136 THE CORNELL READING-COURSES 



A glance at Fig. 77 shows that these heads ranged in length from 6 centi- 

 meters to 11.5 centimeters, the greatest number occurring in the group, 

 or class, between 8.5 centimeters and 9 centimeters in length. Here 

 again will be noted a gradual decrease in frequency on either side of this 

 group. 



Fluctuations are not inherited, that is, they are not handed down from 

 parent to offspring. It has been shown that ears which came from parents 

 measuring 10 inches were not all 10 inches long. In fact, there were 

 more ears 9 inches in length than any other. The type of those ears, 

 in regard to length, was 9 inches. If fluctuations are not inherited, 

 they cannot be of especial value to the breeder. Even though we select 

 an individual that is in advance of or below the type, it will not produce 

 progeny all of which are like itself. It is not known what effect a long- 

 continued selection of this kind will have, but as yet there is no evidence 

 to prove that it will change the type. 



Mutations. The other kind of variation, called mutation, is of the 

 most importance to breeders. Mutations are sometimes called sports or 

 discontinuous variations. They are striking, and are different from the 

 plants among which they have their origin. Mutations are of practical 

 use because they are inherited. This does not mean that all the off- 

 spring of a mutation are exactly like the parent. The offspring, if arranged 

 in a distribution table, will show fluctuating variability just as the parent 

 family did; but the type will be different. A mutation always establishes 

 a new type. For example, in Fig. 76 the type of the ear as to length was 

 9 inches. If a mutation should occur in that particular strain of corn, 

 the type would be changed to, possibly, a ic-inch ear, although there 

 would be a distribution on either side of the type just as there was in 

 the ears arranged in Fig. 76. No one knows what causes mutations to 

 appear. If they could be produced at will, the process of breeding would 

 be greatly advanced. 



The Cupid sweet pea is one of the most striking examples of muta- 

 tion. It appeared in a pure variety of Emily Henderson, which is a tall- 

 growing variety. The dwarf plant suddenly appeared one year and has 

 continued true to its type. Most of our double flowers owe their origin 

 to mutation. A large number of 'varieties of tomatoes also originated 

 in this manner. 



The same form of mutation may occur again and again in a group of 

 plants. The thornless cactus is an example of this. Smooth branches 

 often occur on thorned trees. This form of mutation is known as bud 

 mutation. The haw tree is rich in such mutations. 



Without doubt many of our famous breeds or races of animals owe 

 their origin to some ancestral mutation. There are those who think the 



