516 VARIATIONS 



much concerns those who are interested in improving plants and 

 animals, for unless a variation is inheritable it disappears with the 

 individual having it. It is not transmitted to offspring and thus 

 perpetuated in future generations. Fluctuating variations and 

 mutations deserve to be discussed more fully on account of their 

 importance in animal and plant breeding. 



Fluctuating variations. — Mutations are rare, consequently 

 nearly all variations are the fluctuating type. Fluctuating 

 variations are displayed by all kinds of characters and in all 

 kinds of ways. As their name suggests they are not constant. 

 They are usually due to differences in moisture, food supply, 

 temperature and other environmental influences. They may be 

 present in one generation and absent in another. Thus Corn 

 may mature early one year and late the next. The time required 

 for Corn or any other kind of plant to mature is not constant 

 but fluctuates over a considerable period of time, depending upon 

 the season. The length of heads of Wheat or Oats and the 

 number or kernels produced per head are not constant, but vary 

 widely on different plants and in different seasons. In the color 

 of flowers there are various degrees of intensity, and often on the 

 same plant. Some leaves of a plant are long and others are not 

 so long and between certain limits all degrees of length may be 

 found. Height of plants, thickness of stem, number of leaves 

 per plant, and most all other characters fluctuate, ranging in 

 degree from one extreme to the other and showing all gradations 

 in magnitude between the extremes. Fluctuating variations are 

 also called continuous variations because their degrees of magni- 

 tude grade into each other. 



Quetelet's law. — To the casual observer fluctuating variations 

 seem to follow no system, but students of variations have found 

 that fluctuating variations do follow a rather definite law. They 

 follow the law of Quetelet, named after Quetelet, the Belgian 

 anthropologist who discovered that fluctuating variations follow 

 the law of probabilities. The law can be explained best by illus- 

 trations. Suppose we study the variation in the number of 

 kernels per head in a field of Wheat. Among the various numbers 

 of kernels per head there will be one number more common 

 than any other number. That is, the plants having this number 

 of kernels per head will be more numerous than those plants 

 having more or fewer kernels. The number common to the 



