58 GENETICS 



eye. They are shots directed at a different target 

 altogether. 



To the student of heredity there are two distinc- 

 tions of prime importance with respect to mutations. 

 First, that they usually appear full-fledged without 

 preparatory stages, and second, that they breed true 

 from the start. Fluctuations, on the contrary, ordin- 

 arily "revert" to the parental type in subsequent gen- 

 erations. The great practical importance to the 

 breeder of a knowledge of these heritable mutations 

 is at once apparent. 



3. Freaks 



A further distinction should be made between 

 mutations and so-called freaks or monstrosities, 

 namely, that the former breed true, while the latter 

 do not. A human physical deformity, such as a 

 club-foot, for example, or a humped back, is not a 

 mutation, because it does not reappear as a heritable 

 character. Variations of this kind are not predeter- 

 mined in the germplasm, but are usually instances of 

 something that went wrong during the development 

 of the individual somatoplasm. 



Thus, among normally "right-handed" snails "left- 

 handed" individuals have occasionally been dis- 

 covered which, when bred, were found to produce 

 all normal "right-handed" progeny. They are 

 therefore not mutations at all, but freaks or mon- 

 strosities due probably to some unusual occurrence 

 early in the cleavage stages of the embryo. 



