Heredity 157 



true results of the hybridization are to be discovered. 

 Moreover, the study must be more exact than a mere con- 

 trasting and comparing of plants : character must be com- 

 pared with character. 



Mendel's experiments. The garden pea seemed to fulfill 

 all of the requirements. Mendel chose well-marked hor- 

 ticultural races or varieties. He grew these two years 

 before the experiment proper was begun in order to de- 

 termine their stability or trueness to type. When the 

 experiments were finally begun, he used only normal 

 plants as parents, throwing out such as were weak or 

 aberrant. Peas are self-fertile. It was to be expected 

 that under such conditions the hybrid offspring would 

 show uniformity of action ; and it did. 



In order to study the behavior of the hybrids, it was 

 necessary to choose certain prominent marks or characters 

 for comparison. Seven of these characters were chosen 

 for observation. These marks pertain to seed, fruit, 

 position of flowers, and length of stem, and they may be 

 assumed to be representative of all other characters in 

 the plant. These characters were paired (practically 

 opposites) as long-stem vs. short-stem, round-seed vs. 

 angular-seed, inflated pods vs. constricted pods. They 

 were " constant" and "differentiating." Of course every 

 parent plant possessed one or the other of every pair of 

 contrasting characters ; but in order to facilitate his 

 studies, Mendel chose a special set of parents to illustrate 

 each character. 



The seed-shape characters were roundness and angu- 

 larity the former being the " smooth" pea of gardeners 

 and the latter the " wrinkled" pea. Let us suppose that 



