28 A MANUAL OF MENDELISM 



(15) Conversely, if a hybrid be mated with a recessive 

 and both are pure in their other characters, the number 

 of equal kinds produced indicates in how many pairs of 

 characters the hybrid and the recessive differ. If two 

 kinds are produced, the hybrid and the recessive differ 

 in one pair, if four kinds are produced, the hybrid and 

 the recessive differ in two pairs, and so on. For, if they 

 differed in more than 1, 2, S . , . n pairs, they would 

 form more than 2, 4, 8 . . . 2" different combinations. 



(16) It is obvious that, if the parents differ only in 

 certain characters, the remaining characters in both 

 must be the same. It must not be assumed, however, 

 that every character finds another with which it forms 

 a pair, for there may be species every member of which 

 without exception carries certain characters. For 

 instance, every elephant has a trunk. 



It has to be remembered that the foregoing deductions 

 hold only when characters are distributed normally ; for, 

 since the discovery of Mendel's papers, many cases 

 have been observed in which the characters are not 

 normally distributed. As nothing has been discovered, 

 however, to modify Mendel's law — to suggest that 

 hybrids do not produce equal numbers of the factors in 

 which their parents differed — it follows that abnormal 

 distributions must themselves be the result of inde- 

 pendent disturbing causes, and, when these are identified 

 and allowed for, the foregoing deductions apply. 



The chief disturbing causes are : 



(a) The inseparability of the effect of one factor from 

 that of another. 



(6) The suppression of the effect of one factor by 

 that of another. 



