194 Certain Females normally Hybrid [CH. 



special heterozygosis of females is no rarity* Bred with 

 their own corresponding male type these females would in 

 the ordinary course never reveal that they were hetero- 

 zygous ; for the spurious allelomorphism subsisting between 

 the femaleness and the dominant factor which it repels 

 would preserve the males homozygous and the females 

 heterozygous, and so the impurity would be hidden in 

 perpetuity. The real condition could only be brought to 

 light either by a cross with a distinct race; or by a variation 

 which through the introduction of some new factor or the 

 omission of one previously existing, upsets the equilibrium 

 hitherto maintained among the several elements through 

 the cell-divisions of gametogenesis. 



The recognition of these facts brings us a step, if a small 

 one, nearer to the discovery of the nature of variation. In 

 such a case as that of the Canary, since the ordinary green 

 hens are " hybrids " of Cinnamon, the Cinnamon variety is 

 perceived to have been already in existence, ready to appear 

 if the critical event which could release the variety should 

 occurf. What that critical event may have been we still 

 cann<5t suggest. 



The minute analysis of such cases as these will almost 

 certainly disclose systems of interrelationship between the 

 factors more complex than that we have been considering. 

 In particular, it will be well to keep in view the obvious 

 possibility that sexual dimorphism may be, sometimes at 

 least, a phenomenon in which a compound character plays 

 a part, so that for the production of femaleness, for instance, 

 it may be necessary that two factors should coincide in the 

 same zygote. 



Finally, in a preliminary survey of the subject, the 

 attractiveness of Castle's suggestion is not to be denied. 

 It may well be that the unions of spermatozoa with ova 



* Amongst much evidence which Professor C. O. Whitman most 

 kindly gave me in 1907 concerning the results of his long-continued 

 experiments in crossing the species of Pigeons, was an example which 

 is strongly suggestive of the same heterozygosis of the female. When 

 Turtur humilis . is crossed with a white T. risorius $ , white young may 

 come in F^ and are then always females. 



t The same remarks apply of course also to the moth Abraxas grossu- 

 lariata of which the males are pure to the type, while the females are 

 hybrids of lacticolor. 



