382 castle: role of selection in evolution 



modified race. Even in wild species, such as the skunks, white- 

 spotting is manifestly a variable character, which no doubt will 

 respond to the selective efforts of our skunk farmers, who de- 

 sire an all-black race. Why white-spotting should be a less 

 stable character genetically than some others, it is impossible 

 to say, but the fact is beyond question. Morgan has suggested 

 that in general the genetic' basis of a Mendelian character may 

 be a single molecule, and gives this as a reason for believing in its 

 constancy. But white spotting can hardly fall in with this 

 conception. It seems to me more probably due to a quantitative 

 deficiency in the germ of some substance which normally finds 

 its way into all epidermal cells of the body and which is responsible 

 for the development in them of melanin pigment. Greater and 

 greater deficiencies of this substance cause more and more 

 extensive white areas. 



Complete or total albinism behaves very differently. It 

 results from a complete change in some color factor which may 

 well be a simple molecule since it appears to be incapable either 

 of contamination in crosses or of modification under selection. 

 Nevertheless the color factor (molecule or whatever it may be) 

 evidently is not so simple but that it can assume at least four 

 mutually allelomorphic forms, as shown for the guinea-pig by 

 Wright, a like number of allelomorphs, though not their exact 

 equivalents, being known also in the rabbit. 



As regards the agouti factor in mice, rabbits, and guinea-pigs, 

 this too may assume several different allelomorphic conditions, 

 though it is not certain that any one of these fluctuates or can 

 be modified other than by associating with it unrelated genetic 

 factors. 



The divergent conclusions which students of genetics have 

 reached concerning the stability of Mendelian genes and the 

 consequent effects of selection for their modification are probably 

 due in part to the particular choices which they have made of 

 test cases. A study of albinism alone would lead one to believe 

 in the fixity and constancy of Mendelian genes and the impossi- 

 bility of modifying them by selection. A study of white spotting 

 leaves one with the unshakable conviction that this form of 



