CHAPTER VII 



Fertility 



Darwin's splendid work on cross- and self-fertiliza- 

 tion, his study of the mechanism of cross-fertilization in 

 orchids, and his work on the different forms of flowers 

 of plants of the same species, mark the beginning of 

 the modern study of the problem of fertility and 

 sterility. Darwin carried out studies on the effects 

 of cross-fertilization in comparison with self-fertilization 

 and reached the conclusion that the offspring resulting 

 from cross-fertilization are more vigorous than the 

 offspring from self-fertilization. No one can read his 

 books dealing with these questions without being 

 impressed by the keenness of his analysis and the 

 open-minded and candid spirit with which the prob- 

 lems were handled. Since Darwin's time we have not 

 advanced very far beyond the stage to which Darwin 

 carried these questions. We have more extensive 

 experiments and some more definite ways of stating 

 the results, but Darwin's work still stands as the most 

 important contribution that has been made to this 

 subject. 



The credit of the second advance belongs to Weis- 

 mann. His speculations concerning the effects of 

 mixing of the germ-plasms of the two individuals, 

 that combine at the time of fertilization, not only 

 aroused renewed interest in the nature of the process 

 of sexual reproduction, but brought to light also the 



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