How Can We Explain Changes in the Earth's Population? 



Facts and Explanations According to the records preserved in the 

 rocks, in coal mines, in tar pits, and elsewhere, there can be no doubt that the 

 species of animals and plants which inhabited the earth in the past were suc- 

 ceeded through the ages by different species. This is as truly a matter oi fact 

 as the statement that the men, women and children in a given town have 

 changed in the past ten years, even if the total number remains the same. 

 But while people of ordinary intelligence can grasp and accept the facts, it 

 has been difficult to find an explanation that would satisfy everybody. 



We saw that Georges Cuvier, who helped to establish the historical fact 

 that life forms have changed through the ages, thought of the succession of 

 species as discontinuous (see page 447). That is, he believed that as existing 

 forms were destroyed in one or another cataclysm, other species were created 

 to take their places. 



A different type of explanation was offered by an older contemporary of 

 Cuvier's, the French zoologist and philosopher Jean Baptiste Lamarck (1744- 

 1829). According to Lamarck's view, life has been continuous from the begin- 

 ning, regardless of how living things came to be in the first place. Descendants 

 differ from their ancestors; and when the deviation has reached a certain 

 degree, there is a new species. 



These two types of explanation account for the same facts. Both can be 

 supported by good arguments and by further facts. On the whole, however, 

 the theory that there has been modification with co72tinuity of descent fits in 

 with more kinds of known facts and calls for fewer special assumptions. This 

 theory does not, of course, answer the question How did living beings originate 

 in the first place? Nor does it, of itself, tell us just how modifications have 

 taken place. But it does lend itself better to further experimental exploration 

 than the theory of a new special creation for each species. 



Haifa century later, when explorers and investigators had brought together 

 vast numbers of observations about plants and animals in all parts of the 

 world, the question was again brought into violent controversy by the Eng- 

 lish naturalist Charles Darwin (1809-1882). Darwin, Lamarck and Cuvier all 

 agreed that the living species of today are different from their predecessors 

 of ancient times. But according to Cuvier, the predecessors were not the 

 ancestors, whereas Lamarck and Darwin emphasized that "like begets like", 

 and thought therefore that the succession of forms was continuous. That is, 

 they declared the present species to be descendants of earlier ones, with modi- 

 fications. Yet Darwin and Lamarck did not agree as to how the modifications 

 had come about. 



Lamarck's Explanation Lamarck based his explanations on two familiar 

 sets of facts: (1) As everybody knowS; the development of an organism is in- 



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