To increase the frequency of desirable traits in their stocks, plant and animal breeders employ artificial selection (example of darwin and his pigeon"s on page 74). Darwin realized that a process like artificial selection occurs in nature. His theory of evolution by natural selection suggests that descent with modification is the logical outcome of four postulates: The variations among individuals are, at least in part, passed from parents to offspring. In every generation, some individuals are most successful at surviving and reproducing than others. The survival and reproduction of individuals are not random; instead they are tied to the variation among individuals. The individuals with the most favourable variations, those who are better at surviving and reproducing, are naturally selected. If these four postulates are true then composition of the population changes from one generation to the next (see figure 3. 4 on page 76). The ability of an individual to survive and reproduce in its environment.