HEREDITY AND VARIATION 251 



tends to be slightly different 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 very great degree. This factor in 

 the lives of plants and animals is called variation. Heredity and 

 variation are the cornerstones on which all the work in the improve- 

 ment of plants and animals, including man himself, is built. 



The Bearers of Heredity. — We have seen that somewhere in 

 every living cell is a structure known as a nucleus. In this nucleus, 

 which is a part of the living matter of the cell, are certain very 

 minute structures always present, known as chromosomes. These 

 chromosomes (so called because they take up color when stained) 

 are believed to be the structures which contain the determiners 

 of the qualities which may be passed from parent plant to offspring 

 or from animal to animal ; in other words, the qualities that are 

 inheritable (see page 252). 



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

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

 half as many chromosomes as do the body cells. In preparing 

 for the process of fertilization, half of these elements have been 

 eliminated, so that when the egg and sperm cell are united they 

 will have the full number of chromosomes that the other cells 

 have. 



If the chromosomes carry the determiners of the characters 

 which are inheritable, then it is easy to see that a fertilized egg must 

 contain an equal number of chromosomes from the bodies of each 

 parent. Consequently characteristics from each parent are 

 handed down to the new individual. This seems to be the way in 

 which nature succeeds in obtaining variation, by providing cell 

 material from two different individuals. 



Offspring are Part of their Ancestors. — We- can see that if 

 you or I receive characteristics from our parents and they received 

 characteristics from their parents, then we too must have some of 

 the characteristics of the grandparents, and it is a matter of com- 

 mon knowledge that each of us does have some trait or lineament 

 which can be traced back to our grandfather or grandmotlier. 

 Indeed, as far back as we are able to go, ancestors have added 

 something. 



