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. 
Tue 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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