CHAPTER SIX 



The History of 

 Evolutionary Thought 



It is not proposed here to review the whole history of evokitionary 

 thought. This subject is well covered in works on the historv of biology. 

 The present objective is simply to review some of the main trends in evo- 

 lutionary thought in recent times. 



LAMARCK 



The only really important pre-Darwinian student of evolution was La- 

 marck ( Figure 26 ) , a French biologist ( 1744-1829 ) who began his career 

 as a botanist, but became a zoologist when he was offered an appointment 

 in zoology at the jardin des Plantes (an institute of general biology, in 

 spite of its name). Lamarck's services to general zoology are manifold, 

 althou2;h his name is usuallv associated with an outmoded thcorv of evo- 

 lution. His studies of invertebrates were extensive, and resulted in a 

 greatly improved classification, including the recognition of the inverte- 

 brates and the vertebrates as distinct sections of the Animal Kino;dom. 

 He came very close to the cell theory thirtv-nine years before Schleiden 

 and Schwann formulated it. 



Lamarck's systematic studies convincc^d him that species were not con- 

 stant, but rather were derived from pre-existing species. To account for 

 this, he devised an elaborate theory which may be summarized in four 

 propositions: (1) Living organisms and their component parts tend con- 

 tinuallv to increase in size. (2) Production of a new organ results from 

 a new need and from the new movement which this need starts and main- 

 tains. (3) If an organ is ustul constantb% it will tcMid to become higliK' 

 dc\eloj:)ed, whereas disuse results in degcMieration. (4) Modifications pro- 

 duced by the above principles during the lifetime of an indixidual will 

 be inlierited by its ofispring, w itli llie result that changes are cumulative 

 over a period of time. 



Lamarck first published his tlu^orv in 1802, and he defended it vigor- 

 ously until his death. l*"or it, he suffered both social and scientific ostra- 

 cism, but he had the courage of his convictions. He failed to convince his 



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