PLANT HYBRIDIZATION BEFORE MENDEL 231 



The total number of the crossed plants amounted to 1,101, and 

 of the selfed plants to 1,076. As a result, Darwin found that the 

 plants derived from crosses between different strains of the same 

 species were taller on the average, than plants derived from 

 crosses within the 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 respect to height, de- 

 rived 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 whole plant has come from the same fertilized cell, rarely are 

 widely differentiated, Darwin reasons that the effects of inter- 

 crossing 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 con- 

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

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

 are in one sense distinct individuals, sometimes varying and differing 

 from one another in structure and constitution." (lb, p. 449.) 



"Thus," he concludes, "the proposition that the benefit from cross 

 fertilization depends on the plants which are crossed having been sub- 

 jected during previous generations to somewhat 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, p. 449.) 



Darwin comments also on the reversed situation, where changes 

 in the external conditions result in sterility, for which he seeks to 

 find a logical connection with the condition induced by crossing. 



"For as, on the one hand, slight changes in the conditions of life are 

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

 the size, vigour, and fertility of their offspring, so, on the other hand, 

 certain other changes in the conditions of life cause sterility ; and as 

 this likewise ensues from crossing much modified forms or species, we 

 have a parallel and a double series of facts which apparently stand in 

 close relation to each other." (ic, 2:126.)- 



Darwin appears to hold the ill effects of close fertilization to 

 be due to the fact that the sexual elements in the different flow- 

 ers on the same plant have not differentiated, while in his con- 

 clusion he appears to consider the benefits of cross-fertilization 

 to be due to the individuals involved in the cross having been 

 differentiated through being exposed to different conditions. 



