262 BIOLOGY: GENERAL AND MEDICAL 



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 

 two chief factors, which he calls "natural selection'' and 

 "sexual selection." The former, which he describes as 

 the "struggle for existence," he finds to be identical with 

 Herbert Spencer's doctrine of the "Survival of the 

 fittest." 



Darwin begins by a consideration of the various 

 "breeds" of domestic animals and shows that from 

 a few primitive stocks the many varieties of domestic 

 animals have been cultivated by artificial selection. He 

 points out that among animals there are slight variations 

 in the direction of desirability and undesirability, and 

 that by carefully conserving the desirable and eliminat- 

 ing the undesirable, man has been able to produce the 

 various kinds of cattle, sheep, hogs, horses, dogs, rabbits, 

 fowls, pigeons, etc., so well known to us. If it is to be 

 conceived that natural selection is analogous to artificial 

 selection, it is first necessary to admit that living things 



