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THE AMERICAN NATURALIST [Vol. LIII 



same strain, and taller in the latter case than in the case 

 of the offspring of self -fertilized plants. The average 

 ratio of 620 crossed to 607 selfed plants in height, derived 

 from Darwin's tables, was as 100:86. 



From the fact that flower buds are in a sense distinct 

 individual plant units, which sometimes vary and differ 

 widely from one another, and yet, when on the same plant, 

 owing to the fact that the plant has come from the same 

 fertilized cell, rarely are widely differentiated, Darwin 

 reasons that the effects of intercrossing can be explained. 

 He says: 



The fact that a cross between two flowers on the same plant does no 

 good or very little good, is likewise a strong corroboration of our 

 conclusion; for the sexual elements in the flowers on the same plant can 

 rarely have been differentiated, though this is possible, as flower buds 



Hence, he concludes : 



Thus the proposition that the benefit from cross-fertilization depends 



different conditions, or to their having varied from some unknown 

 cause as if they had been thus subjected, is securely fortified from all 

 sides (lb, 444). 



Darwin comments also on the reversed situation, where 

 changes in the external condition result in sterility, for 

 which he seeks to find a logical connection with the condi- 

 tion induced by crossing. 



On the one hand, slight changes in the conditions of life are favor- 

 able to plants and animals, and the crossing of varieties adds to the 



tail, ..ilicr cliaiiges in the conditions of life cause sterility; and as this 

 likcv\i>e ensues from crossing much modified forms or species, we have 



dole relation to each other (Ic, 2:126). ' 



Darwin's view as to the reason for the good effects of 

 crossing was based upon the long prevalent opinion that, 



