APPENDIX 235 



When to a familiar or to an established system it 

 becomes necessary to add new recessive types diffi- 

 culties arise. This may be illustrated in the case 

 of combs of fowls, the main facts concerning whose 

 inheritance have been discussed on page 216. On 

 the presence and absence scheme a factor gets its 

 name from the effect that that factor produces in the 

 absence of other factors affecting the character. The 

 factor for pea, for instance, got its name from the 

 effect produced when a factor for rose was supposed 

 to be absent; and the formula for single comb, rpS, 

 means that a f ac tor S, for single, produced its particular 

 effect when the two other factors were absent. When 

 a new condition, combless, was met with and added to 

 the series it was represented as due to the loss of the 

 factor (S) for single. The formula for combless 

 became rpsB (B standing for the vestige of a comb, 

 called Breda, that remained). What now is the 

 factor for rose comb? Originally this factor got its 

 name from its effect in the absence of pea but in the 

 presence of S (RpS) ; now the factor for rose, R, should 

 be re-named from its effect in the absence of both 

 factors P and S. The series must then be re-con- 

 structed on a new basis and the same process must 

 be gone through with whenever a new factor is 

 brought into relation with an established system. 



ACKNOWLEDGMENTS 



We wish to express our indebtedness to Miss E. 

 M. Wallace for her skill in making many of the illus- 

 trations, and also to Miss M. L. Hedge who has 



