2l8 



COOK 



Diagram of Methods of Descent. — Concluded. 



Methods of 

 Descent. 



Conjugate 

 hybridism . 



Perjugate 

 hybridism. 



Parent p, Parent S Conjugate a Perjugate S Perjugate 

 Organisms. « Gametes. S Organisms.,' 2 Gametes. ;-S Organisms. 



Diverse 



species. 



Diverse 

 species. 



Diverse. 



Diverse. 



Intermedi- H No perju- 

 ate, sterile, j gate 

 gametes. 



Intermedi- 

 ate or 

 diverse, 

 fertile. 



Alike or 

 diverse. 



No perju- 

 gate organ- 

 isms. 



Inter- 

 mediate or 

 diverse like 

 mutations. 



POSITION OF MENDELISM AS A METHOD OF DESCENT. 



No claim of completeness is made for these condensed sum- 

 maries and tables. They are introduced at this time only to 

 show the position of Mendelism among other methods of descent. 

 It has been customary to describe Mendelism as a form of her- 

 edity, but in reality it constitutes a rather wide departure from 

 the primary concept of heredity, as developed in evolutionary 

 literature. Heredity usually implies the production of like by 

 like, which many have considered the normal process of descent. 



Mendelism is one of the methods of descent in which unlike 

 produce unlike. Mendelism has aroused special interest in the 

 scientific world largely because it seemed to contradict the 

 earlier inferences from the idea of heredity, by showing that 

 contrasted differences are preserved, and not reduced to a uni- 

 form intermediate average. Instead of being a form of heredity, 

 Mendelism is a specialization of heterism ; it is one of the methods 

 of increasing diversity of descent, which sustains the efficiency of 

 the processes of sexual reproduction. The preservation of dif- 

 ferences inside the species by means of sex-inheritance is one 

 of the most familiar phenomena of descent, but the intimate 

 resemblance between Mendelism and sex-inheritance has not 

 been adequately appreciated. 



MENDELIAN " PRINCIPLES " NOT DEMONSTRATED. 



To explain these phenomena of polar inheritance the Men- 

 delian doctrine of pure germ-cells was invented, but the facts 



