462 ELEMENTARY BIOLOGY 



animal is sometimes found among dogs. Peacock fanciers 

 sometimes find a single bird with plain black plumage. Several 

 times whole flocks of such birds have been established from a 

 single freak mated with the normal type. 



483. Mutations. In more recent years special attention has 

 been given to these sports by many biologists, and the Dutch 

 botanist Hugo de Vries has developed a theory to account for 

 the origin of new kinds of plants and animals, based on the 

 fact that such freak individuals are often able to establish dis- 

 tinct lines of descendants. De Vries has himself cultivated many 

 lines of new plants that originated in this sudden manner from 

 various wild and cultivated species. Such suddenly arising de- 

 partures from the ancestral type are called mutations, and the 

 individuals bearing these characters for the first time are called 

 mutants. De Vries's mutation theory does not attempt to ex- 

 plain how it is that such plants or animals originate ; it only 

 tries to show how such mutations may lead to the establishment 

 of new races of organisms (see Fig. 247). 



484. Origin of new characters. There are a number of 

 experiments that throw some light on the origin of mutations. 

 One of the best known is that by W. L. Tower, an American 

 biologist, who subjected the eggs and larvae of potato beetles 

 to various unusual conditions of temperature, moisture, and 

 nutrition. The individuals that developed under these extreme 

 conditions showed no evidence of having been mistreated. But 

 some of the offspring of these beetles departed in a marked way 

 from the usual appearance of their ancestors. In the Frontis- 

 piece are shown some of the new types of beetles that appeared 

 in Tower's experiments. It is supposed that the conditions 

 to which the developing beetles were subjected had no direct 

 effect upon the bodies of the animals, but did have an effect 

 on the chromosomes of the cells that later became the germ 

 cells. On mating some of these new types with the normal 

 ones, it was found that there really were new characters, for 

 it was then possible to establish lines that would breed true. 



