Range and Frequencies of Mutations 385 



dominants that appear as losses are the hornlessness in cattle, 

 the short-fingered hand in human beings, a missing thumb, 

 the bob-tailed cat, and the loss of pigment in certain types of 

 poultry. Some recessive losses are illustrated by albinism in 

 rodents, the loss of color in certain flowers, silky feathers in 

 poultry, and night-blindness among human beings (a condi- 

 tion due apparently to the absence of certain purple pigments 

 from the retina) . 



Examples of additions are supernumerary digits in hu- 

 man beings, duplications of parts of the foot in cats and in 

 cattle, deeper color, longer hair, longer tail, increased stature, 

 increased branching, feathers on the foot of poultry, the 

 persistence of web between the toes in birds and in human 

 beings, and so on. Additions or accentuations may, like 

 losses, also be either recessive or dominant. 



The frequency of mutation is very difficult to estimate 

 since only a rare example of mutation in nature comes to 

 the attention of the scientist. A study has been made of the 

 mutations reported in the most intensively studied animal, 

 the fruit fly, and here it is estimated that between thirty and 

 sixty mutations appear in every hundred thousand. This 

 means that in a population of a hundred thousand that 

 number of individuals will show one or more traits diverg- 

 ing from the parental type and able to reproduce the char- 

 acter in subsequent generations. One can get an idea of 

 how great a frequency this is by comparing it with relatively 

 rare events in a more familiar population such as that of 

 one's own city or state. For example, out of every 100,000 

 human births reported, there are 12 cases of triplets. In a 

 population of 100,000 we should expect to find 42 or 43 

 deaf mutes, and about 60 blind persons (although many of 

 the latter are not born blind) . 



It is very likely that there is a great variation as to 

 mutability of species; that is, some stocks are more stable 

 than others, and it is possible that mutations are not so fre- 

 quent in other species of plants and animals as they are in 

 evening primrose or in fruit flies. Even at this rate of muta- 



