444 THE MAINTKNANCE OF SPECIES 



of reducing the phenomena of inheritance to a measurable mathemat- 

 ical basis. For this purpose he wisely chose for experimentation gar- 

 den peas, which not only are easily grown, but also possess readily 

 recognized constant characteristics. Since peas are normally self- 

 fertilized, they represent at the start comparatively pure hereditary 

 strains. Moreover, hybridization in peas can be controlled from 

 contamination by insects. Since fertilization occurs before the flowers 

 open, the hooded structure of the flowers is such that interference from 

 their chance visits is prevented. As is well known, insects may carry 

 on involuntary hybridization experiments of their own in connection 

 with many plants, by transferring pollen grains from the stamen of 

 one flower to the stigma of another. 



Instead of considering the whole complex individual in the light of 

 a "hybrid" unit, as former hybridizers had done with much result- 

 ing confusion, Mendel focused his attention upon single alternative 

 characters, one pair at a time, that were unlike in the two contributing 

 parents. This simplification of the problem made the collection of 

 data less complicated, and the analysis of results possible. Finally, 

 he not only combined single pairs of characters into hybrids, but he 

 went further and followed up the results obtained by breeding these 

 known hybrids together through several generations, meanwhile tak- 

 ing meticulous pains to account for all the offspring of whatever sort 

 in each case, so that ratios of relative occurrence could be computed. 

 For example, he dealt with seven pairs of alternative characteristics 

 found in different strains of peas, as follows : ^ 



1. Smooth seeds or wrinkled seeds ; 



2. Yellow seed-coats or green seed-coats ; 



3. Tall vines or dwarf vines ; 



4. Colored flowers or white flowers ; 



5. Axial flowers or terminal flowers ; 



6. Inflated pods or constricted pods ; 



7. Green pods or yellow pods. 



In every case when these pairs of characters were put together 

 the hybrids thus produced were not intermediate in appearance, but 

 were alike, and resembled one of the parents and not the other. 

 When these hybrids in turn were interbred with each other, or allowed 

 to be normally self-fertilized, which amounts to the same thing, the 



' Dr. O. E. White, of the Brooklyn Botanical Garden, as early as 1917 reported thirty-four pairs 

 of hereditary characters in peas on which determinative experimental studies have been made. 



