92 THE COTTON PLANT IN EGYPT CHAP. 



Whenever a family is henceforth mentioned, it should 

 be understood to be one of these strains. Commercial 

 "varieties" will be specifically designated as such. 



Since the fluctuation-graphs for parents will necessarily 

 be consulted when dealing later with genetics proper, the 

 reader is referred to these, except where some special 

 point requires to be illustrated by a special figure. 



For the expression of fluctuation in statistical terms, we 

 shall take the percentage probable error, and not the 

 "coefficient of variation." The latter is the "standard 

 deviation " expressed in percentage of the mean. The former 



Vy 7*, 

 f 



and our particular expression is ~ This form is 



Mean 



convenient, since half the observed cases must lie within 

 the limits given. The maximum possible true fluctuation 

 may be taken as odds of about 30:1, or 3*2 times the 

 probable error. 



Whenever a specific figure is given for a family it 

 should be understood as having been checked by com- 

 parison with other families, and therefore considered by 

 the author as being fairly representative of the data 

 available. 



Colour characters. Such characters as the colour of 

 the petal anthers are devoid of appreciable fluctuation. 

 This uniformity is probably apparent rather than real, 

 since a difference of less than 10 per cent, in the 

 intensity of a colour cannot be perceived by the ordinary 

 eye. 



When colours have to be classified, as in hybrid popula- 

 tions, standard flowers, &c. are always used, and the 

 unknowns are matched to them. 



Characters which depend on the presence of antho- 

 cyanin in the tissues, such as the red spot at the junction of 

 petiole with lamina, fluctuate widely. Comparisons can 



