The theory of natural selection is the centerpiece of On the Origin of Species and of the evolutionary theory. It ＿＿（42）＿＿ the adaptation and evolution of organisms. Furthermore, the theory of natural selection explains the divergence of species from common ancestors and the endless diversity of life. According to Darwin, an organism that ＿＿（43）＿＿ favorable characteristics will tend to survive and reproduce in greater numbers than others of their kind, ensuring the ＿＿（44）＿＿ of those characteristics in later generations. Through this passive choosing, this natural selection, the physical features of an entire population come to change over time. For instance, the hardiest plant ＿＿（45）＿＿ the drought while others wither and die; the quickest antelope outruns its predators while others are caught and killed. It can be observed that in any system in which resources are ＿＿（46）＿＿, a competition to acquire those resources arises and the best, or the “fittest”, as Darwin would say, are inclined to secure those resources.