SCOPE OF GENETICS 23 



at once the nature of Variation and Rever- 

 sion. The Reversion occm*s because the two 

 factors that made the height of the old Sweet 

 Pea again come together after being parted : 

 and the Variations by which each of the 

 dwarfs came into existence must have taken 

 place by the dropping out of one of these 

 elements or of the other. 



Conversely there are factors which by 

 their presence can prevent or inhibit the 

 development and appearance of others pre- 

 sent and unperceived. 



For example, all the factors for pigmen- 

 tation may be present in a plant or an 

 animal; but in addition there may be 

 another factor present which keeps the 

 individual white, or nearly so. 



There are cases in which the action of 

 the factors is superposed one on top of the 

 other, and not until each factor is removed 

 in turn can the effects of the underlying 



^mm 



