CHAPTER XXII 

 NATURAL SELECTION 



Many theories of varying importance represent our attempts 

 to explain the evident relationship of organisms and their apparent 

 origin from a common source. Some of these theories were for- 

 mulated without knowledge of some of the important biological 

 principles which science now makes available for their evaluation. 

 It is therefore important to bring to the study of evolutionary 

 theory a sound understanding of the facts of organic relationship 

 presented in the foregoing chapters. 



The theories represent, in the main, two general tendencies 

 which are foreshadowed by the two scientists, Lamarck and 

 Darwin. The former placed emphasis on the environment as a 

 factor in evolution, the latter upon inherent qualities. There is 

 less of a gulf between their views than is indicated by this common 

 treatment, but the fact remains that they have been regarded as 

 the exponents of opposed schools of thought. 



Origin of the Theory of Natural Selection. No theory has 

 played a greater part, and none perhaps contains a greater measure 

 of truth than the theory of natural selection which was formulated 

 almost simultaneously by Charles Darwin and Alfred Russell 

 Wallace and later developed by Darwin in his famous book. The 

 Origin of Species. It was suggested to both men, strangely enough, 

 by Malthus' ideas of overproduction in the human race. It is 

 not to be supposed, however, that this alone was responsible, for 

 the application of the principle to plants and animals and its 

 elaboration to explain the origin of species with their infinite 

 diversity required knowledge of a wide range of scientific facts. 

 Both Darwin and Wallace were well informed in the field of natural 

 science, and the range of material presented by Darwin in his 

 books relating to evolution is truly remarkable. 



Statement of the Theory. The theory of natural selection 

 is based on the fact that more individuals are produced in every 

 species than can survive, and on the occurrence of useful charac- 

 ters in the normal range of variation. It assumes that overpro- 



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