PARENTS AND OFFSPRING 383 



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 cell and the sperm cell are united 

 they will have the same number of chromosomes as the other cells 

 of the body. 



We have already learned that in the process of fertilization the 

 nuclei of the sperm and of the egg cell unite, or fuse, to form a new 

 nucleus in the fertilized egg. If the chromosomes carry the deter- 

 miners of the characters which are inheritable, then it is easy to 

 see that a fertilized egg must contain equal numbers of chromo- 

 somes from the bodies of both parents. In this way characters 

 from each parent are handed down to the new individual. 



Offspring are Part of their Ancestors. — If you receive charac- 

 ters from your parents and they received characters from their 

 parents, then you must have some of the characters of the grand- 

 parents, and as a matter of fact each of us does have some traits or 

 lineaments which can be traced back to a grandfather or grand- 

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

 contributed something. 



Natural Selection. — Charles Darwin was one of the first 

 scientists to suggest how heredity applies to the development of 

 plants and animals. He knew that although animals and plants are 

 like their ancestors, they also tend to vary. In nature, he believed, 

 the variations which best fit a plant or animal for life in its own 

 environment are the ones which are handed down, because those 

 individuals having variations not fitted for life in that particular 

 environment will die. Thus, said Darwin, nature seizes upon 

 favorable variations ; and after a time, as the descendants of each 

 of these individuals also tend to vary, a new species of plant or 

 animal, fitted for that special place, will be gradually formed. 



1 This is not quite exact, for it has been found that in some animals at the time 

 when the chromosomes are reduced in number, there are an even number in the 

 female sex cells but an odd number in the male sex cells. When the male cells 

 divide to reduce the number of chromosomes, some sperm cells receive an odd 

 and some an even number of chromosomes. Therefore, after fertihzation, some 

 eggs have an even and some an odd number of chromosomes. The fertilized egg 

 cells with the odd number of chromosomes develop into male animals ; the cells 

 with the even number of chromosomes become females. The sex-determining 

 chromosome is known as the accessory chromosome and is found in some worms, 

 many insects, myriapods, spiders, and some mammals. 



