No. 578] MULTIPLE ALLELOMORPHS 



9.") 



not's general idea that a series of mutually allelomorphic 

 conditions of spotting exists. Unquestionably in rats, 

 "hooded" and "Irish" are such modal conditions of 

 spotting, allelomorphic with each other and with the un- 

 spotted or self condition (Doncaster, 1905; MacCurdy 

 and Castle, 1907; Castle and Phillips, 1914). The last- 

 named authors find that independent factorial modifiers 

 probably affect the extent of the spotting and yet that, 

 aside from such modifiers, the spotting factor proper may 

 assume relatively stable allelomorphic conditions which 

 Mendelize when crosses are made between stages suffi- 

 ciently distinct. The point of especial interest in allelo- 

 morphic conditions of spotting is that they are not per- 

 fectly stable, but are capable of gradual aud apparently 

 indefinite modification through the selection of fluctua- 

 tions either plus or minus. It would be premature to 

 conclude that similar fluctuations (though perhaps less 

 conspicuous ones) do not occur about the modal condi- 

 tions of other genetic factors which show allelomorphic 

 variation. The black-and-tan form of agouti certainly 

 fluctuates in the amount of ticking found on the sides of 

 the body and the head ; doubtless some of this fluctuation 

 may be due to factors genetically distinct from the chief 

 allelomorphic factor concerned, but there is at present no 

 sufficient ground for supposing the chief factor itself to 

 be incapable of fluctuation. Indeed, it seems highly prob- 

 able, in the light of evidence already obtained, that the 

 present modal condition of the black-and-tan character is 

 one which has been attained only as a result of persistent 

 selection, and that reversed selection will carry it back 

 appreciably nearer to the modal condition seen in gray 

 rabbits. Accordingly, it appears doubtful whether allelo- 

 morphs are themselves perfectly and permanently stable. 

 Moreover, the rapid increase of recognized allelomorphs 

 makes us wonder whether their number is limited and defi- 

 nite. Black-and-tan represents, on the whole, an inter- 

 mediate condition between black and gray. Is it not con- 

 ceivable that intermediates may yet be discovered be- 



