation for two factors 

 (9:3:4), on the assumption that the doubles differ from 

 the singles by one factor, and both from none by an- 

 other factor, is 117 doubles, 39 singles, 52 none. This is 

 not a very bad fit. 



There is one striking result brought out by these curves. 

 There are no 12- or 13-feathered birds without an oil gland. 

 This is an expression of the relation that Darwin sug- 

 gested as due to "correlation" in the sense that more tail 

 feathers suppressed the development of the oil gland. 

 But there is no such obvious solution as is shown by the 

 F 2 group, for there may be a large number of tail feathers 

 present and the oil glands be well developed, single or 

 double. The curves suggest, rather, linkage between 

 a gene for extra feathers, and a gene for absence of oil 

 glands. 



The tails with double, single and no oil glands were also 

 classified according to the four color groups already re- 

 ferred to, viz., blue, white, blue and white, edged white 

 (Figs. 12 and 13). The double and single curves appear 

 to be the same, the no oil gland curve seems significantly 

 different. If so, it means that there is some linkage be- 

 tween white color and absence of oil glands. 



The foregoing evidence makes probable the view that a 

 gene for more than 12 feathers, and the gene for no oil 

 gland, and a gene for white color are linked, i. e., are 

 carried by the same chromosome. The genes for the oil 

 gland and for the number of tail feathers are closer to 

 each other than either is to the gene for white. More 

 data, especially from back-crosses, will be necessary to 

 establish this conclusion. 



Split Feathers 

 Dr. Solley tells me that the split and double feather 

 that occurs at times in the fantails is selected against. 

 It is of not infrequent occurrence in the F 2 and back- 

 crossed birds that I have obtained. In the records these 



