342 THE CHANGING GENERATIONS 



2. The Theory of Natural Selection. This, the essential part of the Darwinian 

 explanation of the causes of evolution, rests upon the following propositions: 



a. Overproduction of offspring. Animals and plants are enormously fertile; 

 the young produced by each generation are many times as numerous as the 

 parents. Yet the number of individuals of each species remains approximately 

 stationary under natural conditions, showing that most of the offspring of each 

 generation must perish. 



b. The struggle for existence. Since more organisms are produced than can 

 survive, there is competition between individuals for food and space. Also, since 

 not all situations are equally well suited for a particular kind of organism, each 

 individual must pass an endurance test set by those factors of the environment 

 that are unfavorable to it. Which individuals survive and which ones die depends 

 upon the outcome of this struggle for existence. 



c. Variation and natural selection. The different individuals of the same species 

 are not all alike. Because of this variation, some will in one way or another be 

 better fitted than others to succeed in the struggle for existence. There will thus 

 be natural selection of the more fit for survival and production of the next 

 generation. 



d. Hereditary transmission of characteristics. The survivors of one generation 

 will tend to transmit their own characteristics to their progeny, including those 

 characters that ensured their own survival. The less fit, most of which will die 

 while young, will in the long run fail to reproduce themselves. 



e. Continued change inevitable. Because of this natural selection, each new 

 generation will show an appreciable increase in average fitness to the environ- 

 ment. But since overproduction of offspring and the struggle for existence operate 

 anew on each successive generation, and since the physical environment does not 

 remain fixed, complete adaptation can never be reached, and continued change must 

 result. 



/. Formation of new species (speciation) . Not only will the descendants of one 

 stock change as time passes and thus become different "species" from their 

 ancestors but also, if some of the members of this stock meet the conditions of 

 life through change in one direction and others through change in other directions, 

 the different groups must end by becoming different species, though descended 

 from a single ancestral species. 



gr. Nonadaptive characters. Variations that prove neither useful nor harmful 

 to their possessors will be unaffected by natural selection and will remain as 

 fluctuating variations about an unchanged average condition. 



3. The Theory of Sexual Selection. Darwin saw that there were differences 

 between the sexes of many species (for example, plumages of male and female 

 birds, presence or absence of antlers in deer, etc.) which could not be adequately 

 accounted for by the theory of natural selection. In an effort to explain such 

 differences he developed the theory of sexual selection, according to which com- 

 petition for mates (through combat between males or through choice exercised 

 by females) determines success in mating and hence perpetuates some charac- 

 teristics at the expense of others. This is a subsidiary theory with which we shall 

 not further concern ourselves. 



