vii NATURAL CROSSING 123 



field. Certainly the error of natural crossing must be less 

 between Hindi and Egyptian than between two Egyptians. 



With a value of 3 per cent, for American and Egyptian, 

 and of 10 per cent, for Egyptian and Egyptian, under the 

 most favourable conditions, the occurrence of such values 

 as 35 per cent., mentioned above, leads us to suspect that 

 the story is not yet complete. 



Before proceeding further we must define the value of 

 our "percentage vicinism" with more exactitude. We are 

 agreed that computation on the basis of the number of 

 ovules is the most precise, but it remains to decide how 

 we shall recognise those ovules. The difficulty which 

 arises in recognition is due to the fact that all Fi hybrids, 

 and most F^ hybrids, germinate much more energetically 

 than their parents. In this peculiarity ■ lies the great 

 weakness of simple " selection methods." The precise 

 reasons for this diff"erence are still not clear, but knowledge 

 of the fact is very old. It is therefore not suflScient to 

 count the number of vieinists in a population raised from 

 contaminated seed ; we shall be nearer to the truth if we 

 take the ratio of vieinists to the number of seeds sown, 

 thus assuming that all the seeds which did not establish 

 themselves were pure-bred. In inter-Egyptian crossing 

 this factor will be insignificant as compared with its 

 importance in crosses of Egyptian with Upland. In the 

 latter case the author has been the victim of such 

 absurdities as the cultivation of a family which contained 

 100 per cent, of vieinists ; the soil tilth was not good, the 

 weather was cool, and between the two perils of 

 mechanical resistance and " sore-shin " not a single selfed 

 embryo survived. 



When attempting to account for amounts of real 

 vicinism which rise as high as 30 per cent, we must 

 consider the nature of the pollen to be found on the 

 breeding-plot. This plot has contained cottons from 

 Egypt, from America, from India; indigenous cottons 



