SELECTIVE FERTILIZATION IN POLLEN MIXTURES. 283 



crosses. The dissimilar unions gave greatly increased numbers 

 in both reciprocal combinations. However, the type of mating 

 did not influence the number of eggs laid and there is no proof 

 that cross-fertilization occurred more readily than self-fertiliza- 

 tion. The results show that the cross-fertilized eggs hatched 

 better and the offspring survived in greater numbers, a result 

 which is easily understood since there were lethal factors involved 

 in the material worked with. 



The only evidence from the animal side of a definite selective 

 action comparable to the many instances of self-sterility in 

 plants is the well-known case of self-impotency in dona intes- 

 tinalis (Castle, 1896). Morgan (1905, '07, '10) has experimented 

 with this organism and has found that the self-sterility is not 

 always complete. Material gathered on the Pacific coast showed 

 somewhat greater receptiveness to the individual's own sperm 

 than eggs of the same species at Woods Hole which were almost 

 entirely unresponsive to sperm from the same individual which 

 produced the eggs. In another ascidian, Cynthia partida, he 

 found that self-fertilization takes place frequently but the sperm 

 of unrelated individuals is more effective. A third species, 

 Molgula manhattensis is self-fertilized as readily as cross-fertilized. 

 From this it seems that incompatibility of uniting gametes as a 

 means of insuring cross-fertilization exists in various grades of 

 effectiveness. Even in extreme cases the degree of self-sterility 

 may be modified by internal and external conditions. In Nico- 

 tiana East and Park find that self-fertilization sometimes takes 

 place towards the end of the growing period when the vigor of the 

 plants is reduced. 



DISCUSSION. 



As far as the writer knows the results obtained from maize 

 stand alone among plants in showing a selective action unfavor- 

 able to fertilization by sperm from individuals of different 

 hereditary constitution. The handicap placed upon the foreign 

 pollen is proportional to the germinal unlikeness. If the unequal 

 effect is due to a slower growth of the pollen tube through the 

 tissues of style the selective action may be restricted to plants, 1 



1 E. C. Miller (Jour. Agric. Research, Vol. 18. pp. 255-266, Dec. 1919), has 

 recently made a detailed study of fertilization in maize and finds that from many 



