CHAPTER 



6 



HEREDITY AND VARIATION 



What any individual is, what it can become, and what its descendants can 

 be Uke is determined by its hereditary constitution, its genetic make-up. It 

 is often said that like reproduces like, and this statement is true in a general 

 sense. Frogs give rise to frogs, not to toads; and dogs reproduce dogs. How- 

 ever, the pups of a litter are not identical with their parents or with one an- 

 other. A new individual resembles its parents, yet differs from them. The 

 phenomena of heredity and variation go hand in hand. Heredity may be 

 defined as the tendency of individuals to resemble their ancestors and rela- 

 tives; variation is the tendency of organisms related by descent to differ in 

 specific ways. New individuals arise during the process of reproduction and 

 develop through an orderly series of changes until they reach maturity. It 

 is obvious that whatever it is that passes from one generation to the next 

 must determine not only the typical sequence of developmental processes but 

 also the characteristics of the adult organism. In other words, the germ 

 cells must carry the mechanism responsible for heredity and variation. The 

 individual ordinarily develops under environmental conditions which are 

 practically uniform for successive generations but which cannot be ignored 

 in seeking the complete answer to questions concerning heredity and varia- 

 tion. Genetics, a great subdivision of zoological science which has developed 

 conspicuously since 1900, has for its province the subject matter relating to 

 the facts and theories of heredity and variation. 



Heredity and variation can be studied by four different methods, all of 

 which have yielded information concerning basic problems. It is possible 

 to observe and analyze resemblances and variations from one generation to 

 another in large groups of individuals as they are found under natural con- 

 ditions. This is the statistical method, or the method of biometry. Contrasted 

 with such mass analysis is the observation of inheritance and variation in 

 animals bred under experimental conditions for generation after generation. 



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