The Making of Species 



to a builder, and variations to his materials. 

 The kind of building that a builder can construct 

 depends very largely on the material supplied to 

 him. The Forth Bridge could not have been 

 built had those who constructed it had no material 

 given them but bricks and mortar. Wallaceians 

 regard natural selection as a builder who is sup- 

 plied with every kind of building material stone, 

 bricks, wood, iron, aluminium, in any quantities 

 he may desire. They therefore regard natural 

 selection as the one and only cause which deter- 

 mines evolution. This, however, is a wrong 

 idea. Natural selection should rather be likened 

 to a builder who is supplied with a limited variety 

 of building materials, so that considerable restric- 

 tions are imposed on his building operations. 

 The doors, windows, fireplaces, etc., are supplied 

 to him ready-made. He merely selects which of 

 these he will use for each building. 



The third factor of evolution which we have 

 considered is sexual selection. As we have seen, 

 sufficient attention has not been paid to this sub- 

 ject, so that we are not yet in a position to say 

 how much, if any, influence it has exercised on 

 the course of evolution. 



In addition to these three factors, there are, 

 we believe, some others. Before proceeding to 

 a consideration of these, it is important to study 

 carefully the modus operandi of natural selection, 

 or, in other words, the nature of the struggle for 



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