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MR G. UDNY YULE ON THE ASSOCIATION 



error. This is the question whether fertilisation of a flower with pollen from another 

 flower on the same plant is any better than strict self-fertilisation. DARWIN came to 

 the conclusion that in only one of the five species tried Digitalis purpurea was 

 there any sensible advantage in crossing different flowers. The point was a difficulty, 

 as there should be an advantage, though a slight one, on his theory that the benefit 

 of cross-fertilisation arises from differences in the general constitution of flowers 

 crossed. 



In only three of the five species tried are the numerical data given in the form 

 required for the present method. They run as below : 



Thus, in the case of Ipomea pwpurea, the association is distinctly negative 

 crossing with another flower of the plant was worse than pure self-fertilisation but 

 in both the other cases it is positive. Mimulus, it is true, offers somewhat doubtful 

 evidence (two experiments having given conflicting results), but the coefficient of 

 association is almost twice its probable error. The foxglove certainly exhibits far the 

 highest and most significant association. Two out of the three species give a positive 

 result, and if all the species are pooled together the result is positive. The fact is we 

 are looking, in all probability, for a very small association, and extensive experiments 

 may be necessary to render its existence certain. Taking the above results together, 

 I should certainly say they gave evidence on the whole of a positive association. The 

 odds against the negative association in the case of Ipomea occurring as a purely 



