PRINCIPLES AND METHODS OF PLANT-BREEDING 141 



The first column of figures to the left, in Fig. 76, represents the different 

 lengths of ears. These different lengths are called classes, and in this 

 instance they differ by one half inch each. The last column to the right 

 contains the frequencies. These are the total number of individuals which 

 are grouped in each class. 



In the above table there is a fairly wide range of variation. The extremes 

 range from 3 inches to 12 inches. This indicates a fair amount of vari- 

 ability. In other words, this particular " population " of corn was not 

 especially uniform. There are mathematical ways of calculating exactly 

 this amount of variation, but for practical purposes these are not necessary. 



To determine type 



What does a breeder mean when he speaks of type? His idea of type 

 is usually the standard which he has in mind. It is his ideal toward which 

 he is selecting. The ideal type should not be confused with the actual 

 type. The distribution table furnishes him a way to know exactly what 

 the actual type is as well as to know how nearly he is approaching his 

 ideal type, which we have called his standard. An inspection of Fig. 

 76, shows that the greatest number of ears were 9 inches long. In other 

 words, the actual " type " or " mode " is 9 inches. This is considered 

 the bes*t measure of type. It is important to know what percentage of 

 the population conforms to the type. To find this, divide the number 

 of individuals at the mode by the total number of individuals. In this 

 particular instance it is 67 -5- 327 = 20. 5. This is called the modal coef- 

 ficient. For this population of corn, 20.5 per cent of the individuals con- 

 formed to the type. 



To measure progress 



There is another coefficient which is of use to the practical man, called 

 the standard coefficient. In this case the breeder might be selecting for 

 a lo-inch ear. To determine what proportion of the individuals conform 

 to his standard, divide the number of individuals in the lo-inch class 

 by the total number. The standard coefficient, then, in this case, is 

 38 -7-327== ii. 6. In this instance, n.6 per cent of the individuals con- 

 formed to the standard of the breeder. If this coefficient increases from 

 year to year he knows that he is progressing. If it remains the same his 

 selection is of no avail. 



The method of measuring variation, type, and progress that has here 

 been explained can be applied to numerous characters of any crop. To 

 those who care to make such investigation, it will prove interesting and 

 valuable. 



