188 EVOLUTION AND ANIMAL LIFE 



Mendel's principal data were derived from the crossing of 

 varieties of peas (Pisum sativum) in which he found several pairs 

 of well-marked contrasting characters. Bateson gives a clear 

 and concise summary account of Mendel's methods and results 

 which we quote in the following paragraphs. For the purposes 

 of his experiments Mendel selected seven pairs of characters as 

 follows : 



1. Shape of ripe seed, whether round; or angular and 

 wrinkled. 



2. Color of "endosperm" (cotyledons), whether some shade 

 of yellow; or a more or less intense green. 



3. Color of the seed skin, whether various shades of gray and 

 gray-brown; or white. 



4. Shape of seed pod, whether simply inflated; or deeply 

 constricted between the seeds. 



5. Color of unripe pod, whether a shade of green; or a bright 

 yellow. 



6. Nature of inflorescence, whether the flowers are arranged 

 along the axis of the plant; or are terminal and form a kind of 

 umbel. 



7. Length of stem, whether about six or seven feet long, 

 or about three fourths to one and one half feet. 



"Large numbers of crosses were made between peas differing in 

 respect of one of each of these pairs of characters. It was found that 

 in each case the offspring of the cross exhibited the character of one 

 of the parents in almost undiminished intensity, and intermediates 

 which could not be at once referred to one or other of the parental 

 forms were not found. 



" In the case of each pair of characters there is thus one which in 

 the first cross prevails to the exclusion of the other. This prevailing 

 character Mendel calls the dominant character, the other being the 

 recessive character. 1 



"That the existence of such ' dominant' and 'recessive' charac- 

 ters is a frequent phenomenon in cross breeding, is well known to all 

 who have attended to these subjects. 



" By letting the cross-breds fertilize themselves Mendel next raised 

 another generation. In this generation were individuals which showed 



1 "Note that by these novel terms the complications involved by the 

 use of the expression /prepotent' are avoided." 



