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ORGANIC EVOLUTION 



simultaneously with that of Wallace before 

 the Linnean Society in London. The fol- 

 lowing year, 1859, he published his classi- 

 cal work, On the Origin of Species by 

 Means of Natural Selection, in which he 

 formulated his theory backed up by abun- 

 dant evidence. Even though both men had 

 arrived at similar explanations of evolution, 

 the credit has gone to Darwin, as it justly 

 should. 



Darwin's book convinced the scientific 

 world of the soundness of evolution and 

 his theory of natural selection provided 

 food for thought for those interested in 

 how it came about. Because the book was 

 written in a manner comprehensible to 

 laymen and because it was staunchly de- 

 fended by such eminent biologists as T. H. 

 Huxley, the dieory of evolution came to be 

 generally accepted by the intelligent world. 

 Natural selection as an explanation as to 

 how evolution came about was immedi- 

 ately attacked by scientists and has con- 

 tinued under fire up to the present, al- 

 though today it is generally accepted, with 

 modifications, as the most tenable of the 

 explanations advanced thus far. 



THE THEORY OF NATURAL 

 SELECTION 



Both Darwin and Wallace had been 

 stimulated by Robert Malthus (1766-1834) 

 for the central theme which culminated in 

 the theory of natural selection. Malthus 

 had written his "Essay on Population" in 

 1798 in which he argued that since human 

 beings reproduce at a rate far outstripping 

 the food supply, there must be positive 

 checks such as wars, disease, and famines 

 to keep them from overrunning the world. 

 This by its very nature involves a "struggle 

 for existence." Darwin seized upon this 

 phrase as a possible explanation for the ori- 

 gin of new species which is the heart of 

 evolution. He reasoned that favorable traits 

 would be preserved and unfavorable ones 

 would be destroyed, thus paving the way 



for the origin of new species. Witii this 

 kernel as a start, Darwin, as well as Wal- 

 lace, developed the theory of natural selec- 

 tion. 



In essence, Darwin's theory of evolution 

 or origin of species through natural selec- 

 tion involves the following steps in reason- 

 ing which are borne out by fact. 



First, he considered variation an innate 

 property of protoplasm because he encoun- 

 tered it in all groups of plants and animals 

 (Fig. 26-2). Any species of living thing 

 varies widely, a fact that he could not 

 account for but for which we have since 

 found an explanation, to be considered 

 presently. This variation within a species 

 provides raw material with which evolu- 

 tion can proceed. Without it there could be 

 no evolution. 



Second, since each organism produces 

 many more offspring than can possibly 

 survive and yet populations remain fairly 

 constant, there must be a continual "strug- 

 gle for existence" within and between 

 groups as well as with the environment. 

 Out of this would come diose best suited 

 to survive under the existing conditions. 

 This could be a passive struggle, such as 

 plants and animals resisting the desicca- 

 tion of desert regions; or it could be ex- 

 tremely active, as in predation, that is, one 

 feeding on the other. In any case, only the 

 most fit would survive, and for this Darwin 

 coined the phrase, "survival of the fittest." 

 Thus the struggle for existence has a selec- 

 tive effect in removing the unfit and pre- 

 serving the fit. Moreover, since only the 

 most fit survive, they alone can perpetuate 

 the traits that made them best suited and 

 hence pass them on to their offspring. This, 

 continuing under changing circumstances 

 of environment, would result in wider and 

 wider differences between organisms, ulti- 

 mately resulting in new species originating. 



It is possible that the changing environ- 

 mental conditions at the extremes of a large 

 area occupied by a single species might be 

 such as to bring about variations around 



