270 



THE AMERICAN NATURALIST 



[Vol. LIV 



Crossed with brown normal yyblbl such a yellow would give the 

 following zygotes : 



Yy BL bl; Yellow heterozygous for black and lethal 



yy BL bl; Black lethal; dies* . 



^' r, ^ ,, ■ , ronininhhj. 



Yy bl bl ; Yellow carrying brown normal 

 yy bl bl ; Brown normal 



Yy bL bl ; Yellow carrying brown and lethal 

 yy bL bl ; Brown lethal ; dies* [ ^ 



Yy Bl bl ; Yellow normal heterozygous for black 

 yy Bl bl ; Black normal 



The death of the rare brown lethal individual would not be 

 noticed, for the common death of black lethals would leave a dis- 

 tinct excess of brown normals. 



This hypothesis is capable of experimental test and involves a 

 lethal mutation in an entirely new factor which presupposes no 

 generality of the process in all yellows and agoutis; and simply 

 assumes that yellow, when present, hampers the action of the 

 lethal in much the same sort of way that it hampers the activity 

 of the black forming factor in the skin and hair. 



The above hypothesis is advanced simply as an additional pos- 

 sibility for test in case something more than chance fluctuation 

 due to random sampling is involved. 



To all who are familiar with the recent advances in our knowl- 

 edge of heredity, which were made possible largely through the 

 investigations of Morgan and others with 'the fly, Drosophila 

 melanogaster, especially to those who have followed the develop- 

 ment of the chromosome theory of heredity with its correlative 

 theories of mutation and evolution, the urgent need of extensive 

 corroborative evidence from other animals and plants must be 

 forcibly clear. Although it appears inconceivable that the con- 

 clusions reached from the drosophila investigations are not ap- 

 plicable in all their essential features to all animals and plants, 



