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 ina 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 operand: of natural selection, 
or, in other words, the nature of the struggle for 
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