400 EVOLUTION AND GENETICS 



Summary. The theory of natural selection is the most impor- 

 tant of the great theories of evolutionary method. It is based 

 on the fundamental principles of variation and influences tending 

 to favor the development of a limited range of the variations of a 

 given species. These influences may be either spatial isolation or 

 the effects of overproduction and resultant crowding. Crowding 

 brings organisms into competition in which useful variations aid 

 in the preservation of the individuals possessing them and harmful 

 variations hasten destruction. The result is the preservation of a 

 limited number of individuals with some uniformity which is 

 preserved in their offspring, and the range of variation in the 

 species is thus shifted to that of the surviving individuals. Darwin 

 interpreted this as a cumulative process which would ultimately 

 bring about a greater development of the characters preserved. 

 Various objections to the theory have been proposed and many 

 of them have been satisfactorily answered, but they lead to the 

 conclusion that natural selection is an effective process of adapta- 

 tion rather than of species-formation. 



REFERENCES 



Wallace, A. R., Contributions to the Theory of Natural Selection, 1870. 



MivART, St. G., On the Genesis of Species, 1871. 



Darwin, C, The Origin of Species, 6th edition, 1880. 



Romanes, G. J., Darwin and After Darwin, 1892. 



Morgan, T. H., Evolution and Adaptation, 1903. 



Kellogg, V. L., Darwinism Today, 1907. 



Lull, R. S., Organic Evolution, 1917. 



Newman, H. H., Readings in Evolution, Genetics and Eugenics, 1921. 



