THE GREAT RELAY RACE 455 



Outbreeding and Hybrid Vigor 



Outbreeding, on the contrary, introduces variety and tends to 

 cover up recessive defects by the introduction of new dominant char- 

 acters, although it does not permanently eliminate the former. 



In nature probably most animals and plants outbreed. Even 

 hermaphroditic animals such as earthworms and snails, in which both 

 sexes are included in one individual, usually mature their eggs and 

 sperm at different times, as already noted, thus insuring outbreeding. 

 The same thing is true to a large extent of the great array of plants in 

 which both pollen grains and ovules are housed in the same flower. 



One of the beneficial results of outbreeding is hybrid vigor, which 

 usually accrues to the first generation of hybrids. This result may be 

 accounted for as the summation of desirable dominant characters 

 from the two diverse parents. The advantages gained by this type 

 of cross, however, do not endure in successive generations, when 

 inbreeding comes in with its leveling effects. The former confusion 

 and uncertainty about the consequences of inbreeding, outbreeding, 

 and hybrid vigor is straightened out when one goes behind the scenes 

 with the insight made possible by Mendel's laws. 



Asexual Propagation 



Another practical way of maintaining desirable hereditary quali- 

 ties, particularly in plants, when once they have been obtained, is 

 by asexual propagation through slipping or grafting. This is the 

 method employed in maintaining strains such as navel oranges, w^hich 

 produce no seeds, and also in plants which do produce seeds whose 

 phenotypes are known desirable somatoplasms but whose genotypes 

 are hidden in unknown problematical seeds. By this procedure of 

 asexual propagation the desired combination is continued, without 

 the introduction of any disturbing germinal modification from the 

 outside. Many of Luther Burbank's famous plant "creations," 

 such as the spineless cactus and the white blackberry, have been 

 established and made available by this method. 



The Germplasmal Method 



The foregoing somatoplasmal methods of approach in studying 

 heredity, although to a remarkable degree successful, are at best only 

 indirect. It is more and more apparent that the most hopeful line 

 H. w. H. — 30 



