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 common knowledge that each of us does have some trait or lineament which can be traced back to our grandfather or grandmother. Indeed, as far back as we are able to go, ancestors have added something. [Illustration: COMPARISON OF SEXUAL AND ASEXUAL CELL REPRODUCTION] Charles Darwin and Natural Selection.--The great Englishman Charles Darwin was one of the first scientists to realize how this great force of heredity applied to the development or evolution of plants and animals. He knew that although animals and plants were like their ancestors, they also tended to vary. In nature, the variations which best fitted a plant or animal for life in its own environment were the ones which were handed down because those having variations which were not fitted for life in that particular environment would die. Thus nature seized upon favorable variations and after a time, as the descendants of each of these individuals also tended to vary, a new species of plant or animal, fitted for the place it had to live in, would be gradually evolved.