112 Plant Genetics 



be modified in certain reactions? Why not admit 

 that MendeHan factors may be modified, and explain 

 Castle's results in this way ? The reason is that when 

 we begin to admit that unit characters and single 

 Mendelian factors may be modified, the whole conception 

 of inheritance becomes chaos. The great advantage of 

 the factor hypothesis is that it furnishes the clearest 

 method of describing breeding results. A statement 

 by East (3) on this point is pertinent. 



Taking into consideration all the facts, no one can well deny 

 that they are well described by terminology which requires 

 hypothetical 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 the environment and 

 by other units, simplicity of description is gained. If, however, 

 one creates a hypothetical unit by which to describe phenomena, 

 and this unit varies, he really has no basis for description. 



In other words, this statement means that the great 

 value of the factor hypothesis is to supply a terminology 

 for describing the facts of inheritance rather than a 

 means of explaining these facts. According to this, 

 therefore, it may be seen that although Castle claims 

 to have modified a unit character by selection, his 

 results can come under the factor hypothesis, and thus 

 our descriptions may be kept clear and uniform. 



There is another piece of work, however, that cer- 

 tainly cannot be explained (that is, described) in terms 

 of the factor hypothesis; on the contrary, it almost forces 

 one to admit that unit characters, even though hypo- 

 thetical, may vary. This is the work of Jennings (4) 



