ANIMAL AND PLANT REPRODUCTION 33 



plants returned to a type of reproduction which held all 

 the advantages of bisexuality by evolving means for pro- 

 moting cross-fertilization. But it is the advantage of 

 cross-fertilization and not the assumed disadvantage of 

 self-fertilization that should be stressed. The Knight- 

 Darwin Law, " Nature abhors perpetual self -fertiliza- 

 tion," should read Nature discovered a great advantage 

 in an occasional cross-fertilization. 



The higher plants made a success of hermaphroditism 

 because there was a return to the advantages of gono- 

 chorism through the development of almost innumerable 

 devices tending to promote frequent crossing between 

 plants of the same and nearly related species. Some 

 species did actually return to true structural gonochorism, 

 but in most cases other means of obtaining cross-fertiliza- 

 tion were developed. There was no advantage, consider- 

 ing their sessile mode of life, in relinquishing the 

 possibility of self -fertilization. 



Some of the various cross-fertilization mechanisms 

 utilized are very reminiscent of those of animals. Monoa- 

 cisrn, the production of male and of female flowers on the 

 same plant, and dichogamy, the maturation of the male 

 and female organs at different times, have their counter- 

 parts in the other kingdom. So also the physiological phe- 

 nomenon self-sterility of which only one instance is known 

 among animals is very common among plants. Some 

 hundred or so species distributed throughout thirty-five 

 or more families have been shown to be self-sterile, al- 

 though the true number is probably many times this 

 figure. Again polygamy, w^here, in addition to hermaph- 

 roditic flowers, either male or female flowers are devel- 



3 



