I2O Heredity and Environment 



SEX-LINKED CHARACTERS* 



Recessive characters, appearing in male when simplex, in female when 

 duplex. 



Normal Gower's Muscular Atrophy 



Normal Haemophilia (Slow clotting of 



blood) 



Normal Color Blindness (Daltonism; in- 



ability to distinguish red from 

 green) 



Normal Night Blindness (Inability to see 



by faint light) 



Normal Neuritis Optica (Progressive 



atrophy of optic nerve) 



SUMMARY 



The principles of heredity established by Mendel are almost as 

 important for biology as the atomic theory of Dalton is for chem- 

 istry. By means of these principles particular dissociations and 

 recombinations of characters can be made with almost the same 

 certainty as particular dissociations and recombinations of atoms 

 can be made in chemical reactions. By means of these principles 

 the hereditary constitution of organisms can be analyzed and the 

 real resemblances and differences of various organisms deter- 

 mined. By means of these principles the once mysterious and 

 apparently capricious phenomena of prepotency, atavism and 

 reversion find a satisfactory explanation. 



Before the establishment of Mendel's principles, heredity was, 

 as Balzac said, "a maze in which science loses itself." Much still 

 remains to be discovered about inheritance, but the principles of 

 Mendel have served as an Ariadne thread to guide science 

 through this maze of apparent contradictions and exceptions in 

 which it was formerly lost. 



* See page 187. 



