CONTENTS Xlll 



CHAPTER PAGE 



XL How Characters behave in Transmission . . . .141 



Characters tend to combine in definite mathematical propor- 

 tions — Characters that do not blend — Mendel's law of 

 hybrids — Dominant and recessive characters — Pure races 

 may spring from crossing — Very few individuals pure — A 

 second method of improvement — Improvement by hybridiza- 

 tion complicated — Mutation and mutants — Origin of new 

 and improved strains 



XII. How the Offspring compares with the Parent, 



or Descent with Modification 154 



The complex nature of heredity — The offspring not like the 

 parent — Mediocrity the common lot, whatever the parentage ; 

 regression — Some offspring better and some worse than their 

 parents — The exceptional parent and his offspring — Pro- 

 gression — The exceptional offspring and his parent — Rever- 

 sion — Degeneracy 



XIII. The Law of Ancestral Heredity 166 



The extent to which the offspring resembles the parent and 

 the extent to which he resembles more remote ancestors — 

 Chance of resembling a particular individual ancestor — The 

 individual a composite — The number " two " 



XIV. Heredity and Environment 171 



Mistaken estimate of environment — All the characters of the 

 race, both good and bad, are transmitted to the individual by 

 his parentage — The function of environment is to assist or 

 to hinder in development — Environment does not add unit 

 characters — Modifications due to environment 



XV. Systematic Improvement of Animals . . . . .178 



Origin of the w pure bred " — Pedigree registers — Advanced 

 registry — Unregistered stock and scrubs — Systems of breed- 

 ing — Source of sires — Herd improvement and breed im- 

 provement — Rational improvement — Choosing the breed — 

 Breed differences slight — Market classes and grades — 

 Knowledge of market requirements needful 



XVI. Systematic Improvement of Plants 198 



Improvement by selection — Crossing to produce new varieties 



— Application of Mendel's law in crossing — Separation of the 

 desired character — Behavior of the recessive — Behavior of 

 the dominant — When more than two characters are involved 



— Systems of planting — Records 



