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TEXTBOOK OF ZOOLOGY 



Charles Darwin (1809-1882), an Englishman, made extensive 

 studies on the problem of the manner and means by which new 

 species of organisms arise. He very effectively developed the thesis 

 that they originate by a process of natural selection. This was based 

 on the idea that no two individuals are exactly alike, that new varia- 

 tions are constantly appearing, and finally that those individuals 

 or groups best suited to their environment would be the ones to 

 persist and produce progeny. His conception of the factors and 



P^ig. 



13. — Agassiz (1807-1873), great American pioneer zoologist. (From Locy, 

 Biology and Its Makers, published by Henry Holt and Company, Inc.) 



limitations determining the development of new species, pictures a 

 constant struggle for existence among organisms, with those whose 

 natural variations happen to fit them best to the changing features 

 of the environment persisting as dominant species and others being 

 crowded out. Those least fitted to the environment would naturally 

 become extinct. 



Darwin did not claim originality in his idea. Lamarck, Buffon, 

 and Erasmus Darwin, grandfather of Charles, had presented similar 



