Heredity 193 



2. It may be nothing but presence in a latent state. 



3. There may not be such a factor as the absence 

 factor. 



Of the three proposed, the first seems to be, Hurst 

 remarks, the simplest, but it is difficult to realize and 

 understand how such an absence factor is originated. 

 Furthermore, he says: " There are many cases where 

 the factor .for presence is in a latent condition." The 

 third explanation meets an objection in the fact that there 

 is no pairing of factors in cross-breeding. Consequently, 

 it follows that, according to this view, it is impossible to 

 explain the phenomenon of segregation. 



Mutations resulting from mendelian segregation and re- 

 combination. It is very probable that many mutations 

 which appear suddenly and remain constant are the result 

 of mendelian segregation and recombination. If many 

 unit-characters are involved, it is easily perceived how 

 certain combinations of these would produce plants 

 of unusual appearance which will be homozygous and 

 breed true. Reference to Table I, p. 176, will show the 

 great possibilities of obtaining apparently new characters 

 by new combinations of old ones. It will be noted that 

 when as many as 10 allelomorphs are involved, and this 

 does not seem to be an impossible number, there is the 

 possibility of producing 1024 different visible types. 



Mutations which mendelize are constant. The effect 

 of swamping of mutations by crossing is prevented be- 

 cause of their continued identity due to the purity of the 

 germ-cells which represent them. 



Mutations may be due to three things : (a) the ac- 

 quisition of one or more new characters, (6) the loss of 

 o 



