CHAPTER XXI 

 INHERITANCE 



So careful of the type . . . 



So careless of the single life. — Tennyson. 



The old adage that 'like begets like' expresses the general fact 

 of heredity. Everyone recognizes that parent and offspring agree 

 in their fundamental characteristics: they 'belong to the same 

 species.' And everyone realizes that the resemblance may be 

 strikingly exact even in details of form or behavior. Family traits 

 reappear. The mere statement of striking resemblances among the 

 individuals of a family is a tacit admission that no two individuals 

 are exactly alike; in other words, heredity is organic resemblance 

 based on descent — inheritance of the characters exhibited by the 

 parents is not complete, there is variation. Indeed ''variation is 

 the most invariable thing in nature," but one must guard against 

 the impression that there is an antithesis between heredity and 

 variation. "Living beings do not exhibit unity and diversity, but 

 unity in diversity. Inheritance and variation are not two things, 

 but two imperfect views of a single process." 



We may now address ourselves to the problems of heredity and 

 variation which are at the basis not only of what organisms have 

 been in the past and are at the present, but also of whatever the 

 future may have in store for them. Variations are the raw materials 

 of evolutionary progression or regression. From a broad point of 

 view, the origin of species and the origin of individuals are essen- 

 tially the same question. If we can solve the relations of parent 

 and offspring, the origin of species will largely take care of itself. 

 As a matter of fact, historically the question of species origin was 

 approached first, and through the work of Darwin became of 

 paramount interest in the latter half of the nineteenth century. 

 The twentieth century finds the individual — the hereditary re- 

 lation of parent and offspring — the center of investigation, and 

 it forms the science of genetics. Organic evolution attempts to 

 establish the general fact that all organisms are related by descent; 

 genetics attempts to show how specific individuals are related. 



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