526 Influence of Nutrition and Selection. 



quarter of the individuals and therefore analogous to the 

 so-called ''probable error." He calls it the Ouartile (Q). 

 There is obviously one quartile on either side of the 

 Median (M) ; these are called Qi and Q2. If the curve 

 is symmetrical, the two quartiles have the same value; 

 otherv^ise the dissimilarity of the empirically determined 

 Qi and Qo is a measure of the degree of symmetry of the 

 curve. If the difference between the two is within the 

 range of the error of observation, their mean value 

 Q=(Qi + 02)/2 is the measure of the amplitude of 

 variation of the material under consideration. 



If we wish to compare the amplitude for different 

 characters together we must reduce them to a common 

 measure. This is done by dividing Q by M /^ 



We see therefore that Qi, M and Q2 are the numbers 

 which have to be determined by observation. The form 

 of the curve is determined by them and any differences 

 between the curves so determined and the actual figures 

 themselves must be ascribed to errors in observation, at 

 any rate in symmetrical curves. The greater the number 

 of observations which go to make a curve the smaller 

 will these differences be. 



In the following sections I shall deduce these val- 

 ues from the data ; and use them as a basis for discussion. 

 One advantage of this will be that it will render drawings 

 of the curves superfluous, or at any rate only useful for 

 the purpose of demonstration; and that it will compress 

 the numerical material into a few figures. 



A few remarks on the subject of construction of these 

 curves (Figs. 115-118) are called for. The number of 

 ordinates is by no means necessarily the same as the 



* Ed. Verschaffelt, Ueher graduelle VariahiUt'dt von pUans-, 

 lichen Eigenschaften, Ber. d. d. bot. Gesellsch., Vol. XII, 1894, p. 350. 



