EFFECTS OF INBREEDING 



productive plants, lacking in vigor. But racial 

 vigor is fully restored by a cross between two 

 depauperate unproductive individuals obtained 

 by self-fertilization, as has recently been shown 

 by Shull. This result is entirely in harmony 

 with those obtained by Darwin, who showed 

 by long-continued and elaborate experiments 

 that while some plants do not habitually cross 

 and are not even benefited by crossing, yet in 

 many other plants crossing results in more 

 vigorous and more productive offspring; that 

 further, the advantage of crossing in such cases 

 has resulted in the evolution in many plants 

 of floral structures, which insure crossing 

 through the agency of insects or of the wind. 



In animals the facts as regards close fer- 

 tilization are similar to those- just described 

 for plants. Some animals seem to be indiffer- 

 ent to close breeding, others will not tolerate 

 it. Some hermaphroditic animals (those which 

 produce both eggs and sperm) are regularly 

 self-fertilized. Such is the case, for example, 

 with many parasitic flat-worms. In other cases 

 self-fertilization is disadvantageous. One such 

 case I was able to point out some fifteen years 



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