144 THE LESSON OF EVOLUTION 



Thus use, although it cannot originate, is a cause 

 of definite variation, and Bering's theory explains 

 the opinion held by Lamarck and his followers , that 

 use causes organs to be developed. 



To judge of the importance of the principle of 

 use-inheritance we must resort to observation ; and 

 here we find considerable difficulty in distinguishing 

 its action from that of natural selection ; for both 

 develop and preserve useful characters only. The 

 great characteristic of natural selection is the diver- 

 sity of its action ; the same end being obtained by 

 various means. Whereas, in use-inheritance the 

 same cause ought to produce the same effect , unless 

 it be hindered by the internal forces of heredity. 



For instance, it is thought by Professor Henslow 

 that the irritation produced by insects when they 

 visit flowers is largely a cause why ordinary actino- 

 morphic flowers have become zygomorphic, or 

 bilateral. But bilateral symmetry is often useful, 

 by forcing the insect to assume a certain position 

 when abstracting the nectar from the flower, and 

 so may have been produced by natural selection ; 

 while it is difficult to see how the hap-hazard visits 

 of insects to actinomorphic flowers could have ori- 

 ginated zygomorphism, for they would alight quite 

 irregularly. The long legs of some dipterous insects 

 could not possibly be due to use, for they are de- 

 veloped in the pupa stage, before they have been 

 used, and do not alter in any way after the perfect 

 insect is produced. Also in many cases we find that 

 the constant use of an organ does not develop it. 

 The short wings of the penguin are constantly in 



