466 Plant Physiology 



282. Fluctuating variation. - The minor differences 

 which all individuals of any population exhibit are com- 

 monly fluctuating, or continuous, variations. The ideal, 

 or type, which these individuals approach is an average 

 individual, with reference to a number of characters. If 

 many individuals be examined with respect to any one 

 character, the result may be given in the form of a curve of 

 variation. An examination, for instance, of a population 

 of the common field daisy would disclose a variation in 

 the number of ray flowers ; thus there might be from 10 to 

 20. This variation in number represents the range, and 

 the numbers 10, 11, 12, 13, etc., constitute the variates or 

 classes. Perhaps the majority of the population would 

 have the same number of ray flowers, say 15, which class 

 would then represent the mode, or class of greatest fre- 

 quency. In a normal curve there would be a diminishing 

 frequency towards both higher and lower classes. Quete- 

 let has shown that this curve, in the main, corresponds 

 with the law of probabilities, or curve of frequency of 

 error. 



invariably appear. The tomato is an excellent example of variability. 

 Little more than a century ago it was introduced as a vegetable. To- 

 day its varieties are numbered by the hundred, and there are a great 

 many well-defined types and forms of fruit, characters of leaf, size, etc. 

 We are thus sure that by one means or another variation has been effected, 

 and in a marked degree. Once several strains or varieties are developed, 

 hybridization is the greatest possible source of variation, or multiplica- 

 tion of forms. 



The use of score cards in judging corn, apples, and other farm and 

 horticultural crops draws special attention in a practical way to standards 

 among economic plants, and at the same time to departure from the 

 standards, - - to variation. The producer is likely to have in mind as 

 an ideal the best or most highly developed type of any variety or strain, 

 and thus in the work of selection he may constantly depart from the 

 old ideal in the direction of a new and improved strain. 



