no THE COTTON PLANT IN EGYPT chap. 



to tallness, while 95. C. 3 and 5 were throwing out 

 shorts. 



The uniformity of the deviation, which does not reflect 

 the heterogeneity of 2 and 3, is due to the fact that the 

 plants were growing in pairs, and had not been checked 

 for stunting. 



Such an experience is the rule rather than the excep- 

 tion. Thus, a series of 75 plants of various varieties 

 were examined in four characters, and their oflFspring 

 compared with them. The results were as follows : 



a. Offspring like parent in thirty-eight cases ; some 

 unlike in thirty-seven cases. 



h. Alterations in each character, twenty-five, fifteen, 

 sixteen, and fourteen cases respectively. 



c. Alteration of all four characters took place in two 

 cases, of three characters in seven cases, of two in fifteen, 

 of only one in thirteen. 



There is no recorded difference between the different 

 commercial varieties in this respect. The newest are 

 equally heterozygous with the oldest, though with less 

 intensity of difference. 



The cause of the impurity — which soon appears even 

 when the original strain was pure — ^is to be found in the 

 act of natural cross-fertilisation, or vicinism. 



The effects of this double impurity need scarcely be 

 elaborated. It is obvious that such a welter of unequal 

 individuals must form an excellent medium in which 

 natural selection can work. The transference of a 

 " commercially pure " variety to a new district will be 

 followed by " acclimatisation " ; such acclimatisation wiU be 

 perfectly genuine, but it wUl not be due to any mysterious 

 impress of the environment on the individuals."' ^* 



Again, if such gametic contamination is continuous from 

 year to year, and if our variety is not isolated upon 

 an island in mid-ocean, we find a steady admixture taking 

 place with other varieties. Such obvious contamination 



