CROSS-BREEDING AND INBREEDING 509 



Similarly, he has shown that some of the hybrids produced by cross- 

 ing the eggs of the sea-urchin Strongylocentrotus purpuratus with the 

 sperm of S. franciscanus hved nearly twice as long under cultural con- 

 ditions as did either pure breed, while the reciprocal cross showed 

 very low viability. 



d) Augmented facility of vegetative propagation. — Many hybrid plants 

 are noted for their success in vegetative propagation. It is believed 

 that plants such as strawberries, brambles, grasses, etc., that propa- 

 gate so successfully by vegetative methods, are the products of hybrid- 

 ization. The vegetative method of reproduction not only maintains 

 a fortunate combination of genetic characters that could hardly be 

 repeated by gametic reproduction, but maintains their hybrid vigor 

 as well. 



These and perhaps some other effects, all of which are essentially 

 beneficial, have been noted in both animals and plants. The follow- 

 ing explanation of hybrid vigor has been given by D. F. Jones: 



^'Explanation of hybrid vigor. — From the illustrations given it is 

 evident that there is a tendency for the features of both parents to be 

 expressed in the offspring. This is the basis for an understanding of 

 the vigor derived from crossing. There is a greater number of differ- 

 ent hereditary factors in a hybrid individual than in either pure parent. 

 Nearly all variations that are recessive are less favorable to the de- 

 velopment of the organism than their dominant mates Since 



crossing brings out those qualities which help the individual in its 

 growth and suppresses the abnormal and unfavorable characters, it 

 is to be expected that hybrids will tend to be strong and vigorous. 

 This will be true, however, only if each parent is able to supply the 

 deficiency of the other, and if the forms crossed are not so diverse 

 that their union is incompatible with normal growth. If the parents 

 are themselves hybrids, further crossing may bring together no great 

 number of dominant favorable growth factors but may even uncover 

 recessive characters. Hence further crossing can not usually increase 

 size and vigor, and in fact may even result in the appearance of weak- 

 nesses. This is clearly understandable from the operation of Mendel's 

 principles of heredity." 



The question now arises as to whether hybridization in general is 

 advantageous or the reverse. Undoubtedly first-generation hybrids 

 are generally an improvement upon either parent-race, especially if 

 the parents belong to races not too distantly related. If we could stop 

 hybridization after one generation, as Nature stops it in the case of the 



