CONTENTS xix 



PAGE 



Causes of unusual sex-ratios 542 



Metabolic theories of sex-determination 544 



Inheritance of unusual sex-ratios 546 



Secondary sexual characters 548 



The effects of castration 548 



CHAPTER XXXV 

 FERTILITY IN ANIMALS 



Introductory 551 



Factors influencing fertility 551 



The Darwinian theory of fertility 552 



Inbreeding not in itself harmful 553 



Fertility as related to Mendelian factors 554 



The chromosomes and fertility Drosophila 555 



Sterility in other animals 555 



Sterility in hybrids 556 



Fertility as related to heterozygosis 559 



Fecundity in fowls 559 



Conclusion 563 



CHAPTER XXXVI 



SOME BELIEFS OP PRACTICAL BREEDERS 



Introductory 564 



Telegony 564 



Harmful effects of hybridization 571 



Infection of the male. Saturation 571 



Maternal impression 572 



Prepotency 573 



The Mendelian interpretation 574 



The relative factor interpretation 575 



The hereditary complex interpretation 575 



Greater prepotency in the male 576 



Conclusions 576 



CHAPTER XXXVII 



METHODS OF BREEDING 



Introductory 577 



Methods means to an end 577 



Phenotypic selection 577 



Limitations of phenotypic selection 578 



Pedigree breeding 580 



Breeding systems based on blood relationship 580 



Inbreeding 581 



Line breeding 582 



Out-breeding 583 



Other systems of breeding 583 



Genotypic selection 584 



