HEREDITY 153 



investigators, it remained for De Vries to demonstrate 

 by scientific investigation the important role played by 

 mutations in the evolution of plants and animals. This 

 theory undertakes to explain the origin of species by sud- 

 den marked variations rather than by continuous or 

 fluctuating variations. De Vries' experiments were suc- 

 cessful not only in demonstrating clearly the remarkable 

 tendency of certain species to vary abruptly, but of even 

 more significance was his demonstration of the fact that 

 such variations were as surely transmitted as were the 

 regular or normal characters of the species. 



It is true, mutations must not be confounded with 

 the appearance of freaks or monstrosities among plants 

 and animals. Often through arrested development or 

 accidental injury during the embryonic existence of ani- 

 mals, certain characteristics may become so modified 

 that they appear in the form of new characters. Such 

 freaks are not transmitted and are not therefore muta- 

 tions. The term mutations is used only to designate 

 those variations which are heritable. 



144. Kinds of mutations. Mutations may be addi- 

 tions or improvements to the life of organic beings, or 

 they may result in diminishing the ability of an animal 

 or plant to live and thrive in an ordinary environment. 

 De Vries has therefore suggested a classification of muta- 

 tions as progressive, regressive and degressive. 



Progressive mutations are those which have contrib- 

 uted something entirely new. It is in reality an addi- 

 tional character. Attention has already been called 

 to cases of variation in which the offspring is provided 

 with extra fingers or toes. An example of this form of 

 mutation is described by Alexander Graham Bell in the 

 case of multi-nippled sheep. 



