The Making of Species 
Darwin paid some attention to the subject. He 
believed that both internal and external causes 
contribute to variation, that variations tend to 
be inherited whether the result of causes within 
the organism or outside it. He believed that 
the inherited effect of use and disuse was a cause 
of variation, and cited, as examples, the lighter 
wing-bones and heavier leg-bones of the domestic 
duck and the drooping ears of some domestic 
animals. He supposed that animals showed a 
greater tendency to vary when under domestica- 
tion than when in their natural state, attributing 
the supposed greater variability to the excess 
of food received, and the changed conditions of 
the life of domestic animals. Nevertheless, he 
was fully alive to the fact that ‘nearly similar 
variations sometimes arise under, as far as we 
can judge, dissimilar conditions; and, on the 
other hand, dissimilar variations arise under 
conditions which appear to be nearly uniform.” 
In other words, the nature of organisms appeared 
to Darwin to be a more important factor in the 
origin of variations than external conditions. 
Evidence of this is afforded by the fact that 
some animals are more variable than others. 
Finally, he frankly admitted how great was his 
ignorance of the causes of variability. Varia- 
bility is, he stated, governed by unknown laws 
which are infinitely complex. 
It will be convenient to deal with each of 
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