86 THE CASE OF PAPILIO POLYTES [ch. 



mimetic is recessive to the mimetic forms — that the 

 mimetics contain a factor which does not occur in the 

 non-mimetics. If this factor, which may be called X, 

 be added to the constitution of a non-mimetic female 

 it turns it into a mimetic. If X be added to a male 

 such an individual, though incapable of itself exhibiting 

 the mimetic jjattern owing to the inhibitory factor 

 always present in that sex, becomes capable of trans- 

 mitting the mimetic factor to its offspring. Expressed 

 in the usual Mendelian way the formulae for these 

 different butterflies are as follows: — 



Mimetic) f 



?? / = I 



where X stands for the mimetic factor and / for the 

 factor which inliibits the action of X. All males are 

 heterozygous for 7, but during the segregation of 

 characters at some stage in the formation of the 

 families only the male-producing sperms come to contain 

 the factor /. It is lacking in aU the female-producing 

 sperms formed by the male. 



(? (1) does not contain the factor for the mimetic 

 condition and gives only daughters of the M form when 

 mated with an if?. ^ (2) on the other hand is homo- 

 zygous for the factor X, and consequently all of his 

 germ cells contain it. This is the male that gives 

 nothing but mimetic daughters with whatever form of 

 female he is bred, c? (3) is heterozygous for X ; that is 

 to say, one half of his germ cells contain it, the other 

 half not. With the Jf ? he must give equal numbers 



