iv PRESENCE AND ABSENCE THEORY 35 



between these two pure forms, though neither of them 

 chould, on Mendel's view, contain the single? An ex- 

 planation which covers the facts in a simple way is that 

 which has been termed the " Presence and Absence" 

 theory. On this theory the dominant character of an 

 alternative pair owes its dominance to the presence of a 

 factor which is absent in the recessive. The tall pea is 

 tall owing to the presence in it of the factor for tallness, but 

 in the absence of this factor the pea remains a dwarf. All 

 peas are dwarf, but the tall is a dwarf plus a factor which 

 turns it into a tall. Instead of the characters of an al- 

 ternative pair being due to two separate factors, we now 

 regard them as the expression of the only two possible 

 states of a single factor, viz. its presence or its absence. 

 The conception will probably become clearer if we follow 

 its application in detail to the case of the fowl's combs. 

 In this case we are concerned with the transmission of the 

 two factors, rose (R) and pea (P), the presence of each of 

 which is alternative to its absence. The rose-combed 

 bird contains the factor for rose but not that for pea, and 

 the pea-combed bird contains the factor for pea but not 

 that for rose. When both factors are present in a bird, 

 as in the hybrid made by crossing rose with pea, the result 

 is a walnut. For convenience of argument we may de- 

 note the presence of a given factor by a capital letter and 

 its absence by the corresponding small letter. The use of 

 the small letter is merely a symbolic way of intimating 



