MENDELISM AND OTHER METHODS OF DESCENT 20I 



same parents are often definitely unlike, thus showing that 

 these differences arise from the method of descent, and do not 

 depend merely upon the influences of external conditions. 



Gciiclic Variation with Unrestricted Descent {JVeism). — 

 Neism is a form of descent in which the parent organisms have 

 the usual individual diversity of the group to which they belong, 

 while the conjugates and perjugates are more diverse than 

 usual, and differ from the remaining members of the group in 

 the manifestation of a new character. 



U)iiforniity of Restricted Descent [Heredity). — Heredity is 

 a method of descent in which the indiscriminate individual 

 diversity of heterism has been eliminated by narrow breeding 

 or selective restriction of descent. In this way relatively great 

 uniformity may be brought about and maintained for a long 

 series of generations. 



To treat heredity as but one among many forms or methods 

 of descent may appear unusual, or even unwarranted. This is 

 because heredity — the production of like by like — has been 

 considered the typical method of descent under theories which 

 hold that differences among the members of a species are not 

 normal phenomena of descent, but due to external conditions. 

 The popularity of this doctrine of normal uniformity in descent 

 has been so great that descent and heredity have often been used 

 as completely synonymous terms, just as it is now proposed by 

 some to extend the inferences drawn from Mendelian inheritance 

 over the whole field of evolution. Under the present interpre- 

 tation of the facts other methods of descent are recognized as 

 more general and more normal than the production of like by 

 like. Descent is a wider term than heredity, since it includes 

 all the methods of propagation by which organic series are 

 maintained. 



Heredity, in the more definite sense, is a fact, but only under 

 conditions of restricted descent. It is only among our narrow- 

 bred domesticated varieties of plants and animals, or among 

 similarly limited groups in nature, that like produces like in any 

 manner which approximates uniformity, so that the descendants 

 may appear " perfectly true to the physical characters of their 

 progenitors." And even this artificial uniformity of restricted 



