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 

 60 



