VARIATION IN GENERAL 1 7 



production, in which results are to be ascribed rather to fortui- 

 tous variations among independent units than to anything like a 

 linking together of the separate characters involved. 



The ultimate unit. The student must maintain a clear vision 

 as to distinctions of this sort. From the biological standpoint 

 we accept as the unit of variability the ultimate physiological 

 units that must reside within the cell ; for purposes of everyday 

 use, however, we assume as a unit that detail of form or function 

 that is important to the breeder, understanding perfectly well 

 that, considered physiologically, it doubtless consists of a num- 

 ber of ultimate units which, like the elements in a chemical com- 

 pound, are entirely capable of other and distinct combinations. 



To repeat, we may assume either one of these conceptions as 

 the unit for study according to the purpose in hand. In the 

 text the word " character " will be used to denote that detail of 

 (racial) structure or function which is important to the farmer 

 and to this study. The character will be considered as the 

 practical unit of variation, knowing perfectly well that the ulti- 

 mate unit of variability lies much farther back in the constitution 

 of the protoplasm itself. When this is in mind the terms " ele- 

 mentary character," " physiological unit," or some equivalent 

 term will be used. 



Whatever the situation, the student must not consider the 

 individual as the unit of variability or he will come to grief both 

 in study and in practice, nor should he become confused by 

 those occasional and remarkable correlations of characters that 

 suggest a unit of variability unduly large. 



SECTION VII DISTINCTIONS AS TO KINDS OF 

 VARIATIONS 



In the critical study of variation it is necessary to observe 

 certain distinctions that are often overlooked, resulting in more 

 or less confusion as to what is really involved in the term 

 "variation." 



Variation quantitative or qualitative. The first question that 

 should arise in the mind of the student touching any deviation is 

 this : Is the difference one in degree merely (quantitative) or 



