No. 551] THE MENDELIAN NOTATION 



651 



Taking into consideration all the facts, no one can 

 deny that they are well described by terminology which 

 requires hypothetical descriptive segregating units as 

 represented by the term factors. What then is the object 

 of having the units vary at will? There is then no value 

 to the unit, the unit itself being only an assumption. It 

 is the expressed character that is seen to vary; and if 

 one can describe these facts by the use of hypothetical 

 units theoretically fixed but influenced by environment 

 and by other units, simplicity of description is gained. 

 If, hotvever, one creates a hypothetical unit by which to 

 describe phenomena and this unit varies, he really has 

 no basis for description. 



The facts obtained when working with pied types are 

 complex. They are evidently not thoroughly understood 

 as is evidenced by a different interpretation made by 

 every worker who has investigated them. Doncaster 

 and Mudge see two types of Irish rat. Why not three or 

 four? Crampe obtained hooded rats from cross of self- 

 colored and albino, the hooded coming only from hetero- 

 zygotes having some white. No adequate explanation 

 has been given. Cuenot concluded regarding pied mice 

 with several degrees of piedness that each was recessive 

 to the other of next higher grade. In fact the behavior 

 of self colors and spotted colors among mammals as 

 among plants is pretty well " confused," as in several 

 species spotted types dominant to self color are known. 



Castle's other' experiments in selection— the forma- 

 tion of a four-toed race of guinea-pigs starting with one 

 animal with a rudimentary fourth toe, and the perfec- 

 tion of a silvered race of guinea-pigs from an animal in 

 which the character was feebly expressed— need not be 

 considered here. Both were necessarily crossed with 

 normals at the start, and gradual isolation of races hav- 

 ing the proper gene complex for complete expression of 

 the characters is to be expected. There have been nu- 

 merous selection experiments of this type— such as those 

 of De Vries, the Vilmorins, the Illinois Agricultural Ex- 

 periment Station, etc.— that have yielded results. 



