Mutations Established Facts 383 



Mutations Experimentally Established 



It may well be that de Vries had committed a serious 

 blunder in mistaking the segregated descendants of a hybrid 

 plant for mutations. The fact of mutation has nevertheless 

 been since established for many other plants and animals. 

 More particularly, it has been shown that with each muta- 

 tion there goes a related change in the chromosomal mechan- 

 ism of the plant or animal. Generally speaking, therefore, 

 Weismann's theory is supported by concrete facts. Variations 

 in the number or arrangement of chromosomal elements are 

 visible under the microscope. Corresponding to these 

 variations in the chromosomal pattern there are to be ob- 

 served variations in the structure or appearance of the indi- 

 vidual organisms. The idea that nature does actually make 

 saltations, that new types arise completely formed from par- 

 ents having different characteristics, is now no longer a 

 theory, but an established fact. The theory is simply 

 that // is in this manner that new species arise. Progres- 

 sive divergence in plant and animal forms is brought 

 about by the occasional or more frequent production of 

 mutations. 



The most thoroughly studied animal from the point of 

 view of heredity and mutations is the fruit fly Drosophila, 

 which shares with Mendel's peas the glory of having served 

 the scientists in establishing fundamental laws of genetics. 

 In this animal hundreds of distinct traits have been studied, 

 many of them having originated while under observation. 

 The scientist was present at the birth of a new species from 

 an old. 



It is commonly supposed that mutations must be very 

 striking departures from the parental pattern, but this im- 

 pression is due to the nature of the material with which 

 the investigators work. They have to select well defined 

 characters to facilitate recognition, recording, classifying and 

 so on. With the increase in experience and with the accumu- 

 lation of information it is seen that there are mutations 



