Xo. 600] PIEBALD BATS AND MULTIPLE FACTOBS 741 



ency of the germ plasm to maintain the status quo. On 

 the basis of the first view, the external influence (selec- 

 tion) would have major importance in defining the 

 course of evolution ; on the second, the internal influence 

 (the inherent nature of the germ plasm itself) would 

 have major importance. In both cases, the nature of the 

 progeny would depend on the nature of the germ plasm 

 of the parents. In both cases selection would be able to 

 modify the race. But in neither case is the origin of the 

 changes in the germ plasm explained. The fundamental 

 causes of evolution are as much a mystery as ever. 

 Grant a certain hypothesis of germinal changes, and 

 selection becomes a more important factor in evolution 

 than when another hypothesis is granted. But even such 

 an increased importance of selection does not give it the 

 value of a fundamental creative cause of evolution. 



There has appeared a theory that would give selection 

 still greater importance by saying that selection has the 

 power to build up unit-factors and induce mutation. 



Unit-characters may arise gradually as the result of repeated selec- 

 tion in a particular direction (Castle, :12b, p. 280). 



In yellow animals, as in blacks, individuals of varing intensity occur 

 the darkest known as reds, the lightest as creams. A complete series 

 of intermediates can be obtained if so desired. If we select any two 

 widely separated stages in this series fairly stable in their breeding 

 capacity and cross these, they Mfendelize, i.e., they behave as if they were 

 a single unit-character difference between them. . . . That difference 

 might equally well be half as great as it is, or a quarter as great, or a 

 thousandth part as great. A monohybrid ratio would result equally in 

 each case, upon crossing the two quantitatively different stages (Castle, 

 :12a, p. 358). 



Now this may be true for yellow guinea pigs, but the 

 rats clearly demonstrate that it is not true in all cases. 

 The two quantitatively different stages of the hooded 

 pattern represented by the plus and minus races do not 

 result in a monohybrid ratio when they are crossed. 



However there has appeared a 1 'unit-character" dif- 

 ference in one of these races of hooded rats. It appeared 

 suddenly, and it Mendelizes when crossed with other 



