196 MENDELISM 



shall find later on that there is some evidence derived 

 from an entirely different class of facts which seems to 

 support the idea that a selective fertilization of this 

 kind really does take place in certain cases. The 

 phenomenon is nevertheless so remarkable that we 

 may have some hesitation in accepting it without 

 further evidence. In the meantime it must be recorded 

 as a distinctly exceptional case, though not, be it 

 noted, as an exception to Mendel's law. The gametes 

 obey the law, as was shown by crossing yellow with 

 non-yellow, and it is only hi their manner of combina- 

 tion that a complication has been introduced. 



We have still to describe a case in which two latent 

 factors, one derived from each parent, give rise, by 

 their simultaneous presence in the zygote produced, 

 to the appearance of an entirely new character. The 

 following example is the first one of the kind to be 

 completely elucidated, and is one of those studied by 

 Messrs. Bateson and Punnett and Miss Saunders. 



The white-flowered variety of sweet -pea known as 

 Emily Henderson was found to exist in two forms, 

 only to be distinguished from one another by the 

 shape of the pollen grains which they produced. In 

 one of the two the shape of the pollen is elliptical 

 (long pollen), in the other it is approximately spherical 

 (round pollen). Sweet-peas normally undergo self- 

 pollination, so that the two types naturally remain 

 distinct. Let us see what happened when the long- 

 and the round-poll ined forms were crossed together. 



The cross-bred plants (FJ had coloured flowers 

 flowers of the old-fashioned purple type known to 



