ANIMAL AND PLANT REPRODUCTION 25 



to the same order, not to mention a transformation of the 

 same individual. 



A few other mechanisms which promote cross-fer- 

 tilization have been found in isolated cases. They are not 

 as widespread as the one just described, but are peculiarly 

 interesting nevertheless. Among certain of the cirripedes, 

 the normal individuals are hermaphroditic, but in addition 

 a few tiny degenerate males are developed. They are little 

 more than bags of sperm and are calculated to make some- 

 what amusing any generalization as to the ** stronger 

 sex.'' Darwin, who discovered them, called them com- 

 plemental males. Another means of preventing continued 

 self-fertilization is self-sterility, a condition in which self- 

 fertilization is very difficult or even impossible through 

 some physiological impediment which is not clearly under- 

 stood. It was demonstrated by Castle ^^ for the American 

 race of Ciona mtestinalis. 



In what appear to be the essential features, the 

 vicissitudes of reproduction have been similar in tlie 

 vegetable kingdom. The problems were solved in differ- 

 ent ways, but the gross results are largely the same. The 

 most striking difference is the varied success of certain 

 mechanisms. In the animal kingdom sexual reproduction 

 wherever instituted practically always displaced asexual 

 reproduction. Only in a few forms which are either fixed 

 or parasitic in their mode of life did the two methods per- 

 sist side by side. In plants, however, where the sessile is 

 the common condition, asexual and sexual reproduction 

 have continued harmoniously side by side clear up 

 through the angiosperms. Again, there is a marked dif- 

 ference in the success of hermaphroditism. In plants 

 hermaphroditic forms became the dominant types in the 



