CHAPTER IX 



EVOLUTION AS THE RESULT OF EXTERNAL AND 

 INTERNAL FACTORS 



We come now to a consideration of other theories that 

 have been advanced to account for the evolution of new 

 forms ; and in so far as these new forms are adapted to 

 their environment, the theories will bear_ directly on the 

 question of the origin of adaptive variations. One school 

 of transformationists has made the external world and the 

 changes taking place in it the source of new variations. 

 Another school believes that the changes arise within the 

 organism itself. We may examine these two points of view 

 in turn. 



The Effect of External Influences 



We have already seen that Lamarck held as a part of his 

 doctrine of transformation that the changes in the external 

 world, the environment, bring about, directly, changes in the 

 organism, and he believed that all plants and many of 

 the lower animals have evolved as the result of a reaction of 

 this sort. This idea did not originate with Lamarck, how- 

 ever, since before him Buffon had advanced the same hy- 

 pothesis, and there cannot be much doubt that Lamarck 

 borrowed from his patron, Buffon, this part of his theory 

 of evolution. 



This idea of the influence of the external world as a factor 

 inducing changes in the organism has come, however, to be 

 associated especially with the name of Geoffroy Saint-Hilaire, 

 whose period of activity, although overlapping, came after 



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