HEREDITY AND VARIATION 



381 



The law of nature which causes a child to possess characters 

 Hke either or both of his parents and like their parents as well, 

 is called heredity. If we trace the workings of heredity in our own 

 individual cases, we shall probably find that we are molded like 

 our ancestors not only in physical characters but also in mental 

 qualities. The ability to play the piano well or to paint well is 

 probably as much a case of inheritance as the color of one's eyes or 

 the shape of one's nose. We are a complex of physical and mental 

 characters, received from 

 all our ancestors. 



Variation. — But no boy 

 in the class is exactly like 

 any other boy; even 

 brothers are different in 

 appearance and in action. 

 In this wonderful mold of 

 nature each one of us 

 tends to be slightly dif- 

 ferent from his or her 

 parents. Each plant, each 

 animal, varies to a greater 

 or lesser degree from its 

 immediate ancestors, and may vary to a great degree. This ten- 

 dency among plants and animals is called variation. Heredity 

 and variation are the corner stones on which all the work in the 

 improvement of plants and animals, including man himself, is 

 built. 



The Bearers of Heredity. — We have seen that a cell contains 

 a nucleus, in which are certain very minute structures known as 

 chromosomes (because they take up color when stained). When 

 cells divide, each chromosome divides by splitting lengthwise, so 

 that equal amounts of each chromosome are thus carried into the 

 new cells formed from the original cell. These chromosomes are 

 believed to be the structures which contain the determiners of the 

 qualities which may be passed from parent to offspring ; in other 

 words, the qualities that are inheritable (see pages 47-49, 396). 



The Germ Cells. — But it has been found that certain cells of 

 the body, the egg and the sperm cells, before uniting contain only 



Example of variation in heads of roosters. 



