MUTATION 59 



4. Kinds of Mutation 



De Vries has classified mutations according to 

 their component units into three categories: pro- 

 gressive, regressive, and degressive. 



Progressive mutations are signaHzed by the addi- 

 tion of a new character to the sum of complex char- 

 acters making up the individual. If rumor may be 

 believed, iViine Boleyn, the second in the interesting 

 series of wives of Henry VIII, was a progressive mu- 

 tant with respect to at least three characters, for she 

 is said to have been possessed of an extra finger on 

 each hand, supernumerary mammae, and extra teeth. 

 Evidences that each of these three characters occur 

 as heritable mutations is presented in Davenport's 

 *' Heredity in Relation to Eugenics." 



Regressive mutations are characterized by the 

 dropping out of something. Thus albinism is caused 

 by the absence of pigment or color. Albinic mutants 

 which breed true are well known, particularly among 

 mammals, such as rats, mice, rabbits, cats, guinea- 

 pigs, and even man himself. 



Degressive mutations include cases of the return 

 of a character which was formerly present in the 

 past history of the race, but which has for generations 

 been absent or latent. Castle's four-toed race of 

 guinea-pigs furnishes an example of this class of 

 mutations. In 1906 Professor Castle discovered a 

 newly born guinea-pig in one of his pens with four 

 toes on each hind foot, from which he has successfully 

 established a four-toed race. The hypothetical an- 



