202 COOK 



descent is not permanent, but gives way in time to the diversity 

 of mutative degeneration. 



Abrupt Variation in Restricted Descent [Hfutation). — Mu- 

 tation is a method of descent in which the parent organisms are 

 alike, but abrupt differences appear among the offspring, usu- 

 ally as the result of variations of the parent gametes. 



The parent organisms are not only alike with respect to some 

 particular character, but belong to the same series of like indi- 

 viduals, in which like has produced like for numerous gener- 

 ations. 



If the new character is dominant it becomes apparent among 

 the conjugates, but if recessive it can not attain expression un- 

 less two recessive gametes can be brought together, which is 

 not possible until the perjugate generation. Mutations differ 

 from genetic variations in the conditions of uniformity and 

 restricted descent in which they appear, and usually also in the 

 greater amplitude of the differences acquired by mutation. 



Mutative variations are usually preserved by polar inheri- 

 tance, and the new character may be either dominant or reces- 

 sive, quite as in Mendelian crosses of mutative varieties. The 

 condition of restricted descent in which mutations appear is also 

 the condition in which polarity of inheritance is most pronounced, 

 as though an effort were being made to restore the normal diver- 

 sity of the group, by preserving all of the mutative differences. 



Atavism. — Atavism is a method of descent in which the off- 

 spring diverge from the parents and their immediate relatives 

 in the expression of a character in some more remote ancestor. 

 Thus many mutative variations of narrow-bred domesticated 

 varieties are in the direction of characters of the wild type of 

 the species. Atavism is related to reversion, but a distinction 

 may be made by calling those cases reversion in which the 

 return to the ancestral character has been occasioned by the 

 interbreeding of members of two or more narrow-bred varieties. 



Descent with Discriminate Diversity. — Discriminate inher- 

 itance includes those forms of descent in which the parent 

 organisms represent narrow-bred groups, so that their differ- 

 ences are not merely individual or indiscriminate. The differ- 

 ent methods of descent in discriminate inheritance can be con- 



