246 SCIENCE AND THE HUMAN MIND 



natural selection was sufficient alone to explain the 

 formation of new species, undertook a series of experi- 

 ments on the hybridization or cross-breeding of peas. 

 He published his results in the volumes of the local 

 scientific society, where they lay buried for forty years. 

 Their rediscovery, confirmation and extension by 

 William Bateson and other workers marks the first 

 step in the recent development of heredity as an 

 exact experimental and industrial science. 



The essence of Mendel's discovery consists in the 

 disclosure that in heredity certain characters may be 

 treated as indivisible and apparently unalterable 

 units, thus introducing what may perhaps be termed 

 an atomic conception into the field of biology. An 

 organism either has or has not one of these units ; 

 its presence or absence are a sharply contrasted pair 

 of qualities. Thus the tall and dwarf varieties of the 

 common eating pea, when self-fertilized, each breed true 

 to type. When crossed with each other, all the hybrids 

 are tall and outwardly resemble the tall parent. 

 Tallness is therefore said to be " dominant " over 

 dwarf ness, which is called " recessive." But when 

 these tall hybrids are allowed to fertilize themselves 

 in the usual way, they are found to be different in 

 genetic properties from the parents whom they re- 

 semble outwardly. Instead of breeding " true," their 

 offspring differ among themselves ; three-quarters are 

 tall and one-quarter are dwarf. The dwarfs in turn 

 all breed " true," but of the tails only one-third breed 

 true, giving rise solely to tall plants, while in the 

 next generation the remaining two-thirds repeat the 

 phenomena of the first hybrid generations, again 

 giving birth to pure dwarfs and mixed " tails." 



