PROBLEM 4. Theories about the Origin of Kivds of Living Things 559 



Fig. 508 The okapi {close rehuive 

 giraffe) feeds fro?/! the lower branches of trees 

 in the forest. Note its short neck. (American 



MUSEUM OF NATURAL HISTORY) 



Darwin's theory summed up. Accord- 

 in(T to Darwin's tiieorv: Animals and 



D 



plants normally produce more offspring 

 than there is room for. Since there is 

 overproduction, there is naturally a 

 struggle for existence. In this struggle 

 the fittest survive. There will always be 

 some organisms that are more fit than the 

 others because no two organisms are 

 alike; in other words there is variation. In 

 this way organisms are "naturally" se- 

 lected for survival. That is why Darwin 

 called his theory the theory of natural 

 selection. If, however, the theory is to 

 explain how new species arise, one must 

 assume that these variations are inherited. 

 To review what you have read do Exer- 

 cise 5. 



De Vries modified Darwin's theory. 

 When De Vries found that certain varia- 

 tions (mutations) can be inherited, he 

 fitted this idea into Darwin's earlier 

 theory. According to the De Vries 

 theory, there is a struggle for existence 



Fig. 509 The giraffe eats leaves froitt higher 

 branches of trees. How ?night De Vries have 

 explained the origin of the giraffe's long neck? 



(AMERICAN MUSEUM OF NATURAL HISTORY) 



because most organisms tend to repro- 

 duce in enormous numbers. When a 

 mutation occurs which makes the or- 

 ganism more fit than others of its spe- 

 cies, this mutant will be more likely to 

 survive. Since the mutation can be in- 

 herited, the desirable change will con- 

 tinue. Thus, after long periods of time, 

 new species become established. To test 

 your understanding, do Exercise 6. 



As modified by De Vries, the theory 

 of natural selection is acceptable to most 

 biologists. There are still many unsolved 

 questions about many of the details of 

 natural selection and even about the im- 

 portance of the various factors that Dar- 

 win listed. Most biologists today believe 

 that species originate by natural selec- 

 tion of individuals that vary in heredi- 

 tary characteristics. 



Lamarck had a theory which is not gen- 

 erally accepted. The Chevalier de La- 

 marck in 1809, long before Darwin's 

 time, offered one explanation for the 



