454 ELEMENTARY BIOLOGY 



grafts, or by means of bulbs or tubers. The Burbank potato, for ex- 

 ample, which is the best-known potato in this country, is propagated 

 by means of the tuber. Seedless varieties of grapes, apples, oranges, 

 plums, and so on would be propagated by grafts or cuttings ; and 

 hybrids of various kinds of fruits, although they may have seeds, 



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 ^ 



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31 



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f 



died of 

 heart 

 both affected disorder 



Fig. 243. Heredity of human traits 



(Squares represent males; circles represent females.) /, pedigree of a family showing 

 artistic (dark upper portion), literary (right section), and musical (left section) ability; 

 2, family with digestive troubles ; j, family with heart disorders ; 4, family with feeble- 

 mindedness (/, died in infancy; ?, uncertain mental condition). (/, 2, and j, after 



Davenport ; 4, after Goddard) 



would be propagated in the same way. From what we know of the 

 fact of segregation, we can understand how unreliable the seeds of 

 most common fruits are likely to be, when we consider that most 

 seeds are probably hybrids. 



477. Heredity in man. So far as reliable facts are available, 

 heredity seems to follow the same course among human beings 

 as among other organisms. 



Human beings take a comparatively long time to mature, 

 so that in order to get complete records for many generations 



