Reproduction 361 



upon the stigma of the same flower, but rather to struc- 

 tural difficulties preventing pollination; that is to say, 

 it appears that the flowers may be self-fertile; nevertheless, 

 the effect is that the best seed production requires insect 

 visits. In consequence, to produce clover seed economi- 

 cally, this crop should be permitted to flower only in the 

 season when the bumblebees are abundant and active. 

 The first crop is too early, so that it is commonly cut for 

 hay, and the second crop is permitted to develop seed. 



210. Darwin's conclusions. A study of the remark- 

 able morphological devices in many flowers pollinated by 

 insects suggested to Darwin the importance of determin- 

 ing, with respect to the offspring, the comparative physio- 

 logical effects of cross and self fertilization. He made 

 fertility and constitutional vigor of the offspring his field 

 of investigation. As a result of his extensive experiments 

 with some familiar plants throughout a period of years, 

 Darwin concluded that " cross-fertilization is generally 

 beneficial, and self-fertilization injurious." In drawing 

 these conclusions, he made careful comparisons of the 

 offspring with respect to height, weight, constitutional 

 vigor, and fertility. 



Darwin recognized some general exceptions, in which, 

 for instance, self-fertilization was often more effective for 

 a generation or two (as in tobacco and Petunia) than 

 crossing with relatively closely related individuals. 

 Nevertheless, in the case of tobacco a cross with wholly 

 fresh stock was invariably more effective than self-fer- 

 tilization. The most notable exception to his general 

 statement, quoted above, occurred in a vigorous individual 

 of the morning glory (Ipomcea purpurea), Hero, whose 



