DIVERGENCE 289 



an essay "On the Law which has Regulated the Intro- 

 duction of New Species." In February, 1858, he wrote 

 a famous essay " On the Tendency of Varieties to Depart 

 Indefinitely from the Original Type." The appearance 

 of this essay led to the publication of a preliminary 

 essay by Darwin, and the papers of both authors were 

 published in the Proceedings of the Linnean Society of 

 London, August, 1858. To the credit of Darwin it 

 should be said that, finding himself anticipated by a 

 friend, he expressed his complete willingness to withdraw 

 from the field, but was dissuaded from pursuing this 

 course by the friends who knew the wealth and value 

 of the material he had collected. Wallace parallels 

 Darwin in discussing the nature of varieties, the struggle 

 for existence, the law of perpetuation, of useful and use- 

 less variations, and the partial reversion of domesticated 

 varieties, but though his writings contain the same 

 fundamental thoughts, Wallace did not support them 

 with the cogency and thoroughness of Darwin and so 

 has been eclipsed by the greater light. 



Darwin taught that the origin of species depends 

 upon a number of factors which may be summarized as 

 follows: 



1. Over-production. All plants and animals pro- 

 duce more offspring than can possibly survive 

 under natural conditions. 



2. Variation. Among the offspring no two are 

 precisely alike, and often there are striking dis- 

 similarities, which may or may not be advanta- 

 geous. 



3. Struggle for existence. Life is a continual con- 

 test or struggle in which the strongest or "fittest" 

 survive. Among the variations of every species, 



. those better adapting the individual to succeed 

 in the struggle for existence will tend to be per- 

 petuated, those unfitting him for the struggle, 

 will bring about extinction, so the species will 

 tend to vary and new species gradually arise. 



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