WHAT EVOLUTION IS 135 



whole set of ^lendelian principles has 

 proved in the hands of the experi- 

 mentalist little short of marvelous in 

 their application. Exceptional cases 

 often occur, but when these are 

 worked out, they are commonly found 

 to be in essential agreement with the 

 general principles. Thus when the 

 red-flowered variety of the common 

 garden four o'clock is crossed with 

 the white-flowered form, as shown 

 on the lower half of page 131, the off- 

 spring are neither red-flowered nor 

 white-flowered but have flowers of an 

 intermediate tint, pink. If now these 

 offspring are bred amongst them- 

 selves three classes result: twenty- 

 five per cent pure whites, twenty-five 

 per cent pure reds, and fifty per cent 

 pinks, a state of affairs that may be 

 described as parallel to that of the 

 guinea pigs, so far at least as purity 

 of the germ is concerned but without 



