302 GENETICS 



the mating of the two, some offspring may be produced 

 that have all the superior qualities of the two original stocks : 

 strength, agility, acute sight and hearing. The result may 

 thus be Individuals that are superior to any that have before 

 existed. At the same time of course there will be produced 

 other Individuals that combine the poor qualities of the 

 parent stocks; in the case above suggested, they will com- 

 bine weakness and slowness with poor senses. Such indi- 

 viduals are at a great disadvantage; In time they die or fail 

 to propagate. There may be thus left only the individuals 

 that combine the superior qualities of the original stocks. 

 A superior stock has been produced. 



In such ways, further, the variety among organisms is 

 greatly increased. From two stocks, each with certain com- 

 binations of characters, there are produced, by crossing, 

 many stocks with different combinations of characters. 



2. By recombinations of genes, single characteristics 

 such as strength, vigor or development, dimensions, ability 

 in special directions, and the like, may be increased beyond 

 the degree In which they before existed. 



We have seen In the preceding pages how such character- 

 istics may be increased by crossing, beyond the degree in 

 which they existed in the two parent stocks. It may thus 

 result that they will be Increased beyond the degree in which 

 they have ever before existed, in the given species of or- 

 ganism. 



This results, as we have seen, on the one hand by the 

 contribution of supplementary genes on the part of the 

 two parents, as illustrated on pages 219 and 291 and fig- 

 ures 47 and 60. It results likewise from the recombination 

 in the offspring of the genes that were in earlier genera- 

 tions separated in different ancestors, as illustrated on 

 page 296 and figures 62 and 62. A concrete example is 

 the following: 



East and Hayes crossed two varieties of tobacco.^ One 



